UCC28881: Isolated flyback design - troubles with output power - following

Hi,

An expert will reply to you.

Hi Zdnek,

Thank you for reaching out again.

Can you please repost the schematic and circuit input / output voltage again as this has been some time?

Regards,

Harish

HI Harish,

Input voltage 88VAC - 300VAC

Output 24VDC / 0.15 A

Oscillograms were made with cca 90VAC at input. Limiting is coming earlier with higher voltage.

Thanks

BR Zdenek

Hi Zdenek,

Thank you for the details.

Hopefully you are better now after the injury.

From your waveforms attached in last can please confirm the following:

CH1: Output voltage (yellow)

CH3: switch node/ drain voltage or aux voltage (red)??

CH4: drain current (green)

It looks like the drain current is not correct. Could you please attach the zoomed waveforms and clarify? Also how is Vdd being supplied here?

Regards,

Harish

Hi Harish,

You are right. Yellow - Vout; Red - Vdrain; Green - Idrain; Blue - Vdd

I can see that Vdd is maybe too much disturbed. Probe had an influence on behavior, when I was measuring it. It also seems that transformer is in saturation when I see Id curves. But there is 3.6mH of Lpri now. Vdd is suplied by HV input and Vdd is supported by 100nF capacitor. I use Vdd also for controlling of FB as I tested it should not be a trouble.

BR

Zdenek

Hi Zdenek,

Thank you for clarifying.

It seems the drain current definitely looks saturated. Also the demag time of the Vds waveform does not look ok.

I would recommend checking the saturation current of transformer and revisit the snubber components as well.

Please refer any flyback reference design using UCC28881 from the link below to start with:

https://www.ti.com/reference-designs/index.html#search?keyword=UCC28881&tlogy=Flyback,Flyback-%20Non%20Sync,Flyback-%20DCM,Flyback-%20Active%20Clamp,Flyback-%20CCM,Flyback-%20Integrated%20Switch,Flyback-%20Transition%20Mode,Flyback-%20Quasi%20Resonant,Flyback-%20Synchronous,Flyback-%20Multiphase,Flyback-%20PFC

Regards,

Harish

Hi Harish,

I'm struggling with insufficient room for a properly working transformer. I know that transformers I'm trying to use are quite unsatisfactory for design. I assume that I'm beyond the limit of transformer parameters at this frequency. I was also experimenting with snubber.. I'm able to get better Vds peaks, It has no big influence on transformer behavior though. I will check the designs you recommends me, but I think I will move to IC with higher switching frequency to get better transformer situation for me.

Thank you for your support and help here.

BR Zdenek