Problem with PWM hot wire cutter

[Geoff_S]

I'm getting depressed that I can't make a simple circuit work...

I'm trying to drive a hot wire foam cutter with a PWM signal (to modulate the heat output/temperature of the cutter).

The schematic below shows what I came up with, based on various reference circuits from datasheets and other designs I've seen around on the net. I've made a single-sided PCB using press'n'peel and tested it this morning. It seems to work (in that the heat of the wire does vary as the duty cycle of the PWM signal is changed), but my probem is the mosfet is getting extremely hot. Too hot to touch (but no smoke, yet) after a few seconds.

The power supply to the hot wire is a 24 VDC SMPS, rated at 10 amps.

The hot wire cutter bow itself is a length of stainless steel wire, approx 1m long by 0.3mm dia. With 24 VDC applied to it directly, the measured current is around 2 amps.

The PWM signal is coming from a CNC controller, via the parallel port of my PC. The output voltage of the parallel port card is 5V. I've checked this with a DVM (5V when the pin is high, 0 when low). The frequency of the PWM signal is 50 Hz.

I set the value of R1 to give me 10mA drive current to the optocoupler.

The mosfet (IRFZ40) is just something I had lying around. I figured it had more than ample current capacity, and reasonably low Rds (0.028 ohms). With peak of 2A, I figured it would only be dissipating I*I*R = 0.1W. I've put a small heatsink on it regardless.

I added a flyback diode just in case there were some inductive spikes when the fet switched off (even though I expect the inductance of the hot wire to be very small). I added some biggish capacitors on the output to smooth the output.

I'm at a bit of a loss to explain why the fet is getting so darn hot. Do I need a proper mosfet driver, or have I made a simple mistake somewhere else ?? Any help would be greatly appreciated !
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[pebe]

I think your problem is the result of fitting the 2 x 4700µF caps. They defeat the idea of PWM which is to turn on and off the full voltage to the load at a certain rate. Assuming you need 12V 1A to keep your wire at the correct temperature, then you need to switch the 24V supply on and off at a 50% duty cycle.

But with the caps present, the FET when ‘on’ will charge them to about 13V and they will then discharge through the wire when ‘off’. At 1A, the caps will discharge about 2V during the ‘off’ period, so the wire will get about 12V mean with 2Vpp ripple. Because the caps present a very low impedance to the FET, the charging current will be very high for a short period – followed by a longer period off. The result might easily be 10A on for 10% of the time followed by an off period of 90%. That would still give a mean current of 1A at 12V.

But while the caps are charging at 10A there would still be about 12V across the FET. That would give a mean dissipation of 12W in the FET.

Could that be the cause of your trouble?

[Geoff_S]

Thanks Pebe. It's been suggested on another forum that the caps were unnecessary and probably causing issues. The main issue pointed out was that the voltage being applied to the gate of the mosfet (24v) exceeds its spec sheet rating (+-20V). I'm in the process of revising the schematic. Thanks.