Switching power supply manufacturers share switching power supply maintenance method steps and experience summary! At the beginning of the maintenance of the switching power supply, we first need to use a multimeter to check whether there is a breakdown short circuit in each power device, such as whether the power rectifier bridge stack, switch tube, high-frequency high-power rectifier tube, high-power resistor to suppress inrush current If the resistance of each output voltage port is abnormal, we need to replace it with a new one. After completing the above tests, if it still does not work after turning on the power, then we need to test the Power Factor Module (PFC) and Pulse Width Modulation Module (PWM), consult related materials, and be familiar with each pin of the PFC and PWM modules. The function and its modules are necessary for proper operation.
For a power supply with a PFC circuit, you need to measure whether the voltage across the filter capacitor is about 380VDC. If there is a voltage of about 380VDC, the PFC module is working properly. Then check the working status of the PWM component and measure its power input VC and reference voltage output VR, start control Vstart / Vcontrol terminal voltage is normal, use 220VAC / 220VAC isolation transformer to power the switching power supply, use an oscilloscope to observe whether the waveform of the CT terminal to the ground of the PWM module is a linear sawtooth wave or triangle, such as TL494 CT terminal is sawtooth The CT end of the FA5310 is a triangular wave. Whether the waveform at the output V0 is an orderly narrow pulse signal.
In the practice of switching power supply maintenance, there are many switching power supplies that use UC38 ×× series 8-pin PWM components. Most of the power supplies cannot work because the power supply startup resistance is damaged or the chip performance is reduced. When there is no VC after R is disconnected, the PWM component cannot work, and the resistor with the same power resistance value needs to be replaced. When the startup current of the PWM component increases, the value of R can be reduced until the PWM component can work normally. When repairing a GEDR power supply, the PWM module was UC3843. No other abnormality was found during testing. After a 220K resistor was connected in parallel with R (220K), the PWM component worked and the output voltage was normal. Sometimes the 5V voltage on the VR side is 0V and the PWM component does not work because of a peripheral circuit failure. When repairing the power supply of the Kodak 8900 camera, in this case, disconnect the external circuit connected to the VR side, and VR changes from 0V to 0V. 5V, PWM component works normally, 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. VC, Vstart / control, CT and RT waveforms and V0 waveforms are all normal. The G-pole of the field effect power switch has no V0 waveform. Because FA5331 (PFC) is a chip component, there is a virtual gap between the V0 end and the board after a long time Welding, the V0 signal is not sent to the G pole of the FET. Weld the V0 terminal to the solder joint on the board, and measure the filter capacitor with a multimeter with a voltage of 380VDC. When the Vstart / control terminal is at a low level, the PFC also cannot work, so it is necessary to detect the related circuits whose endpoints are connected to the periphery.
In short, the switching power supply circuit is easy, difficult, large or small, and the output voltage is diverse. As long as you grasp its core, that is, you are fully familiar with the basic structure of the switching power supply and the characteristics of the PFC and PWM modules, and the basic conditions of their work. According to the above steps and methods, you can perform maintenance on the switching power supply and quickly remove the switch Power failure to achieve twice the result with half the effort. When the switching power supply does not start, we usually need to check whether the switching frequency is correct, whether the protection circuit is blocked, the voltage feedback circuit, the current feedback circuit is no problem, whether the switch tube is broken, etc. The switching power supply transformer generates heat or emits a "beep" sound, usually because the switching frequency is incorrect. The output voltage of the switching power supply and the power indicator light are usually short-circuited on the secondary side.
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