A316J optocoupler driving characteristics and the damage caused by faults to IGBT modules

As a high-performance inverter module driver IC, A316J has been applied in many types of frequency converters. The OC signal fed back to the CPU through A316J can provide fast and effective protection for the IGBT module. The protection mechanism is as follows: during the driving pulse setting period of the CPU (the IGBT module should also be in conduction period), A316J forms a closed loop with the C and E poles of the driven IGBT tube through the peripheral circuits of pin 14 and pin 16. When the voltage drop of the IGBT tube is detected to be greater than 7V, A316J blocks the output pulse and sends an OC alarm signal (effective at low level) to the CPU from pin 6, causing it to implement protection shutdown action. If the fault state needs to be resolved, the CPU will send a reset signal to pin 5 of A316J (effective at low level), and A316J will release the pulse lock and enter the working state.

The internal resistance of IGBT tubes is within the normal current range, and their voltage drop is generally within 3V. When abnormal overcurrent occurs, the voltage drop of the tubes rises sharply, and the tubes face the edge of breakdown damage. At this time, quick break protection must be implemented. This task is completed by A316J sending OC signals to the CPU.
There are three situations during the work process that can cause A316J to report OC signals:

1. Abnormal load causes excessive operating current (about 3 times higher than the rated current), resulting in a voltage drop of more than 7V in the IGBT tube;
2. IGBT pipes have open circuit damage;
3. The poor driving circuit causes underexcitation of the IGBT transistor. At this time, although the output current is small, the voltage drop of the transistor is greater than the action threshold due to the transistor being in a slightly conductive and randomly turned off state, which will also send an OC signal.
   When repairing, when it is necessary to judge the quality of A316J and pulse circuit (when the quality of IGBT tubes is not known, disconnect the power supply of the inverter circuit), the 14 pin external resistance of A316J can be short circuited with the diode series circuit to A316J’s 16 pin, to remove the OC alarm and pulse locking functions, which is conducive to detecting the quality of A316J itself and pulse input and output circuits.

An example of A316J driver malfunction:
Repair a low-power frequency converter, replace the damaged IGBT module, power on and test the machine. When the frequency rises to 20Hz or above, the motor produces a “click click” sound, accompanied by the vibration of the motor body. During operation, there may be an OC shutdown phenomenon. Detect the output voltage and there is also jitter phenomenon. It is determined that one of the tubes in the IGBT module has poor conductivity or performance, or that one of the driving circuits is faulty.

1. When the 6 pins of the W upper arm drive A316J are removed, the OC fault will no longer jump during no-load operation. Check the peripheral components of pin 14 of A316J and find no abnormalities. After replacing A316J, the fault still persists;
2. Individual replacement of the IGBT tube on the upper arm of W was tested, but the fault still persists;
3. The fault is still in the peripheral circuit of A316J. Remove the two 100uF/25V capacitors from the A316J power supply, and the detection capacity is only a few micrometers. I also checked the capacitors of other driving power supplies and found that there was a loss of capacity or a serious decrease in capacity. The cause of the damage to the inverter module is here.
   After the capacitor of the driving power supply fails, the load carrying capacity of the power supply is greatly reduced. This is manifested in the low speed no-load operation of the load (low current operation), where the IGBT tubes in the module still maintain a small conduction voltage drop, and the motor can still operate stably. When the frequency rises or operates under load, due to the capacitor failure of the driving power supply and the decrease in driving capacity, the IGBT tube cannot be opened well, forming a large conducting resistance, resulting in serious three-phase imbalance and causing motor jumping. Furthermore, due to the continued increase in tube voltage drop, the A316J detection circuit outputs an OC signal and protects the shutdown. If the filtering capacitor with a positive driving voltage loses capacity, it will only cause underexcitation, trip the OC signal, and shut down the machine; So the loss of capacitance of the filtering capacitor with a negative cut-off voltage can bring dangerous consequences! Insufficient cut-off negative pressure can easily cause IGBT tubes to fail to cut off effectively after opening, leading to the possibility of a common short circuit between the upper and lower arm tubes of a certain phase, resulting in the explosion and damage of the inverter module!

The loss of cut-off negative pressure is the first killer of IGBT module damage, and the above reasons should be the second killer. The loss of capacity of the DC circuit energy storage capacitor is the third killer of IGBT module damage. So for machines with damaged inverter modules, we cannot ignore the inspection of the filtering capacitor of the driving power supply, especially for machines that have been used for a long time and low-power models with limited space and poor heat dissipation!