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TIDA-00951: Snubber requirement on the synchronous rectifier side

Eswar Kumar48
Prodigy 60 points

Part Number: TIDA-00951

Hi,

While the converter is operating in buck mode (Charging mode) do we not require any snubbers for the mosfets on the LV side. Because the mosfet body diode reverse recovery current will cause a spike on the mosfet.

Also can you please briefly explain how the active clamp circuit is helping in reducing the spike and what exactly is causing this spike in the discharging mode.

  • 0
    TI__Intellectual 2815 points
    Hi Eswar,

    The di/dt of the current through the mosfet body diode is limited by the leakage inductor. Hence we don't see any significant reverse recovery on the LV mosfet.

    During discharge mode, the design works as a current-fed converter.
    In "traditional" current-fed converters (unlike voltage fed converters), there wont be any decoupling cap at the input of the switching power stage. Also the inductor current is continuous. As a result when a diagonal mosfet pair is turned off, due to the stray (mosfet lead, PCB trace) inductance there would be huge voltage spike on the mosfet.

    In this design the active clamp diverts the current out of the mosfet diagnol pair at turn-off hence elminates the voltage spike at turn off as the energy in the stray inductance will be very very low at turn off.

    Best Regards,
    Ram
  • 0 in reply to Ramkumar S
    Prodigy 60 points

    Hi Sir,

    Thanks for your reply.

    The operation of active clamp is still not clear to me. Lets consider the below image

    have just drwan the current fed side of the converter with
    parasitic inductances.

    During boost mode all the switches are turned on and the indcutor charges and discharges when one diagonal
    pair is turned off.

    Lets consider the switches 1 and 4 are off. Before turning off the Lps at these Mosfets are carrying current
    when the mosfets are turned off, the stored energy will be dumped into the Coss of the Mosfets which will
    result in spike.

    Now my doubt is how come the presence of the clamp circuit present near the boost inductor will snub the spike caused by
    parasitic inductance?

    I hope I expressed doubt clearly.

    Will be awaiting your reply.

    Thanks,
    Eswar

  • 0 in reply to Eswar Kumar48
    TI__Intellectual 2815 points
    Hi Eswar,

    When the mosfet switches 1 & 4 turn-off, initially the COSS of these mosfets will start charging. When the COSS voltage exceeds the voltage of the active clamp capaitor C, the body diode of Sc gets forward biased and this the active clamp capacitor C starts charging. This results in clamping the "spike" across the mosfet 1&4 as the value of actie clamp cap C is much higher than the COSS of the mosfet 1&4, there by a singificant portion of the mosfet turn of current will be diverted into charing the active clamp cap C resulting in very low voltage spike on mosfet 1&4.