A brief discussion on the maintenance of switching power supplies: practice leads to true knowledge

　　When a switching power supply has a problem, how should it be repaired ? This article specifically describes the methods and steps for repairing a switching power supply and some experience summaries.

　　Specific methods for switching power supply maintenance

　　When repairing a switching power supply, we first need to use a multimeter to check whether each power device has a breakdown short circuit, such as whether the power rectifier bridge stack, switching tube, high-frequency high-power rectifier tube, and high-power resistor that suppresses surge current are burned out, etc. Then we need to check whether the resistance of each output voltage port is abnormal. If the above devices are damaged, we need to replace them with new ones.

　　After completing the above tests, if it still cannot work normally after turning on the power, we need to test the power factor module (PFC) and pulse width modulation component (PWM), consult relevant information, and familiarize ourselves with the functions of each pin of the PFC and PWM modules and the necessary conditions for the normal operation of the modules.

　　For power supplies with PFC circuits, it is necessary to measure whether the voltage across the filter capacitor is about 380VDC. If there is a voltage of about 380VDC, it means that the PFC module is working properly. Then detect the working status of the PWM component, measure whether the voltage of its power input terminal VC, reference voltage output terminal VR, and start control Vstart/Vcontrol terminal is normal, use a 220VAC/220VAC isolation transformer to power the switching power supply, and use an oscilloscope to observe whether the waveform of the CT terminal of the PWM module to the ground is a well-linear sawtooth wave or triangle, such as the TL494 CT terminal is a sawtooth wave, and the FA5310 CT terminal is a triangle wave. Whether the waveform of the output terminal V0 is an orderly narrow pulse signal.

　　In the practice of switching power supply maintenance, many switching power supplies use UC38×× series 8-pin PWM components. Most power supplies cannot work because the power supply startup resistor is damaged or the chip performance is degraded. When R is disconnected and there is no VC, the PWM component cannot work and needs to be replaced with a resistor with the same power resistance as the original. When the starting current of the PWM component increases, the R value can be reduced until the PWM component can work normally. When repairing a GEDR power supply, the PWM module is UC3843, and no other abnormalities are found during the inspection. After connecting a 220K resistor to R (220K), the PWM component works and the output voltage is normal. Sometimes due to a peripheral circuit failure, the 5V voltage at the VR end is 0V, and the PWM component does not work. When repairing the Kodak 8900 camera power supply, this situation is encountered. Disconnect the external circuit connected to the VR end, VR changes from 0V to 5V, the PWM component works normally, and the output voltage is normal.

　　When there is no voltage of about 380VDC on the filter capacitor, it means that the PFC circuit is not working properly. The key detection pins of 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, there is no 380VDC voltage on the filter capacitor on the test board. The VC, Vstart/control, CT and RT waveforms and the V0 waveform are all normal. There is no V0 waveform when measuring the G pole of the field effect power switch tube. Since FA5331 (PFC) is a patch component, the V0 terminal and the board have a cold solder joint after the machine has been used for a long time, and the V0 signal is not sent to the G pole of the field effect tube. Solder the V0 terminal to the solder joint on the board, and use a multimeter to measure the filter capacitor to have a 380VDC voltage. When the Vstart/control terminal is at a low level, the PFC cannot work either, so it is necessary to detect the relevant circuits connected to its terminals and the periphery.

　　In short, the switching power supply circuits are easy or difficult, the power is large or small, and the output voltage is varied. As long as you grasp the core things, that is, fully familiar with the basic structure of the switching power supply and the characteristics of the PFC and PWM modules, their basic working conditions, follow the above steps and methods, and do more hands-on maintenance of the switching power supply, you can quickly eliminate the switching power supply fault and achieve twice the result with half the effort.

　　Switching power supply maintenance experience

　　When the switching power supply fails to start oscillation, we usually need to check whether the switching frequency is correct, whether the protection circuit is blocked, whether the voltage feedback circuit and current feedback circuit are OK, whether the switching tube is broken down, etc.

　　If the switching power supply transformer heats up or makes a "hissing" sound, it is generally because the switching frequency is incorrect.

　　If the output voltage power indicator of the switching power supply flashes, it usually means that there is a short circuit on the secondary side.