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MOSFET output current limited by Inductor Saturation current?

  1. Apr 14, 2011 #1
    So the MOSFET's output current in a simple buck converter with a PWm a driver and a MOSFET and LC circuit. The Inductor's saturation or Irms current limits the MOSFET's current output, correct? I heard that if it exceeds saturation current that the MOSFETs could be damaged.

    is this true?

  2. jcsd
  3. Apr 14, 2011 #2


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    Yes it is true. Once the inductor saturates the mosfet current will increase very rapidly which could damage it.
  4. Apr 14, 2011 #3
    it will damage the MOSFET, correct?

    Will there be any damage to the inductor or the bulk output capacitors?
  5. Apr 15, 2011 #4


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    We need to step back and talk more about fundamentals here, IMO.

    How do you model an real inductor; with what circuit components? (What series components? We don't need to consider parallel winding capacitance for your present question)

    And given that model, how does the model's impedance change when the current exceeds the saturation current? And is it usually a sharp knee or a soft knee?

    And why would the MOSFET or Bipolar high-side switch be damaged? What sizing considerations go into each component in a DC-DC converter's design? And what kind of current limiting schemes should you consider in your design? Nothing should get damaged if the output of your DC-DC gets shorted out...
  6. Apr 15, 2011 #5
    I am just trying to analyze a circuit. I am not at that level yet for some of your questions. But I read that once saturation point is exceeded that the inductor losses inductance as the current rises significantly.

    Each phase, or driver with one pari of high-side and pair of low-side FET outputs to an inductor. The MOSFETs can output 40A, has Rdson of 3.4mOhm and output capacitance of 7.7nF. Come in a canPAK package. THere are 4 MOSFETs and one driver per phase along with a single inductor for both pairs of MOSFETs, i know its an odd design. Totally overkill considering the current rating of the Inductor.

    but the inductor has a saturation current of 28A and Irms of 28A as well. Metal alloy.
    DRC is 0.0012ohm typical and 0.0014 with 0.68uH inductance.

    the PWM probably has current limiting but I have no idea what it is set too.

    I just want to know if there is was way that the inductor limits the current output, in this case would a good engineer design the circuit so that current is limited at 28A.

    I want to know, or better understand how it is going to work if lets say there is NO current limiting and the MOSFETs output all together 30-40A through the inductor.
  7. Apr 15, 2011 #6


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    Wow, monster stuff!

    That's a tiny resistance for the inductor. You would calculate the resistive heating for the wire in the inductor, to figure out if the inductor would actually fry with that current.
  8. Apr 15, 2011 #7
    Wouldn't the MOSFET fry first? or what? which equation?
  9. Apr 15, 2011 #8


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    Staff: Mentor

    You just have to calculate the temperature rise of each, and see if that would exceed the damage threshold for the device.

    And where is all this current going? If the inductor saturates, it's not likely that the DC-DC can stay in regulation. Other things may fry too...
  10. Apr 15, 2011 #9
    The current will go to the bulk output capacitor bank with something like 5000uF-9000uF, then to high frequency ceramic caps and then to the CPU(processor).

    Some of the Intel processors, all of the i7 900 series have an 100A load step from idle to load. We overclock which means we take 2.8ghz processors to 4 or 5 or 6 or 7ghz depending on the cooling.

    The VRM (voltage regulator module) in question have 16 pairs of these caps, but only 8 of these inductors, which is weird, usually pair of high and low side MOSFETs have their own inductor, but this VRM has a weakness, and I want people to be informed.

    In our community we had very high end $600 dollar GPUs very new ones that would have their 35A DriverMOSFETs or 30A low Rdson MOSFETs blow up because of their inductors, that was the theory of why.

    Our processors have a 130watt TDP, which is their thermal spec, so they pull about 105watts and then loss a bunch to efficiency. but the GPUs have 200-300 watt TDPs.

    Motherboards now have very sophisticated SMPS. Some have 24 phase designs, some 8 some even as little at 6, and everywhere in between. The top end GPUs I have seen one with 12 phases and the ones blowing up have 5.
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