THYRISTOR FAILURES AND TESTING
Recognizing Failed SCR or Diode
It is easy to recognize a failed SCR or diode because it normally (99.9 percent) becomes a short circuit. Any fuse in the circuit is usually blown due to the overcurrent. Shorted SCRs or diodes give a resistance around 10 D when measured by a volt-ohmmeter. The remain- ing 0.1 percent, the gate of an SCR, is open. The gate to cathode resistance of a good SCR is normally 15 to 30 D while that of an open-gate SCR is infinity.
Another failure mechanism occurs near the end of the useful life of the device. The leak- age will increase due to degradation of all junctions. This results in operating problems such as numerous overcurrents. These symptoms should be taken as an indication that the devices should be replaced.
Testing of SCRs or Diodes
If the device is not experiencing problems such as intermittent overcurrent trips or blowing fuses, it should not be tested. The chance of damaging the devices by disconnecting and testing them is higher than that of finding a suspicious one. In general, if the device is oper- ating properly, do not test it.
Comments about Failure Rates
1. The lifetime of all semiconductors can be calculated.
2. The thermal fatigue life decreases when
● The temperature changes increase
● The size of the device is larger
● The device is soldered rather than press-packed
3. Damage to the device would occur at a fault current, resulting in 300°C change.
4. Fault coordination for each application should be done properly in the design phase to prevent explosion of the devices.
5. The failure rate depends on
● Temperature
● PIV
● Bridge failure mode (catastrophic or not)
6. Failed devices are normally shorted (less than 10 D).
7. There is no need to routinely test the devices if the drive is operating properly.