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What is the reason for the low voltage output of the switching power supply?

Article Source：Kinri Energy | Author：Kinri Energy | Issuing Time：2024.05.22
The reason for low voltage output of switching power supply


24V 20A 480W Switching Power Supply

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

(2) The presence of overcurrent in the load circuit causes an increase in the load of the switching power supply, resulting in a decrease in output voltage.

(3) Switching error on/off. When the scanning circuit first starts working, the power switch is in standby mode. This type of fault is applicable to machines without a backup power source. The CPU power source is taken from the same power source and not provided by a secondary power source.

(4) The end of the on/off interface circuit is in a state between startup and standby due to a malfunction, resulting in the output voltage of the switch power supply being lower than the normal value and higher than the standby value.

(5) The end of the protection circuit enters a conductive state due to a fault, causing the power supply to enter a weak vibration state and causing a decrease in the output voltage of the switching power supply.

(6) The diode, filtering capacitor, and current limiting resistor in the rectifier output circuit are damaged, causing low output voltage.

(7) Pulse width modulation circuit malfunction, unable to respond correctly to changes in the output voltage of the switching power supply, causing incorrect adjustment direction of the switching transistor base voltage, resulting in low output voltage of the switching power supply.

(8) The positive feedback resistance value in the positive feedback circuit changes, the performance of the freewheeling diode deteriorates, or the constant current source malfunctions, resulting in insufficient positive and negative feedback, leading to longer oscillation periods and a decrease in oscillation frequency, thereby causing low output voltage of the switching power supply.

(9) The excitation switching power supply operates in a low-frequency state due to the lack of reverse pulse, resulting in low output voltage.

Methods and steps for diagnosing faults

From the above analysis, it can be seen that the cause of low voltage involves various parts of the switching power supply itself and all circuits related to the switching power supply. When repairing, the fault range should be narrowed down first.

(1) First, measure the voltage at pole C of the switch tube to confirm that the power supply to the switch tube is normal.

(2) Determine the fault based on the voltage at each output terminal of the switching power supply.

Some output terminals of switch mode power supplies have normal voltage, while others are below normal values. The fault lies in the rectifier output circuit with low output voltage. The current limiting resistor, rectifier diode, and filtering capacitor in the circuit should be checked and replaced. If the current limiting resistor is hot, it indicates that the load is overcurrent. Check the load.

All outputs of the switching power supply are low. This situation indicates that both the load and rectifier output circuits are normal, and the fault lies in the positive feedback circuit, pulse width adjustment, on/standby circuit, and protection circuit of the switching power supply.

Some output voltages have a large decrease in proportion, while others have a small decrease in proportion. The measurement results indicate that the fault is in the circuit with a large proportion of output voltage drop. At this point, this load can be disconnected. If the disconnected circuit is a line circuit, a dummy load should be connected. After disconnecting the load, measure the voltage at each output terminal of the switching power supply. If it returns to normal, it can be determined that there is overcurrent phenomenon in the load of the disconnected circuit. If it is still abnormal, it indicates that the fault is in the rectification and filtering circuit.

Disconnect the main load, connect the light bulb, and determine if there is a load fault.

For some machines with flashing receiving diagrams and unstable voltage after loading, it is difficult to identify whether the fault is in the power supply or load. The "borrowing method" can be used to load another machine of the same size and B+voltage with this power supply for judgment.

Reserve the startup, positive feedback, soft start, and negative feedback circuits.

Cancel various protection circuits and standby control circuit terminal transistors one by one. Start up and observe whether the fault has been eliminated to gradually narrow down the scope of the fault.

Attention: Circuits that have both voltage stabilizing effects cannot be disconnected (such as optocouplers). Be cautious when disconnecting the protective circuit and take measures to prevent voltage rise.

Using alternative methods and repairing the pulse width adjustment circuit. Replace the original sampling circuit with a self-made sampling circuit to determine the fault range.

(1) After replacement, the voltage returned to normal, indicating that the fault lies in the sampling circuit and optocoupler circuit.

(2) If the voltage is still low, disconnect the original sampling circuit B+connection point. If the voltage is still low, check the B+filtering capacitor. After confirming that it is good, the fault can be traced to the hot bottom plate section. First, check if the soft start circuit has diverted the B pole of the switch tube. Still not working, check the positive and negative feedback circuits.

The negative feedback method for checking the hot bottom plate is similar to the method for checking high voltage, using the idea of forcing B+to output high (note: changing the working point should not cause B+to be too high and expand the fault).

In short, in the maintenance of power supplies, reverse thinking can be used when the voltage is unstable. When the voltage is high, it becomes low, and when the voltage is low, it becomes high. If necessary, manual changes to the working point voltage can be used. To facilitate the identification of fault points, it is important for maintenance personnel to have a flexible grasp.
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