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power MOS advice

Discussion in 'Electronic Design' started by :: Marcolino ::, Oct 12, 2004.

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  1. Hi!
    I ask you an advice witch MOS to use for pwm (20Khz) signal for my load
    (Peltier cell) on 15V 7A
    I run it with IRLZ14 but crashed in 2 weeks.

    Thanks,

    Marco


    p.s.
    sorry for my english, but I'm foreign :)
     
  2. colin

    colin Guest

    couldnt find the specs for that device off hand, but although replacing it
    with a biger heftier device may solve the problem, maybe it would be better
    to check that the mosfet is being driven on and off quickly and hard enough
    and the peak switching voltage is controlled.

    Colin =^.^=
     
  3. Ken Smith

    Ken Smith Guest

    What sort of failure did you have?

    Can you discribe the circuit?

    If it ran for 2 weeks it could be that it wasn't the MOSFETs fault. Some
    other part may be the first failure or a design error may from time to
    time cause bad things to happen.
     
  4. http://www.irf.com/product-info/datasheets/data/irlz14.pdf

    That's a pretty wimpy old MOSFET, only 10A Id rating (7A at 100'C),
    and 200m Ohm Rds(on) (around 300 hot). Something like the IRLZ44
    (50A/60V/28mOhm cold) or the Fairchild RFP30N06LE (30A/60V/47mOhm
    cold) would be better, and not expensive.

    http://www.irf.com/product-info/datasheets/data/irlz44.pdf
    http://www.fairchildsemi.com/ds/RF/RFP30N06LE.pdf

    What does the actual current through the MOSFET look like?

    I do hope you're not going directly into the Peltier with raw PWM.
    Nto a prblem.
     
  5. Hi, this is circuit:
    http://www.gallicisalpini.it/img/peltier.jpg

    see the image if correct?


    Marco
     
  6. Fred Bartoli

    Fred Bartoli Guest

    Foreign fingers Spehro ?
     
  7. Guest

    I just had an wild idea.. maybe the capacitor C6 or a reverse peltier effect
    (heat diff -> reverse potential) is causeing a "back emf". And could be solved
    by haveing a regular diode as protection for the mosfet.

    Sufficient cooling is an absolute must however. How have you solved this?

    And you could possible investigate the use of "IBGT" semiconductor to switch
    the peltier.

    Beware thermal stresses applied to the peltier through pwm.
     
  8. You are probably getting a lot of unnecessary I^2*R heating in the
    Peltier. If the Peltier is actually designed for, say, 9 Vmax and
    you're hitting it with 15V pulses, for example, the average current
    (as measured by an inexpensive multimeter) may look okay but the
    efficiency will be less than it should be because of the current
    spikes (much higher RMS current), and can even cause thermal runaway
    in a closed-loop system. If you add a series inductor, shunt capacitor
    (low impedance type) and a fast catch diode you can reduce the peak
    current in both the MOSFET and the Peltier device. Usually module
    manufacturers recommend < 10% ripple.
     
  9. legg

    legg Guest

    I'd suspect the PIC, oscillator and drive methods first.

    To pwm from a PIC, you need to be very sure that SW or clock faults
    result in a switch that is off and a program that can recover or
    self-reset into a safe to run state.

    Check the drive, at the gate, and look for well defined drive voltage
    levels.

    The PIC outputs can go Hi-Z, if I'm not mistaken, so perhaps a smaller
    pull down resistor is in order, to avoid long glitches at supply
    turn-on.

    If the mosfet is any distance from the PIC, a local drive buffer might
    be advisable at the gate.

    RL
     
  10. Ken Smith

    Ken Smith Guest

    I can't see anything in the schematic that would account for the
    transistor failing.

    Things to check:

    Does the input power have spikes on it?

    Can the PIC ever rattle the line up and down very fast or go into a
    tri-state condition such that the MOSFET's gate gets some half way voltage
    for a long period of time?
     
  11. Tim Wescott

    Tim Wescott Guest

    Where I've done this we've put in dead-processor detectors that shut off
    the PWM if it stops -- this prevents you from driving it 100% on, but it
    also keeps you from letting the smoke out. Unfortunately I did not
    design the circuits, so I can't recommend anything.

    This should just be a convenience at software design time -- if you
    release to production while you're still having software faults in the
    amount of code that you can fit into a PIC then you _really_ have QA
    problems.
     
  12. Once the PWM module is enabled, it should only output 0V/Vdd unless
    it's disabled in software or reset. It's conceivable that the WDT
    could be resetting the chip and thus tristating the output, but even
    making generous assumptions, I'm not sure that ~half a joule
    worst-case is all that bad, unless it was happening very frequently.

    What's the case temperature on the MOSFET in normal operation?

    If it's running @50% duty cycle, 6A average, the power dissipation
    could be in the 20W neighborhood without even considering the
    switching losses.
     
  13. There's a 6A fuse in series with the load in his circuit. Presumably
    the zener and resistor are there to keep a MOSFET with a G-D short
    from taking out the PIC.
     
  14. Joerg

    Joerg Guest

    Hi Marco,
    One of the posters already hinted at it: If you run a PWM to obtain an
    average current of 7A (6A fuse?) and the peak current is above the FET's
    rating the FET might slowly burn up inside. As Spehro suggested, try a
    bigger FET.

    As said before, a Peltier might not like to be operated in this way. You
    may want to think about a low pass before it, along with a faster PWM to
    save on the L and C.
    Hey, we are all foreigners. At least in most places of the world.

    Ciao, Joerg
     
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