Maintenance of Switch Power Supply Fault in Taian N2-1013 VSD

Upon powering on, there was an OC fault. It was detected that the inverter output module was not damaged, and most of the six inverter drive ICs were damaged. Further inspection revealed a peculiar phenomenon in the switch mode power supply: when the CPU motherboard was disconnected for power supply,+5V was measured to be normal, but the power supply of other branches was higher than normal, such as+15V being+18V, and the driving power supply of 22V being 26V. When the wiring block of the CPU motherboard was connected,+5V was measured to be normal, but the power supply of other branches showed an abnormal increase! If the driving power supply of 22V even rises to nearly 40V (the maximum supply voltage of PC923 and PC929 is 36V), the damage to the driving IC is caused by this.

Key inspections were conducted on the voltage stabilization process, and peripheral circuits such as IC202 and PC9 showed no abnormalities. Further investigation revealed no abnormalities in other circuits, and maintenance was deadlocked.
Analysis: The voltage stabilizing part of the circuit works. The voltage sampling of the voltage regulator circuit is taken from the+5V circuit. When the wiring block of the CPU motherboard is unplugged, it is equivalent to a light load or no load of+5V. The rising trend of+5V increases the negative feedback of the voltage, reduces the duty cycle of the power switch driver pulse, reduces the excitation current of the switch transformer, and the output voltage of other branches is relatively low; When inserted into the wiring block of the CPU motherboard, it is equivalent to a+5V load or overload. The decreasing trend of+5V reduces the negative voltage feedback, increases the duty cycle of the power switch driver pulse, and increases the excitation current of the switch transformer, causing the output voltage amplitude of other branches to increase. The current situation is that when the+5V circuit is unloaded, although the output of other power supplies is lower, it is still higher+ After 5V loading, other power supply branches exhibit abnormally high voltage output! The faulty link is either due to a malfunction of the power supply itself causing a decrease in load capacity, or an abnormality in the load circuit. Both abnormalities have caused the voltage regulator circuit to undergo conscientious “misregulation”, resulting in the maintenance of the “voltage stability” of the+5V faulty circuit and the occurrence of “abnormal voltage changes” in other power supply branches!

Start repairing the+5V circuit, unplug the power filter capacitor C239220u10V, and check that the capacity is only a dozen microfarads, with obvious leakage resistance. The failure of a capacitor perfectly satisfies two conditions: a decrease in capacity reduces the power supply’s carrying capacity, and leakage causes the load to become heavier.
After replacing this capacitor, the test run was normal.