H-Bridge has a path to ground. How ?

Thread Starter


Joined Jul 21, 2020
a while back I designed with the help of you a H-Bridge that as far as I remember worked well. Now I tried to power it back up again, and saw that it was shorting somewhere. It was powered by my power supply, which went into constant current as soon as I supplied the circuit.

I attached the circuit below. I disconnected the inductor, so that the drain's are kept open. Also (what I think the problem is) the oscillator inputs are open/floating. The circuit is powered by a 20kHz square wave 0-3.3V.

I think the problem is that Q4 and Q1 are driven at the same time. But I don't understand how that is possible.

Why is it drawing every current possible when the oscillator pins are floating ? And how can I fix it ?



Audioguru again

Joined Oct 21, 2019
You should NEVER leave the gate of a Mosfet floating because then it could easily store some voltage on the high capacitance of its gate to source and be turned on all the time.
Simply add a resistor from the two gates that are joined together and to ground. Try 10k ohms.