Causes And Maintenance Methods Of Low Output Voltage Of Switching Power Supply

Reasons for low output voltage of switching power supply:

(1) The working voltage provided by the 220V AC voltage input and the rectification filter circuit to the switching tube is insufficient, which exceeds the control range of the pulse width adjustment circuit.

(2) The overcurrent in the load circuit causes the load of the switching power supply to increase and the output voltage to drop.

(3) The on/off switch is wrong. After the online scanning circuit starts to work, the switching power supply is in standby state immediately.

(4) The terminal of the on/off interface circuit is in a state between startup and standby due to a fault, which causes the output voltage of the switching power supply to be lower than the normal value and higher than the standby value.

(5) Due to a fault, the end of the protection circuit enters a conduction state, causing the power supply to enter a weak vibration state, resulting in a drop in the output voltage of the switching power supply.

(6) The diode, filter capacitor and current limiting resistor in the rectification output circuit are damaged, and the output voltage is low.

(7) The fault of the pulse width modulation circuit cannot correctly respond to the change of the output voltage of the switching power supply, and the direction of the base voltage of the switching tube is not adjusted correctly, resulting in a low output voltage of the switching power supply.

(8) The value of the positive feedback resistor in the positive feedback circuit changes, the performance of the freewheeling diode decreases or the constant current source fails, resulting in insufficient positive feedback, resulting in a longer oscillation period, lower oscillation frequency, and lower output voltage of the switching power supply .

Methods and steps for determining faults

The cause of the low voltage involves all parts of the switching power supply itself and all circuits related to the switching power supply, and the scope of failure should be reduced during maintenance.

(1) First measure the voltage of the C pole of the switch, and confirm that the switching power supply is normal.

(2) Determine the fault according to the voltage of each output terminal of the switching power supply.
If the fault is in the rectified output circuit with low output voltage, check and replace the current limiting resistor, rectifier diode and filter capacitor in the circuit. If the current limiting resistor gets hot, the load is overcurrent and the load has been checked.
Each output of the switching power supply is low. This situation indicates that the load and the output circuit of the rectifier are normal, and the fault may be caused by the positive feedback circuit of the switching power supply, the pulse width adjustment, the conduction/standby circuit and the protection circuit.

Some output voltages have a larger rate of decrease and some output voltages have a smaller rate of decrease. The measurement results show that the output voltage drop caused by the circuit fault is relatively large. At this point, the load can be disconnected. If the circuit is broken, a dummy load should be connected. After disconnecting the load, measure the output voltage of the switching power supply again. If it returns to normal, it can be inferred that the circuit load has overcurrent. If it is still abnormal, it means that the fault is in the rectification and filtering circuit.
Replace the original sampling circuit with a self-made sampling circuit to determine the fault range.

(1) After the replacement, the voltage returned to normal, indicating that the fault was in the sampling circuit and the end coupler circuit.

(2) If the voltage is still low, disconnect the B+ access point of the original sampling circuit. If the voltage is still low, check the B+ filter capacitor. Once confirmed to be good, troubleshoot in the hot baseplate section. First check to see if the soft start circuit is shunting the B pole of the switch. If it still doesn't work, check the positive and negative feedback circuits.
The method of checking the negative feedback of the thermal bottom plate is similar to the method of checking the high voltage, and the method used is to force the B+ output to a high level (note: changing the operating point will not cause B+ to be too high). high, leading to fault expansion.

In the maintenance of the power supply, you can use reverse thinking when the voltage is unstable, reduce the voltage when the voltage is high, increase the voltage when the voltage is low, and artificially change the operating point voltage if necessary. In order to find the point of failure, maintenance personnel must be quick thinking.