MOSFET Drive Issue

Discussion in 'General Electronics Chat' started by R!f@@, May 16, 2011.

  1. R!f@@

    Thread Starter AAC Fanatic!

    Apr 2, 2009
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    See the attached.

    The circuit is a Chinese treadmill board HV +B and driver side diagram.

    The MOSFET used is what I think would be since the original one is burnt to a crisp and the number is not even slightly visible.
    It happened due to it not being mount to the heatsink, the mounting bracket was missing. How ? not for now.

    What I like to know is how the Mosfet is wired.
    The motor is between source and ground. I presume the FET is N channel since the center pin has HV B+ Connected. Which would mean it is the Drain Terminal.
    Now from what I have mainly seen, the Motor is in the Drain circuit, with the feedback resistor at the Source.

    My question is will the MOSFET work wired like shown in the diagram or could it be an IGBT.

    Since the motor is at the Source(emmiter), it is in the Switch Drive path, isn't it.
    What I mean is the Motor Voltage will effect the Gate wouldn't it?

    or Am I wrong?
     
    Last edited: May 17, 2011
  2. tom66

    Senior Member

    May 9, 2009
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    That's an unusual configuration. It looks like you have B+ and B- swapped around. It's rare to see a MOSFET on the high side as that requires the gate to be at a high potential relative to ground.
     
  3. R!f@@

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    Exactly what I thought

    And I did not gave the supply wrong, it is how the traces are.
    Besides if the B+ is switched, it would not go to drain wouldn't it.

    May be why a 16V zener is their to prevent gate Voltage from going above the gate maximum voltage.
     
  4. tom66

    Senior Member

    May 9, 2009
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    The 16V zener is relative to eath, not the source pin. This will quickly destroy the gate as the gate pin will be at about -330V (assuming a 16V gate and 350V B+.)
     
  5. R!f@@

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    On the contrary this was working, the Mosfet overheated due to less contact with the heatsink.

    So what do you think shud be there

    about the Zener, it is corrected, I made a booboo, image update in the first post

    please note that HV GND and Drive circuit GND is isolated
     
    Last edited: May 17, 2011
  6. ErnieM

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    Apr 24, 2011
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    First, when you edit your post like that you remove all sense of the previous replies, so next time just make a new entry.

    You don't show how PWM Drive or the Current Sense are generated or used, and I suspect you left out a few important connections from the power switch you show and the control module.

    We've used such a scheme in the past to make what we call a "high side switch." The connection between PWM Drive and Current Sense IS the gate-source of the FET, and the control module would just apply the gate voltage thru that path.

    To make that work the control module typically gets its own floating supply referenced to Motor-1. Seen that way then D2 is just a gate voltage limiter; FETs die instantly when the gate is over driven.

    The sense resistor is typically connected in a kelvin (4-wire) connection, so if you ohm the board out you'll find what look like redundant connections at Q1-S and Current Sense, and the other side of R4 and Motor-1. These are to separate the high motor current from the sense and control signals.
     
  7. R!f@@

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    Guess what.

    It works, the gate Voltage is 0.3 to 1V with respect to source.

    {ed}

    If the MOS blows this time, I will use two in parallel.

    The one in use is IRFP460
     
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