Hi,
I am designing a high power MOSFET switch for a battery disconnect. I have my design requirements set at around 90 V and 150 A. I realize, that the continous operation of the MOSFET would require a proper thermal design, i.e. a good heatsink and low Rds(on).
The trouble I have now is switching the goddamn thing. The switching speed doesn't actually matter all that much, I need to find a good balance between switching losses and inductive spiking.
I've tried the gate driver IC NCP81074, with the typical circuit schematic, driving the gate at 12 V (gate charge and discharge resistors both equal to 2.7R, then i changed them to 10R, didn't seem to make much difference):
https://www.onsemi.com/pub/Collateral/NCP81074-D.PDFI tried the circuit with several MOSFETs - IAUT300N10S5N015, IRF150P220 and the IXFH220N20X3, all of which should be easily able to switch this sort of load. I don't have a PCB made yet for this project, I am using a combination of a bare copper laminate with a prototype board for the gate driver circuit, the whole thing looks like this:
I know, the grounding for the gate driver looks horrible, but the actual gate drive waveform looks quite nice.
Anyway, I am loading this transistor with a high power PSU switching it at 40 V and 40 A (1/8th of the required load). If I dare to go closer to 50 V Vds, the transistor fails short after turn on.
The turn on waveform looks like this:
I can see that there is some body diode reverse conduction, the Vds dips below 0V for a brief moment, then it spikes up and swithes on very slowly. I am not 100 % sure what is causing this effect, possibly the parasitic inductance of the PSU? Remember, after cranking up the Vds, the transistor fails short on turn on.
The turn off also looks terrible with a huge spike of the Vds (above 100 V):
Thing is, this spiking looks a bit different to what I saw online, usually the spikes settle down on the high Vds voltage (the transistor is off by then). However, here the spikes are damped after a short time and only then the Vds starts to rise (it is not visible on the images).
I managed to cure the spikes by adding a 1 uF capacitor between the drain and source, but also radically increasing the turn off time:
This circuit will be quite safety critical, since the control circuit has to be able to disconnect the battery at full load, the bad thing about MOSFETs is that they usually fail short. Basically, I am trying to build a semiconductor contactor (a semicontactor?
).
I have thought about using multiple MOSFETs in parallel, however, for now, the big problem seems to be the Vds voltage, not the drain current. I am leaving the thermal design in continous conduction for later, first I need to have good confidence in the switching circuit.
TBH I am not sure what is causing these effects, since altering the switching speeds doesn't seem to do much to this effect (could some sort of snubber help?). I never had to worry about damaging MOSFETs, since I was switching a couple of watts at best. Also, I intend to measure the drain current, I should have a current probe available in a few days time.