IRFLZ44N Cooling Requirements

Discussion in 'General Electronics Chat' started by tracecom, May 24, 2010.

  1. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
    I previously posted a switching circuit using an IRFZ44N to switch about 12-14 vdc at 5 amps maximum. Thanks to several forum members, I have made some changes to the circuit and the result is attached. As you may note, the IRFZ44N has been changed to an IRLZ44N, the voltage requlator has been changed from a 7812 to a 7809, and a couple of ceramic caps have been added to the regulator to suppress transients.

    As some of you know, I am a tinkerer and not an engineer; my math skills are poor. So today, after making the changes to the prototype, I decided to do some "shade tree" experimentation in an effort to determine what the cooling requirements would be on the MOSFET.

    Gate voltage: 7.67v
    Load voltage: 13.36v
    Load current: 2.73a
    Duty cycle: 2 seconds off, 8 seconds on for 1 hour
    Ambient temperature: 24 deg. C
    Max. tab temperature of IRLV44N: 33 deg. C (mostly still ambient air and no heat sink)

    My hope is that even with a full load of 5a, no additional cooling will be required, but I don't know. I intend to build a 5a power supply and load, but I haven't done so yet. In the interim, what do you think? Will I be able to put this circuit in a small enclosure without a heat sink or a fan?

    Thanks in advance for your help.
  2. bertus


    Apr 5, 2008

    What load are you switching?
    Why is the led on the +9 Volts line?
    The led will get reversed powered when the fet is "open" through the load.

  3. Bychon


    Mar 12, 2010
    I was OK until you got to the "small box with no fan". I have so messed up doing that!

    5 amps instead of 2.73 times 9 degrees heating = 40.48C
    Not bad...until you confine it.:(
  4. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
    The MOSFET is switching ground to the LED and the 2.73a load (purely resistive.) The LED and the load share a common ground, but the LED is powered by the regulated 9v.
  5. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
    I know I should at least have a ventilated enclosure, but it seems that most of the project boxes are plastic and even the metal ones aren't ventilated. The entire circuit is on a perfboard 3.625" by 1.75". I need to check the datasheet and see if the tab on the MOSFET is isolated; if so, I might mount it to the metal box or a metal front panel on a plastic box.
  6. retched

    AAC Fanatic!

    Dec 5, 2009
    A simple drill motor and bit will turn a non-ventilated enclosure into a ventilated one.

    You dont really have to be a craftsman to do it either. There are fan covers that can attach to internal or externally mounted fans. Add a covered intake on the other end and your set. Nice and easy.