Maintenance Methods and Techniques for Switching Power Supplies
1. When repairing a switching power supply, first use a multimeter to check whether each power component is broken down or short circuited. The casing of the switching power supply, such as the power rectifier bridge stack, switch tube, and high-frequency high-power rectifier tube, should be checked; Does the high-power resistor that suppresses surge current burn out. Re check whether the resistance of each output voltage port is abnormal. If the above components are damaged, they need to be replaced.
2. After completing the steps, if the power supply is still not working properly, it is necessary to check the power factor module (PFC) and pulse width modulation component (PWM), refer to relevant information, and familiarize oneself with the functions of each pin of the PFC and PWM modules and the necessary conditions for their normal operation.
3. Then, for power supplies with PFC circuits, it is necessary to measure whether the voltage at both ends of the filtering capacitor is around 380VDC. If there is a voltage of around 380VDC, it indicates that the PFC module is working properly. Next, the working status of the PWM component is checked, and the power input terminal VC and reference voltage output terminal VR are measured to start the control Vstart/VControl terminal voltage. Use a 220VAC/220VAC isolation transformer to supply power to the switching power supply. Use an oscilloscope to observe whether the waveform of the PWM module CT terminal to ground is a linear sawtooth wave or a triangle. For example, TL494 CT terminal is a sawtooth wave, and FA5310 CT terminal is a triangle wave. Is the waveform of output V0 an ordered narrow pulse signal. Maintenance Methods and Techniques for Switching Power Supply
4. In the practice of repairing switch mode power supplies, many switch mode power supplies use UC38 XXX series 8-pin PWM components. Most switch mode power adapters cannot work because the power starting resistor is damaged or the chip performance is reduced. When there is no VC after R circuit breaker, the PWM component cannot work and needs to be replaced with a resistor with the same power resistance as the original one. 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 GE DR power supply, the PWM module was UC3843 and no other abnormalities were detected. After connecting a 220K resistor to R (220K), the PWM component worked and the output voltage was normal. Sometimes, due to peripheral circuit faults, the VR terminal's 5V voltage is 0V and the PWM component does not work. When repairing the Kodak 8900 camera power supply, if this situation is encountered, the external circuit connected to the VR terminal is disconnected, and the VR changes from 0V to 5V. The PWM component works normally and the output voltage is normal. Maintenance Methods and Techniques for Switching Power Supply
