UCC28950: SLUA560C - Rectification MOSFET Loss Calculation

Hello Torben

I'm afraid that you are correct - there is an error in the app note.

SR Turn-on for QE and QF is lossless - it is done with zero current in the part and zero volts across it.

SR turn-off for both QE and QF is a ZVS transition - there is no voltage across the device when it is turned on.

There is however current in the MOSFET as it is turned off. This current then flows in the body diode until it decays to zero - this is the interval during which the transformer secondary current is transitioning out of one secondary winding and into the other. This will incur Vf*I(t) losses - (Vf is the body diode forward voltage)

The SR switching is shown in the images below.

Now, let's look at EQ71 - below

The first term is simply the I^2R losses in the MOSFET

The second (yellow) term is I think in error - The losses during the time when the current is moving out of one SR and into the other are I^2R losses as the current rises in one SR (red trace for QF below) and I^2R losses in QE until the time when QE is turned off and the losses then become Vf*i(t) until the current has decayed to zero. (blue trace for QE below). Of course the QF and QE losses will swap over during the following switching cycle.

The red term is in error and should be ignored - as you pointed out the SRs are ZVS

The Green term is the energy needed to charge and discharge the gate - this term is correct

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The SRs are switched at zero voltage so that there is no loss associated with the discharge of the Coss. There are however other losses - the energy needed to charge and discharge the gate capacitance at each switching cycle and the losses associated with the body diode conduction in the interval between the drain of the MOSFET being driven down to zero volts and the MOSFET channel being turned on.

Please let me know if you need any further information

Regards  Colin

Hi Colin,

thank you for your quick and detailed answer!

In case you are going to correct the app note it may also be worth checking the SLUC222 UCC28950 Excel Design Tool. I think it uses the same calculations as in the app note.

Regards Torben