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Maintenance method of switching power supply

1. When repairing the switching power supply, first use a multimeter to check whether each power component has a breakdown and short circuit, such as the power rectifier bridge stack, switch tube, high-frequency high-power rectifier tube; whether the high-power resistor that suppresses the surge current is blown. Then check whether the resistance of each output voltage port is abnormal. If the above components are damaged, they need to be replaced.

2. After the first step is completed, it still can’t work normally after turning on the power. Next, check the power factor module (PFC) and the pulse width modulation component (PWM), consult the relevant information, and be familiar with the functions and functions of each pin of the PFC and PWM modules. A prerequisite for its modules to work properly.

3. Then, for the power supply with PFC circuit, 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 normally. Refer to the voltage output terminal VR, start the control to check whether the Vstart/Vcontrol terminal voltage is normal, use a 220VAC/220VAC isolation transformer to supply power to the switching power supply, and use an oscilloscope to observe whether the waveform from the CT terminal of the PWM module to the ground is a good linear sawtooth wave or triangle, such as TL494 The CT terminal is a sawtooth wave, and the CT terminal of the FA5310 is a triangular wave. Whether the waveform of the output terminal V0 is an ordered narrow pulse signal.

4. In the practice of switching power supply maintenance, many switching power supplies use 8-pin PWM components of the UC38XX series. Most of the power supplies cannot work because the starting resistance of the power supply is damaged or the chip performance is degraded. When there is no VC after R is disconnected, the PWM component cannot work, and the resistor with the same value as the original power resistance needs to be replaced. When the startup current of the PWM component increases, the R value can be reduced until the PWM component can work normally. When repairing a GE DR power supply, the PWM module was UC3843, and no other abnormalities were found. After connecting a 220K resistor to R (220K), the PWM component worked and the output voltage was normal. Sometimes due to the failure of the peripheral circuit, the 5V voltage of the VR terminal is 0V, and the PWM component does not work. When repairing the power supply of the Kodak 8900 camera, when this happens, disconnect the external circuit connected to the VR terminal, and the VR changes from 0V to 0V. 5V, the PWM components work normally, and the output voltages are normal.

5. 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. There is no V0 waveform at the G pole of the field effect power switch tube. Since the FA5331 (PFC) is a SMD component, there will be a virtual gap between the V0 end and the board after the machine is used for a long time. Soldering, the V0 signal is not sent to the G pole of the FET. Weld the V0 terminal to the solder spot on the board, and use a multimeter to measure the filter capacitor with a voltage of 380VDC. When the Vstart/control end is low level, the PFC can't work either, so it is necessary to detect the related circuit connected with its end point and the periphery.

In short, the switching power supply circuit is 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, you are fully familiar with the basic structure of the switching power supply and the characteristics of the PFC and PWM modules, the basic conditions for their work, and the maintenance of the switching power supply is carried out according to the above steps and methods, and you can quickly eliminate the switch. Power failure, to achieve a multiplier effect.

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