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Discussion in 'Electronic Basics' started by Fritz Oppliger, Jan 26, 2005.

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  1. I run a heater wire, 3 ohms at 13VDC, using IRFZ44 FET. On some units the
    FET gets hot, on others it does not. Given resistance and voltage and duty
    cycle are the same, why the difference?
  2. John Larkin

    John Larkin Guest

    What's the gate drive: voltage, frequency?

  3. That fet is certainly big enough to handle the 4 amp load
    without much heat if it is fully on.
    Either it is not being turned completely on
    with enough gate voltage, it is transitioning between on and off
    too slowly, or it is on the verge of oscillating.
  4. Thanks for the responses!

    The IRFZ44 gate is driven by a ULN2003A against a 10K pullup to the same
    ULN2003A is driven by 8051 port against 4k7 pullup to 5VDC.
    Frequency... 200Hz max.
  5. Then there is a fourth possibility, in addition to the three I
    listed. The darlington transistors do not pull all the way to zero
    when they are on, so if some mosfets have a lower turn on threshold
    than others, they may leak a considerable current in the off state
    (when the darlington pulls down) and dissipate heat. You can test
    this by forcing the darlington on, continuously (disconnect the
    processor line) and see if the transistor still gets warm.
  6. peterken

    peterken Guest

    OR some of them might oscillate due to layout, this being continuously in
    the analog region
  7. John Larkin

    John Larkin Guest

    Take a look at the gate risetime with a scope. Fets like that have a
    lot of capacitance, so the fet may be turning on very slowly when
    pulled up by a wimpy 10k resistor. Try a lower gate pullup resistor or
    an active pullup.

    I bet the gate waveform looks like...

    ----------- 13v

    and the plateau is a killer for dissipation.

    But that wouldn't very well explain fet-to-fet differences. So see if
    the gate drives look different some other way. Check the heatsinking,

  8. John Larkin

    John Larkin Guest

    Could well be; that might explain the differences. A small (47 ohms,
    maybe) resistor in series with the gate lead, real close, usually
    fixes that.

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