SEPIC converter with coupled inductors

I've built a SEPIC converter and I'm having trouble making it work with coupled inductors. It works well with separate inductors, but with the coupled inductor I get a lot of ringing and the voltage drops and can't go up with a load attached.
Schematic:

sepic1.jpg

Waveform at the inductor

DS1Z_QuickPrint1.jpg

Waveform at the gate of the switch

DS1Z_QuickPrint4.jpg

The converter is built on perfboard so there's some expected ringing. The controller is UC3845 and the switching frequency is 80kHz.



I suspect it might need some kind of snubber at the drain of the switch, but there's the issue of the voltage dropping.
 

Silvio

Member
Hi Bogdan, I seen a lot of coverters like yours and they all have some kind of snubber on the drain. Ringing is inevitable when the fet switches off and this must be suppresed somehow to a safe level otherwise the fet will not last very long due to exceeding the rated voltage of the fet.
Simple is good so simple does not work at tmes.
 
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