I recently started to start working on a project where I started using a half-bridge to turn on and off a device that normally plugs into a wall outlet (120 VAC, 60 Hz). In the future, I am looking to use two of these in an H-Bridge configuration, but for simplicity, I am just using a half bridge for now.
As I was designing the circuit, I started to think of a potential issue regarding the power-up state of the half bridge. When the device is powered up, and the biases for the gate driver / half-bridge are brought up, I was concerned that I might not know the ON/OFF state of each FET. To combat this, I was considering adding a relay to the circuit supplying the voltage to the half-bridge so that I can let all of the low-voltage control circuitry power up, and after I can establish the state of the half bridge, then I flip the relay to supply the voltage to it as show below (please ignore the values on the FET's):
Here is what I was wondering:
Is this really necessary? Is this dangerous to do, as the FET's have some drain to source capacitance, so could this potentially temporarily cause a surge on the 170V rectified AC wall voltage signal? I considered trying to solve this with either a choke/low valued resistance on the 170V input, but again if this is a non-issue, than I won't worry too much. I have a prototype of this device made, and I powered up without the relay with no issue, but I want to make sure I was not just getting lucky, and now I am just waiting for this thing to fail.
If it helps at all, these are the gate driver and FET's that I am using:
FET - IRLS640A
Gate Driver - FAD6263M1X
Thank you so much!