We are using the IRF6648 FET as a sync FET in a sync buck with vin=28v vout=1.5v, iout=17.1A, fsw=200khz.
The Qrr for IRF6648 is 56nC.
Therefore the dissipation in the lower FET due to this Qrr is ..
(Qrr * Vin * Fsw)/2
Thus equals 0.155W
However, the datasheet conditions for Qrr measurement involved a di/dt of 100A/us.
In our sync Buck converter the di/dt would be about 17.1A/8ns which equals 2137A/us.
Given this difference, how much more would the FET dissipation be due to this reverse recovery?
IRF6648 FET datasheet
https://www.infineon.com/dgdl/irf6648pbf.pdf?fileId=5546d462533600a4015355ec6e561a59
PS: There's little point in using a schottky in parallel with the sync fet because sufficiently voltage derated schottkys for such purpose have high vf and would not sufficiently stop the intrinsic fet diode from conducting. -The intrisic fet diode would still conduct very significantly even with a paralleled schottky.
The Qrr for IRF6648 is 56nC.
Therefore the dissipation in the lower FET due to this Qrr is ..
(Qrr * Vin * Fsw)/2
Thus equals 0.155W
However, the datasheet conditions for Qrr measurement involved a di/dt of 100A/us.
In our sync Buck converter the di/dt would be about 17.1A/8ns which equals 2137A/us.
Given this difference, how much more would the FET dissipation be due to this reverse recovery?
IRF6648 FET datasheet
https://www.infineon.com/dgdl/irf6648pbf.pdf?fileId=5546d462533600a4015355ec6e561a59
PS: There's little point in using a schottky in parallel with the sync fet because sufficiently voltage derated schottkys for such purpose have high vf and would not sufficiently stop the intrinsic fet diode from conducting. -The intrisic fet diode would still conduct very significantly even with a paralleled schottky.