I'm trying to come up with a way to do overcurrent protection for the following circuit: My first thought here was to monitor the increase in voltage drop over one of the mosfets (M1) and via a zener (D5) (that sets the cutoff level) activate a transistor that drains the gate. The zener is a 1.8V one and combined with the 1 voltage drop of D4 i thought it would cutoff at 2.8V which would be 3-4A for a irf740. But it's not cutting off anything.. any thoughts on how this function could be implemented into this circuit?
After much thought about this circuit.
First concentrate on the dynamic switching times of the two IRF740. Their switching on/off times are under 50nSec. Operational failure modes of most all AC switching supplies is transient damage.
That said do not couple the gate control direct to the AC connections. The time constant of your current limit circuit is way to slow. Go back and think of a simple LC current lag network. It could trigger an off state for U1 for example.
You could use something like a positive peak detector (simple RC circuit) to monitor the voltage across the switch and compare it to a fixed reference. Fixed reference can be formed using a simple voltage divider.
This circuit might need some blanking time initially (I'm talking about the case where voltage across the switch goes from the peak to normal saturation voltage).