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Switched Mode Power Supply Overshoot problem

Discussion in 'Electronic Design' started by [email protected], Feb 16, 2007.

  1. Guest

    Hi, all-
    I hope someone could offer me some advice.
    My design has a buck switched mode power supply (12v to 1.8v,
    MAX1993).
    On startup, there's an overshoot on the voltage, and the contoller
    shuts down on
    the over-voltage protection.
    What could be the causes? maybe wrong values on the output inductor,
    or capacitor?
    maybe wrong ESR value on the cap? maybe layout design problem?
    Thanks for any help, guys,
    Harpoon
     
  2. A copy of the schematic would be very helpful. But
    increasing the capacitance of the output filter may help.
    The best cure is probably in the feedback divider and filter
    section of the circuit.
     
  3. Genome

    Genome Guest

    Good luck with that one. It is a highly complex, read I've never really
    looked very hard at that stuff much before, controller. Working out what it
    thinks its transfer function might be could be hard. It's probably some
    wierd thing that is not suited to 'normal' linear analysis.

    The dirty sheet does mention possible instabilities due to the capacitor ESR
    zero being too high. Voltage feedback is a simple divider and there is no
    feedback available to tailor the response of the internal error amplifier if
    there is one in there.

    That might suggest that the overall voltage loop including the capacitor is
    second order up to the ESR zero where it becomes first order and crosses
    over at some higher frequency. If the ESR zero is too high then is crosses
    over second order and is unstable...... BTW this is speculation.

    Anyway, one possibility is that with such a large VIN/VOUT differential you
    can rapidly ramp up your filter inductor current to some excessive level and
    when the output voltage reaches the set level the controller switches things
    off BUT you have still put energy into the inductor and that gets delivered
    to the output causing the overvoltage.

    Cures for that would be reducing the absolute current limit level and/or
    increasing the inductor value so it doesn't store too much excess energy.

    If I'm right about the loop behaviour (pigs might fly) then you might split
    the feedback resistor from the output node into two in series with a ten to
    one ratio and add a capacitor across the larger resistor..... and then
    experiment with the value of the capacitor. That has two effects.

    The first is it introduces a low frequency zero at the input making the loop
    first order up to the frequency where the smaller resistor cancels that
    zero. This is good because is makes the loop less conditionally stable.

    The second is that such a response results in a more damped behaviour and,
    in particular, a less snappy start up. Rather than starting up balls to the
    wall it should bring the output voltage up 'gradually'.

    Further than that I wouldn't like to guess. All of the above is speculation
    based on 'normal' controllers and should but might not apply to this one.

    Good Luck

    DNA
     
  4. legg

    legg Guest

    Certainly is a convoluted architecture.

    This thing claims to have a built-in slow start. With a constant
    on-time topology, this is frequency modulated. Are you starting up by
    using the enable line, or just whanging the input with 12V?

    Without a well established 5V bias supply, you can't count on the slow
    start working. This is also the supply that is needed to switch from
    2V5 to 1V8 regulation.

    At lower output voltage settings, the app circuit has the output
    terminal directly returned to the FB input, preventing the
    conventional use of output feed forward.

    Are you using the current sensing terminals?

    Are you using the application circuit layout?

    RL
     
  5. Guest

    Hi all,
    Thanks for all the help!
    Just wanted to inform you that the problem was solved, with help from
    MAXIM support:
    The current-limit setting in my design was wrong: I supply 4A max, and
    my limit was
    set to 14A, so the initial current was too much for the controller.
    the fix was to set the current limit threshold via divider resisors,
    and change the low
    side mosfet to one with higher Rds resistance, as per the datasheet.
    Thats all,
    thanks again for all the help!
    Harp.
     
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