PSU repair

Максим Исаев
Reading time: 12 min
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The PSU is designed to convert the 220V mains voltage into the voltages necessary for the stable operation of the consumers connected to it. PSU also provides stabilization of output voltages, provides protection against short circuits, and provides the necessary power, depending on the connected load.
This material was prepared by Bgacenter teachers, as part of the course – repair of switching power supplies.

Switching power supply

To ensure the load of the miners, UPSs of various capacities are used. This material discusses in detail the PSU used for different ASIC models.

The APW7 design includes:

  • housing – from a shielded metal box
  • UPS printed circuit board has radio components installed
  • the cooling system consists of a forced fan
  • wires required to connect the load

The main function is performed by the board with the elements located on it.

Сторона монтажа APW7
Mounting side APW7

Elements located on the printed circuit board of the UPS:

  1. fuse fuse
  2. Varistor
  3. Line filter capacitor
  4. Throttle
  5. Bypass capacitors
  6. Smoothing capacitor
  7. Filter Capacitors
  8. Power transistors
  9. Fan connector
  10. Smoothing capacitors of the synchronous rectifier
  11. Output transformer
  12. Diode
  13. PFC transistor
  14. NTC thermistors
  15. Relay
  16. PFC circuit choke
  17. Diode bridge
Сторона печати APW7
Print side APW7

How a PSU works

So, the APW7 switching power supply works according to the following principle:

  1. Over voltage and short circuit protection circuit. The circuit consists of a varistor and a fuse in a heat shrink tube. When the voltage exceeds 350 V, the varistor is activated (breaks through), the fuse blows, protecting the UPS board from overvoltage. In this case, the repair consists of replacing the fuse.
  2. The next block is the network filter circuit. It includes a capacitor, two chokes, another capacitor and a number of blocking capacitors designed to eliminate mains interference and noise emissions from the power supply to the network. With minor voltage surges, the inductor tries to increase its magnetic field, as a result of which all the increased voltage coming from the network is abruptly extinguished on it. Capacitors smooth out surges from the operation of the switching converter and prevent penetration into the network.
  3. After the mains filter, there are negative resistance thermistors (NTC), which work to reduce the resistance when heated. This is necessary to limit the current through the diode bridge at the initial moment of charging the smoothing filter capacitors after the diode bridge.
  4. Then comes the rectifier diode bridge, on it we get a constant voltage from the alternating one. This voltage is initially smoothed out by high-capacity filter capacitors 470 uF at 450 V each. At this point in time, a voltage of about 315 V appears on the capacitors.
  5. Since the UPS, in addition to active power, has reactive power, which negatively affects the operation. Structurally, this is eliminated by the PFC (Power Factor Correction) scheme – Power Factor Correction. In this UPS, it is designed on the master chip of pulses and a field effect transistor. A powerful high inductance choke is installed in front of the transistor. As a result of the operation of this circuit, the voltage on the filter capacitors increases to 390 volts and it is now the main one for powering the DC converter circuits.
  6. For the PWM controller to work, it is necessary to use a constant voltage of +12 Volts. This voltage is formed on an auxiliary transformer and rectified by diodes. Also, this voltage is necessary to power the cooling system (fan).
  7. A 12 volt auxiliary source feeds the PWM controller circuit, which generates pulses for a DC-DC converter, consisting of a power transformer and two field-effect transistors. The pulses are fed from the PWM controller to the master oscillator. And already from the master oscillator, the pulses arrive at the gates of the transistors that control the power transformer.
  8. The impulse voltage received on the secondary winding of the transformer, due to the operation of a single-cycle direct converter, is fed to the synchronous rectifier circuit. Where the voltage is smoothed by a synchronous filter built on capacitors and fed to the output terminals to power the hash boards. Feedback and voltage stabilization is carried out through the PWM controller circuit.
  9. The synchronous rectifier is controlled by a DC driver circuit.

UPS faults

Switching power supplies are characterized by the following malfunctions:

  • diode bridge failure
  • PFC transistor failure
  • breakdown of power transistors
  • short-circuited turns of the power transformer or its breakage
  • synchronous rectifier burnout
  • changing the capacitance rating of the synchronous rectifier filter
  • lack of trigger pulses in PWM controllers, auxiliary source 12V and main
  • relay malfunction (clicks are heard, but the UPS does not turn on)
  • burnout of terminal contacts of connected load
  • can’t hold a load
  • the cooling system does not work with a good auxiliary source of 12 V
  • breakage of SMD resistors supplying PWM microcircuits
  • malfunction of SMD transistors in matching stages

UPS Diagnostics

Repair of the APW7 power supply begins with an external inspection. Attention should be paid to the presence of mechanical damage and previously performed repairs. By the absence of a sealant and an unwashed flux, it can be assumed that a repair was carried out earlier – the board was soldered. 

We start diagnosing the board by finding the power filter capacitors. They are usually large in size. We look at its ratings, as we see from the inscription on the capacitor, it has parameters of 450V 470 microfarads each.

Фильтрующие конденсаторы диодного моста
Diode bridge filter capacitors

For further repair, it is necessary to discharge it, regardless of the time it has been out of service. With a tester in the DC voltage measurement mode, we make sure that there is no voltage at the terminals of the capacitors. To do this, we connect the device from the side of the printed circuit board. If there is voltage, we discharge the capacitor with a 60W incandescent lamp and check again with a tester for the absence of voltage.

Only after this procedure can further repairs be carried out. To facilitate troubleshooting, we make sure indirectly that there is no short circuit in the main power supply circuit of the synchronous rectifier along the + 12V main circuit.

To do this, we apply the black probe to the output located at the bottom, and the red probe to the output located at the top, we should see serviceable field-effect transistors (the readings of the MS-319 multimeter (pointer) should be about 20 ohms). 

Замер выходного сопротивления по цепи 12V
Measurement of output resistance in a 12V circuit

Меняем местами щупы, происходит заряд конденсаторов и сопротивление увеличилось, это говорит о We swap the probes, the capacitors are charged and the resistance has increased, this indicates the health of the rectifier.

Разряд конденсаторов выходного выпрямителя
Discharge of output rectifier capacitors

We continue the repair, proceed to the diagnosis of the power unit. With a tester from the mains power connector in the continuity mode, we check the input of one wire to the diode bridge (variable designation input). The tester should show 0 (or emit a kind of beep), which immediately indicates the health of one inductance filter circuit and the integrity of the printed conductor and fuse.

Проверка целостности предохранителя и LC фильтра до входа диодного моста
Checking the integrity of the fuse and LC filter before the input of the diode bridge

Similarly, we check the second wire, but to a different output of the diode bridge. This indicates the health of the second conductor.

Проверка провода и LC фильтра
Checking wire and LC filter

Repair is necessary if the multimeter shows non-zero readings. In this case, we are looking for a break, we eliminate it. In this case, everything is fine.

Next, we check the resistance between the two network control entry points. It should be high (tester in MOhm mode). The measurement showed high resistance in this case. This indicates the absence of a short circuit at the input and the serviceability of the varistor. After making sure that the input unit is working, we check the diode bridge.

Проверка варистора на отсутствие КЗ
Checking the varistor for the absence of a short circuit

The method for checking the diode bridge is standard, the diode continuity mode. After making sure that it is working, we examine the PFC block and its circuits. We check MOSFET (field effect transistor). We put the probes between the gate and the source, then the gate and the drain – the resistance should be high and the tester does not show us anything. It is right.

Измерение сопротивления между затвором и стоком
Measuring resistance between gate and drain

Next, check the drain-source. When a red probe is applied to the source and black to the drain, we will see a voltage drop across the diode of the order of 0.470 mV. In the reverse application of the probes, we will not see any drops. We conclude that the transistor is in good condition.

Проверка Сток-Исток полевого транзистора
FET Drain-Source Test

To measure the control pulses at the gate of this transistor, it is necessary to use an oscilloscope. If there are pulses, we conclude that the microcircuit is in good condition and the pulse is applied to the gate of the field-effect transistor.

Проверка ШИМ PFC
PFC PWM test

Next, we check the circuit of the auxiliary power supply + 12V, assembled on the ICE2QR4765 microcircuit indicated according to the circuit diagram. To do this, in the diode continuity mode, we put one probe on the + high-voltage capacitor, and the second on pin 4 of this PWM chip, we make sure that the auxiliary source transformer winding is intact.

Проверка цепи питания микросхемы ШИМ и целостностности обмотки трансформатора вспомогательного источника питания
Checking the power supply circuit of the PWM chip and the integrity of the winding of the transformer of the auxiliary power supply

Repairs must be carried out with a discharged high-voltage capacitor and the UPS disconnected from the mains!

After that, we check the operation of the main PWM and its power supply circuits according to the electrical circuit diagram. Next, we check the semi-bridge circuit on MOSFET transistors. They are checked with a multimeter in diode continuity mode. Initially, for each transistor, we check the Gate-Source junction, the multimeter should show OL, this indicates that the junction is working (it is not broken).

Измерение Затвор-Сток
Gate-Drain Measurement

The next step, in the resistance measurement mode, check the Gate-Source. At the same time, we check the resistance of the matching driver. The resistance of a good output should be between 10 and 20 ohms.

Проверка Затвор-Исток
Gate-Source Test

As can be seen from the measurements, this indirectly indicates the health of the transistors. In case of doubt about the serviceability of the transistors, they must be soldered and checked separately. The LCR-T4 digital meter is used to test transistors.

LCR-T4
LCR-T4

Then we check the Drain-Source junction for a short circuit. To do this, set the red probe of the multimeter to the Source, and connect the black one to the Drain. The voltage drop in the diode continuity mode should be 0.434 V. This indicates the health of the field effect transistor.

Измерение перехода Сток-Исток
Drain-Source Transition Measurement

When applying the probes in the opposite direction, the multimeter shows OL.

Проверка перехода в обратном направлении
Checking the transition in the opposite direction

How to check the power supply

After successfully identifying faults and repairing damaged elements, the unit must be tested. To do this, the UPS is connected via an isolation transformer to the mains supply.

Then, an electronic load of the required power is connected to the PSU to check for operability. Testing is performed within 1-2 hours. To avoid repeated repairs, it is not recommended to turn on the UPS without a connected load. 

Soldering power supplies

During the repair of the UPS, it becomes necessary to check the elements. To do this, you need to solder the corresponding element from the printed circuit board. Soldering is important to be done carefully, using a soldering iron of the required power:

  • from 80 watts – for the repair of power elements: transformer, power transistors, output diodes, diode bridge, smoothing capacitors;
  • up to 60 watts (or hot air soldering station) – for repairing low and medium power components.

If the UPS worked in violation of temperature conditions (overheated), then when the compound is removed, the SMD components can be torn off the printed circuit board. It is important to remember this, and with further repairs, restore the strapping to the board.

When repairing the UPS, Rose alloy is used to reduce the temperature of the factory solder and prevent damage to the lead wires.

During installation it is necessary to solder on:

  • solder paste with a melting point of 183 degrees Celsius – low power elements
  • POS 61-63 (Pb 61-63/ Sn 40) – power electronic components.

After repair, before taking measurements on transistors, it is important to lower the temperature of the UPS, since in a heated state, the keys are open.

Before soldering newly installed components (transistors), their leads must be cleaned and tinned.

After soldering, it is necessary to wash with alcohol or other cleaner those places where soldering was performed.

UPS repair equipment

Equipment used during UPS repair, consumables:

  • Screwdriver Set
  • side cutters
  • pliers
  • discharge lamp
  • multimeter
  • oscilloscope
  • LCR-T4
  • SMD tester 3910
  • soldering iron from 80 watts
  • dryer hot air station Quick 857DW+
  • microscope CM0745
  • boron machine
  • desoldering pump electric
  • tweezers
  • dental probe (wooden toothpicks)
  • soldering flux
  • BGA paste
  • Falcon 530 board cleaner
  • Toothbrush
  • copper braid 1.5 and 3.0 mm wide
  • Rose alloy
  • acetone polish remover

Conclusions:

  • UPS is a complex electronic device. Repair of a switching power supply in the event of a malfunction must be carried out knowing the principle of its components and element base
  • It is important to follow the safety precautions to determine the failure of the UPS, as there are dangerous voltages from 300 to 400 V, depending on the design of the unit (without PFC 300V with PFC 390V)
  • Repair is sometimes complicated by the presence of a moisture-proof coating that is difficult to remove. In its design, the UPS has powerful outputs of the power transformer. Soldering it requires a soldering iron with more power. When repairing, care must be taken not to damage the conductive paths.
  • The UPS for verification, after the repair has been completed, must be switched on under load. With the load disconnected, power switches may fail.
  • In order to learn how to repair switching power supplies for miners, we invite you to the appropriate course at Bgacenter
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