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FLYBACK SMPS PROBLEM

Discussion in 'General Electronics Chat' started by ravilite, Oct 29, 2017.

  1. ravilite

    ravilite New Member

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    hello guys ,
    Here is the situation, I am working on a 90 watts , 1050 mA , 72 V flyback based constant current smps which is used for outdoor LED lights,
    Mosfet used 12A 800V
    O/P diode used - Average forward current 8 A , Peak revers voltage 400V

    In this design diode and Mosfet are getting damaged in pairs with in a month of operation. I din't find any burnt marks or any cracks on the diode or on the Mosfet they just look like a fresh components.
    To protect the the diode and Mosfet I have used RCD snubber circuit. but still they are getting damaged.

    I am unable to understand the reason of Mosfet and diode failure.
    can anyone please tell me the possible reasons of output diode failure in flyback SMPS.
     
  2. kubeek

    kubeek Well-Known Member

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    Welcome to the forum, please show your schematic. Without that it is very hard to say anything.
     
  3. ravilite

    ravilite New Member

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    This is the schematic , used diode and Mosfet I have mentioned above
    upload_2017-10-23_23-56-33.png
     
  4. dave

    Dave New Member

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  5. kubeek

    kubeek Well-Known Member

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    What is the ratio of the transformer?
     
  6. ci139

    ci139 Active Member

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  7. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    It is hard to know if the diodes or the MOSFET died first. Probably the diodes. Diodes=short will cause great stress on the MOSFET.
    What is the temperature of the diodes and MOSFET?
    Do you know if the core (transformer) is near saturation?
    ----------------
    diode current. I think one diode will work. Average current is 1A and you have 3x5A of diodes.
    I can not see how much ring there is on the primary and secondary.

    Are you in continuous or discontinuous mode?
     
    Last edited: Oct 29, 2017
  8. ravilite

    ravilite New Member

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    Transformer turns are
    Primary turns : 45
    Secondary turns : 28
    Transformer core is PQ2620

    The diode I am using U16C40C diode having T-220 package which have two internal diodes out of which only one is getting damaged .

    Constant current power supply is working in critical conduction mode.

    How can I check if the transformer is getting saturated. And how the transformer saturation can damage the Mosfet and output diode.
     
  9. ravilite

    ravilite New Member

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    The power supply I am Using have short circuit protection, whenever output gets shorted input watts drops t0 0.5 - 0.2 watts .
    I have measured voltage across the mosfet drain and source during normal operation voltage is around 180V and when output is shorted voltage climbs to 310V .
    The mosfet I am using is of 800 voltage .
     
  10. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Here is a picture of primary transformer current. (or the voltage across the Source resistors) I added a red line. If there is any hint of the current going up at the end then you have saturation. (gets worse at high load and after the transformer gets hot)
    upload_2017-10-30_7-2-35.png
    If the core saturates the transformer inductance drops, current gets high, and out of control current. .....
    This is hard on the MOSFETs.
     
  11. schmitt trigger

    schmitt trigger Active Member

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    Waveforms, we need waveforms!
    At least a pair: the Vdrain to ground, and I drain (measured as a voltage across the sense resistors).
    If these can be made simultaneously with the scope in two channel mode, even better.

    Goes without saying: an isolation transformer is required to make these readings safely.

    Also make sure that:
    1) your probes have been compensated and are in 10X mode
    2) use short, short, short grounding leads to avoid picking up stray noise.
    3 don't crowd the screen with readings. What we want to see is the waveform's shape, with as much detail as possible.
     
  12. schmitt trigger

    schmitt trigger Active Member

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    The second set of measurements will be more difficult: Measuring how the output current shares across the three paralleled output diodes.

    I suspect that the problem could lie there. Most likely they are not sharing current equally, across all the possible operating conditions.
    Could you replace these diodes with a single, higher capacity one?
    Or; could you add a very low value, 0.1Ω resistor in series with each individual diode?
     
  13. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    There are three 5A diodes. It should work with only one. I do not see "sharing" is a problem.

    There are snubbers on the primary and secondary but with out scope pictures we don't know if they are good.
    Maybe the transformer is hand wound.
    Probably this is a bread board with long wires. (we just don't know)
     
  14. ravilite

    ravilite New Member

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    I have not used Three separate diodes instead I have used U16c40c diode which has 2 internal diodes and out of which one is getting damaged
     
  15. ravilite

    ravilite New Member

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    Yes I agree with you current sharing is not the problem, I have used the same diode ( U16C40C ) even in higher current smps and it just worked fine.

    The Board I am working on is a well developed PCB ( I am not doing nothing on bread board ), and even the transformer is also professionally made its not hand wounded.

    We have have made around 50 such SMPS out of which 30 got failed and all have the same problem, diode , mosfet and PWM IC got failed.

    I will try to upload the waveforms photo as soon as possible.
    Can you guys tell me which are the important location where I should check the waveforms.
     
  16. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    U1 pin2 Vcc please test if the voltage is too high.
    U1 Pin2 to Pin1 should have a 0.1uf capacitor on the supply. Capacitor must be close to IC.
     
  17. ravilite

    ravilite New Member

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    Can PWM IC damage the mosfet , if yes then what are the possible reasons
     
  18. kubeek

    kubeek Well-Known Member

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    It could drive the mosfet with more than Vgsmax. What type is that IC?
     
  19. ci139

    ci139 Active Member

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    if the event is not 100% - perhaps it's just the figure for particular mosfet / diode time to live variations
     
  20. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Question:
    Answer:
    Very likely the voltage is OK. I did not design the transformer so I don't know. The supply of U1 would have to be more than 20 volts to kill the MOSFET.

    Most likely the MOSFET shorts G-D and sends 100s of volts to U1. So the MOSFET kills the IC. I think the diodes are dead first.
     
  21. kubeek

    kubeek Well-Known Member

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    Also, does the diode have a heatsink? How big is it? What type is the mosfet?
     
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