1. When repairing the switching power supply, first use a multimeter to check whether the power components are short-circuited, such as the power rectifier bridge, the switch, the high-frequency high-power rectifier; whether the high-power resistor that suppresses the inrush 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 will not work normally after the power is turned on. Then, the power factor module (PFC) and the pulse width modulation component (PWM) should be detected. Check the relevant information and familiarize with the functions of each pin of the PFC and PWM modules. The prerequisites for the normal operation of the module.
3. Then, for a power supply with a PFC circuit, it is necessary to measure whether the voltage across the filter capacitor is about 380 VDC. If there is a voltage of about 380 VDC, the PFC module works normally, and then the working state of the PWM component is detected, and the power input terminal VC is measured. The reference voltage output terminal VR starts to control whether the voltage of the Vstart/Vcontrol terminal is normal. The 220VAC/220VAC isolation transformer is used to supply power to the switching power supply. The oscilloscope is used to observe whether the waveform of the PWM module CT to ground is a linear sawtooth wave or triangle, such as TL494. The CT end is a sawtooth wave, and the FA5310 has a CT wave at the CT end. Whether the waveform of the output terminal V0 is an ordered narrow pulse signal.
4. In the practice of switching power supply maintenance, there are many switching power supplies that use UC38×× series 8-pin PWM components. Most power supplies cannot work because the power-on resistors are damaged or the performance of the chips is degraded. When there is no VC after R is disconnected, the PWM component cannot work, and the same resistance as the original power resistance needs to be replaced. When the PWM component startup current increases, the R value can be reduced until the PWM component can operate normally. When repairing a GE DR power supply, the PWM module is UC3843, and no other abnormalities are detected. After a 220K resistor is connected to R (220K), the PWM component works and the output voltage is normal. Sometimes due to peripheral circuit failure, the VR terminal 5V voltage is 0V, the PWM component does not work. When the Kodak 8900 camera power supply is repaired, this situation is encountered. The external circuit connected to the VR terminal is disconnected, and the VR changes from 0V. 5V, PWM components work normally, and the output voltage is normal.
5. When there is no voltage of 380VDC on the filter capacitor, the PFC circuit does not work normally. The key detection pin of the PFC module is 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, test that there is no 380VDC voltage on the filter capacitor on the board. VC, Vstart/control, CT and RT waveforms and V0 waveforms are normal. The measured field effect power switch tube G has no V0 waveform. Since FA5331 (PFC) is a patch component, the machine will appear between the V0 terminal and the board after a long time. Solder, V0 signal is not sent to the FET of the FET. Solder the V0 end to the solder joint on the board, and measure the filter capacitor with a 380 VDC voltage with a multimeter. When the Vstart/control terminal is low and the PFC is not working, the relevant circuit whose terminal is connected to the periphery is detected.
In short, the switching power supply circuit is easy to have, the power is large and small, and the output voltage is various. 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, the basic conditions for their operation, according to the above steps and methods, more hands-on maintenance of the switching power supply, you can quickly eliminate the switch Power failure, achieve twice the result with half the effort.
The general situation is:
1. The switching power supply does not start, generally 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 switching tube is broken or not.
2, the transformer heats up or emits a "beep" sound, generally the switching frequency is wrong;
3, the output voltage power indicator flashes a flash is generally short-circuited on the secondary side.
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