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Should MOSFETs be open all the way?

Discussion in 'General Electronics Discussion' started by stevenmahoney, Mar 11, 2013.

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  1. stevenmahoney

    stevenmahoney

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    Mar 11, 2013
    I have a question, perhaps a simple one.

    In the following schematics driver send a signal to the gates of three MOSFETs. I expected a square signal in the output, but it is straight DC. It looks like one or all MOSFETs is open.

    I replaced the driver IC, driver transistors, all MOSFETs, but result is the same.

    What am I missing?

    [​IMG]
     

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  2. Harald Kapp

    Harald Kapp Moderator Moderator

    11,517
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    Nov 17, 2011
    Your gate signal is mostly High, therefore turning on the MOSFET. There are only small Low going pulses turning off the MOSFETs.
    Your yellow LED should show this by being almost fully lit.
    Your driver signal probably needs to be inverted?

    You can test the MOSFETs by removing R7 from the circuit and short-circuiting Z3. Your MOSFETs should now be off.
     
  3. stevenmahoney

    stevenmahoney

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    Mar 11, 2013
    I understand that and feared that the duty of the signal is too high, however, service manual listed this signal as correct one:

    [​IMG]
     
  4. henrypereira

    henrypereira

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    Nov 7, 2012
    have you tested the signal before zener diode?
    or at the output of the driver ic?

    those will be the steps to identify the culprit of the fault as i can see one of your components its losing its value causing the effect you have.
     
    Last edited by a moderator: Mar 11, 2013
  5. Electrobrains

    Electrobrains

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    5
    Jan 2, 2012
    It's not easy to say what is wrong, or even if something is wrong. You must give more information about what your circuit does and why do you think the duty cycle should be lower?
    It seems you have some kind of voltage or pulse regulator loop. Is it a pre-regulator for a SMPS or some Power Factor Correction circuit?
    The full ground circuit is not shown (the drains of the MOSFETs hang on the positive, rectified rail). Where does the branch from node P5.Y2 go?
    Perhaps your error is at a totally different place.
     
  6. stevenmahoney

    stevenmahoney

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    Mar 11, 2013
    Yes, signal is perfect all the way yo the gates.
     
  7. stevenmahoney

    stevenmahoney

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    Mar 11, 2013
    Electrobrains, this is an electronic ballast for HMI light (film industry). This circuit should produce a 20kHz square wave. The power factor looks OK to me, because that is what service manual said it should be on the gates. What I do not understand why I cannot see a square signal at output.
     
  8. Electrobrains

    Electrobrains

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    5
    Jan 2, 2012
    So in fact there is no problem? The unit works?
    It seems you have a square wave on the output. It's just high Duty Cycle. That's normal if you have a regulating PWM circuit involved.
     
  9. stevenmahoney

    stevenmahoney

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    Mar 11, 2013
    Electrobrains, I do not know if there is a problem. The attached oscilloscope clip is a square wave at the gates of MOSFETs. Still getting a straight DC at the output... Could it be because I run it without a load?
     
  10. Electrobrains

    Electrobrains

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    Jan 2, 2012
    1. Definitely Yes! Ballast circuits often need a load to work.
    2. The "output" in your diagram would need a load, otherwise you will measure DC. Your multimeter has too high resistance. C13 will discharge through the MOSFETs and will hardly be able to get any current through the multimeter. Thus C13 has 0V over it and you measure straight onto the dc supply.
     
    Last edited: Mar 11, 2013
  11. stevenmahoney

    stevenmahoney

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    Mar 11, 2013
    It is marked "OUTPUT" in the right bottom corner.
     
  12. Electrobrains

    Electrobrains

    259
    5
    Jan 2, 2012
    Sorry, I saw it too late but edited the message.
     
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