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Synchronous rectifier switching losses in phase shift ZVS

Ashok Chakravarthi
Prodigy 125 points

Hi,

As per SLUA560B, voltage- current over lapping switching losses are considered while calculating the losses of synchronous rectifier. Are these losses are really applicable over here ?.

I think it won't because mosfet body diode will start conduct before the gate turn on and turn off. Please clarify me.

Regards,

Ashok

  • 0
    TI__Mastermind 31580 points
    Hi Ashok,

    I have contacted the applications engineer for this product he should be in contact soon.

    Regards

    Peter
  • 0 in reply to Peter Meaney
    TI__Mastermind 25570 points

    Hello Ashok

    Ideally there should be no turn-on switching losses in the SRs because they are switched at zero volts. Turn-off losses are significantly more complicated to calculate because they depend on the current carried in the SR at the time of turn-off. During the off (1-D) time when the primary has zero volts across it, the output inductor current is shared between the two SRs but the problem is that this sharing is unequal and depends on transformer leakage inductance. When the transformer primary is re-energised during the on time (D) one of the SRs is left on - it won't see any switching loss. The other SR is turned off during the ZVS transition on the primary. This SR will see some cross conduction losses.

    An accurate calculation of the SR loss is very complex and the calculation shown in the App note you mention is really just an approximation. The conduction losses are 50% of the total in any case, and even these are an underestimate because they don't take account of the increase in MOSFET Rds_on with temperature.

    Hope this clarifies things

    Regards
    Colin