BTA41-600BRG stuck ON in some cases

[t_jonas]

We are using BTA41-600BRG triacs on our temperature control boards to turn on 240V tank heaters. The tank heaters are 1500W, 3000W, or 4500W depending on the unit model and operate at 240VAC. The circuit we are using is attached. We use a FOD4108 driver to activate the triac.

We have seen at least 2 cases recently where the triac continues to conduct even after the trigger for the driver goes off. In both cases, when we move the PCB to a different unit with the reduced load of 1500W heater, the triac works normally and turns off when we turn off the trigger. In another simulation with the failed triac on the larger size heater, if we turn off the unit while near the setpoint temperature, then turn the unit back on, the PID is working and we pulse the heaters; and in this case it is working okay also (triac shuts off when trigger shuts off). But if we turn on the unit when the temperature is far from the setpoint and the heater is on 100%, it never turns off when its gets close and the trigger pulses or turns off completely once we pass the setpoint.

The triacs that fail are brand new and the problem is observed before we send it to our customer.

In the first case we replaced the triac and the problem went away. In the second case we have not made any change yet.

Do you have any suggestions as to why this might be happening? We did not make any changes on the seconds PCB with this problem yet because we would like to troubleshoot to figure out what is going on.

[Peter BENSCH]

Welcome @t_jonas, to the community!

There could be several reasons for what you've noticed, including an incorrectly sized snubber. R26 (39.2Ω)  seems to have the right value für your big loads, but I'm not sure if 1W is enough for the maximum power dissipation. At 10nF, C19 certainly does not have sufficient capacity to filter fast dv/dt changes and would probably need to be increased to around 330nF...680nF or even bigger at R26=39.2Ω.

Furthermore, the inductive components of the supply line can also contribute to the problem and delay the current, causing the current to remain above the holding current when switching off. When the current then reaches zero crossing and the triac wants to block, the triac can be reignited by the resulting dv/dt, partly because the snubber capacitor C19 is too small.

In addition, at 3kW or 4.5kW, the power loss in the triac leads to excessive self-heating, which further exacerbates the sensitivity to the above-mentioned reasons. Adequate cooling can counteract this effect.

Hope that helps?

Regards
/Peter