1,When repairing the switching power supply, first use a multimeter to detect whether the power components are short-circuited.


Such as power rectifier bridge stack, switching tubes, high-frequency high-power rectifier; inhibit inrush current of high-power resistors are burned. Then test the output voltage port resistance is abnormal, the above components need to be replaced if damaged.

2, after the completion of the first step, the power supply can not work properly, then to detect the power factor module (PFC) and pulse width modulation components (PWM), consult the relevant information, familiar with the PFC and PWM module, the function of each foot and its modules work properly necessary conditions.

3, then, for the power supply with PFC circuit is required to measure the filter capacitor voltage is about 380VDC, such as about 380VDC voltage, indicating that the PFC module is working properly, and then detect the PWM components of the working state, measuring its power input VC, reference voltage output VR, start control Vstart/Vcontrol terminal voltage is normal, the use of 220VAC / 220VAC / 220VAC / 220VAC components (PWM). 220VAC/220VAC isolation transformer to the switching power supply, using an oscilloscope to observe the PWM module CT terminal to ground waveform is a good linear sawtooth wave or triangle, such as the TL494 CT terminal for the sawtooth wave, the FA5310 CT end of the triangle wave. Whether the waveform of output V0 is an orderly narrow pulse signal.

4, in the switching power supply maintenance practice, there are many switching power supplies using UC38 × × series 8-pin PWM components, the

Most power supplies do not work because the power supply start-up resistor is damaged, or chip performance degradation. When R is broken without VC, the PWM component can not work, you need to replace the resistor with the same value as the original power resistance. When the PWM component startup current increases, the R value can be reduced until the PWM component can work normally. In the repair of a GE DR power supply, PWM module for the UC3843, testing did not find any other abnormalities in the R (220K) on the parallel connection of a 220K resistor, PWM components work, the output voltage are normal. Sometimes due to peripheral circuit failures, resulting in the VR terminal 5V voltage of 0V, PWM components do not work, in the repair of the Kodak 8900 camera power supply, encountered this situation, connected to the VR side of the external circuit to disconnect the VR from 0V to 5V, PWM components work normally, the output voltage are normal.

5, when the filter capacitor on the voltage of 380VDC or so, indicating that the PFC circuit does not work properly.