Just curious if anyone out there has built an H-bridge at a hundred volts or more, and used it to drive at least a few hundred mA with PWM. I’ve got one working now using four 100V-compatible FETs, but at 0.1mA the circuit tends to destroy one of the P-FETs. All of the devices are rated for much more current than that, but I suspect that the pulsing is driving current in and out of the drain on that P-FET (while it is off) and causing it to overheat.
I should also mention that I’m using this to drive a non-inductive load - so current spikes shouldn’t be a problem. But if anyone is running an H-bridge like this successfully for any application, let me know! Thanks.
Having both N and P FETs on the same side turn on at the same time is bad. This happens when one turns on too fast or the other turns off too slowly. The current is substantial! Both FETS get hot and the P side always blows first.
Exceeding the Gate to Source voltage limits destroys the gates real fast. Most FETs are rated at 20 volt max Gate to Source voltage. At 21 volts they are gone. Even a small voltage spike will kill them. Nothing gets hot, they just stop working. How are you driving the P side? Perhaps a 15 volt zener across the Gate - Source junction would help.
Sorry about the slow reply! I actually do have a zener diode in there right now, and my first thought was that it was failing somehow and breaking the FET. But the diode still tests okay after the FET breaks.
With the Zeners across the Gate-Source junction, I think that we can eliminate the gate as the problem. You did say that the FETs were overheating, so I would expect that you are turning on one leg before turning off the other. One characteristic of this problem would be the H bridge functioning properly at full speed in either direction and overheat at any intermediate speed.
Try this: Work on one side at a time, one P and one N FET. Instead of wiring the two drains togather, use a resistor to limit current. With 100 Volt supply, I’d start with a 10Kohm. Watch the voltage across the resistor with an O-scope and look for spikes at the transistion. If you get spikes, then work on your gate drive circuit to do the “turn-off” faster or “turn-on” slower.
I assume that you not using one of the gate-driver chips and that you are using one pin off of a PIC to control both the P and N FETs on one half of the bridge.
I think I can eliminate the shorting possibility, as well. I have a millisecond delay in my program between turning off one direction and turning on the other, the part that blows out is consistently a P-FET rated for 19A instead of the N-FET it would be shorting through, and, most importantly, it works perfectly fine at low loads (<100mA). It’s run fine at low loads for a total of about fifty hours now…
But I should give your suggestion a try just to be sure it’s not the problem!
