Circuit design of power supplies for personal computers. Encyclopedia of radio electronics and electrical engineering

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Switching power supplies (UPS) for personal computers have important advantages - small size and weight. However, they are built according to rather complex schemes, which makes troubleshooting difficult. The author of the proposed article, talking about the circuitry of these blocks, relies on experience with UPS, the so-called AT format.

UPS of household computers are designed to operate from a single-phase AC network (110/230 V, 60 Hz - imported, 127/220 V, 50 Hz - domestic production). Since the network 220 V, 50 Hz is generally accepted in Russia, the problem of choosing a unit for the desired mains voltage does not exist. You just need to make sure that the mains voltage switch on the unit (if any) is set to 220 or 230 V. The absence of a switch indicates that the unit is capable of operating in the mains voltage range indicated on its label without any switching. UPSs designed for 60 Hz operate flawlessly on a 50 Hz network.

The UPS is connected to AT-format motherboards with two wire harnesses with sockets P8 and P9, shown in fig. 1 (view from nests). The wire colors shown in brackets are standard, although not all UPS manufacturers strictly follow them. To correctly orient the sockets when connecting to the motherboard plugs, there is a simple rule: the four black wires (GND circuit) that go to both sockets must be located side by side.

The main power supply circuits of ATX format motherboards are concentrated in the connector shown in fig. 2. As in the previous case, view from the side of the outlet sockets. UPSs of this format have a remote control input (PS-ON circuit), when connected to a common wire (COM circuit - "common", equivalent to GND), the unit connected to the network starts working. If the PS-ON-COM circuit is broken, there is no voltage at the UPS outputs, except for the "on duty" +5 V in the + 5VSB circuit. In this mode, the power consumption from the network is very low.

ATX UPSs are available with an additional output socket shown in fig. 3.

The purpose of its circuits is as follows:

FanM - output of the fan speed sensor that cools the UPS (two pulses per revolution);

FanC - analog (0...12 V) input for controlling the rotation speed of this fan. If this input is disconnected from external circuits or more than 10 V DC is applied to it, the fan performance is maximum;

3.3V Sense - +3,3 V voltage regulator feedback signal input. It is connected by a separate wire directly to the microcircuit power pins on the system board, which allows you to compensate for the voltage drop on the supply wires. If there is no additional outlet, this circuit is brought out to socket 11 of the main outlet (see Fig. 2);

1394R - minus of the 8...48 V voltage source isolated from the common wire to power the IEEE-1394 interface circuits;

1394V - plus the same source.

UPS of any format must be equipped with several outlets for powering disk drives and some other computer peripherals.

Each "computer" UPS outputs a logic signal, called R G. (Power Good) in AT units or PW-OK (Power OK) in ATX units, which indicates that all output voltages are within acceptable limits. On the "motherboard" board of the computer, this signal is involved in the formation of the system reset signal (Reset). After turning on the UPS, the signal level RG. (PW-OK) stays low for some time, disabling the processor until transients are completed in the power circuits.

In the event of a mains power failure or a sudden malfunction of the UPS, the logic level of the PG signal (PW-OK) changes before the output voltages of the unit fall below acceptable values. This causes the processor to stop, prevents corruption of data stored in memory, and other irreversible operations.

UPS interchangeability can be assessed by the following criteria.

The number of output voltages to power an AT-format IBM PC must be at least four (+12V, +5V, -5V, and -12V). The maximum and minimum output currents are regulated separately for each channel. Their usual values ​​for sources of different power are given in Table. 1.

ATX format computers additionally need +3,3 V and some other voltages (they were mentioned above).

Please note that the normal operation of the unit when the load is less than the minimum is not guaranteed, and sometimes this mode is simply dangerous. Therefore, it is not recommended to turn on the UPS without load in the network (for example, for testing).

The power of the power supply unit (total for all output voltages) in a consumer PC fully equipped with peripheral devices must be at least 200 watts. It is practically necessary to have 230 ... 250 W, and when installing additional "hard drives" and CD-ROM drives, more may be required. PC malfunctions, especially those that occur when the electric motors of the mentioned devices are turned on, are often associated precisely with overloading the power supply. Computers used as information network servers consume up to 350 watts. UPS of low power (40 ... 160 W) is used in specialized, for example, control computers with a limited set of peripherals.

The volume occupied by the UPS usually grows by increasing its length towards the front of the PC. Mounting dimensions and mounting points of the unit in the computer case remain unchanged. Therefore, any (with rare exceptions) block can be installed in place of the failed one.

The basis of most UPS is a push-pull half-bridge inverter operating at a frequency of several tens of kilohertz. The inverter supply voltage (approximately 300 V) is rectified and smoothed mains. The inverter itself consists of a control unit (a pulse generator with an intermediate power amplification stage) and a powerful output stage. The latter is loaded on a high-frequency power transformer. Output voltages are obtained using rectifiers connected to the secondary windings of this transformer. Voltage stabilization is performed using pulse-width modulation (PWM) of pulses generated by the inverter. Usually, only one output channel is covered by the stabilizing feedback, usually +5 or +3,3 V. As a result, the voltages on the other outputs do not depend on the mains voltage, but remain subject to the influence of the load. Sometimes they are additionally stabilized using conventional stabilizer microcircuits.

NETWORK RECTIFIER

In most cases, this node is performed according to a scheme similar to that shown in Fig. 4, the differences are only in the type of rectifier bridge VD1 and more or less protective and safety elements.

Sometimes the bridge is assembled from individual diodes. With the switch S1 open, which corresponds to the power supply of the unit from the network 220 ... 230 V, the rectifier is a bridge, the voltage at its output (capacitors C4, C5 connected in series) is close to the amplitude of the mains. When powered from a network of 110 ... 127 V, by closing the contacts of the switch, they turn the device into a rectifier with a voltage doubling and receive a constant voltage at its output, twice the amplitude of the mains. Such switching is provided in the UPS, the stabilizers of which keep the output voltages within acceptable limits only if the mains deviates by ± 20%. Units with more efficient stabilization are able to operate at any mains voltage (usually from 90 to 260 V) without switching.

Resistors R1, R4 and R5 are designed to discharge the rectifier capacitors after it is disconnected from the network, and C4 and C5, in addition, equalize the voltages on the capacitors C4 and C5. Thermistor R2 with a negative temperature coefficient limits the amplitude of the charging current surge of capacitors C4, C5 at the moment the unit is turned on. Then, as a result of self-heating, its resistance drops, and it practically does not affect the operation of the rectifier. Varistor R3 with a classification voltage greater than the maximum amplitude of the mains protects against surges of the latter. Unfortunately, this varistor is useless if the block with the closed switch S1 is accidentally connected to the 220 V network. The replacement of resistors R4, R5 with varistors with a classification voltage of 180 ... Sometimes varistors are connected in parallel with the specified resistors or only one of them.

Capacitors C1 - C3 and a two-winding inductor L1 form a filter that protects the computer from interference from the network, and the network from interference generated by the computer. Through capacitors C1 and C3, the computer case is connected by alternating current to the wires of the network. Therefore, the touch voltage to an ungrounded computer can reach half the network voltage. This is not life-threatening, since the reactance of the capacitors is quite large, but it often leads to failure of the interface circuits when peripheral devices are connected to the computer.

POWERFUL INVERTER CASCADE

On fig. 5 shows part of the schematic diagram of a common GT-150W UPS.

The pulses generated by the control unit are fed through the transformer T1 to the bases of the transistors VT1 and VT2, opening them in turn. Diodes VD4, VD5 protect transistors from reverse polarity voltage. Capacitors C6 and C7 correspond to C4 and C5 in the rectifier (see Fig. 4). The voltages of the secondary windings of the transformer T2 are rectified to obtain output. One of the rectifiers (VD6, VD7 with L1C5 filter) is shown in the diagram.

Most powerful UPS stages differ from the one considered only in the types of transistors, which can be, for example, field-operated or contain built-in protective diodes. There are several versions of the base circuits (for bipolar) or gate circuits (for field-effect transistors) with different numbers, ratings and switching circuits of elements. For example, resistors R4, R6 can be connected directly to the bases of the respective transistors.

In steady state, the inverter control unit is powered by the output voltage of the UPS, but at the moment of switching on, it is absent. There are two main ways to get the supply voltage needed to start the inverter. The first of them is implemented in the scheme under consideration (Fig. 5). Immediately after turning on the unit, the rectified mains voltage is supplied through the resistive divider R3 - R6 to the base circuits of the transistors VT1 and / T2, opening them slightly, and the diodes VD1 and VD2 prevent shunting of the base-emitter sections of the transistors by the windings II and III of the transformer T1. At the same time, capacitors C4, C6 and C7 are charging, and the charging current of capacitor C4, flowing through winding I of transformer T2 and through winding II of transformer T1, induces voltage in windings II and III of the latter, opening one of the transistors and closing the other. Which of the transistors will close and which will open depends on the asymmetry of the characteristics of the cascade elements.

As a result of the action of a positive OS, the process proceeds like an avalanche, and the pulse induced in the winding II of the transformer T2 through one of the diodes VD6, VD7, the resistor R9 and the diode VD3 charges the capacitor C3 to a voltage sufficient to start the operation of the control unit. In the future, it is fed through the same circuit, and the voltage rectified by diodes VD6, VD7, after smoothing by the L1C5 filter, is supplied to the output + 12 V of the UPS.

The variant of the initial start-up circuits used in the LPS-02-150XT UPS differs only in that the voltage to the divider, similar to R3 - R6 (Fig. 5), is supplied from a separate half-wave mains voltage rectifier with a small filter capacitor. As a result, the inverter transistors open slightly before the main rectifier filter capacitors (C6, C7, see Fig. 5) are charged, which provides a more confident start.

The second way to power the control unit during start-up involves the presence of a special low-power step-down transformer with a rectifier, as shown in the diagram in Fig. 6 applied in UPS PS-200B. The number of turns of the secondary winding of the transformer is chosen in such a way that the rectified voltage is slightly less than the output in the +12 V channel of the unit, but sufficient for the operation of the control unit. When the output voltage of the UPS reaches its nominal value, the diode VD5 opens, the bridge diodes VD1 - VD4 remain closed during the entire period of alternating voltage, and the control unit switches to the output voltage of the inverter, without consuming more energy from the "starting" transformer.

In high-power inverter stages started in this way, there is no need for an initial bias on the bases of the transistors and positive feedback. Therefore, resistors R3, R5 are not required, the diodes VD1, VD2 are replaced with jumpers, and the winding II of the transformer T1 is performed without a tap (see Fig. 5).

OUTPUT RECTIFIERS

On fig. 7 shows a typical diagram of a four-channel UPS rectifier assembly.

In order not to violate the symmetry of the magnetization reversal of the magnetic circuit of a power transformer, rectifiers are built only according to full-wave circuits, and bridge rectifiers, which are characterized by increased losses, are almost never used. The main feature of UPS rectifiers is smoothing filters, starting with an inductance (choke). The voltage at the output of a rectifier with a similar filter depends not only on the amplitude, but also on the duty cycle (the ratio of the duration to the repetition period) of the input pulses. This makes it possible to stabilize the output voltage by changing the duty cycle of the input. Used in many other cases, rectifiers with filters starting with a capacitor do not have this property. The process of changing the duty cycle of pulses is usually called PWM - pulse width modulation (PWM - Pulse Width Modulation).

Since the amplitude of the pulses, proportional to the voltage in the supply network, at the inputs of all the rectifiers in the unit changes according to the same law, stabilization using PWM of one of the output voltages stabilizes all the others. To enhance this effect, the filter chokes L1.1 - L1.4 of all rectifiers are wound on a common magnetic circuit. The magnetic connection between them additionally synchronizes the processes occurring in the rectifiers.

For the correct operation of a rectifier with an L-filter, it is necessary that its load current exceed a certain minimum value, which depends on the inductance of the filter inductor and the pulse frequency. This initial load is created by resistors R4 - R7 connected in parallel with the output capacitors C5 - C8. They also serve to accelerate the discharge of capacitors after the UPS is turned off.

Sometimes a voltage of -5 V is obtained without a separate rectifier from a voltage of -12 V using an integrated stabilizer of the 7905 series. Domestic analogues are KR1162EN5A, KR1179EN05 microcircuits. The current consumed by computer nodes in this circuit usually does not exceed a few hundred milliamps.

In some cases, integral stabilizers are also installed in other UPS channels. This solution eliminates the influence of a changing load on the output voltages, but reduces the efficiency of the unit and, for this reason, is used only in relatively low-power channels. An example is the diagram of the PS-6220C UPS rectifier assembly, shown in fig. 8. Diodes VD7 - VD10 - protective.

As in most other blocks, Schottky barrier diodes (VD5 assembly) are installed in the +6 V voltage rectifier, which are distinguished by a lower voltage drop in the forward direction and a reverse resistance recovery time than conventional diodes. Both of these factors are favorable for increasing efficiency. Unfortunately, the relatively low allowable reverse voltage does not allow the use of Schottky diodes in the +12 V channel either. However, in the node under consideration, this problem is solved by connecting two rectifiers in series: the missing 5 V is added to 7 V by a rectifier on the assembly of Schottky diodes VD5.

To eliminate voltage surges dangerous for diodes that occur in the transformer windings at the fronts of the pulses, damping circuits R1C1, R2C2, R3C3 and R4C4 are provided.

CONTROL UNIT

In most "computer" UPSs, this node is built on the basis of a TL494CN PWM controller chip (domestic analogue - KR1114EU4) or its modifications. The main part of the diagram of such a node is shown in Fig. 9, it also shows the elements of the internal device of the said microcircuit.

(click to enlarge)

The sawtooth voltage generator G1 serves as the master. Its frequency depends on the ratings of the external elements R8 and C3. The generated voltage is supplied to two comparators (A3 and A4), the output pulses of which are summarized by the OR element D1. Further, the pulses through the elements OR-NOT D5 and D6 are fed to the output transistors of the microcircuit (V3, V4). Pulses from the output of the element D1 also arrive at the counting input of the trigger D2, and each of them changes the state of the trigger. Thus, if a log is applied to pin 13 of the microcircuit. 1 or it, as in the case under consideration, is left free, the pulses at the outputs of the elements D5 and D6 alternate, which is necessary to control the push-pull inverter. If the TL494 chip is used in a single-cycle voltage converter, pin 13 is connected to a common wire, as a result, trigger D2 is no longer involved in the work, and pulses appear at all outputs simultaneously.

Element A1 is an error signal amplifier in the UPS output voltage stabilization circuit. This voltage (in this case - +5 V) through the resistive divider R1R2 is fed to one of the inputs of the amplifier. At its second input, there is an exemplary voltage obtained from the A5 stabilizer built into the microcircuit using a resistive divider R3 - R5. The voltage at the output A1, proportional to the difference between the input, sets the threshold for the operation of the comparator A4 and, consequently, the duty cycle of the pulses at its output. Since the output voltage of the UPS depends on the duty cycle (see above), in a closed system it is automatically maintained equal to the exemplary one, taking into account the division factor R1R2. The R7C2 chain is necessary for the stability of the stabilizer. The second amplifier (A2) in this case is turned off by applying appropriate voltages to its inputs and is not involved in the work.

The function of comparator A3 is to ensure that there is a pause between pulses at the output of element D1, even if the output voltage of amplifier A1 is out of range. The minimum trigger threshold A3 (when pin 4 is connected to common) is set by the internal voltage source GV1. As the voltage at pin 4 increases, the minimum pause time increases, therefore, the maximum output voltage of the UPS drops.

This property is used to soft start the UPS. The fact is that at the initial moment of operation of the unit, the filter capacitors of its rectifiers are completely discharged, which is equivalent to closing the outputs to a common wire. Starting the inverter immediately "at full power" will lead to a huge overload of the transistors of a powerful cascade and their possible failure. The C1R6 circuit provides a smooth, overload-free start of the inverter.

At the first moment after switching on, the capacitor C1 is discharged, and the voltage at pin 4 of DA1 is close to +5 V, obtained from the stabilizer A5. This guarantees a pause of the maximum possible duration, up to the complete absence of pulses at the output of the microcircuit. As the capacitor C1 is charged through the resistor R6, the voltage at pin 4 decreases, and with it the duration of the pause. At the same time, the UPS output voltage rises. This continues until it approaches the exemplary one and the stabilizing feedback takes effect. Further charging of the capacitor C1 does not affect the processes in the UPS. Since capacitor C1 must be completely discharged before each switching on of the UPS, in many cases, circuits for its forced discharge are provided (not shown in Fig. 9).

INTERMEDIATE CASCADE

The task of this cascade is to amplify the pulses before they are fed to powerful transistors. Sometimes the intermediate stage is absent as an independent node, being part of the master oscillator microcircuit. A diagram of such a cascade used in the PS-200B UPS is shown in fig. 10. The matching transformer T1 here corresponds to the one of the same name in fig. 5.

The APPIS UPS uses an intermediate stage according to the scheme shown in fig. 11, which differs from the one discussed above by the presence of two matching transformers T1 and T2 - separately for each powerful transistor. The polarity of switching on the windings of the transformers is such that the transistor of the intermediate stage and the powerful transistor associated with it are in the open state at the same time. If special measures are not taken, after several cycles of inverter operation, the accumulation of energy in the magnetic cores of transformers will lead to saturation of the latter and a significant decrease in the inductance of the windings.

Let's consider how this problem is solved, using the example of one of the "halves" of the intermediate stage with transformer T1. When the transistor of the microcircuit is open, the winding Ia is connected to a power source and a common wire. A linearly increasing current flows through it. A positive voltage is induced in winding II, which enters the base circuit of a powerful transistor and opens it. When the transistor in the microcircuit is closed, the current in the winding Ia will be interrupted. But the magnetic flux in the magnetic circuit of the transformer cannot change instantly, therefore, a linearly decreasing current will appear in the winding Ib, flowing through the opened diode VD1 from the common wire to the plus of the power source. Thus, the energy accumulated in the magnetic field during the pulse is returned to the source during the pause. The voltage on the winding II during the pause is negative, and the powerful transistor is closed. In a similar way, but in antiphase, the second "half" of the cascade with transformer T2 works.

The presence of pulsating magnetic fluxes with a constant component in the magnetic circuits leads to the need to increase the mass and volume of transformers T1 and T2. In general, the intermediate stage with two transformers is not very successful, although it has become quite widespread.

If the power of the transistors of the TL494CN chip is not enough to directly control the output stage of the inverter, a circuit similar to that shown in fig. 12, which shows the intermediate stage of the KYP-150W UPS. Half of the winding I of the transformer T1 serve as collector loads of transistors VT1 and VT2, which are alternately opened by pulses coming from the DA1 microcircuit. Resistor R5 limits the collector current of the transistors to approximately 20 mA. With the help of diodes VD1, VD2 and capacitor C1 on the emitters of transistors VT1 and VT2, the voltage of +1,6 V is maintained necessary for their reliable closing. Diodes VD4 and VD5 dampen oscillations that occur at the moments of switching transistors in the circuit formed by the inductance of the winding I of the transformer T1 and its own capacity. Diode VD3 closes if the voltage surge at the middle terminal of winding I exceeds the supply voltage of the cascade.

Another version of the intermediate stage circuit (UPS ESP-1003R) is shown in fig. 13.

In this case, the output transistors of the DA1 chip are connected according to a common collector circuit. Capacitors C1 and C2 are forcing. Winding I of transformer T1 does not have an average output. Depending on which of the transistors VT1, VT2 is currently open, the winding circuit is closed to the power source through a resistor R7 or R8 connected to the collector of a closed transistor.

TROUBLESHOOTING

Before repairing the UPS, it must be removed from the computer system unit. To do this, disconnect the computer from the network by removing the plug from the outlet. Having opened the computer case, release all the UPS connectors and, having unscrewed the four screws on the rear wall of the system unit, remove the UPS. Then remove the U-shaped cover of the UPS case by unscrewing the screws securing it. The printed circuit board can be removed by unscrewing the three "self-tapping" screws that secure it. A feature of the boards of many UPSs is that the printed conductor of the common wire is divided into two parts, which are connected to each other only through the metal case of the unit. On the board removed from the case, these parts must be connected with a hanging conductor.

If the power supply was disconnected from the power supply less than half an hour ago, it is necessary to find and discharge oxide capacitors 220 or 470 uF x 250 V on the board (these are the largest capacitors in the unit). During the repair process, it is recommended to repeat this operation after each disconnection of the unit from the network, or temporarily shunt the capacitors with resistors of 100 ... 200 kOhm with a power of at least 1 W.

First of all, they inspect the parts of the UPS and identify obviously faulty ones, for example, burned out or with cracks in the case. If the failure of the unit was caused by a malfunction of the fan, you should check the elements installed on the heat sinks: powerful inverter transistors and Schottky diode assemblies of the output rectifiers. During the "explosion" of oxide capacitors, their electrolyte is sprayed throughout the block. To avoid oxidation of metal current-carrying parts, it is necessary to wash off the electrolyte with a slightly alkaline solution (for example, by diluting Fairy with water in a ratio of 1:50).

Having connected the unit to the network, first of all, all its output voltages should be measured. If it turns out that the voltage in at least one of the output channels is close to the nominal value, the fault should be sought in the output circuits of the faulty channels. However, as practice shows, output circuits rarely fail.

In the event of a malfunction of all channels, the troubleshooting procedure is as follows. Measure the voltage between the positive terminal of the capacitor C4 and the negative C5 (see Fig. 4) or the collector of the transistor VT1 and the emitter VT2 (see Fig. 5) If the measured value is significantly less than 310 V, you need to check and, if necessary, replace the diode bridge VD1 (see Fig. 4) or the individual diodes that make it up. If the rectified voltage is normal, and the unit does not work, most likely one or both transistors of the powerful inverter stage (VT1, VT2, see Fig. 5), which are subject to the greatest thermal overloads, have failed. With serviceable transistors, it remains to check the TL494CN chip and the circuits associated with it.

Failed transistors can be replaced with domestic or imported counterparts that are suitable for electrical parameters, overall and installation dimensions, guided by the data given in Table. 2.

(click to enlarge)

Replacement diodes are selected according to the table. 3.

(click to enlarge)

The rectifier diodes of the mains rectifier (see Fig. 4) can be successfully replaced by domestic KD226G, KD226D. If capacitors with a capacity of 220 uF are installed in the mains rectifier, it is advisable to replace them with 470 uF, a place for this is usually provided on the board. To reduce interference, it is recommended to shunt each of the four rectifier diodes with a 1000 pF capacitor for a voltage of 400 ... 450 V.

Transistors 2SC3039 can be replaced by domestic KT872A. But the PXPR1001 damping diode to replace the failed one is difficult to purchase even in big cities. In this situation, you can use three KD226G or KD226D diodes connected in series. It is possible to replace a failed diode and a powerful transistor protected by it with a transistor with a built-in damping diode, for example, 2SD2333, 2SD1876, 2SD1877 or 2SD1554. It should be noted that in many UPS manufactured after 1998, such a replacement has already been made.

To improve the reliability of the IED, it can be recommended to connect chokes with an inductance of 7 μH in parallel with resistors R8 and R5 (see Fig. 4). They can be wound with a wire with a diameter of at least 0,15 mm in silk insulation on any ring magnetic cores. The number of turns is calculated according to known formulas.

A trimming resistor for adjusting the output voltage (R3, see Fig. 9) is not available in many UPSs; a constant one is installed instead. If tuning is required, it can be done by temporarily installing a tuning resistor, and then again replacing it with a constant value found.

To improve reliability, it is useful to replace imported oxide capacitors installed in the filters of the most powerful + 12 V and + 5 V rectifiers with K50-29 capacitors equivalent in capacitance and voltage. It should be noted that not all capacitors provided by the circuit are installed on the boards of many UPSs (apparently out of economy), which negatively affects the characteristics of the unit. It is recommended to install the missing capacitors in the places intended for them.

When assembling the unit after repair, do not forget to remove the temporarily installed jumpers and resistors, and also connect the built-in fan to the corresponding connector.

Literature

1. Kulichkov A. Switching power supplies for the IBM PC. - M.: DMK, series "Repair and Service", 2000.
2. Guk M. IBM PC hardware. - S.-Pb.: Peter, 2000.
3. Kunevich A.. Sidorov I. Inductive elements on ferrites. - S.-Pb.: Lenizdat, 1997.
4. Nikulin S. Reliability of elements of radio-electronic equipment. - M.: Energy, 1979.

Author: R. Aleksandrov, Maloyaroslavets, Kaluga Region

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