UCC3809-1: diode selection question in flyback slua086 (U-165)

Hello Yi

1/ "The peak repetitive forward current is equal to the reflected primary peak current" and you are of course correct that the two currents do not flow at the same time. Let's say you have 1A peak current in the primary, then the secondary current would be 1*Np/Ns.  Here's a diagram - showing a typical primary waveform (blue) and a secondary one (red) - the duty cycle is about 50% and the turns ratio is 2:1 so the secondary current peak is twice that of the primary.

Regards

Colin

Thanks Colin, I understand.  By the way, do you have any advice on my question 2 ? 

Hello Yi

My apologies - I missed that one.

The reverse recovery of the diode comes into play when the MOSFET is turned on and the diode current drops to zero while the MOSFET current increases - at the time arrowed below.

It's all very difficult to analyse fully but the basic message is that 'Faster is Better'. When the MOSFET turns on the voltage on the diode reverses. The diode will conduct in the reverse direction for a little while. The losses associated with this can be significant because the reverse voltage across the diode is high.

So - 'Faster is Better' - typically one would use so-called 'Fast' or 'Ultra Fast' diodes as the 'default' option (trr < 50ns). At low output voltages Schottky diodes are a good choice - like the MBR2535CTL device you mentioned. The reason is that they are majority carrier devices with very fast reverse recovery.

At high voltages you can consider wide bandgap devices like SiC

Regards

Colin

Hi Colin, thanks for helping us on this forum for more than once .

By the way , should you have spare time, would you please help take a look at this thread on my confusions about flyback mosfet drive peak current, and maybe share some of your insights? Thanks again.

https://e2e.ti.com/support/power-management/f/196/t/898815