Killed half bridge - what went wrong?

throbscottle said:

I've been having nice success using a IR21531S as a speed control for a 24vdc motor which draws around 3A running and over 10A startup. A time-switched resistor limits the start current (3R3 for about 200mS, then 0R1 to feed a tacho in the future). The motor is the spindle motor for my PCB drill, finally moving from perf-board prototype to a proper board. The high and low side transistors are FDP047AN08A0 (75V, 80A, 4.7mOhm)

I've been running it from an old wall-wart with the speed turned down, for testing, and it's worked very well. Since today is the day I was going to use the prototype to drill the PCB which will replace it, I connected an unregulated supply consisting of transformer, rectifier and 3300uF capacitor (just what I happened to have lying around), 27v no-load, so it can run at full speed.

But what actually happened was that the high side transistor blew s/c, apparently at switch on. The only thing that was different was I'd missed off the on/off control connection to the IR21531S after previous tinkering, so it went straight to "on" instead of starting up in shutdown, but I don't really see why this would have caused the failure.

So can anyone suggest what might have actually happened at switch-on, here?

As always, thanks in advance

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throbscottle said:

Schematic attached. It's part of a larger drawing so is a bit squashed up. Mostly pre-biased "digital" transistors are used for the switching, Q15's bias resistors do double-duty as the timing resistor in the inrush limiter on the right. The 24v supply goes to the terminals PL1 marked "motor HV". If it wasn't used there would be a jumper on PL9 giving a couple of internal options. The IR21531S is a self-oscillating half bridge driver with 6uS dead time. D16 and D18 enable duty other than 50% to be selected (RV4 is 100K btw). The switch on the left gives options of always on, turn off after about 15 seconds, or on/off on every cycle. R16 on the left gets it's input from a comparator, Q17 causes the input from the footswitch to be delayed until the inrush limiter has finished (it feeds into a flip-flop, not shown). D17 stops the 15.6v Vcc of the bridge driver from feeding into the 5v logic Vcc (Q13 collector must always have a supply or strange things happen).

"Stop" is for an emergency stop switch

It's currently built on perf-board but I tried to keep the bridge circuit as tight as possible and used a star ground.

It's the high side bridge transistor that blew.

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