Hi. Below is my circuit. It is to be a motor controller but at the moment I'm testing with a 300W lamp in a domestic environment. I have marked the max. voltage ratings in blue. the problem is Q1 (FGD3N60LSD) keeps failing. C-E becomes permanently low impedance ~100ohm. The mains is 240VAC, so that's 330Vpk. To allow for spikes I have picked devices rated at 600V for BRIDGE1, D1 and Q1. Q2 is omitted at the moment. Fortunately the Q1 failure doesn't affected the rest of the circuit.
The failure happens immediately when the circuit is connected to the mains. I've programmed it to flash for 10ms every 500ms. A few times it has worked when first connected, then the next time I connect it to the mains, it fails. The duty cycle is low so nothing's getting hot when it's working correctly. When Q1 fails, Q1 quickly gets hot.
I tried a TVS rated at ~400V across Q1's C-E junction. This allowed the circuit to work for about 10 connections, then it failed on the 11th. so I think the failure is due to a voltage spike across Q1 C-E. But I don't understand why my overrating of 600V isn't sufficient to prevent this nor why the TVS (400V breakdown) didn't offer permanent protection. Also, why does it only fail when initially connected to the mains? I'm thinking it might have something to do with switch bounce and stray inductance. Any help/advice gratefully received.
The failure happens immediately when the circuit is connected to the mains. I've programmed it to flash for 10ms every 500ms. A few times it has worked when first connected, then the next time I connect it to the mains, it fails. The duty cycle is low so nothing's getting hot when it's working correctly. When Q1 fails, Q1 quickly gets hot.
I tried a TVS rated at ~400V across Q1's C-E junction. This allowed the circuit to work for about 10 connections, then it failed on the 11th. so I think the failure is due to a voltage spike across Q1 C-E. But I don't understand why my overrating of 600V isn't sufficient to prevent this nor why the TVS (400V breakdown) didn't offer permanent protection. Also, why does it only fail when initially connected to the mains? I'm thinking it might have something to do with switch bounce and stray inductance. Any help/advice gratefully received.