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I am currently designing a circuit that includes a nichrome coil used as a heating element. The wall adapter is 25V 5A DC, which leads to different buck converters that takes it down to the appropriate voltages. The Buck converter that leads to the coil is set to have an output of 15A. The Flow to each component including the heating coil is controlled by a number of MOSFETS.

I have noticed that upon testing the heating coil (in testing I have separated the component being tested from the others so in this situation the power goes straight from adapter to buck to coil to MOSFET then back to buck), the MOSFET (which is rated at 30A and 60V) is overheating and melting. I have tried using heat sinks, splitting up the current between multiple MOSFETS, and added PWM to allow the MOSFETS time to cool down, but I can only make the PWM rest periods so long before I can no longer achieve the desired heat. Still with all of this, the MOSFETS heat up and begin smouldering.

Because of the amount of MOSFETS I've ruined and amount of dangerous fumes I've inhaled, I feel it is time to turn to the experts for insight.

My assumption is the circuit is in essence a short circuit, although I would have assumed the coil would have supplied ample impedance.

Does the fact that the buck converter is lowering the voltage mean that excess amperes are being drawn?

On a side but still related note, if the circuit is in essence acting like a short and drawing uncontrolled current, will this risk damaging the adapter?

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    \$\begingroup\$ What is the actual resistance of the coil? What voltage is your buck converter applying to the coil? How are you driving the gates of the MOSFETs? Show us a circuit diagram. Your question is far too vague to answer as-is. \$\endgroup\$ – Dave Tweed Aug 3 '14 at 19:18
  • \$\begingroup\$ If your heater is creating more heat than desired, reduce the voltage. If it is only doing its job, use a more capable switching circuit. Make sure the MOSFET's are fully "on" (and not already blown), heat sink them, parallel them, add a fan, etc That is a non-trivial amount of power you are working with. \$\endgroup\$ – Chris Stratton Aug 3 '14 at 19:19
  • \$\begingroup\$ The impedance of the inductor is directly proportional to the applied frequency. Since you are driving the coil with DC the reactance of the coil is 0, leaving only the resistance of the windings. impedance = resistance + reactance \$\endgroup\$ – sherrellbc Aug 3 '14 at 19:53
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    \$\begingroup\$ Show your circuit. \$\endgroup\$ – Andy aka Aug 3 '14 at 20:34
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A circuit diagram of your setup, particularly including your MOSFET driver would be really nice. However, I'll go out on a limb here and suggest that you need to look closely at your drive circuits. It is entirely possible that you are driving your MOSFET gates from 3.3 volt logic. This is perfectly doable - as long as you are using MOSFETs with logic-level gate thresholds. A lot of power MOSFETs need a minimum of 4 volts on the gate to ensure full turn-on. If you're only giving it 3.3 volts, it will only turn on partially, and will dissipate too much power. It's important that you realize that operating a transistor within both voltage and current limits can still kill it if you don't get rid of the heat dissipated, and at high currents it's easy to generate too much heat.

There is a quick check for this (if you want physical proof): Get ready to sacrifice one more MOSFET, but hey, who's counting, right? Drive a heating element full on. Quick like a bunny, measure the voltage across the nichrome and the voltage across the MOSFET. If your MOSFET voltage is not less than about 10% of the nichrome voltage, you're doing something wrong, and less is better in this case. You don't mention your drive voltage, but it has to be less than 25 volts. Let's say it's 20 volts, and let's say the current is 10 amps - this is just to illustrate, OK? Then total power dissipated is 200 watts, and the effective resistance of the total load is 2 ohms. If your MOSFET is fully on, I'd expect an Rds of .1 ohms or less (and this will give a MOSFET voltage about 5% of the nichrome voltage). This would provide a MOSFET dissipation of 10 watts. Without a heat sink, this would kill the MOSFET, so you need a heat sink in any case. And this better not be one of those little U-shaped jobbers, either. You need real heat sink, possibly with a fan. With more airflow you can use a smaller heat sink.

You need to consult the data sheet for your MOSFET to determine both Vgs(th),the threshold gate voltage, and Rds(on), the on-resistance when the gate is properly driven. Then you will need the specs on your heat sink, specifically the thermal resistance to ambient. You will also need to do a little research on how to specify a heat sink.

As for some of your other questions, consider the two voltages you measured. If they both add up to the nominal voltage of your drive converter, you are not drawing too much current, and you don't have a short. You are just fatally abusing your MOSFETs.

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