Calculating FET turn-on switching losses in offline flyback?

[Flyback]

Hello,
As you know, when investigating FET (turn ON) switching losses in offline flybacks, it is nice to be able to calculate the miller charge time, since for the duration of the miller plateau interval, the FET is in the linear region, and the energy dissipated is 0.5 x Vds(off) * I(pri_pedestal) * [duration of miller plateau].
The way to calculate the miller plateau interval, would be using dt = (C.dv)/I
…where
dv = ds(off)
I = current from gate driver.
C = Crss = Cgd
However, “Crss” is highly variable as the voltage across it varies, the magnitude of Crss to be used in the above equation would be a research issue.
Given that we are using the FET IP65R280E6, what value would you use for the value of “Crss” in the above equation?
Page 12 of the datasheet gives the value of Crss in relation to the voltage across it, but as you can see, it varies over several orders as Vds varies down to zero volts
IP65R280E6 datasheet:
https://www.infineon.com/dgdl/Infin...n.pdf?fileId=db3a304328c6bd5c0128d3bdf1ca009e

 

[Tony Stewart]

There are many more parameters required for a good design, such as ripple, standby power, output power max. And selection of MOSFET based on lithography micron size affects Coss, Ciss*RdsOn product, which affects gate driver RdsOn RC time constant etc. air gap eddy current losses etc. choice of efficiency.

http://www.power-eetimes.com/en/ste...yback-design.html?cmp_id=71&news_id=222903558

 

[crutschow]

I would use the total gate charge rather than the capacitance to calculate the switching time.

 

[Tony Stewart]

Yes Charge Q is an alternative method .to compute discharge time constants and associated losses for L and Fet. More specifically it is change in Q or change in CV, where Q and C are inversely related to Imax and RdsOn and volume of the junction., so SiN is better but cost more than Si and IGBT are preferred when ratings get over 800A and/or Volts some exceeding 1Meg VA product, but used for MegaWatt forward inverters and flyback for <100W.

Due to much lower RdsOn's this choice gets even more tricky....search those keywords...