Quick question regarding protection of MOSFET gates - for the device in the following datasheet (which is actually an IGBT, but of course has a MOSFET 'front end' if you like) the gate features a back to back Zener and normal diode setup from the collector to the gate, and from the gate to ground a back to back Zener setup.http://www.fairchildsemi.com/ds/FG/FGD3440G2.pdf
Now, the Zeners from collector to the gate - I believe are what are responsible for the self clamping of the device at 400V, when the collector reaches 400V the topmost Zener conducts - turning the gate on allowing any inductive spike to dissipate through the collector to the emitter, whilst the normal diode stops intended gate voltage conducting to the collector.
However, the arrangement that clamps the gate voltage to a safe level for the gate, why the back to back Zeners - I don't see why there are back to back Zeners? When gate voltage exceeds the safe threshold, the top of the two Zeners should just conduct the excess voltage to ground, surely?
Or perhaps, the bottom most Zener is there to protect against negative ground transients - where suddenly there would be a greater than safe potential between ground and the gate?