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Measuring the Output Impedance of a (Large Capacity) Voltage Source

Discussion in 'Electronic Design' started by Anand P. Paralkar, Dec 29, 2013.

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  1. Hi everyone,

    I am trying to test a MOSFET H-bridge at around 400V DC input. I need
    to observe the MOSFET Drain-Source waveforms for ringing during the on
    to off state (and vice versa) switching.

    I observe that the drain to source waveform is affected by the DC source
    that I use to supply in the input DC voltage to the MOSFET H-bridge. We
    have three different sources (including one that I built using a simple
    variac, rectifier and huge capacitor bank) and I feel that there is a
    particular DC source (bought from a vendor) that does not work well
    (gives horrible drain to source waveforms).

    In order to be sure, I would like to see that the one that gives a bad
    waveform has something in its output impedance that makes it misbehave
    while the other two have something "good" about their output impedance
    that make them give good switching waveforms.

    Is there a (simple) method to measure the output impedance of a 600V,
    15A DC voltage source?

    I will use the suggested method to evaluate the three sources that I
    have and hopefully, I will find the evidence to prove that the source
    that gives the bad waveforms has something "bad" in its output
    impedance. :)

  2. Wimpie

    Wimpie Guest

    El 29-12-13 10:55, Anand P. Paralkar escribió:
    I had such things in the past also (but with less power). You may
    observe the transient (step) response of the supply by switching a
    large low inductance resistor with a mosfet or fast IGBT. It was
    strange to see that some power supplies were close to instability.

    You could also use small signal measurement, but the large signal
    response may be different from the small signal response.

    Somewhat OT: I think it is good to have sufficient decoupling close
    to the H-bridge so that the DC power supply doesn't "see" the
    switching transients.
  3. Fred Abse

    Fred Abse Guest

    If you mean what I mean by "large capacity", why aren't
    you using IGBTs?
    How are you loading the H Bridge? Motor?

    If there's any inductance in the load, you need freewheeling diodes across
    the FETs, don't trust the parasitic substrate diodes to do the job.
    Firstly, look at the waveform at the power supply. It should be DC, with
    no more than 20% P-P ripple.
    Measure its open circuit voltage, then resistively load it for, say 1 amp,
    and measure the voltage again. Calculate the source resistance from the
    reduction in voltage, and the value of your load resistor. It's just a
    resistive voltage divider.
    Describe, or preferably show us, the waveforms.
  4. The one you're seeing unacceptable behavior is most likely a switching
    supply or one with a very tight regulator in it.
    The ringing or even sudden changes of current loads to no loads for
    example is making it a little jumpy.

    inductive ringing can generate more voltage than your supply and it's
    also possible you are hitting a safety shut down because the caps being
    used in the supply are small, since they are a switcher at most likely
    much higher frequency than the ring you have, it makes it easy for your
    H-bridge to upset it..

    Try hanging some large low ESR caps on the output side of the supply or
    load it down with non inductive load.

  5. Den søndag den 29. december 2013 16.08.54 UTC+1 skrev Fred Abse:
    unless you disable the parasitic diodes with a series diode I don't see
    external diodes helping much

    It seems most power fets have similar rating for the diode as for the fet
    but they can be slow

    maybe that is the problem, shoot-through triggering a current limit in the supply

  6. Fred Abse

    Fred Abse Guest

    That's the point.
  7. Add a series low ESR capacitance to the output (high voltage type). Use a network analyzer to sweep the impedance coupled into the capacitor (if you do not have an impedance analyzer, apply a function generator with series resistance, and plot values a regular frequencies.

    If the load is really low impedance, use a HiFi amplifier as the source. Ifthat proves to feed to little current into the output to measure the impedance, add a transformer to boost the current.

    Sub 10mohm impedance measurements can be done this way


  8. miso

    miso Guest

    (if you do not have an impedance analyzer, apply a function generator
    with series resistance, and plot values a regular frequencies.
    If that proves to feed to little current into the output to measure the
    impedance, add a transformer to boost the current.
    Seems to me you should to meditate a bit about charging this series cap.
    The instantaneous connection would put the full 400v into the network
    analyzer. Perhaps a precharge is in order.
  9. miso

    miso Guest

    The first thing to consider that the power supply isn't linear. If it is
    a switcher, then for sure it isn't linear. If it is a "linear" supply,
    well linear circuits have slew limitations, and if you are slewing, you
    are not linear. Further, there is usually protection circuitry in the
    loop. Linear supplies have multiple control loops.

    Thus you probably have to whack the supply with a load pulser and
    observe the voltage via a scope with probe AC connected. But then the
    next issue is are you testing the power supply or the load pulser. Most
    electronic loads are pretty crappy, so you are watching the electronic
    load settle as well as the power supply.

    For datasheets, where you want to be 100% sure all the ugliness (and
    hopefully lack thereof) is due to the device under test, you use as
    passive of a load pulser as possible. Typically you go from no load or
    in some cases minimum load current (i.e. high value resistor) to a low
    value resistor switching it in by a power mosfet. I've built these
    pulsers using PCB strips to solder a number of parallel carbon film
    resistors in order to make a low inductance resistor. Generally one
    mosfet is enough. You just make sure the on resistance is low compared
    to the resistor array.

    Use this somewhat passive load pulser to perturb your supply and see
    what happens.
  10. Precharge would be a good idea for this voltage. I have used it for lower voltage (30V), and the inrush surge current is manageable at that voltage


  11. We have a MOSFET that can handle 20A and 1200V. That was large, atleast
    for me.
    No, we currently have only lamp loads (50 * 100W incandescent bulbs).
    Surprisingly, the voltage, atleast as seen at the input of the H-bridge
    PCB shows only a DC voltage.
    I am not too sure this will hold good when we are using the DC to
    produce a switching output (as in a PWM sine or rectangular wave).
    Will try.

    Thanks for replying and wish you a happy new year.

  12. Fred Abse

    Fred Abse Guest

    That's small to medium capacity, to me. I'd still use IGBTs, though.

    40A, 1200V IGBT FGA20S120M $2.24 at Newark.

    IGBTs don't have parasitic diodes, BTW.
    Lamps have very low resistance cold. You'd be better using 5kW worth of
    heating elements. 50 hundred watt lamps, in parallel, will be a fraction
    of an ohm cold.

    Never start an investigation thinking you know the answer. That's what
    police do. Not engineers.
    Under load or open circuit?
    Measure at different load currents, up to maximum. Plot the results.

    You need a pulsed load to go any further, so you can see rise times. Maybe
    use your H bridge as the load.
  13. Only at 25C otherwise, it's a 20 amp unit.
    and also, they do have diodes.

  14. Guest

    With an infinite heatsink.
    With an infinite heatsink. Good luck with any practical cooling to
    25C while dissipating 30W.
  15. Guest

    Good grief. ...and cool that with LN2? <sheesh!>
  16. Fred Abse

    Fred Abse Guest

    Dissipation is dependent on duty cycle.

    Typical VCEsat 1.75V at IC=20A, TC=125C That's 35W at 100% duty.
    PDmax is 348W at TC=100C

    Note temperature quoted as TC, not TJ.
  17. Fred Abse

    Fred Abse Guest

  18. Fred Abse

    Fred Abse Guest

    Sorry, that should be 174W at 100C
  19. Guest

    Sorry, try doing that in the real world.
  20. Den onsdag den 1. januar 2014 21.32.10 UTC+1 skrev John Fields:
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