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    push pull dc dc converter

    Dear guys

    I am going to work on a high frequency DC-DC converter. Due to 50kHz high switching frequency i am facing continues Spikes and Ringing signals on the Drains of Mosfet, these spikes gradually damps out , but due to this spike my Mosfets gets too much hot.

    If any body could share any idea, I welcome !!

    Regards
    Iftikhar Abid

    •   Alt7th May 2009, 04:47

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    push pull converter

    Use the snubber to reduce.



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    push pull dc to dc converter

    it also gets heat up... and broken down........


    1 members found this post helpful.

    •   Alt7th May 2009, 07:04

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    push pull dc/dc

    Hi,
    Wind Transformer properly because with high frequency, leakage inductance is creating problem.
    Use proper snubber to control peak.
    Use small resistors inbetween driver and mosfet gate to reduce ringing.
    Use an input inductor of proper value.
    If you tell detail regarding power requirement and other data, I can help you and can provide you a detail circuit. Thanks.



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    push pull dcdc

    Check ringing frequency .refer to Unitrode app note on snubber design you can calculate values of res. & cap.

    First check leakage inductance of your transformer.

    Increase value of gate resistor to reduce rise time / fall time. This will also reduce ringing. But increase heating in MOSFET so don't increase beyond 33E/47E



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    snubber push pull

    Let me to get back to your original question:
    but due to this spike my Mosfets gets too much hot.
    The conclusion is most likely incorrect.

    And regarding the suggested snubbers:
    it also gets heat up... and broken down......
    Snubbers are dedicated to absorb energy. So it's basically normal, that they heat up. The trick is in adjusting the time constant and impedance in a way, that they only (mostly) absorb unwanted high frequency oscillation energy and little at the SMPS operating frequency. Because the ideal converter waveform is a square-wave with unlimited frequency spectra, this is impossible in principle, so the snubber dimensioning is always a compromise.

    As long as the spikes must not be feared to damage the switching devices, they are mainly a problem of generating interferences, but they don't cause excessive losses.

    Recent MOSFET have typically integrated zener-diodes, that clamp spikes above the maximum Vds. This way, the energy stored in the leakage inductance is turned into losses. Absorbing the energy in a snubber can only increase the total losses. You either have to use a transformer with less leakage inductance or a switcher topology, that recovers the energy at the primary. A classical two-transistor push-pull converter can't, unfortunately.



    •   Alt11th May 2009, 06:52

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    push pull dc to dc converters

    I think we need to see schematic then only we can draw some conclusion.



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    Re: High Frequency Push-Pull DC-DC converter Designing

    Tahmid my input voltages are 385 volts output is 145 volts the spikes are almost 800 volts what suggestion you can make and give your email id so i can email you my schematic



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