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Need help regarding flywheel diodes

  1. Jun 18, 2009 #1
    Hi all. This is my first post here. Really hope someone can help :)

    Im facing some problem regarding the design of my little switch mode boost regulator. It uses the max631, which has a built it catch diode.

    The input voltage to the max631 comes from a logic controlled p channel mosfet. Vsupply to the drain; one end of the inductor that the max631 requires is connected to the mosfet's source, and the other end to Vin of the chip.

    The max631 has a built in "catch" diode. Since im not too sure how smps and coils really work, I do not really know if a flywheel diode parallel to the inductor is needed or not. To the best of my understanding, I think flywheel and catch diode mean the same thing.

    Is it needed?
    Would the addition of a parallel flywheel diode lower the switching effiency?

    I've uploaded a brief sketch. Do take a look.
    Many thanks in advance!
  2. jcsd
  3. Jun 18, 2009 #2


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    Staff: Mentor

    Welcome to the PF. A boost up-converter has an output rectifying diode, not a flywheel diode. Buck down-converters are the ones that use a flywheel diode to keep the output current flowing when the high-side switch element is opened up. You should not put the catch diode in that you show across the boost inductor.

    Here's a good tutorial from Maxim that discusses buck, boost and other DC-DC converter topologies:


  4. Jun 18, 2009 #3
    I shouldnt put the catch diode across the inductor?
    May I know why?
    Im concerned that when the supply is cut, the back current induced would damage my mosfet. Or will it not?

    Many thanks again.
  5. Jun 18, 2009 #4
    If anything, put a capacitor from the output of your mosfet to ground to protect it. If you think you REALLY need the reverse diode, put it across your mosfet. The red diode in your figure across the 330 uHy inductance will prevent proper functioning of your SMPS.
    Get datasheet at http://www.datasheetarchive.com/MAX631-datasheet.html
    Last edited: Jun 18, 2009
  6. Jun 18, 2009 #5
    Alright. Now that I've read a bit more on SMPS, specifically the boost types, am I right to say that a flywheel diode is not needed in my circuit?

    When the mosfet switches off, the induced current flows toward the smps and not back to the mosfet because the inductor is placed after the mosfet.

    However, if i choose to place my inductor between Vsupply and mosfet, the there will be a problem.

    Did i get it right?
  7. Jun 18, 2009 #6
    The 330 uH inductor should be attached directly to the MAX631. The other end should be attached to your mosfet switch and bypassed to ground to give a good low-impedance high-frequency connection to ground. A flywheel diode across your mosfet will prevent reverse biasing it. The MAX631 switching frequency is about 40 to 60 kHz, so your bypass cap should be about 1 uF so it won't interfere with the 330 uH inductance operation. The datasheet also shows a 100 uF cap on the MAX631 output.

    Get datasheet at http://www.datasheetarchive.com/MAX631-datasheet.html
  8. Jun 18, 2009 #7
    Alright i understand how a diode placed across the mosfet protects it frm reverse currents, but how does a bypass cap at the output of the mosfet achieve the same thing?

    Dont mind explaining it to me?

    Thank you!
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