Hopefully an easy FET question - selecting a new FET

Discussion in 'General Electronics Chat' started by zero10, Feb 11, 2013.

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  1. zero10

    Thread Starter New Member

    Sep 20, 2012
    I am working on a controller for heated seats I am adding to my van. I have a trivial circuit made using thermistors, resistors, potentiometers and comparators that I was using to drive a FET, and was switching an LED to indicate the status of the seats and it is doing exactly what I want: when the temperature is below the set point the FET is "on" and when it is above the fet is "off". However, when I connected the seat heaters themselves I found that the FET heated up very very quickly. Thinking it was an error in driving the FET and I was spending too long in switching (as I verified I was providing enough gate voltage) I reduced the circuit to the following:


    The lower resistor is just a low value (500 ohm) pull-down, and the top right resistor is the 55W heating element, I just didn't have a good symbol in my library for it.

    Still, with this simple setup the FET will heat up VERY quickly. Googling the problem led me to believe the problem is related to a high Rds(on) value. This FET (IRF840) has an Rds(on) of about 1.0ohms, so when the heater elements are trying to pass ~4.5A of current through it that means 4.5W of power, and with a junction to ambient thermal resistance of 62*C/W that means in open air the junction would (theoretically) reach 279*C above ambient, not really an ideal temperature.

    I started searching around and came across the Alpha & Omega AOx516, which is very affordable and claims to have an Rds(on) of just 5 milli-ohms (10V gate voltage 20A current) with a similar thermal resistance, at 4.5A it would only be producing 0.0225W of heat and should only reach about 1.4*C above ambient in open air (although I will be heat sinking it to the enclosure of the controller). Unfortunately this shows as being a logic level FET, but it does also state that it allows up to 20V gate-source voltage and I can guarantee that I never exceed 16V gate-source in my application.

    My concerns are as follows:

    Have I made a huge mistake somewhere here with my math?
    Will switching to this FET from the one I was using allow me to run the FET without any heat sink at all in an enclosed case?
    Is there a better FET I could be using for this purpose?
    Am I being really stupid in selecting a logic level FET?
    Is the data sheet wrong? Can the FET really have an Rds(on) of just 0.005 ohms?

    Thanks in advance for any help, I am kind of a newbie when it comes to analog electronics, most of my experience is with microcontrollers.
  2. takao21203

    Distinguished Member

    Apr 28, 2012
    Doh. You must sink off the heat, screwing it to the enclosure, or forcing it with a fan.

    Even a small piece of wire has considerable resistance.

    The flywire I use has...some 1.5 Ohms for 2 metres approx.

    0.005 Ohms wire does not really exist.
  3. takao21203

    Distinguished Member

    Apr 28, 2012
    If you are concerned about the gate voltage, use a voltage divider to scale it down. Also for a non-switching application (not SMPS), you do not need a low gate resistor such as 470 Ohms.

    A heater could tolerate even some 100 milliseconds delay without any effect.

    Logic level MOSFET is not a limitation as such, it makes no difference, just the threshold is much lower. Always observe the max. voltages from the datasheet. If you switch inductive stuff, you would some day see MOSFETs exploding in prototype circuits for various reasons.

    The hotter they get, and the more close you approach the max. voltage, the higher the chance for explosion. The more you approach these limits, the less reserves for inrush currents, spikes, back EMF, higher than normal peak voltages, and all that.

    Theory only gives you so and so as much information, if you saw them exploding at some point, you know more.
  4. zero10

    Thread Starter New Member

    Sep 20, 2012
    Thanks for the information!

    There are definitely no inductive loads present here, only a resistive heater connected to each FET.
    The low gate resistor is only for my testing circuit, the one I made up under my first assumption that my driving circuitry was to blame for the heating, the actual circuit uses a totem pole to drive the gate of the FET. If I run into any problems with that part of the circuit down the road I'll post a full diagram of it as well but at this point I think it'll just muddy the waters.

    I also thought 0.005 ohms Rds(on) sounded suspiciously low, most other vendors are citing values between 0.007 and 0.050 ohms. I will be heat sinking the FETs to the project enclosure (using isolating heat sinks of course) just in case something happens, like a heater dead shorts so that the fuse can be the limiting factor and not my FET, but I would prefer not to be sinking very much heat at all into the enclosure under normal circumstances as it will be located up inside the dashboard - not in a good area for airflow.

    Interesting point about exploding FETs, I hope I don't have to deal with that :( I see this one has a maximum gate-source voltage of +/-20V and I can guarantee that I will never exceed 16V on the gate as I have added over-voltage protection. If you think that is too close a margin to run I can change the circuit to apply a lower gate voltage. This choice was only made because it was simpler to operate the FET at what is essentially the vehicles operating voltage (14.5V nominal, although I have seen it hit 14.8V once).
  5. bertus


    Apr 5, 2008

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