atx motherboard size in cm. Microatx dimensions

To date, there are four prevailing motherboard sizes - AT, ATX, LPX and NLX. In addition, there are smaller versions of the AT (Baby-AT), ATX (Mini-ATX, microATX) and NLX (microNLX) formats. Moreover, an extension to the microATX specification has recently been released, adding a new form factor, FlexATX, to this list. All these specifications, which determine the shape and size of motherboards, as well as the location of components on them and the features of cases, are described below.

AT

The AT form factor is divided into two modifications that differ in size - AT and Baby AT. A full-size AT board is up to 12" wide, which means that such a board is unlikely to fit in most of today's cases. Mounting such a board will most likely be hindered by a drive bay and hard drives and power supply. In addition, the location of the board components at a large distance from each other can cause some problems when working on large clock speeds. Therefore, after motherboards for the 386 processor, this size is no longer found.

Thus, the only AT-form-factor motherboards available on the market are the Baby AT-formatted motherboards. The size of the Baby AT board is 8.5" wide by 13" long. In principle, some manufacturers may reduce the length of the board to save material or for some other reason. Three rows of holes are made in the board to fix the board in the case.

All AT boards have common features. Almost all have serial and parallel ports attached to the motherboard via connector brackets. They also have one keyboard connector soldered onto the board at the back. The processor socket is installed on the front side of the board. The SIMM and DIMM slots are in different locations, although they are almost always located on the top of the motherboard.

Today, this format is slowly disappearing from the scene. Some companies still release some of their models in two versions - Baby AT and ATX, but this happens less and less. Moreover, more and more new features provided by operating systems are implemented only on ATX motherboards. Not to mention just the convenience of work - for example, most often on Baby AT boards all the connectors are assembled in one place, as a result of which either cables from the communication ports stretch almost through the entire motherboard to the back of the case, or from the IDE and FDD ports to the front. Sockets for memory modules, calling almost under the power supply. With limited freedom of action inside a very small space MiniTower, this is, to put it mildly, inconvenient. In addition, the issue of cooling was unsuccessfully resolved - the air does not flow directly to the part of the system that needs to be cooled - the processor.

LPX

Even before the advent of ATX, the first result of attempts to reduce the cost of PCs was the LPX form factor. Designed for use in Slimline or Low-profile cases. The problem was solved by a rather innovative proposal - the introduction of a rack. Instead of plugging expansion cards directly into the motherboard, this option puts them in a vertical rack that connects to the board, parallel to the motherboard. This made it possible to significantly reduce the height of the case, since usually it is the height of expansion cards that affects this parameter. The payback for compactness was the maximum number of connected cards - 2-3 pieces. Another innovation that has begun to be widely used on LPX boards is a video chip integrated on the motherboard. Case size for LPX is 9 x 13" and for Mini LPX is 8 x 10"".

After the advent of NLX, LPX began to be supplanted by this form factor.

ATX

Not surprisingly, the ATX form factor in all its modifications is becoming increasingly popular. This is especially true for boards for processors on the P6 bus. So, for example, out of LuckyStar motherboards for these processors, which are being prepared for release this year, 4 will be made in the Mini-ATX format, 3 - ATX, and only one - Baby AT. And if we also take into account that there are much fewer motherboards for Socket7 today, if only because of the much smaller number of new chipsets for this platform, then ATX wins a convincing victory.

And no one can say that it is unfounded. The ATX specification, proposed by Intel back in 1995, is aimed precisely at correcting all those shortcomings that have emerged over time in the AT form factor. And the solution, in fact, was very simple - rotate the Baby AT board 90 degrees, and make appropriate adjustments to the design. By that time, Intel already had experience in this area - the LPX form factor. The best aspects of both Baby AT and LPX were embodied in ATX: extensibility was taken from Baby AT, and high integration of components was taken from LPX. Here is the result:

• Integrated I/O port connectors. On all modern boards, I/O port connectors are present on the board, so it seems quite natural to place their connectors on it, which leads to a rather significant reduction in the number of connecting wires inside the case. In addition, at the same time, among the traditional parallel and serial ports, a keyboard connector, there was a place for beginners - PS / 2 and USB ports. In addition, as a result, the cost of the motherboard has slightly decreased due to the reduction of cables in the kit.
• Significantly increased ease of access to memory modules. As a result of all the changes, the memory module slots have moved further away from the motherboard slots, from the processor and power supply. As a result, increasing the memory has become in any case a matter of minutes, while on Baby AT motherboards sometimes you have to take a screwdriver.
• Reduced distance between board and disks. The connectors of the IDE and FDD controllers have moved almost close to the devices connected to them. This allows you to reduce the length of the cables used, thereby increasing the reliability of the system.
• Separation of the processor and slots for expansion cards. The processor socket has been moved from the front of the board to the back, next to the power supply. This allows you to install full-size boards in expansion slots - the processor does not interfere with them. In addition, the problem with cooling was solved - now the air sucked in by the power supply blows directly over the processor.
• Improved interaction with the power supply. Now one 20-pin connector is used, instead of two, as on AT boards. In addition, the ability to manage motherboard power supply - switching on right time or upon the occurrence of a certain event, the ability to turn on from the keyboard, turn off operating system, etc.
• Voltage 3.3 V. Now the 3.3 V supply voltage, which is very widely used by modern system components (take PCI cards, for example!) comes from the power supply unit. In AT-boards, a stabilizer installed on the motherboard was used to obtain it. On ATX boards, it is not needed.

The specific size of motherboards is described in the specification largely based on the convenience of developers - from a standard plate (24 x 18'') you get either two ATX boards (12 x 9.6''), or four - Mini-ATX (11.2 x 8.2'') . By the way, compatibility with old cases was also taken into account - the maximum width of an ATX board, 12 '', is almost identical to the length of AT boards, so that it is possible to use an ATX board in an AT case without much effort. However, today it is more related to the field of pure theory - the AT case still needs to be managed to be found. Also, to the extent possible, the mounting holes in the ATX board fully correspond to the AT and Baby AT formats.

microATX

The ATX form factor was developed back in the heyday of Socket 7 systems, and much of it today is somewhat out of date. For example, a typical slot combination, based on which the specification was compiled, looked like 3 ISA / 3 PCI / 1 adjacent. Somewhat irrelevant not today, right? ISA, no AGP, AMR, etc. Again, 7 slots are not used 99 percent of the time anyway, especially today with chipsets such as the MVP4, SiS 620, i810, and other upcoming products like this. In general, for cheap PC ATX is a waste of resources. Based on such considerations, in December 1997, the specification of the microATX format was presented, a modification of the ATX board, designed for 4 slots for expansion cards.

In fact, the changes, compared to ATX, were minimal. The board size has been reduced to 9.6 x 9.6” so that it is completely square, and the size of the power supply has also been reduced. The block of I/O connectors has remained unchanged, so the microATX board can be used in an ATX 2.01 case with minimal modifications.

NLX

Over time, the LPX specification, like Baby AT, ceased to meet the requirements of the times. New processors came out, new technologies appeared. And she was no longer able to provide acceptable spatial and thermal conditions for new low-profile systems. As a result, just as the Baby AT was replaced by the ATX, just as in 1997, as the development of the LPX idea, taking into account the emergence of new technologies, the NLX form factor specification appeared. A format aimed at low-profile cases. Its creation took into account both technical factors (for example, the advent of AGP and DIMM modules, the integration of audio / video components on the motherboard) and the need to provide greater serviceability. So, for assembly / disassembly of many systems based on this form factor, a screwdriver is not required at all.

As you can see in the diagram, the main features of the NLX motherboard are:

• Rack for expansion cards, located on the right edge of the board. Moreover, the motherboard can be freely detached from the rack and pulled out of the case, for example, to replace the processor or memory.
• The processor is located in the left front corner of the board, directly opposite the fan.
• Generally, a grouping of tall components like the processor and memory at the left end of the board to allow for full size expansion cards to be stacked on the rack.
• Single-height (in the area of ​​expansion cards) and double-height I/O connector blocks at the rear end of the board to accommodate the maximum number of connectors.

In general, the stand is a very interesting thing. In fact, this is one motherboard, divided into two parts - the part where the actual system components, and a part connected to it via a 340-pin connector at an angle of 90 degrees, where all kinds of input / output components are located - expansion cards, port connectors, data drives, where power is connected. Thus, firstly, serviceability is increased - there is no need to access unnecessary this moment components. Secondly, as a result, manufacturers have greater flexibility - they make one model of the main board, and a rack for each specific customer, with the integration of the necessary components on it.

In general, does this description remind you of anything? A motherboard-mounted rack that carries some I/O components instead of being integrated onto the motherboard, all of which serves to simplify maintenance, give manufacturers more flexibility, etc.? That's right, some time after the release of the NLX specification, the AMR specification appeared, describing a similar ideology for ATX boards.

Unlike other rather strict specifications, NLX provides manufacturers with much greater freedom in making decisions. NLX motherboard sizes range from 8 x 10″ to 9 x 13.6″. An NLX package must be able to handle both these two formats and everything in between. Usually, boards that fit into the minimum dimensions are designated as Mini NLX. It is also worth mentioning an interesting detail: the NLX case has USB ports located on the front panel - very convenient for identification solutions like e.Token.

It remains only to add that, according to the specification, some places on the board must remain free, providing opportunities for expanding the functions that will appear in future versions of the specification. For example, to create motherboards for servers and workstations based on the NLX form factor.

WTX

However, on the other hand, powerful workstations and servers of the AT and ATX specifications are also not entirely satisfactory. There are problems where the cost plays not the most important role. At the forefront are the provision of normal cooling, the placement of large amounts of memory, convenient support for multiprocessor configurations, a large power supply, the placement of more ports for storage controllers and I / O ports. Thus, in 1998, the WTX specification was born. Designed to support dual processor motherboards of all configurations, support for today's and tomorrow's video card and memory technologies.

Particular attention, perhaps, should be given to two new components - Board Adapter Plate (BAP) and Flex Slot.

In this specification, the developers tried to move away from the usual model, when the motherboard is attached to the case through mounting holes located in certain places. Here it is attached to the BAP, and the mounting method is left to the conscience of the board manufacturer, and the standard BAP is attached to the case.

Apart from the usual things like board dimensions (14 x 16.75""), power supply specifications (up to 850W), etc., the WTX specification describes the Flex Slot architecture - in a sense, AMR for workstations. Flex Slot is designed to improve serviceability, give more flexibility to developers, reduce motherboard time to market. The Flex Slot card looks something like this:

Such cards can accommodate any PCI, SCSI or IEEE 1394 controllers, sound, network interface, parallel and serial ports, USB, system monitoring tools.

Samples of WTX boards should appear around June, and production samples - in the third quarter of 1999.

FlexATX

Finally, just as the Baby AT and LPX evolved into the ATX, the microATX and NPX specifications evolved into the FlexATX form factor. This is not even a separate specification, but just an addition to the microATX specification. Looking at the success of the iMac, which is essentially nothing new but appearance and it wasn't, PC makers decided to go that route as well. And just Intel became the first, in February at the Intel Developer Forum it announced FlexATX - a motherboard with an area of ​​25-30 percent smaller than microATX.

Theoretically, with some modifications, the FlexATX board can be used in cases that comply with the ATX 2.03 or microATX 1.0 specifications. But for today's cases there are enough motherboards without it, it was just about elaborate plastic structures, where such compactness is needed. There, at IDF, Intel demonstrated several possible variants of such cases. The fantasy of the designers went wild - vases, pyramids, trees, spirals, which were not offered. A few turns from the specification to deepen the impression: "aesthetic value", "greater satisfaction in owning the system." Not bad for describing the form factor of a PC motherboard?

Flex - that's why it's flex. The specification is extremely flexible, and leaves many things to the manufacturer's discretion that were previously strictly described. So, the manufacturer will determine the size and placement of the power supply, the design of the I / O card, the transition to new processor technologies, methods for achieving low-profile design. In practice, only the dimensions are more or less clearly defined - 9 x 7.5 "". By the way, regarding new processor technologies - Intel at IDF demonstrated a system on a FlexATX board with a Pentium III, which until autumn is still declared only as Slot-1, and see for yourself in the photo, and the specification emphasizes that FlexATX boards are only for Socket processors...

And finally, another interesting revelation from Intel - in three years, in the following specifications, the power supply may even be located outside the PC case.

Introduction

An integral part of every computer is the power supply. It is as important as the rest of the computer. At the same time, the purchase of a power supply is quite rare, because. A good PSU can power several generations of systems. Given all this, the purchase of a power supply must be taken very seriously, since the fate of a computer is directly dependent on the operation of the power supply.

To implement galvanic isolation, it is enough to make a transformer with the necessary windings. But to power a computer, you need a lot of power, especially for modern PCs. To power a computer, a transformer would have to be made, which would not only be large in size, but also weigh a lot. However, with an increase in the frequency of the supply current of the transformer, to create the same magnetic flux, fewer turns and a smaller cross section of the magnetic circuit are needed. In power supplies built on the basis of a converter, the frequency of the transformer supply voltage is 1000 or more times higher. This allows you to create compact and lightweight power supplies.

The simplest switching power supply

Consider a block diagram of a simple switching power supply, which underlies all switching power supplies.

Block diagram of a switching power supply.

The first block converts the AC voltage to DC. Such a converter consists of a diode bridge that rectifies the alternating voltage, and a capacitor that smooths out the ripple of the rectified voltage. This bokeh also contains additional elements: mains voltage filters against pulse generator ripples and thermistors to smooth the current surge at the moment of switching on. However, these elements may be omitted in order to save on cost.

The next block is a pulse generator that generates pulses at a certain frequency that feed the primary winding of the transformer. The frequency of the generating pulses of different power supplies is different and lies in the range of 30 - 200 kHz. The transformer performs the main functions of the power supply: galvanic isolation from the network and lowering the voltage to the required values.

The alternating voltage received from the transformer is converted by the next block into direct voltage. The block consists of voltage rectifying diodes and a ripple filter. In this block, the ripple filter is much more complex than in the first block and consists of a group of capacitors and a choke. In order to save money, manufacturers can install small capacitors, as well as chokes with low inductance.

First impulse block power supply was a push-pull or single-cycle converter. Push-pull means that the generation process consists of two parts. In such a converter, two transistors open and close in turn. Accordingly, in a single-cycle converter, one transistor opens and closes. Schemes of push-pull and single-cycle converters are presented below.

Schematic diagram of the converter.

Consider the elements of the scheme in more detail:

X2 - circuit power supply connector.

X1 - the connector from which it is removed output voltage.

R1 - resistance that sets the initial small offset on the keys. It is necessary for a more stable start of the oscillation process in the converter.

R2 is the resistance that limits the base current on the transistors, this is necessary to protect the transistors from burning.

TP1 - The transformer has three groups of windings. The first output winding generates the output voltage. The second winding serves as a load for the transistors. The third one forms the control voltage for the transistors.

At the initial moment of switching on the first circuit, the transistor is slightly ajar, because. A positive voltage is applied to the base through resistor R1. A current flows through the ajar transistor, which also flows through the second winding of the transformer. The current flowing through the winding creates a magnetic field. The magnetic field creates voltage in the remaining windings of the transformer. As a result, a positive voltage is created on winding III, which further opens the transistor. The process continues until the transistor enters saturation mode. The saturation mode is characterized by the fact that as the applied control current to the transistor increases, the output current remains unchanged.

Since the voltage in the windings is generated only in the event of a change in the magnetic field, its growth or fall, the absence of an increase in current at the output of the transistor, therefore, will lead to the disappearance of the EMF in the windings II and III. Loss of voltage in the winding III will lead to a decrease in the degree of opening of the transistor. And the output current of the transistor will decrease, therefore, the magnetic field will also decrease. Reducing the magnetic field will create a voltage of opposite polarity. The negative voltage in winding III will begin to close the transistor even more. The process will continue until the magnetic field disappears completely. When the magnetic field disappears, the negative voltage in winding III will also disappear. The process will start repeating again.

A push-pull converter works on the same principle, but the difference is that there are two transistors, and they open and close in turn. That is, when one is open, the other is closed. The push-pull converter circuit has the great advantage of utilizing the entire hysteresis loop of the transformer's magnetic conductor. Using only one section of the hysteresis loop or magnetization in only one direction leads to many undesirable effects that reduce the efficiency of the converter and degrade its performance. Therefore, basically, a push-pull converter circuit with a phase-shifting transformer is used everywhere. In circuits where simplicity, small size, and low power are needed, a single-cycle circuit is still used.

ATX form factor power supplies without power factor correction

The converters discussed above, although they are finished devices, are inconvenient to use in practice. Converter frequency, output voltage and many other parameters “float”, change depending on the change: supply voltage, converter output load and temperature. But if the keys are controlled by a controller that could carry out stabilization and various additional functions, then you can use the circuit to power devices. The power supply circuit using a PWM controller is quite simple, and, in general, is a pulse generator built on a PWM controller.

PWM - pulse width modulation. It allows you to adjust the amplitude of the signal of the passed low-pass filter (filter low frequencies) with a change in the duration or duty cycle of the pulse. The main advantages of PWM are the high efficiency of power amplifiers and great application possibilities.

Scheme of a simple power supply with a PWM controller.

This power supply circuit has a low power and uses a field-effect transistor as a key, which allows you to simplify the circuit and get rid of additional elements necessary for control transistor switches. In power supplies high power The PWM controller has control elements ("Driver") for the output key. IGBT transistors are used as output keys in high-power power supplies.

The mains voltage in this circuit is converted into a constant voltage and fed through the key to the first winding of the transformer. The second winding is used to power the microcircuit and generate voltage feedback. The PWM controller generates pulses with a frequency that is set by the RC circuit connected to pin 4. The pulses are fed to the input of the key, which amplifies them. The duration of the pulses varies depending on the voltage on pin 2.

Consider a real ATX power supply circuit. It has a lot more elements and there are more additional devices in it. The red squares of the power supply circuit are conditionally divided into main parts.

ATX power supply circuit with a power of 150-300 watts.

To power the controller chip, as well as to generate a standby voltage of +5, which is used by the computer when it is turned off, there is another converter in the circuit. In the diagram, it is designated as block 2. As you can see, it is made according to the single-cycle converter circuit. The second block also has additional elements. Basically, these are surge absorption circuits that are generated by the converter transformer. Chip 7805 - the voltage regulator generates a standby voltage of + 5V from the rectified voltage of the converter.

Often, low-quality or defective components are installed in the standby voltage generation unit, which causes the frequency of the converter to decrease to the audio range. As a result, a squeak is heard from the power supply.

Since the power supply is powered by AC 220V, and the converter needs DC power, the voltage must be converted. The first block performs rectification and filtering of the alternating mains voltage. This block also contains a blocking filter against interference generated by the power supply itself.

The third block is the TL494 PWM controller. It performs all the basic functions of the power supply. Protects the power supply from short circuits, stabilizes the output voltage and generates a PWM signal to control transistor switches that are loaded on the transformer.

The fourth block consists of two transformers and two groups of transistor switches. The first transformer generates a control voltage for the output transistors. Since the TL494 PWM controller generates a low power signal, the first group of transistors amplifies this signal and passes it to the first transformer. The second group of transistors, or output ones, are loaded on the main transformer, which forms the main supply voltages. Such more complex scheme control of the output keys is used due to the complexity of controlling bipolar transistors and protecting the PWM controller from high voltage.

The fifth block consists of Schottky diodes that rectify the output voltage of the transformer, and a low-pass filter (LPF). The low-pass filter consists of electrolytic capacitors of considerable capacity and chokes. At the output of the low-pass filter there are resistors that load it. These resistors are necessary so that after turning off the capacitance of the power supply, they do not remain charged. There are also resistors at the output of the mains voltage rectifier.

The remaining elements that are not circled in the block are chains that form “serviceability signals”. These chains carry out the work of protecting the power supply from short circuit or monitoring the health of output voltages.

200W ATX power supply.

Now let's see how the elements are located on the printed circuit board of the 200 W power supply. The figure shows:

Capacitors that filter the output voltages.

Place unsoldered output voltage filter capacitors.

Inductors that filter output voltages. The larger coil not only plays the role of a filter, but also acts as a ferromagnetic stabilizer. This allows you to slightly reduce voltage distortions with uneven loading of various output voltages.

Chip PWM stabilizer WT7520.

A radiator on which Schottky diodes are installed for voltages + 3.3V and + 5V, and ordinary diodes for voltage + 12V. It should be noted that often, especially in older power supplies, additional elements are placed on the same radiator. These are voltage stabilization elements + 5V and + 3.3V. In modern power supplies, only Schottky diodes are placed on this radiator for all basic voltages or FETs, which are used as a rectifier element.

The main transformer, which performs the formation of all voltages, as well as galvanic isolation from the network.

A transformer that generates control voltages for the output transistors of the converter.

Converter transformer that generates standby voltage + 5V.

The radiator, on which the output transistors of the converter are located, as well as the transistor of the converter that forms the standby voltage.

Mains voltage filter capacitors. They don't have to be two. To form a bipolar voltage and form a midpoint, two capacitors of equal capacity are installed. They divide the rectified mains voltage in half, thereby forming two voltages of different polarity connected at a common point. In single supply circuits, there is only one capacitor.

Network filter elements from harmonics (interference) generated by the power supply.

Diode bridge diodes that rectify the AC voltage of the network.

350W ATX power supply.

The 350 W power supply is equivalent. Immediately striking is the large board, enlarged heatsinks and a larger converter transformer.

Output voltage filter capacitors.

A heatsink that cools the diodes that rectify the output voltage.

PWM controller AT2005 (similar to WT7520), which performs voltage stabilization.

The main transformer of the converter.

A transformer that generates a control voltage for the output transistors.

Standby voltage converter transformer.

A radiator that cools the output transistors of the converters.

Mains voltage filter from power supply interference.

diode bridge diodes.

Mains voltage filter capacitors.

The considered scheme has long been used in power supplies and is now sometimes found.

ATX format power supplies with power factor correction.

In the considered circuits, the load of the network is a capacitor connected to the network through a diode bridge. The charge of the capacitor occurs only if the voltage on it is less than the mains. As a result, the current is pulsed, which has many disadvantages.

Bridge voltage rectifier.

We list these shortcomings:

• currents introduce higher harmonics (interference) into the network;
• large amplitude of consumption current;
• a significant reactive component in the consumption current;
• mains voltage is not used during the entire period;
• The efficiency of such schemes is of little importance.

The new power supplies have an improved modern circuit, it has one more additional unit - a power factor corrector (PFC). It performs power factor improvement. Or more plain language removes some of the shortcomings of the mains voltage bridge rectifier.

Total power formula.

The power factor (KM) characterizes how much of the total power of the active component and how much of the reactive. In principle, we can say why take into account reactive power, it is imaginary and does not benefit.

Power factor formula.

Let's say we have a certain device, a power supply, with a power factor of 0.7 and a power of 300 watts. It can be seen from the calculations that our power supply has a total power (the sum of reactive and active power) more than indicated on it. And this power should be given by a 220V power supply network. Although this power is not useful (even the electricity meter does not fix it), it still exists.

Calculation of the total power of the power supply.

I.e internal elements and network wires should be rated for 430W, not 300W. And imagine the case when the power factor is equal to 0.1 ... Because of this, the City Network forbids the use of devices with a power factor of less than 0.6, and if any are found, the owner is fined.

Accordingly, the campaigns were developed new power supply circuits that had KKM. At first, a large inductance choke included at the input was used as a PFC, such a power supply is called a power supply with PFC or passive PFC. Such a power supply has an increased KM. To achieve the desired KM, it is necessary to equip the power supplies with a large choke, since the input resistance of the power supply is capacitive due to the installed capacitors at the rectifier output. Installing a throttle significantly increases the mass of the power supply, and increases the KM to 0.85, which is not so much.

400 W power supply with passive power factor correction.

The figure shows a 400W FSP power supply with passive power factor correction. It contains the following elements:

Rectified line voltage filter capacitors.

A choke that performs power factor correction.

Transformer of the main converter.

Transformer that controls the keys.

Auxiliary converter transformer (standby voltage).

Mains voltage filters from power supply ripples.

The radiator on which the output transistor switches are installed.

Radiator on which diodes are installed that rectify the alternating voltage of the main transformer.

Fan speed control board.

The board on which the FSP3528 PWM controller (analogous to KA3511) is installed.

Group stabilization inductor and output voltage ripple filter elements.

1. Output ripple filter capacitors.

Turn on the throttle to correct the KM.

Due to the low efficiency of the passive KKM, the power supply was introduced new scheme KKM, which is built on the basis of a PWM stabilizer loaded on a choke. This scheme brings many advantages to the power supply:

• extended operating voltage range;
• it became possible to significantly reduce the capacitance of the mains voltage filter capacitor;
• significantly increased CM;
• reduction in the weight of the power supply;
• increase the efficiency of the power supply.

This scheme also has disadvantages - this is a decrease in the reliability of the PSU and incorrect operation with some uninterruptible power supplies when switching battery / mains operating modes. The incorrect operation of this circuit with a UPS is due to the fact that the capacitance of the mains voltage filter has significantly decreased in the circuit. At the moment when the voltage disappears for a short time, the current of the KKM increases greatly, which is necessary to maintain the voltage at the output of the KKM, as a result of which the protection against short circuit (short circuit) in the UPS is activated.

Scheme of an active power factor corrector.

If you look at the circuit, then it is a pulse generator that is loaded on the inductor. The mains voltage is rectified by a diode bridge and supplied to the key, which is loaded with an L1 choke and a T1 transformer. The transformer is introduced for the feedback of the controller with the key. The voltage from the inductor is removed using diodes D1 and D2. Moreover, the voltage is removed alternately with the help of diodes, then from the diode bridge, then from the inductor, and charges the capacitors Cs1 and Cs2. Key Q1 opens and inductor L1 accumulates the energy of the desired value. The amount of accumulated energy is regulated by the duration of the open state of the key. The more energy stored, the more voltage the inductor will give. After turning off the key, the accumulated energy is returned by the inductor L1 through the diode D1 to the capacitors.

This operation allows you to use the entire sinusoid of the alternating voltage of the network, in contrast to circuits without PFC, and also to stabilize the voltage supplying the converter.

In modern power supply circuits, dual-channel PWM controllers are often used. One microcircuit performs the work of both the converter and the PFC. As a result, the number of elements in the power supply circuit is significantly reduced.

Scheme of a simple power supply on a two-channel PWM controller.

Consider a simple 12V power supply circuit using a dual-channel PWM controller ML4819. One part of the power supply generates a constant stabilized voltage + 380V. The other part is a converter that generates a constant stabilized voltage + 12V. KKM consists, as in the case considered above, of the key Q1, the inductor L1 of the feedback transformer T1 loaded on it. Diodes D5, D6 charge capacitors C2, C3, C4. The converter consists of two keys Q2 and Q3, loaded on the transformer T3. The impulse voltage is rectified by the diode assembly D13 and filtered by the inductor L2 and capacitors C16, C18. With the help of the cartridge U2, the output voltage regulation voltage is formed.

GlacialPower GP-AL650AA power supply.

Consider the design of the power supply, in which there is an active KKM:

1. Current protection control board;
2. Inductor, which acts as a voltage filter + 12V and + 5V, and the function of group stabilization;
3. Voltage filter choke +3.3V;
4. Radiator on which rectifier diodes of output voltages are placed;
5. Main Converter Transformer;
6. Transformer that controls the keys of the main converter;
7. Auxiliary converter transformer (forming standby voltage);
8. Power factor correction controller board;
9. Radiator, cooling diode bridge and keys of the main converter;
10. Line voltage filters against interference;
11. Choke power factor corrector;
12. Mains voltage filter capacitor.

Design features and types of connectors

Consider the types of connectors that may be present on the power supply. On the rear wall of the power supply is a connector for connecting a network cable and a switch. Previously, next to the power cord connector, there was also a connector for connecting the monitor's network cable. Other elements may optionally be present:

• indicators of mains voltage, or the status of the power supply;
• fan control buttons;
• button for switching the input mains voltage 110 / 220V;
• USB ports built into the USB hub power supply;
• other.

On the rear wall, less and less fans are placed, pulling air from the power supply. The entire fan bowl is placed at the top of the power supply due to the larger fan mounting space, allowing for a large and quiet active cooling element. On some power supplies, even two fans are installed both at the top and at the back.

Chieftec CFT-1000G-DF power supply.

A wire with a power connector for the motherboard comes out of the front wall. In some power supplies, modular, it, like other wires, is connected through a connector. The figure below shows the pinout of the pins of all the main connectors.

You can see that each voltage has its own wire color:

• Yellow color - +12 V,
• Red color - +5 V,
• Orange color - + 3.3V,
• Black is common or ground.

For other voltages, the colors of the wires for each manufacturer may vary.

The figure does not show the auxiliary power connectors for video cards, since they are similar to the auxiliary power connector for the processor. There are also other types of connectors that are found in brand-name computers from DelL, Apple, and others.

Electrical parameters and characteristics of power supplies

The power supply has many electrical parameters, most of which are not marked in the passport. On the side sticker of the power supply, usually only a few basic parameters are noted - operating voltages and power.

Power supply power

Power is often indicated on the label in large print. The power of the power supply characterizes how much it can give electrical energy to the devices connected to it (motherboard, video card, HDD and etc.).

In theory, it is enough to sum up the consumption of the components used and select a power supply unit with a slightly higher power for the reserve. To calculate the power, you can use, for example, the site http://extreme.outervision.com/PSUEngine, the recommendations indicated in the video card passport, if any, the thermal package of the processor, etc., are also quite suitable.

But in fact, everything is much more complicated, because. the power supply unit produces various voltages - 12V, 5V, -12V, 3.3V, etc. Each voltage line is designed for its own power. It was logical to think that this power is fixed, and their sum is equal to the power of the power supply. But there is one transformer in the power supply to generate all these voltages used by the computer (except for the standby voltage + 5V). True, it is rare, but you can still find a power supply with two separate transformers, but such power supplies are expensive and are most often used in servers. Ordinary ATX PSUs have one transformer. Because of this, the power of each voltage line can float: it increases if other lines are lightly loaded, and decreases if the other lines are heavily loaded. Therefore, the maximum power of each line is often written on the power supplies, and as a result, if they are summed up, the power will come out even more than the actual power of the power supply. Thus, the manufacturer can confuse the consumer, for example, by declaring too much rated power, which the PSU is not capable of providing.

Note that if an insufficient power supply is installed in the computer, this will cause non-radical operation of devices (“freezes”, reboots, clicks of hard disk heads), up to the impossibility of turning on the computer. And if a motherboard is installed in the PC, which is not designed for the power of the components that are installed on it, then the motherboard often functions normally, but over time, the power connectors burn out due to their constant heating and oxidation.

Burnt connectors.

Permissible maximum line current

Although this is one of important parameters power supply, often the user does not pay attention to it when buying. But when the permissible current on the line is exceeded, the power supply turns off, because. protection is triggered. To turn it off, turn off the power supply from the mains and wait for a while, about a minute. It is worth considering that now all the most voracious components (processor, video card) are powered by the + 12V line, so you need to pay more attention to the values ​​\u200b\u200bof the currents indicated for it. For high-quality PSUs, this information is usually placed in the form of a plate (for example, Seasonic M12D-850) or a list (for example, FSP ATX-400PNF) on the side sticker.

Power sources that do not have such information (for example, Gembird PSU7 550W) immediately cast doubt on the quality of performance and the conformity of the declared power with the real one.

The remaining parameters of the power supplies are not regulated, but no less important. It is possible to determine these parameters only by conducting various tests with the power supply.

Operating voltage range

Under the operating voltage range is meant the range of mains voltage values ​​at which the power supply unit maintains its performance and the values ​​of its passport parameters. Now more and more often power supplies are produced with AKKM (active power factor corrector), which allows you to expand the operating voltage range from 110 to 230. There are also power supplies with a small operating voltage range, for example, the FPS FPS400-60THN-P power supply has a range of 220 up to 240. As a result, this power supply, even when paired with a massive uninterruptible power supply, will turn off when the voltage drops in the network. This is because a conventional UPS stabilizes the output voltage in the range of 220V +/- 5%. That is, the minimum voltage for switching to the battery will be 209 (and given the slow switching of the relay, the voltage may be even lower), which is lower than the operating voltage of the power supply.

Internal resistance

Internal resistance characterizes the internal losses of the power supply when current flows. By type, internal resistance can be divided into two types: conventional for direct current and differential for alternating current.

The equivalent circuit of the power supply.

DC resistance is the sum of the resistances of the components that make up the power supply: wire resistance, transformer winding resistance, inductor wire resistance, circuit board track resistance, etc. Due to the presence of this resistance, the voltage drops with increasing workload of the power supply. This resistance can be seen by plotting the cross-load characteristic of the PSU. To reduce this resistance, various stabilization schemes work in power supplies.

Cross-load characteristic of the power supply.

Differential resistance characterizes the internal losses of the power supply when alternating current flows. This resistance is also called electrical impedance. Reducing this resistance is the most difficult. To reduce it, a low-pass filter is used in the power supply. To reduce the impedance, it is not enough to install large capacitors and high inductance coils in the power supply. It is also necessary that the capacitors have a low series resistance (ESR), and the chokes are made of thick wire. It is very difficult to implement this physically.

Output voltage ripple

The power supply is a converter that converts the voltage from AC to DC more than once. As a result, there are ripples at the output of its lines. Ripple is a sudden change in voltage over a short period of time. The main problem of ripples is that if the circuit or device does not have a filter in the power circuit or it is bad, then these ripples pass through the entire circuit, distorting its performance. This can be seen, for example, if you turn the volume of the speakers to the maximum during the absence of signals at the output sound card. Various noises will be heard. This is the ripple, but not necessarily the noise of the power supply. But if there is no great harm in the operation of a conventional amplifier from ripples, only the noise level will increase, then, for example, in digital circuits and comparators, they can lead to false switching or incorrect perception of input information, which leads to errors or inoperability of the device.

The form of the output voltages of the Antec Signature SG-850 power supply.

Voltage stability

Next, consider such a characteristic as the stability of the voltages produced by the power supply. In the course of work, no matter how ideal the power supply would be, its voltages change. An increase in voltage causes, first of all, an increase in the quiescent currents of all circuits, as well as a change in the parameters of the circuits. So, for example, for a power amplifier, increasing the voltage increases it output power. Some electronic parts may not withstand the increased power and burn out. The same increase in power leads to an increase in the power dissipated by electronic elements, and, consequently, to an increase in the temperature of these elements. Which leads to overheating and / or change in characteristics.

Reducing the voltage, on the contrary, reduces the quiescent current, and also degrades the characteristics of the circuits, such as the amplitude of the output signal. When it drops below a certain level, certain circuits stop working. Hard drive electronics are especially sensitive to this.

Permissible voltage deviations on the power supply lines are described in the ATX standard and should not exceed ± 5% of the line rating on average.

For a complex display of the magnitude of the voltage drop, a cross-load characteristic is used. It is a color display of the voltage deviation level of the selected line when two lines are loaded: selected and +12V.

Efficiency

Now let's move on to the coefficient of efficiency or abbreviated efficiency. Many remember from school - this is the attitude useful work to spent. Efficiency shows how much of the consumed energy has turned into useful energy. The higher the efficiency, the less you have to pay for the electricity consumed by the computer. Most high-quality power supplies have a similar efficiency, it varies in the range of no more than 10%, but the efficiency of power supplies with PKKM (PPFC) and AKKM (APFC) is much higher.

Power factor

As a parameter that you should pay attention to when choosing a PSU, the power factor is less significant, but other quantities depend on it. With a small value of the power factor, there will be a small value of efficiency. As noted above, power factor correctors bring many improvements. A higher power factor will result in lower currents in the network.

Non-electrical parameters and characteristics of power supplies

Usually, as for electrical characteristics, not all non-electrical parameters are indicated in the passport. Although the non-electrical parameters of the power supply are also important. We list the main ones:

• Operating temperature range;
• reliability of the power supply (time between failures);
• the noise level generated by the power supply during operation;
• power supply fan speed;
• weight of the power supply;
• length of supply cables;
• ease of use;
• environmental friendliness of the power supply;
• compliance with state and international standards;
• power supply dimensions.

Most of the non-electrical parameters are clear to all users. However, let's focus on more relevant parameters. Most modern power supplies are quiet, they have a noise level of about 16 dB. Although even a power supply unit with a nominal noise level of 16 dB can be equipped with a fan with a speed of 2000 rpm. In this case, when the power supply is loaded at about 80%, the fan speed control circuit will turn it on at maximum speed, which will lead to significant noise, sometimes more than 30 dB.

It is also necessary to pay attention to the convenience and ergonomics of the power supply. There are many advantages to using modular power cable connections. This is more convenient connection of devices, less occupied space in the computer case, which in turn is not only convenient, but improves the cooling of computer components.

Standards and certificates

When buying a PSU, first of all, you need to look at the availability of certificates and its compliance with modern international standards. On power supplies, you can most often find an indication of the following standards:

RoHS, WEEE - does not contain harmful substances;

UL, cUL - certificate for compliance with its technical characteristics, as well as safety requirements for built-in electrical appliances;

CE - a certificate that shows that the power supply meets the strictest requirements of the directives of the European Committee;

ISO - international quality certificate;

CB - international certificate of conformity to its technical characteristics;

FCC - compliance with the standards of electromagnetic interference (EMI) and radio interference (RFI) generated by the power supply;

TUV - Certificate of Compliance international standard EN ISO 9001:2000;

CCC - China certificate of safety, electromagnetic parameters and environmental protection.

There are also computer standards of the ATX form factor, which define the dimensions, design and many other parameters of the power supply, including the permissible voltage deviations under load. Today there are several versions of the ATX standard:

• ATX 1.3 Standard;
• ATX 2.0 Standard;
• ATX 2.2 Standard;
• ATX 2.3 standard.

The difference between the versions of the ATX standards mainly concerns the introduction of new connectors and new requirements for the power supply lines of the power supply.

When it becomes necessary to purchase a new ATX power supply, you first need to determine the power that is needed to power the computer in which this PSU will be installed. To determine it, it is enough to sum up the power of the components used in the system, for example, using the calculator from outervision.com. If this is not possible, then we can proceed from the rule that for an average computer with one gaming graphics card a 500-600 watt power supply is enough.

Considering that most of the parameters of power supplies can only be found out by testing it, the next step is to strongly recommend that you familiarize yourself with the tests and reviews of possible contenders - power supply models that are available in your region and satisfy your requests at least in terms of power provided. If this is not possible, then it is necessary to choose according to the compliance of the power supply with modern standards (the larger the number, the better), while it is desirable to have an AKKM (APFC) circuit in the power supply. When purchasing a power supply, it is also important to turn it on, if possible, right at the place of purchase or immediately upon arrival home, and see how it works so that the power supply does not emit squeaks, buzzes or other extraneous noise.

In general, you need to choose a power supply that is powerful, well-made, with good declared and actual electrical parameters, and also turns out to be easy to use and quiet during operation, even with a high load on it. And in no case should you save a couple of dollars when buying a power supply. Remember that the stability, reliability and durability of the entire computer mainly depends on the operation of this device.

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how to match cases to the motherboard and got the best answer

Answer from Andrey Bobrovsky[guru]
Form Factor
Motherboard form factor.
The form factor determines the dimensions, mounting holes, power connectors of the motherboard, as well as the requirements for the cooling system. When choosing components for your computer, you must remember that the computer case must support the form factor of the motherboard. Possible motherboard form factors: ATX, microATX, EATX, BTX, mBTX, mini-ITX, SSI EEB, SSI CEB, custom.
ATX (Advanced Technology eXtended) is one of the most common PC motherboard formats, ideal for building home computer. ATX boards are 30.5 x 24.4. cm and support seven expansion slots. The main connector for connecting the power supply on an ATX standard motherboard can have 20 or 24 pins. Almost all new motherboard models have a 24-pin connector.
MicroATX (mATX) is a slightly smaller ATX standard. Suitable for building office computers when you do not need many slots for system expansion. The microATX boards measure 24.4 x 24.4 cm and support four expansion slots. The main power supply connector on a microATX motherboard can have 20 or 24 pins. Almost all new motherboard models have a 24-pin connector.
FlexATX is a form factor that should eventually replace microATX. Currently, it has not received much popularity. FlexATX boards are 22.9 x 19.1 cm and have a maximum of 3 expansion slots.
EATX (Extended ATX) motherboards differ from ATX in size (up to 30.5 x 33.0 cm) and are mainly used for servers.
BTX (Balanced Technology Extended) - new standard, which replaces ATX. When developing this form factor, great attention was paid to efficient cooling of the elements installed on the board. BTX is ideal for building miniature computers. BTX motherboards measure 26.7 x 32.5 cm and support seven expansion slots.
mBTX (micro BTX) is a smaller version of BTX. The dimensions of such boards are 26.7 x 26.4 cm. mBTX supports four expansion slots.
mini-ITX is a motherboard form factor developed by VIA Technologies. Electrically and mechanically compatible with the ATX form factor. Mini-ITX motherboards are small (17 x 17 cm).
SSI EEB (Server Standards Infrastructure Entry Electronics Bay). Motherboards of this standard are usually used to build servers. Power supply connectors have 24+8 pins. The dimensions of such boards are 30.5 x 33.0 cm.
SSI CEB (SSI Compact Electronics Bay). Motherboards of this standard are usually used to build servers. Power supply connectors have 24+8 pins. The dimensions of such boards are 30.5 x 25.9 cm.
Sometimes you can find motherboards of a non-standard form factor (Proprietary). They are designed to be installed in a special, compatible housing.

Linear and switching power supplies

Let's start with the basics. The power supply in the computer performs three functions. First of all, alternating current from a household power supply must be converted to a permanent one. The second task of the PSU is to lower the voltage of 110-230 V, which is redundant for computer electronics, to standard values required by power converters for individual PC components - 12 V, 5 V and 3.3 V (as well as negative voltages, which we will talk about a little later). Finally, the PSU plays the role of a voltage stabilizer.

There are two main types of power supplies that perform these functions - linear and switching. The simplest linear PSU is based on a transformer, on which the AC voltage is reduced to the required value, and then the current is rectified by a diode bridge.

However, the PSU is also required to stabilize the output voltage, which is due to both the instability of the voltage in the household network and the voltage drop in response to an increase in current in the load.

To compensate for the voltage drop, in a linear power supply, the transformer is dimensioned to provide excess power. Then, at a high current in the load, the required voltage will be observed. However, the overvoltage that will occur without any means of compensation at low current in the payload is also unacceptable. Excessive voltage is eliminated by including a non-useful load in the circuit. In the simplest case, this is a resistor or transistor connected via a Zener diode. In a more advanced one, the transistor is controlled by a microcircuit with a comparator. Be that as it may, excess power is simply dissipated in the form of heat, which negatively affects the efficiency of the device.

In the switching power supply circuit, another variable appears, on which the output voltage depends, in addition to the two already available: the input voltage and the load resistance. In series with the load there is a key (which in the case of interest to us is a transistor), controlled by a microcontroller in pulse-width modulation (PWM) mode. The higher the duration of the open states of the transistor in relation to their period (this parameter is called the duty cycle, in Russian terminology the inverse value is used - the duty cycle), the higher the output voltage. Due to the presence of a key, a switching power supply is also called Switched-Mode Power Supply (SMPS).

No current flows through a closed transistor, and the resistance of an open transistor is ideally negligible. In reality, an open transistor has resistance and dissipates some of the power in the form of heat. Also, the transition between transistor states is not perfectly discrete. And yet, the efficiency of a pulsed current source can exceed 90%, while the efficiency of a linear PSU with a stabilizer in best case reaches 50%.

Another advantage of switching power supplies is a radical reduction in the size and weight of the transformer compared to linear power supplies of the same power. It is known that the higher the frequency of alternating current in the primary winding of the transformer, the smaller the required core size and the number of turns of the winding. Therefore, the key transistor in the circuit is placed not after, but before the transformer and, in addition to voltage stabilization, it is used to obtain high-frequency alternating current (for computer PSUs, this is from 30 to 100 kHz and higher, and as a rule - about 60 kHz). A transformer operating at a frequency of 50-60 Hz, for the power required by a standard computer, would be ten times more massive.

Linear PSUs today are mainly used in low power applications where the relatively complex electronics required for a switching power supply is a more costly expense than a transformer. These are, for example, 9 V power supplies, which are used for guitar effects pedals, and once - for game consoles, etc. But chargers for smartphones are already completely pulsed - here the costs are justified. Due to the significantly lower amplitude of the voltage ripple at the output, linear power supplies are also used in areas where this quality is in demand.

⇡ The general scheme of the ATX standard power supply

The desktop computer PSU is a switching power supply, the input of which is supplied with the voltage of a household power supply with parameters of 110/230 V, 50-60 Hz, and at the output there are a number of DC lines, the main of which have a rating of 12, 5 and 3.3 V In addition, the PSU provides the -12V and, at one time, the -5V required for the ISA bus. But the latter was at some point excluded from the ATX standard due to the termination of support for ISA itself.

In the simplified diagram of a standard switching power supply presented above, four main stages can be distinguished. In the same order, we consider the components of power supplies in the reviews, namely:

1. EMI filter - electromagnetic interference (RFI filter);
2. primary circuit - input rectifier (rectifier), key transistors (switcher) that create high-frequency alternating current on the primary winding of the transformer;
3. main transformer;
4. secondary circuit - current rectifiers from the secondary winding of the transformer (rectifiers), smoothing filters at the output (filtering).

⇡ EMI filter

The filter at the PSU input serves to suppress two types of electromagnetic interference: differential (differential-mode) - when the interference current flows in different directions in the power lines, and common-mode (common-mode) - when the current flows in one direction.

Differential noise is suppressed by a CX capacitor (large yellow film capacitor in the photo above) connected in parallel with the load. Sometimes a choke is additionally hung on each wire, which performs the same function (not in the diagram).

The common mode filter is formed by CY capacitors (blue teardrop-shaped ceramic capacitors in the photo), at a common point connecting the power lines to ground, and the so-called. common mode choke (common-mode choke, LF1 in the diagram), the current in the two windings of which flows in the same direction, which creates resistance to common mode noise.

In cheap models, a minimum set of filter parts is installed; in more expensive, the described schemes form repeating (in whole or in part) links. In the past, it was not uncommon to see PSUs without an EMI filter at all. Now this is rather a curious exception, although when buying a very cheap PSU, you can still run into such a surprise. As a result, not only and not so much the computer itself will suffer, but other equipment included in the household network - pulsed power supplies are a powerful source of interference.

In the area of ​​\u200b\u200bthe filter of a good PSU, you can find several details that protect the device itself or its owner from damage. There is almost always a simple fuse for short circuit protection (F1 in the diagram). Note that when the fuse blows, the protected object is no longer the power supply. If a short circuit has occurred, then it means that the key transistors have already broken through, and it is important to at least prevent the ignition of the electrical wiring. If a fuse suddenly blows in the PSU, then it is most likely pointless to change it to a new one.

Separately, protection against short-term voltage surges using a varistor (MOV - Metal Oxide Varistor). But there are no means of protection against a prolonged increase in voltage in computer power supplies. This function is performed by external stabilizers with their own transformer inside.

The capacitor in the PFC circuit after the rectifier can retain a significant charge after being disconnected from the power supply. So that a careless person who puts his finger into the power connector is not shocked, a high-value discharge resistor (bleeder resistor) is installed between the wires. In a more sophisticated version - along with a control circuit that prevents the charge from leaking when the device is in operation.

By the way, the presence of a filter in the PC power supply (and in the monitor power supply and almost any computer technology it is also there) means that buying a separate "surge protector" instead of a conventional extension cord, in general, is useless. He has the same inside. The only condition in any case is normal three-pin wiring with grounding. Otherwise, the CY capacitors connected to ground will simply not be able to perform their function.

⇡ Input rectifier

After the filter, the alternating current is converted to direct current using a diode bridge - usually in the form of an assembly in a common housing. A separate radiator for cooling the bridge is strongly welcomed. A bridge assembled from four discrete diodes is an attribute of cheap power supplies. You can also ask what current the bridge is designed to determine if it matches the power of the PSU itself. Although this parameter, as a rule, there is a good margin.

⇡ Active PFC block

In an AC circuit with a linear load (such as an incandescent lamp or electric stove), the flowing current follows the same sinusoid as the voltage. But this is not the case with devices that have an input rectifier, such as switching power supplies. The power supply passes current in short pulses, roughly coinciding in time with the peaks of the voltage sine wave (i.e., the maximum instantaneous voltage), when the rectifier smoothing capacitor is recharged.

The distorted current signal is decomposed into several harmonic oscillations in total with a sinusoid of a given amplitude (an ideal signal that would occur with a linear load).

The power used to perform useful work (which, in fact, is the heating of PC components) is indicated in the characteristics of the PSU and is called active. The rest of the power generated by harmonic current oscillations is called reactive power. It does no useful work, but heats up wires and puts a strain on transformers and other power equipment.

The vector sum of reactive and active power is called apparent power. And the ratio of active power to full power is called the power factor (power factor) - not to be confused with efficiency!

A switching PSU has a rather low power factor initially - about 0.7. For a private consumer, reactive power is not a problem (fortunately, it is not taken into account by electricity meters), unless he uses a UPS. The uninterruptible power supply just bears the full power of the load. On the scale of an office or a city network, the excess reactive power generated by switching power supplies already significantly reduces the quality of power supply and causes costs, so it is being actively combated.

In particular, the vast majority of computer PSUs are equipped with active power factor correction (Active PFC) circuits. The unit with active PFC is easily identified by the single large capacitor and inductor installed after the rectifier. In essence, Active PFC is another switching converter that maintains a constant charge on the capacitor with a voltage of about 400 V. In this case, the current from the mains is consumed by short pulses, the width of which is chosen so that the signal is approximated by a sinusoid - which is required to simulate a linear load . To synchronize the current demand signal with the voltage sine wave, the PFC controller has special logic.

The active PFC circuit contains one or two key transistors and a powerful diode, which are placed on the same radiator with the key transistors of the main PSU converter. As a rule, the PWM controller of the main converter key and the Active PFC key are one chip (PWM/PFC Combo).

The power factor of switching power supplies with active PFC reaches 0.95 and higher. In addition, they have one additional advantage - they do not require a 110/230 V mains switch and a corresponding voltage doubler inside the PSU. Most PFC circuits digest voltages from 85 to 265 V. In addition, the PSU's sensitivity to short-term voltage dips is reduced.

By the way, in addition to active PFC correction, there is also a passive one, which involves installing a high inductance inductor in series with the load. Its effectiveness is low, and you are unlikely to find this in a modern PSU.

⇡ Main transducer

The general principle of operation for all pulsed power supplies of an isolated topology (with a transformer) is the same: the key transistor (or transistors) creates an alternating current on the primary winding of the transformer, and the PWM controller controls the duty cycle of their switching. Specific circuits, however, differ both in the number of key transistors and other elements, and in quality characteristics: efficiency, signal shape, interference, etc. But here too much depends on the specific implementation to be worth focusing on. For those interested, we present a set of diagrams and a table that will allow them to be identified in specific devices by the composition of parts.

In addition to the above topologies, in expensive PSUs there are resonant (resonant) versions of Half Bridge, which are easy to identify by an additional large inductor (or two) and a capacitor forming an oscillatory circuit.

⇡ Secondary circuit

The secondary circuit is everything that is after the secondary winding of the transformer. In most modern power supplies, the transformer has two windings: 12 V is removed from one of them, and 5 V is removed from the other. The current is first rectified using an assembly of two Schottky diodes - one or more per bus (on the most heavily loaded bus - 12 V - there are four assemblies in powerful power supplies). More efficient in terms of efficiency are synchronous rectifiers, which use field-effect transistors instead of diodes. But this is the prerogative of truly advanced and expensive PSUs that claim the 80 PLUS Platinum certificate.

The 3.3V rail is typically derived from the same winding as the 5V rail, only the voltage is stepped down with a saturable choke (Mag Amp). A special winding on a 3.3 V transformer is an exotic option. Of the negative voltages in the current ATX standard, only -12 V remains, which is removed from the secondary winding under the 12 V bus through separate low-current diodes.

PWM key control of the converter changes the voltage on the primary winding of the transformer, and therefore on all the secondary windings at once. At the same time, the current consumption by the computer is by no means evenly distributed between the PSU buses. In modern hardware, the most loaded bus is 12-V.

Additional measures are required for separate voltage stabilization on different buses. The classic method involves the use of a group stabilization choke. Three main tires are passed through its windings, and as a result, if the current increases on one bus, then the voltage drops on the others. Suppose the current increased on the 12 V bus, and in order to prevent a voltage drop, the PWM controller reduced the duty cycle of the key transistors. As a result, the voltage on the 5 V bus could go beyond the permissible limits, but was suppressed by the group stabilization inductor.

The 3.3V rail voltage is additionally regulated by another saturable choke.

In a more advanced version, separate stabilization of the 5 and 12 V buses is provided due to saturable chokes, but now this design in expensive high-quality PSUs has given way to DC-DC converters. In the latter case, the transformer has a single secondary winding with a voltage of 12 V, and the voltages of 5 V and 3.3 V are obtained through DC converters. This method is most favorable for voltage stability.

Output filter

The final stage on each bus is a filter that smooths out the voltage ripple caused by the key transistors. In addition, pulsations of the input rectifier, whose frequency is equal to twice the frequency of the mains, break through to the secondary circuit of the power supply unit to one degree or another.

The ripple filter includes a choke and large capacitors. High-quality power supplies are characterized by a capacitance of at least 2,000 microfarads, but manufacturers of cheap models have a reserve for savings when they install capacitors, for example, of half the value, which inevitably affects the ripple amplitude.

⇡ Standby power supply +5VSB

A description of the components of the power supply would be incomplete without mentioning the standby voltage of 5 V, which makes it possible to sleep the PC and ensures the operation of all devices that must be turned on all the time. "Duty room" is powered by a separate pulse converter with a low-power transformer. In some power supplies, there is also a third transformer used in the feedback circuit to isolate the PWM controller from the primary circuit of the main converter. In other cases, this function is performed by optocouplers (LED and phototransistor in one package).

⇡ Power supply testing methodology

One of the main parameters of the PSU is voltage stability, which is reflected in the so-called. cross-load characteristic. KNKH is a diagram in which the current or power on the 12 V bus is plotted on one axis, and the total current or power on the 3.3 and 5 V buses is plotted on the other. At the intersection points, for different values ​​of both variables, the voltage deviation from the nominal by one tire or another. Accordingly, we publish two different KNX - for the 12 V bus and for the 5 / 3.3 V bus.

The color of the dot means the deviation percentage:

• green: ≤ 1%;
• light green: ≤ 2%;
• yellow: ≤ 3%;
• orange: ≤ 4%;
• red: ≤ 5%.
• white: > 5% (not allowed by the ATX standard).

To obtain CNC, a custom-made power supply test stand is used, which creates a load due to heat dissipation on powerful field-effect transistors.

Another equally important test is to determine the range of ripples at the PSU output. The ATX standard allows ripples within 120 mV for a 12 V bus and 50 mV for a 5 V bus. There are high-frequency ripples (at twice the frequency of the main converter key) and low-frequency ones (at twice the mains frequency).

We measure this parameter using the Hantek DSO-6022BE USB oscilloscope at the maximum load on the power supply unit specified by the specifications. On the oscillogram below, the green graph corresponds to the 12 V bus, yellow - 5 V. It can be seen that the ripples are within the normal range, and even with a margin.

For comparison, we present a picture of ripples at the output of the PSU of an old computer. This block wasn't great initially, but clearly hasn't gotten any better over time. Judging by the range of low-frequency ripples (note that the voltage base division is increased to 50 mV to fit the oscillations on the screen), the smoothing capacitor at the input has already become unusable. High-frequency ripple on the 5 V bus is on the verge of an acceptable 50 mV.

The following test determines the efficiency of the unit at a load of 10 to 100% of the rated power (by comparing the output power with the input power measured with a household wattmeter). For comparison, the graph shows the criteria for different categories of 80 PLUS. However, it does not arouse much interest these days. The graph shows the results of the top Corsair PSU in comparison with the very cheap Antec, and the difference is not that very big.

A more pressing issue for the user is the noise from the built-in fan. It is impossible to directly measure it near the roaring power supply test bench, so we measure the speed of rotation of the impeller with a laser tachometer - also at power from 10 to 100%. In the graph below, you can see that at low load on this PSU, the 135mm fan maintains a low RPM and is hardly audible at all. At maximum load, the noise can already be distinguished, but the level is still quite acceptable.

ATX (Advanced Technology Extended) is a desktop form factor. Since its introduction to the market in 2001, this form factor has been the leading standard in the market for mass-produced form factors for computer systems.

ATX defines the following motherboard parameters:

• Motherboard geometry;
• Basic requirements for the position of connectors and openings on the case;
• The shape and location of some connectors (mainly power connectors);
• The geometry of the dimensions of the power supply;
• The location of the power supply on the case;
• Electrical parameters of the power supply;

Board sizes

Story

The ATX form factor was created and made public to computer system manufacturers in 1995. The author of the development is Intel. The ATX standard acted as a logical alternative and evolutionary replacement for the long-standing and already obsolete AT standard.

In addition to Intel, other OEM equipment suppliers began to actively produce motherboards and power supplies for them (as well as other components) in the new ATX form factor. The global displacement of the old standard took place at the end of 1999 - the beginning of 2001. At that time, other modern standards ( microATX, flexATX, mini-ITX), for the most part, retained the imprint key features standard ATX, changing only the size of the boards and the number of slots.

In the course of its development, the ATX specification went through the following evolution of standards:

• ATX 1.0 standard.
• ATX 1.1 standard.
• ATX 1.2 standard.
• ATX 1.3 standard.
• ATX 2.0 standard.
• ATX 2.1 standard.
• ATX 2.2 standard.
• ATX 2.3 standard.

In 2003, Intel released a new standard called BTX. It was created in order to increase the level and intensity of cooling of the system unit. The replacement of ATX was driven by the increasing thermal power of computer components. First of all, it was the processors. A new stage of transition to a new format began, which, however, soon ceased. Majority representatives computer industry abandoned the mass distribution of the new format due to a reduction in the power dissipated by PC components.

To this day, ATX and its derivatives are the most common form factors on the market, and a more interesting alternative is not planned for the foreseeable future.

Key differences between ATX and AT

• The motherboard is responsible for powering the processor. To ensure the operation of the control unit, as well as some peripheral devices, a standby voltage of 5 / 3.3 volts is sent to the board. Despite the fact that many instructions strongly recommend unplugging the power cord to safely replace components, many ATX power supplies are equipped with an isolating switch mounted directly on the case.
• The fan located on the back of the power supply can be supplemented or replaced by a 12/14 cm fan that is installed on the bottom of the power supply. This makes it possible to create a large volume of airflow at lower speeds, which, accordingly, leads to a decrease in the noise level. The location of the elements on the motherboard is carried out in such a way that the processor heatsink is installed in the path of the air flow from the power supply fan.
• The power connector has changed. In order to prevent incorrect connection of two similar power connectors (as was the case in the previous standard), the ATX standard is equipped with a keyed connector that cannot be connected incorrectly. Due to the increase in power consumption, the number of pins in the ATX power connector increased first to 20, and then to 24.
• received an upgrade and back panel corps. The AT standard had only a hole for the keyboard connector on the rear panel. Other devices were connected by means of special boards with connectors installed on the motherboard and attached to special slotted slots. The ATX standard differs in that the keyboard (and mouse) connectors are traditionally located on top, the rest of the place is occupied by a fixed-sized rectangular hole, which, depending on the motherboard manufacturer, can be filled with various connectors in any order. The motherboard comes with a special "plug" with slots for a specific motherboard. This is very convenient, since the user has the opportunity to use the same case with motherboards equipped with completely different sets of connectors. Also, this "stub" has some other functions: it reduces the radiated EMI and forms a single chassis ground loop.

Connectors and plug

The metal "plug" located at the back of the case performs a very important function. Thanks to it, motherboard manufacturers, in the process of integrating various interface devices into their products, can freely arrange connectors without having to coordinate their position with case manufacturers.

The only requirement for the plug is the external geometric dimensions:

• width: 158.75 ± 2 mm;
• height: 44.45 ± 2 mm;
• thickness ranging from 0.94 to 1.32 mm;
• rounding of the panel is not more than 0.99 mm.

The standard connectors in an ATX case are:

• PS / 2 connector for connecting a keyboard and mouse. Some cases have a universal connector that supports both devices. But at present, there is a general trend to change this connector to a modern USB interface. However, among budget boards, these connectors are still used.
• 3.5mm connectors (from 3 to 6 pieces) of the integrated sound card. They include:
  • line output (green);
  • line input (blue);
  • microphone input (pink);
• USB connectors (4 - 8);
• Connector for connecting to a local network.

In addition, the following connectors can be installed:

• Parallel communication port;
• Serial port (1-2) - simple 9-pin connector;
• Game port for connecting a joystick or synthesizer;
• Digital audio outputs (coaxial and/or optical);
• Built-in video adapter;
• Integrated video output (D-sub, S-Video, DVI or HDMI);
• Second port for integrated network cards;
• IEEE 1394 interface;
• Connector for WiFi antenna;
• BIOS quick reset button.