SMPS switching problem

Discussion in 'The Projects Forum' started by roro36, Oct 29, 2010.

  1. roro36

    Thread Starter Member

    Sep 7, 2010
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    Design: Two swith flyback method switched mode power supply. Has about 45volts as the DC across the switches. I'm using PWM for a micro, through a mosfet driver, through a high side driver and onto the gate.
    Results: The clear pulses in the last picture is of the gate source voltage/signal. It is correct. The next is the drain source voltage. Wrong, basically it should show a scaled inverted version of the input signal. As when the signal goes high and the switch turns on, the voltage drop D-S will be zero, and when the switch is off, it should be high. Obviosly not expecting it to follow theory perfectly, but this is not right. Testing circuit is attched to show how it is setup for testing.

    Not sure why the MOSFET is not switching as expected.:confused:
     
    Last edited: Oct 29, 2010
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    First off.
    what the heck is the first attachment. :confused:

    To ur problem.
    What the heck are u trying to do...
    If u are showing schema's, then show complete schema's. Do not expect u can get help on partially explained problems.
    U need to give us more info if u want any help
     
    roro36 likes this.
  3. roro36

    Thread Starter Member

    Sep 7, 2010
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    Ok, changed the picture. My bad. The whole schematic is not needed as I only have this section connected.
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    No
    Show us the schema...

    I have gone thru ur posts and I smell something fishy.
    What are u upto?

    If u don't come clean, no one here will help u.
    We need more info from ur side.
    tell us everything
     
  5. roro36

    Thread Starter Member

    Sep 7, 2010
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    What? I cant believe you're serious. Here is the full thing.
     
  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Serious? Do u think we here for fun.

    This means u are not doing anything worth our time.
    People come here asking for help in real world problems, not to have fun.

    What are u trying to do?

    That circuit will never work in a real world.
     
  7. roro36

    Thread Starter Member

    Sep 7, 2010
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    Well, I'm not sure why it would not work. When I have done the research, this is what it shows to work?
     
  8. roro36

    Thread Starter Member

    Sep 7, 2010
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    The only thing is that I couldn't find the right chips for the mosfet driver in the program and the AC ahould be down to 42V. The SMPS should provide about 20-5V at output.
     
  9. eblc1388

    Senior Member

    Nov 28, 2008
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    Have some confidence in yourself. The result might not be too far off actually. :)

    See here: Flyback converter

    [​IMG]
     
    roro36 likes this.
  10. roro36

    Thread Starter Member

    Sep 7, 2010
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    Hey man thanks, why did the other guy say it wouldn't work? Thanks again.
     
  11. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    That's better. Ur schema says 240Vrms, bridge that and u get what? 42V...

    U have 75V diodes and low power fets.
    U need to use something better to simulate

    {ed}
    I said it won't work concerning ur voltage levels. U did not explain ur self first
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    What MOSFETs are you actually using?
    2N7002's are logic level N-channel MOSFETs, but have an Rds(on) of around 6 Ohms, and are limited to around 150mA current (without looking at a datasheet; but that's in the ballpark).

    They wouldn't work very well for this kind of application.

    You don't have snubber circuits on the MOSFETs. The first time one of them switched off, they'd be blasted right out of the circuit due to the reverse EMF.
     
  13. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    U know what Sarg.....that is exactly what I meant in real world.
    But....
     
  14. roro36

    Thread Starter Member

    Sep 7, 2010
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    I'm using IRF840 mosfets.
     
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