Why Do MOSFETS Blow?

I've got a lot of ideas/thoughts...but let's see what can be determined first.

Have you been able to replace FETs and restore functionality on a blown controller?  (Or are you just replacing the entire controller?)  If so, are there any other damaged parts that also need replaced?

I ask because after blowing through over 150 FETs ($$$), I discovered a connection between load-based FET blowouts...and failed gate resistors.  Which makes logical sense: if the Miller spike has a significant strength (i.e. more kickback/dirty brushes), it will put a significantly higher load on the gate resistor.  In my case, said resistors were 0603 SMD (I know, dumb), and apparently they will just fail open-circuit without warning.  This of course turns the FET loose to deadshort the battery and any other sort of concluding end results.

You might consider a ferrite choke on the motor leads to try to absorb some of the brush noise (if not already present.)

Thanks for the response. I'll detail more about the drives and what I've done so far. Yes, I have replaced the FETs in one of the drives to restore it to working order. When I had the board out on the bench, I followed the signal path through each quadrant. There is a strong 12 volt drive signal on both sides of the 1/2 watt gate resistor. There is an isolated gate drive supply for all quadrants. It is a bought device made by Sola Electric. A transformer with six independent secondaries supplies six rectifiers and filters which then feed linear regulators. Four of these are gate drive supplies, one is control power and one is unused. The power supplies are very solid and steady. The gate signal at the FET is a sharp-ish square that has a positive spike never exceeding 14 volts and a negative spike of about -2 volts. I put it back in service with new FETs and it blew about 2 weeks later.

Another identical drive died and I gave it the same treatment, except I added 1 watt 16 volt zener diodes from source to gate of the FETs and a shcottky diode in reverse parallel to the gate resistor. A page out of the inverter builder's playbook. Turns out the zener begins to conduct at less than 16 volts and the gate signal at the FET never gets over 12 volts. It is still working, but it could go any time. I did the mods as a shot in the dark. I have implored the people with money to just replace them all, but they keep the purse strings tight.

Do you have a good DSO?  (digital storage scope.)  Helps considerably to put the probes in 1x mode (reduces amplifier noise and cable ring).

So if you just have to replace the FETs, it sounds like the gate resistors aren't blowing 😉

Approximately what is the H-bridge bus voltage?

And...what's the FET part number?

Is the H-bridge drive mechanism push-pull?  (In other words, when the H-MOS is turned OFF in the PWM cycle, does the driver turn on the L-MOS at this point?)

If not...we might be dealing with the body diodes on the FETs.  (OR...failing bus voltage capacitors that aren't absorbing the spikes well.  You'd be able to see this with a 'scope on the bus voltage.)  I have read that the body diodes on the FETs are NOT ultra-fast recovery diodes.  In other words, if the dirty motor kickback spikes are very high speed (which is probable!), the FET body diodes might not be conducting fast enough to short the spikes to the power rail--in other words, allowing them to overshoot the rail and (possibly) the FET voltage rating.  (This could result in long-term FET degredation and random failure.)

With the blown FETs usually being diagonal on the 'bridge, it does NOT sound like a Miller spike problem.  (That's ALWAYS on the same side, as the FETs can only cross-conduct across the power supply on one side--not diagonally.)

If you can provide some details about how the motor drive utilizes the H-bridge, that would help narrow down the possible failure modes.

The bus is 24 VDC. The original FETs were/are IRFZ44n. I replaced them with IRF3205 from stock on hand. There is no info about the driver other than "PM Driver D-250 1997. It behaves as a push-pull. It has a DIP switch that configures it as either a stepper or H-bridge driver. It has a signal input terminal and isolated supply terminals.

Worth noting that the IRF3205 has about double the gate capacitance than the IRFZ44...not a problem if the gate signals aren't rounded, though.

Since the failure modes seem to be rather random, I would suggest you check the following with a 'scope:

• voltage across the DC bus to the H-bridge (to see if the power supply filter caps are absorbing any kickback/spikes from the motor).
• voltage across one (active) FET quadrant to see if the body diodes aren't being fast enough (i.e. brief high voltage spikes during switching)

At this point, this is largely an academic question because I hope they all blow so we can upgrade the whole lot.😂

Can a fluctuation of the bus voltage cause a FET blow up even if the drive signal is solid? I discovered yesterday that the building circuit powering the DC bus supply had a partial meltdown in a junction box. None of the machines were running so it was very quiet. I leaned a ladder against a column and suddenly heard the relays all simultaneously chatter in the controls cabinets nearby. So I did it again just to make sure I was crazy. At the top of the column is a junction box containing the line side circuit. I opened it to discover a melted wirenut. Apparently slight vibration is enough to rapidly open and close the circuit. Only the 24 volt bus is powered by that circuit. The controls are on a different circuit.

If the bus voltage ends up transienting beyond the max FET voltage rating...absolutely.