i am mainly referring to mosfets of the 90's mainly rated at voltages 200v and up and amps of 1amp and above with gate capacitance in range of .1 to 333 microfarads. it does not seem to be dependent on current rating meaning rated at even 20 amps and 100watts seem to fail about just as bad as the lower current lower wattage. i used them mainly as an n channel in series with series with p channel but also n channels alone in other types of circuits usually with vcc at around 170 vdc. This was mainly many years ago but now i tried again in a basic symmetrical multivibrator with 2 n chanels collector or drain as u may call it of 30 k ohm and base resistors of 200k ohm and collector to base capacitors of 0.1 to 333 microfarads and i used a vcc of about 166 volts. I think there is no question about the simple circuit but i drew it here anyway. And as what i thought should be a protection from excessive gate voltages i used 2 zeners back to back but not official zeners. Most all if not all regular diodes have a reverse breakdown 'zener' i call voltage even if not officially designated as a zener diode. I just measured the voltage of the ones in series with 100 k ohm which broke down at say around 14 volts and for the other one i used a led of which the ones i used had a reverse breakdown of about 14 volts and the usual forward 'lit' voltage drop of about 2.5 volts and i put them in series so that means in one polarity it would have about .6+14volt and the other way about 14+2.5 volts. Bit the mosfets kept failing. Why ? could it be that in fact the dynamic very non linear resistance at the 'knee' at the point in question was still too high at some point even and allowed the gate to go higher than 20 or 30 volt with respect to the emitter or source i guess u call it. Many different types of mosfets all rated at above 200volts failed. In particular the irf630 one after the other kept failing. The fault being an essentially non infinite gate emmiter or source resistance which should be say near infinity for all practical purposes was now below 44k ohm in both polarities of gate to source. Does anyone know why this kept happening.is there a general reason why mosfets keep failing when all common sense would indicate they should not fail. ok because of a complaint i am going to try and draw the circuit diagram atleast at most a part of as i can in text since it is impossible to draw it using the tools here. In the drawing Rc means collector resistor except here attached to the drain and Rb is base resistor except here attached to gate. C is capicitor coupling drain of one of mosfets to the other mosfet gate. And there is no connection at the crossed vertical/horizontal lines of the left mosfet drain to the right mosfet gate. s,g,d are mosfet source, gate and drains. You can now see why i said the basic symmetrical basic multivibrator circuit except for the addition of the gate to source connections of two zener diodes back to back. The zener voltage was of the order of 14 volts for all the diodes and one of the diodes in each of the 2 identical series gate to emitter lines i used a led diode, forward conduction drop of the usual 2.5v and i measured the reverse zener breakdown at about 14v using 100k series resistor to 170v to determine all 4 zener breakdown voltages. I tried all different values of other components values like in the range Rc 5k to 50k, Rb 60k to 200k and C 1uf to 333 microfarads. And used n channel mosfets irf630 and several others all spec'd to withstand the voltages/currents in the above circuit. As i recall most all failed at some point or another - did not get hot as far as i remember. [
