In the practice of switching power supply maintenance, there are many switching power supplies that use the UC38×× series 8-pin PWM components. Most power supplies cannot work because the power-on resistors are damaged or the chip performance 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 GEDR 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.
When there is no voltage of about 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 end and the board after a long time. Solder, the 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, the PFC will not work. It is necessary to detect the relevant circuits whose terminals are connected to the periphery.
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.