Help in snubber circuit design for a H-bridge

Discussion in 'The Projects Forum' started by chrislee84, Mar 29, 2008.

  1. chrislee84

    Thread Starter Member

    Dec 8, 2007
    14
    0
    The image below is the H-bridge that i have design and make out.

    [​IMG]

    [​IMG]

    This h-bridge is a 24V and use to drive a DC brush motor which is 24V, 150W. After i test it, the driver can work but when i switch the direction, the lower part N-mosfet will burn. Because of this, I want to include snubber circuit to protect my mosfet and the motor but i don't know how to design it. Is the RC snubber is suitable or the RCD snubber is suitable And how to calculate the value of R,C and/D?

    Can anyone help ? What actually parameter that i have to find to design the snubber circuit?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    A diode across the FET's in reverse bias is usually enough for protection. Your image is thumbnail size, so I can't make out any details.

    Are your FET's mounted on heat sinks? Are you certain they switch cleanly between conduction and turn-off? The current is not that high, so I would suspect something like operating in the linear region might be the problem.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Here's the full-size image:
    [​IMG]

    Still not terribly legible, I'm afraid.

    Can you post a schematic with a higher resolution?
     
  4. chrislee84

    Thread Starter Member

    Dec 8, 2007
    14
    0
    sorry, the image i upload previously got something wrong, the below one is the right one:

    [​IMG]

    At the center of the upper and the lower mosfet is connect with the motor which i do not show.

    Dear beenthere, what u mean by this?
    The mosfet is mouted with heatsink already. Does i include the reverse bias diode, i no need to design the snubber circuit for the protection purposes?

    The N-mosfet i use is the IRFP260N and the P-mosfet i use is the FQA47P06 and below is the link of the datasheet of both mosfet:

    N-mosfet(IRFP260N): http://www.irf.com/product-info/datasheets/data/irfp260n.pdf

    P-mosfet(FQA47P06): http://www.tranzistoare.ro/datasheets3/fairchild/FQA47P06.pdf

    As shown in the datasheet, both of the mosfet have the internal reverse diode, does this enough for my application and i do not have include the snubber circuit to protect the mosfet and the motor?
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    There is an internal diode intrinsic to the structure of a power FET, but it is not intended for the kind of conduction necessary to protect the device from a high induced reverse voltage. It takes an external diode to do that.
     
  6. chrislee84

    Thread Starter Member

    Dec 8, 2007
    14
    0
    Hi beenthere,

    Actually how i want to chose the correct diode for the reverse diode?
    What i want to consider is it the current that the diode can sustain?
    The battery i use is 24V/9Ah, so the current may be shoot very high.
    So, how i wanna chose the right one for the diode.
    As i know, is it the schottkey diode is better?

    Hope to hear your reply soon.
     
  7. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    The body diode can usually handle the current but I think it is the reverse recovery time that is the problem-- an external Schottky is usually better.
     
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