Another MOSFET failure thread

Discussion in 'The Projects Forum' started by KiloOne, Jun 21, 2012.

  1. KiloOne

    Thread Starter New Member

    Mar 5, 2009
    2
    0
    BSC057N08NS3 G 80v 100a Infineon Mosfet Failures

    I am driving a single fet with a Supertex HT0740 driver.

    Here is gate signal into the HT0740 (yellow) and gate signal from the HT0740 (green) with the fet removed from the circuit:

    [​IMG]

    Here is gate signal into the HT0740 (yellow) and out of the HT0740 (green) measured with the scope probe attached directly to the gate on a trace less than 1mm from the gate.

    Note, the input to the HT0740 (yellow) now has a 1V/div scale.

    In this case the fet does not fail as the load is disconnected from the circuit.

    [​IMG]

    And here is the fet failure when a load is connected.

    The load is a motor controller that draws a few hundred milliamps at rest and has about 1000 uf of capacitance to charge when fet closes:

    [​IMG]

    What could be the cause of this failure?

    I suspect it has to do with the slow gate turn on time but it appears that the fet is slowing the gate down as the gate signal without the fet is turning on in microseconds.

    Thanks,
    Dale
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    Looks like the driver isn't keeping up. MOSFETs are a voltage controlled device so if you read the gate isn't being charged then it means the device isn't turning on (as you seem to know). What you may not know is your driver chip not only has to charge the gate to source capacitance but it also has to charge the gate to drain capacitance.

    The gate to drain capacitance is typically far larger, as the drain voltage is changing as the gate changes, such that the drain to source capacitance (ie, the load) Cs is reflected back to the gate as Cs*(1+G), or the gain (plus 1) of the stage (FET) times the capacitance.

    Generally this is known as the Miller effect after the guy who first published this effect (in 1920 for a tube circuit).

    Looks like you'll need a stiffer driver chip. The one you selected is rated for only a 600pF load.
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    As ErnieM notes you need a gate driver with more drive. The MOSFET you are using has a maximum of 3900pF input capacitance and requires the transfer of a maximum of 56nC gate charge to switch states.
     
  4. KiloOne

    Thread Starter New Member

    Mar 5, 2009
    2
    0
    Thanks guys,

    False alarm for now, at least until I have to drive 16 of these in parallel.

    Turns out I had put an unneeded diode in my keyswitch line that disabled my anti-spark resistor. What should happen is, when my ignition key is turned into the spring loaded start engine position, this places a 47ohm 5w anti-spark resistor across the fets. By the time my PIC has booted and sends a signal to turn on the fets there should be no volts across them and then you can release the ignition key.

    Since I only have to turn these fets on and off once per vehicle use, it does not matter if they turn on slow, as long as it does not damage them. I am hoping the HT0740 will turn all 16 on in parallel.

    I will be back if they do not.

    Thanks,
    Dale
     
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