Hi
Thanks for your help!
I think the reason for my question in the start of this thread was a bit hasty and based on some lack of knowledge
I am working with a flyback controller based on TI's UCC28C54Q1 controller, with a transformer with an auxiliary winding giving 15,5v, no opto-coupler from the secondary side. 230VAC input and 12V 7A DC output.
The controller has a voltage feedback, 2,5V and a current sense input, max 1V.
The controller has overcurrent limiting.
So I would have thought that the voltage feedback from the aux winding would help keep the output voltage at the expected 12V, and the current sense would manage the overcurrent protection part.
For a while there I thought I had it working pretty good, set at 13,5v at no load dropping to about 12v at full load and dropping voltage with 7A current at overload, with the exception of to much powerloss in the snubber circuit. So I swapped the 700v mosfet switch transistor for a 1700 volt SiC Mosfet one and recalculated the snubber circuit. And now the snubber circuit powerloss is not causing any overheating at least. But I seem to have totally lost the voltage regulation as it drops below 10 V even at 30w load, which is less than half of the max output power.
at no load I measure 13,6 volt output, Vaux 16,4, Vcs = 100mV
at 0,8A load I measure 11,65 volt output, Vaux 16,4, Vcs = 300mV
at 2,3a load I measure 9,75 volt output, V aux 16,4, Vcs = 550mV
The original MOSFET was the IPX65R190CFD
The SiC MOSFET is the IMBF170R450M1
I adapted the design so that I am using the sense output from the SiC MOSFET. Sense resistor connected to source and then the MOSFET sense pin connected to the controller sense input via the filter
My design is basically the same as the application example i the UCC28C54Q1 datasheet, but with different component values of course. I am running a switch frequency of 42kHz.
It was originally based on a Webench design that was totally wrong, so I have struggled quite a bit to get where I am right now.
Any ideas on what I should look for to find the reason for the voltage drop so I can find a solution to that?
MOSFET Datasheets
https://www.mouser.se/datasheet/2/196/Infineon_IPX65R190CFD_DS_v02_07_en-1485079.pdfhttps://www.mouser.se/datasheet/2/196/Infineon_IMBF170R450M1_DataSheet_v02_03_EN-3362317.pdfUCC28C54Q1 Datasheet
https://www.ti.com/lit/ds/symlink/ucc28c54-q1.pdf?ts=1712065214192&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FUCC28C54-