SMPS transformer switching

Discussion in 'The Projects Forum' started by roro36, Nov 18, 2010.

  1. roro36

    Thread Starter Member

    Sep 7, 2010
    52
    0
    This is what the input to my transformer looks like. Basically I have a 2 switch flyback SMPS which switches roughly 75V DC to the transformer, which is what I've shown. It should be on, then off, with some energy discharge into the flyback diodes. Why is there such a large negative spike before the positive one?
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    Post the schematic.
    Then we can take a look at it.

    Bertus
     
  3. roro36

    Thread Starter Member

    Sep 7, 2010
    52
    0
    This is the circuit.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Because the current was still flowing through the flyback diodes when the MOSFETs turned on. Rather than the diodes, you really need more like a snubber arrangement.

    Not sure why you decided to switch both the high and low side of the transformer primary; you might as well have used a full bridge and made a push-pull configuration, or just stuck with switching the low side. As it is now, you're getting losses from the Rds(on) of two MOSFETs instead of just one.
     
  5. hasanferoz

    New Member

    Mar 8, 2011
    25
    0
    roro36 i was looking at ur circuit and was wondering what is the function of R3 and C8 i then figured out that they are actually saving the the transfomer from being saturated if this is write can u tell me the theory behind it and how can i calculate them
     
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