Repair of Switch Power Supply Fault in TECO 7200GA-30kW Inverter

After being damaged and repaired by lightning strikes, the frequency converter has been running for over a month and has experienced strange malfunctions: there is a random shutdown phenomenon during operation, which may occur every few days or every few hours; Difficulty starting, capacitor charging short circuit contactor jumping during the starting process, starting failed, but the operation panel does not display a fault code. After successfully starting with some effort, it can run for a period of time again.

Remove the control board from the site and short circuit the terminals of the thermal relay to prevent it from entering the thermal protection state and unable to test the machine; Short circuit the contact detection terminal of the capacitor charging contactor to prevent it from entering a low voltage protection state. The machine cannot be tested, and a comprehensive inspection was conducted. No abnormalities were found during the inspection, all of which are good.
Install the control board back into the machine, power on and test the machine. When starting, the contactor jumps and cannot start. After unplugging the connection of the 12CN plug cooling fan, the situation greatly improved and the success rate of starting increased. Upon careful observation, the brightness of the display panel decreased during the startup process, indicating that the fault was due to poor load capacity of the control power supply.
When each power supply output is unloaded, the output voltage is normal. Connect resistive loads to the output of each power source, and slightly reduce the voltage value+ After 24V is connected to the cooling fan and relay load,+5V drops to+4.7V, and the screen display and other operations are normal at this time. But if the frequency converter is put into the startup state, the relay will jump, and occasionally fault codes such as “low DC voltage” and “communication interruption between CPU and operation panel” will appear, causing the operation to fail. In measurement, when+5V drops below+4.5V, the frequency converter will immediately change from starting state to standby state. Detailed inspection of the load circuits of each power supply shows no abnormalities.
Analysis: The judgment of poor load capacity of the control power supply is correct. Due to the strict requirements of the CPU for power supply, it can still barely work at no less than 4.7V; But when it is below 4.5V, it is forced to enter “standby mode”; When the voltage is between 4.7V and 4.5V, a fault alarm will be issued to detect the operation of the circuit.
But unexpectedly, the maintenance of this malfunction was quite tricky, and after checking all the relevant components of the switch power supply, none of them were damaged! Helpless, I attempted to conduct a parallel resistance test on R1 (5101), one of the reference voltage divider resistors of U1 (KA431AZ), with the aim of changing the divider value to increase the output voltage. The measured output voltage has slightly increased, but the load capacity is still poor. Upon closer inspection of the circuit board, it appears that there are welding marks on the diversion adjustment tube Q1, but it appears that its model is the original one. Even if it is replaced, it will still be removed and replaced from similar machines. The switch transistor Q2 of this machine is a bipolar transistor with high back pressure and high amplification, which is difficult to purchase in the market, and the circuit has strict requirements for the parameters of these two transistors. Combined with fault analysis, the working point of the shunt adjustment tube is offset, causing too strong a shunt on the Q2 base current, which will result in poor load capacity of the power supply. Try to connect a resistor R6 (330 ohms) in series with a voltage feedback optocoupler and a 47 ohm resistor to reduce the base current of Q1, thereby reducing its shunt ability towards Q2 and enhancing the load capacity of the power supply. Power on test machine, regardless of loading or starting operation,+5V stable output 5V, troubleshooting!

Fault inference: The Q1 switch tube has aging phenomenon and the amplification ability has decreased. Therefore, the insufficient Ib value after shunt makes it fully conductive (increasing the conduction resistance), resulting in a decrease in the power supply’s carrying capacity; There is a characteristic deviation phenomenon in the shunt branch, which leads to excessive shunt and poor driving of the switching tube, resulting in poor load carrying capacity of the power supply.