Other Parts Discussed in Thread: UCC28710, UCC28720, UCC28722
Dear All,
I need to design a flyback isolated converter with Vin=150-500Vdc and Vout=48Vdc/100mA.
Since the UCC28911 has a 700V mosfet inside, I thought: why don't use it?
Unfortunately the webench cannot help because my input data are out of its limits, so I started to design following the example in the datasheet: I fixed the pri-sec ratio to 2.5 in order to avoid Vds to exceed 700V and the sec-aux ratio to obtain about 20V.
From my calculations I found Lpri_min=3,5mH, so I made a transformer N87-EFD25 (quite over-dimensioned) with Npri=54 turns (Bmax around 200mT).
I measured Lpri with a LCR meter and I found 6mH (>3,5mH).
I put the transformer in a simple board with UCC28911 configured for a flyback converter but the results was not very encouraging: the behavior is very unstable, it starts only with particular input voltage and output loads and with Vin>180 it stops (but Vcc never reachs 25V!!)
Using a transformer Wurth with Lpri=1mH the behavior is much better, but it starts only with no load (once started I can add load) and the output voltage changes depending the load.
Someone can tell me what's wrong??
Thanks,
Alberto.