I'm looking for some help in further understanding how this active surge clamp works. The nominal input voltage for the system is 28V and the surge voltage is 80V for 100ms. The output is clamped to ~36V to protect everything downstream.
The Vth of the N-FET is 4V. I've ran some simulations in SIMetrix and I can see that it clamps the surge as desired. I was confused that it didn't show the N-FET conducting during nominal operation...
Klaus, I know what V_gs represents. I can subtract the Vs from Vg in the graph I posted above. If you don't have any helpful advice towards my question, can you please refrain from responding to my post in such a condescending manner? Thanks.
Not really a clamp per se but a voltage limiter (blocking).
Liable to be pretty crude and the clip level will be very
dependent on the load I-V characteristic (and FET VT
& subthreshold slope variation).
The gate and drain are at the same voltage (when it's not clamping) and the gate must be 4V above the source to turn on the fet. So the drain must be 4V above the source too.
This could be reduced with a lower Vgs mosfet. But the bottom line is that this is an incredibly simple circuit and that voltage drop is among the downsides.
I'd consider this a simplistic voltage regulator. You could use an actual voltage regulator IC which would be another easy way to reduce the voltage drop.