Common faults of UPS power supply and their solutions

1. When the utility power is available, the UPS output is normal, but when there's no utility power, the buzzer sounds and there is no output.

Diagnosis: Based on the symptoms that the battery or inverter might be partially faulty, you can follow these steps to identify the issue:

1) Check the battery voltage to determine if it is not fully charged. If the battery isn’t charged, inspect whether the battery itself is faulty or if the charging circuit is malfunctioning.

2) If the battery voltage is normal, check the inverter drive circuit to see if it is operating correctly. If the drive circuit is working, then the inverter may be damaged.

3) If the drive circuit is not functioning properly, test whether the waveform generation circuit is producing a PWM control signal. If the signal is present, the problem lies in the drive circuit.

4) If no PWM signal is generated by the waveform circuit, check 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 the waveform circuit still doesn’t produce a PWM signal, the waveform generation circuit is likely faulty.

These troubleshooting steps can also be reversed for faster fault detection in some cases.

2. The battery voltage is low, and even after turning the battery on for more than ten hours, it still does not charge.

Diagnosis: This could indicate a faulty battery or charging circuit. Follow these steps:

1) Check if the input and output voltages of the charging circuit are normal.

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

3) If the charging circuit still doesn't work, it is likely faulty.

4) If the charging circuit works normally after disconnecting the battery, the battery may have been damaged due to long-term undercharging, over-discharging, or reaching the end of its lifespan.

3. A pair of power transistors in the inverter’s power stage are damaged. After replacing them with the same type, the new transistors burn out quickly due to excessive current. Possible causes include:

1) The overcurrent protection system is not functioning. When an overcurrent occurs, the protection circuit fails to respond.

2) A failure in the PWM component causes the two complementary waveforms to be asymmetrical, resulting in one transistor conducting longer than the other. This imbalance can lead to both transistors being damaged.

3) There is a significant difference in the parameters of the power transistors. Even with a symmetrical input, the output becomes unbalanced, causing magnetic flux imbalance in the transformer. This can lead to saturation and sudden current spikes, damaging both transistors.

4. After turning on the UPS, there is no display on the panel, and the unit does not operate.

Diagnosis: The issue may lie in the mains input, battery, or the battery voltage detection circuit.

1) Check if the mains input fuse is blown.

2) If the mains fuse is intact, check the battery fuse. Some UPS systems disable all outputs and displays if the battery voltage is not detected during self-test.

3) If the battery is in good condition, verify the operation of the mains detection circuit. If it is not working, the UPS will not start from the mains.

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

5. When the mains is connected, each time the UPS is turned on, the relay repeatedly clicks. The panel shows a low battery voltage, and the buzzer sounds continuously.

Based on this symptom, the issue is likely due to low battery voltage preventing successful startup. Remove the battery and perform equalization charging (charge all batteries in parallel). If the issue persists, the battery must be replaced.

6. When the backup UPS is connected to the mains, it operates normally. However, when the mains is disconnected, the inverter produces output, but the voltage is low, and the transformer makes loud noise.

Diagnosis: The inverter output indicates that the final drive circuit is mostly functional. The transformer noise suggests an imbalance in the push-pull circuit. Follow these steps:

1) Check if the power supply is normal.

2) If the power is normal, check the output signal from the pulse width modulation circuit.

3) If the PWM output is normal, check the drive circuit output.

7. When the mains is available, the UPS turns on, the inverter indicator flashes, and the buzzer sounds intermittently. The UPS can only operate in inverter mode and cannot switch back to mains power.

Diagnosis: The inability to switch from inverter to mains power indicates a problem in the conversion circuit.

1) Check if the mains input fuse is damaged.

2) If the fuse is okay, check the output of the mains rectifier and filter circuit.

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

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

8. When a backup UPS is close to full load, and the mains is available, the battery fuse blows when the battery is used as a power source.

Diagnosis: A blown battery fuse suggests that the battery is supplying too much current. Check the following:

1) Whether the inverter is damaged.

2) Whether the battery voltage is too low.

3) If the battery voltage is low, check if the charging circuit is functioning properly.

4) If the charging circuit is working, check the battery voltage detection circuit.

9. The UPS can only be powered by the mains and cannot switch to inverter power.

Diagnosis: The failure to switch to inverter power suggests an issue in the mains-to-inverter conversion process.

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

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

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