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Calculate switching loss in Mosfet

Discussion in 'Electronic Design' started by Mook Johnson, May 28, 2007.

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  1. Mook Johnson

    Mook Johnson Guest

    I asked a broader question before and it must have been too much.

    Here is a simpler version and more to the point of what I'm after.

    Using the above MOSFET in a switching application with the following

    Configuration: half bridge
    Bus Voltage: 500V
    peak flat top current 1.5A (start of ramp on step is 1A, Anti-parallel diode
    freewheels the inductaive load)
    May place an external diode if the internal diode causes too much

    switching freq = 10kHz 90% duty cycle

    I can figure out the losses due to the Rds_on but I'd like to get an
    estimate of the switching losses.

    The FETs will be driven with a TC4427 1.5A fet driver with a small ~5 ohm
    resistor to damp the resonant tank between the pin an trace inductance and
    the gate capacitance.

    Can someone help me with this please?
  2. As an approximation, take the switching times (on/off) and assume that the
    FET resistance goes linearly from 0 to infinity (or vice versa) in that
    time. Integrate over the power; that'll give you the energy that each
    switching process leaves in the FET. Multiply by the number of switches per
    second, and you get the average switching power loss in the FET.

  3. Donald

    Donald Guest

    Please help me understand, why is this information important ??

  4. J.A. Legris

    J.A. Legris Guest

  5. D from BC

    D from BC Guest

    I thought I wrote down the formula...but can't find it...anyways.. I
    had to Google for it. It's out the wild...somewhere..
    D from BC
  6. colin

    colin Guest

    worst case with inductive load, assume the current doesnt change untill the
    top freewheel diode starts conducting,
    vds rises linearly limited by the gate current charging cdg,
    the power at switch off will then be 1/2 V * I * tfall * freq
    power at switch on depends on the initial current.

    dont forget the gate energy required too, although it should be much less.

    Colin =^.^=
  7. Sometimes, when you're switching fast enough, the switching losses are the
    ones that determine the necessary heat dissipation. Or the available
    dissipation determines the maximum switching frequency.

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