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Thread: IR21844S Variable Capacitance Load Driver Problem

  1. #1
    Beginner Beginner DanBrunermer is on a distinguished road
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    IR21844S Variable Capacitance Load Driver Problem

    Hi, the company I work for designed this circuit to drive a variable capacitance load. The ideal waveform shape would be a trapezoidal pulse with a controlled upward slope (about 60V/us) a width, and a controlled downward
    slope of the same magnitude. Here is the relevant schematic section:

    Click image for larger version

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    And I of course have the bootstrap:
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    The slope is supposed to be independent of the load's capacitance, but on the scope it clearly isn't. I suspect it's because as the C increases, it takes longer to charge, which takes longer to feedback thru the bootstrap, and etc.
    Is there any way for me to make the high-voltage output independent of the driver? Like, if I'm switching a high voltage of 96V, could I connect, say, 108V directly to the BS input, and not have that feedback? My understanding
    is the need for bootstrap is because the gate on the MOSFET has to go so much higher than the voltage being switched. Can I do something else?

    Thanks! Dan B.

  2. #2
    Beginner Beginner DanBrunermer is on a distinguished road
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    About this question

    I have a 12V DC-DC Isolated converter coming in, and I'm going to try my idea of plugging the minus of that to the top of my hi voltage and feeding the output directly into VBST. But now I'm wondering, if I do that, do I still need VS connected to the output stage, or should I leave VS disconnected?
    Thanks, Dan

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