Common faults of UPS power supply and their solutions

1) Check the battery voltage to see if it is undercharged. If the battery isn’t fully charged, inspect the battery itself or the charging circuit for faults.

2) If the battery voltage is normal, test the inverter drive circuit. If the drive circuit outputs correctly, the inverter may be damaged.

3) If the drive circuit is not functioning properly, check if the waveform generation circuit produces a PWM control signal. If a signal is present, the issue lies in the drive circuit.

4) If no PWM signal is generated, determine if the protection circuit has blocked the output. If so, identify the cause of the protection trigger.

5) If the protection circuit is not active and the voltage is normal, but no PWM signal is present, the waveform generation circuit is likely faulty.

These steps can also be reversed in some cases, which may lead to quicker fault identification.

2. The battery voltage is low, and even after turning on the system for over ten hours, it still doesn't charge. Fault Analysis: This suggests a possible issue with the battery or charging circuit. Follow these checks:

1) Verify that the input and output voltages of the charging circuit are within normal range.

2) If the input is normal but the output is not, disconnect the battery and retest.

3) If the output remains abnormal, the charging circuit is likely faulty.

4) If the charging circuit functions normally without the battery connected, the battery may be damaged due to long-term disuse, deep discharge, or reaching the end of its lifespan.

3. A pair of power transistors in the inverter’s power stage is damaged. After replacing them with the same type, the new transistors burn out shortly after. This could be due to excessive current, caused by:

1) A faulty overcurrent protection circuit that fails to activate during an overload condition.

2) A malfunctioning PWM component causing asymmetrical waveforms. One transistor conducts longer than the other, leading to imbalance and potential simultaneous conduction, resulting in damage.

3) Large parameter differences between the transistors. Even with symmetrical input, the output becomes unbalanced, causing magnetic flux imbalance in the transformer. This leads to saturation, sudden current spikes, and eventual transistor failure.

4. After turning on the UPS, there is no display on the panel, and the system does not operate. Fault Analysis: This indicates an issue with the mains input, battery detection, or battery voltage monitoring circuits.

1) Check if the mains input fuse is blown.

2) If the fuse is intact, inspect the battery fuse. Some UPS systems will shut down all output and display if the battery voltage is not detected during self-test.

3) If the battery is in good condition, verify the functionality of the mains detection circuit. A faulty circuit may prevent the UPS from starting on mains power.

4) If the mains detection circuit is working, check the battery voltage detection circuit for proper operation.

5. When the mains is connected, the UPS turns on, but the relay repeatedly clicks, the battery voltage is too low, and the buzzer sounds continuously. Fault Analysis: This indicates a low battery voltage preventing successful startup. Remove the battery and perform equalization charging (charge all batteries in parallel). If this doesn’t work, replace the battery.

6. In a backup UPS, when utility power is available, it operates normally. However, when the power fails, the inverter provides output, but the voltage is low, and the transformer emits loud noise. Fault Analysis: The inverter output suggests the final drive circuit is functioning. The transformer noise indicates asymmetry in the push-pull circuit. Perform the following checks:

1) Confirm that the power supply is stable.

2) If the power is normal, check the pulse width output circuit for correct signal output.

3) If the pulse width output is normal, verify the drive circuit output.

7. When the mains is normal, the UPS turns on, the inverter indicator flashes, and the buzzer sounds intermittently. The system only works in inverter mode and cannot switch to mains power. Fault Analysis: The inability to switch to mains power suggests a problem in the conversion process from inverter to mains.

1) Check if the mains input fuse is damaged.

2) If the fuse is intact, test the output of the mains rectifier and filter circuit.

3) If the rectifier/filter circuit is working, check the mains detection circuit.

4) If the mains detection circuit is normal, verify the output of the inverter-to-mains conversion control circuit.

8. In a backup UPS, when the load is close to full capacity, the battery fuse blows when the system is running on battery power. Fault Analysis: Blown battery fuse indicates excessive current from the battery. Perform the following checks:

1) Check if the inverter is damaged.

2) Ensure the battery voltage is not too low.

3) If the voltage is low, confirm the charging circuit is functioning properly.

4) If the charging circuit is normal, check the battery voltage detection circuit for proper operation.

9. The UPS can only be powered by the mains and cannot switch to inverter power. Fault Analysis: This suggests a failure in the conversion from mains to inverter power.

1) Check if the battery voltage is too low and if the battery fuse is intact.

2) If the battery is partially functional, test the battery voltage detection circuit.

3) If the detection circuit is normal, verify the output of the mains-to-inverter conversion circuit.