Repair of switch power supply malfunction in Jialing VSD JP6C-9

Power on, there is no display on the operation panel, and the resistance of the input and output terminals of the main circuit is normal. Diagnosed as a power failure on the control board switch. Listening carefully, there are slight intervals of clicking and clicking sounds, which clearly indicate difficulty in starting the power supply. According to experience, this phenomenon is often caused by abnormal power load. Check the rectification, filtering, and load circuits of each power supply, and there are no abnormalities; Disconnect the power supply branches with high current, such as the cooling fan power supply, inverter drive power supply, and operation panel display power supply, but the fault still persists.

Check the peak voltage absorption network in parallel with the primary winding of the switch transformer (connected in series with the diode after parallel connection of the resistor and capacitor). Use a pointer multimeter to measure the forward and reverse resistance of the diode, both of which are 15 ohms, and feel abnormal. Disassemble and test the two parallel diodes, they are normal. Upon closer observation, there are slight cracks in the capacitor. Upon testing its pins, it was found that it was a 2kV 103 capacitor breakdown short circuit. After replacement, the machine returned to normal.
It is rare for this capacitor short circuit to cause switching power supply to have difficulty starting vibration.

The setting of this voltage peak voltage absorption network was originally intended to absorb the abnormal peak voltage generated during the cut-off period of the switching tube, which endangers the safety of the switching tube. However, after capacitor breakdown, the primary winding of the switching transformer is equivalent to parallel connection of diodes. For switch transformers, the energy absorbed during the conduction period of the switch tube is quickly discharged by the diode during the cut-off period of the switch tube, which cannot accumulate oscillation energy. At the same time, the diode acts as an excessive load on the switch transformer, causing difficulties in starting the switch power supply.。