TRIAC burn out (half burn)

[Ario87]

Hi,
In a dishwasher machine main board we have used 107NN0 TRIAC for controlling ac loads such as valves, pumps, dispenser, etc. In about 2 to 3 percent of products sold to customers, one of TRIACs that has a parallel RC equivalent load is half-burned (TRIAC is short-circuited in one direction) when the machine gets plugged in. Here’s the schematic of a single TRIAC on mainboard. All 8 TRIACs have the same configuration. One TRIAC has RC load and others have inductive load. Surprising point is that we couldn’t reproduce the fault in lab under different test conditions such as Surge or EFT noise, input voltage connection in different angles of sine wave. We’re glad if anyone can help us about this problem.

 

[rjenkinsgb]

Gate current rise too slow?

Thyristors & triacs are usually triggered by a fast pulse; your circuit has 100nF between the driver emitter and 0V, which may slow the gate current rise enough to cause partial conduction & excess dissipation??

It's a wild guess, but other than an external fault causing an overcurrent, it's the only thing I can think of so far.