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High Speed Design ----> Knee in the falling edge?

Discussion in 'Electronic Design' started by EdV, Mar 29, 2007.

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  1. EdV

    EdV Guest

    I worked on this problem a couple of years ago and never did
    understand what the deep down root of the problem was beyond the
    vendor shrunk the die on an optocoupler we used to drive an IGBT and
    we needed to slow down the propagation times.

    The weird thing was the manifestaion of the problem:

    A kink or knee in the middle of the falling edge would turn into a
    "ring". Big time! The IGBT would then be driven in the linear region
    for a a half second or so. After this happens umpty ump times the
    IGBT would become an open.

    So what is with the knee thing in the middle of the falling edge? I
    have seen and have an intuitive feel for ringing at the bottom or top
    of a square wave near the level transitions. But in the middle?

    I was reminded of this by the falling edge (of all things) a PID
    temperature controller. Going from 100C to -40C there it was a knee
    right in the middle.

    It has got to have something to do with some sort of level transition
    for the temp controller case I would imagine SW. The opto coupler
    though. . .

    Sorry if I got rambling here.

    Ed V.

    PS - yeah, I know I should say the part numbers and edge rates but hey
    it was two years ago. : )
  2. James Arthur

    James Arthur Guest

    That's Miller effect--when the collector starts to move, a current
    opposing the base drive flows through the collector-base (in this case
    collector-gate) capacitance. That causes the "knee" in the drive
    waveform--the transistor's fighting back.

    Since the transistor's collector has started to move, the thing is
    linear, and prone to oscillate.

    An opto-coupler driving an IGBT is kinda wimpy--that's not what we'd
    usually call high-speed design!

    A heftier driver (i.e., spend less time in the danger zone), a
    damping resistor / ferrite bead at the gate, or some combination
    thereof might be the cure.

    James Arthur
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