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Coss losses for ZVS converters (UCC28950)

Florin Ciortea
Prodigy 90 points
Other Parts Discussed in Thread: UCC28950

Hello,


I would like to know if the Coss related losses are encountered for ZVS phase shift converters. I read some articles, from several suppliers, that are highlighting the need of counting these losses too. However, in UCC28950 application guide these losses are omitted and many other suppliers are saying the other way around - that the losses do not exist due to resonant charge/discharge of the capacitance. My belief is that there are some losses due to the fact that the resonating inductance must provide the required energy for soft switching (even if through resonance).  However, I do not have the grasp on the subject to know if this energy is practically dumped within the resonating tank, or is rail recovered (as the body diode clamps the switching node to rail voltage prior to on transition). Even if the losses are low in comparison to turn-off or conduction losses, I would like to know just for my general knowledge  if the Coss losses exist or the entire energy is recovered.

Many thanks for your support!

  • 0
    TI__Mastermind 31580 points
    Hi Florin,

    I have forwarded the post to my colleague who will answer this tomorrow.

    Regards

    Peter
  • 0 in reply to Peter Meaney
    TI__Mastermind 25570 points

    Hello Florin

    In an ideal ZVS system the Coss losses are eliminated. However any real system won't be ideal and I would expect that the following factors would come into play.

    The system may not actually achieve ZVS because there is insufficient energy in the resonanating inducance to discharge the switched node all the way to zero. The residual energy at the switched node will be dissipated when the switches turn on. The term here is 'Valley switching'

    There may be too much energy in the resonating inductance and a significant amount of body diode conduction (assuming SiMOSFETs) can happen. This will be more dissipative than conduction when the MOSFET is turned on.


    Any real resonant circuit will have a certain amount of resistance - part of the resonant tank energy will be lost in this resistance

    The 'dead time' allowed for the resonant transition will reduce the effective duty cycle - it depends of course on the relative proportions of dead time to cycle time. The currents in the power transfer part of the switching cycle will have to be (slightly) correspondingly larger than they would otherwise have to be. This will slightly increase the circuit loss.


    Generally, ZVS is worthwhile for off line systems where a 400V PFC stage output has to be down converted to a more useable voltage.


    Regards

    Colin