Switching Power Supply Repair Methods and Practical Experience
Switching Power Supply Repair Methods and Practical Experience
Switching Power Supply Repair Methods and Practical Experience I. Specific Methods for Switching Power Supply Repair 1. When repairing a switching power supply, we first need to use a multimeter to check whether each power component has a short circuit or breakdown. For example, check if the power rectifier bridge, switching transistors, high-frequency high-power rectifier diodes, and high-power resistors for surge current suppression are burnt out. Then, check if the resistance at each output voltage port is abnormal. If any of these components are damaged, we need to replace them.
2. After completing the above tests, if the power supply still does not work properly, we need to check the power factor correction module (PFC) and pulse width modulation (PWM) module. Consult relevant documentation to familiarize ourselves with the function of each pin of the PFC and PWM modules and the necessary conditions for their normal operation. 3. For power supplies with PFC circuits, measure the voltage across the filter capacitor to ensure it is approximately 380VDC. If it is, the PFC module is functioning correctly. Next, check the PWM module's operating status by measuring the voltage at its power input terminal VC, reference voltage output terminal VR, and start/control terminals Vstart/Vcontrol. Power the switching power supply using a 220VAC/220VAC isolation transformer and observe the waveform at the PWM module's CT terminal relative to ground using an oscilloscope. Check if the waveform is a linear sawtooth or triangular wave (e.g., sawtooth wave at the TL494 CT terminal, triangular wave at the FA5310 CT terminal). Also check if the output V0 waveform is an ordered, narrow pulse signal.
4. In switching power supply repair practice, many power supplies use UC38×× series 8-pin PWM modules. Most power supply malfunctions due to a damaged start-up resistor or degraded chip performance. When R is open-circuited, there is no VC, and the PWM module cannot function. Replace it with a resistor of the same power rating. If the PWM module's start-up current increases, reduce the value of R until the PWM module functions normally. When repairing a GEDR power supply, the PWM module was a UC3843. No other abnormalities were found during testing. After connecting a 220K resistor in parallel with R (220K), the PWM module worked, and the output voltages were normal. Sometimes, due to external circuit faults, the 5V voltage at the VR terminal was 0V, and the PWM module did not work. This situation was encountered when repairing a Kodak 8900 camera power supply. Disconnecting the external circuit connected to the VR terminal changed VR from 0V to 5V, and the PWM module worked normally, with all output voltages normal.
5. When there is no voltage of approximately 380VDC on the filter capacitor, it indicates that the PFC circuit is not working properly. The key detection pins for the PFC module are the power input pin VC, the start pin Vstart/control, the CT and RT pins, and the V0 pin. When repairing a Fuji 3000 camera, testing revealed no 380VDC voltage on the filter capacitor on one board. The waveforms VC, Vstart/control, CT, RT, and V0 are all normal. However, there is no V0 waveform when measuring the gate (G) of the MOSFET. Since the FA5331 (PFC) is a surface-mount component, a poor solder joint may develop between the V0 terminal and the board after prolonged use, preventing the V0 signal from reaching the MOSFET's G. After resoldering the V0 terminal, a multimeter was used to measure 380VDC in the filter capacitor. When the Vstart/control terminal is low, the PFC also fails to operate, requiring inspection of its terminals and related external circuitry.
In summary, switching power supply circuits vary in complexity, power output, and output voltage. By grasping the core concepts—familiarizing oneself with the basic structure of switching power supplies and the characteristics of PFC and PWM modules, as well as their operating conditions—and by following the steps and methods described above and practicing switching power supply repairs, one can quickly troubleshoot switching power supply faults and achieve twice the result with half the effort.
II. Experience in Repairing Switching Power Supplies 1. When a switching power supply fails to start, we usually need to check if the switching frequency is correct, if the protection circuit is blocked, if the voltage feedback circuit and current feedback circuit are working properly, and if the switching transistor is short-circuited.
2. If the switching power supply transformer overheats or makes a "hissing" sound, it generally indicates an incorrect switching frequency.
3. If the power indicator light on the switching power supply output voltage is flashing, it generally indicates a short circuit on the secondary side.
