UCC28730: Flyback power supply doesnt start

Hi Ciril,

Thank you for reaching out.

Webench should give a good starting point for the desgin. The IC does not startup as it experiencing a fault condition preventing from starting. The CS pin waveforms are not scaled properly. If you see one pulse at startup most likely it could be due to fault on CS pin like minimum voltage level not reached with 4us and sees it as short. 

Please use the following app note to help you with your debug.

https://www.ti.com/lit/an/slua783/slua783.pdf

Regards,

Harish

Dear mr. Harish

Thank you for reply to my question.

I found error on my test setup, CS resistor was 1k instead 2k8.

Now I have different problems.

First TVS diode in snubber circuit failed due to overload. It is 3W 150V   BZG03C150-M3-08.

Below is voltage on trafo primary winding while it still operates:

Next, gate signal is present in few ms long pulse bursts with gate frequency at cca 240 kHz, which is strange due to the fact that max frequency should be 90 kHz according to datasheet:

Voltage at output is also in form of short pulses.

In between I also try different strategies:

- changing CS resistor

- adding snubber to mosfet

- RCD snubber on primary winding

- additional gate capacitance

- no serial resistor on auxilary voltage supply

- additional cap at VDD

No improvement so far.

Any further help will be much appericated.

Sincerely

Ciril M.

Hi Mr. Ciril,

It's hard to tell what is going on with the waveforms shown.

1. Please probe the transformer aux winding properly with correctly polarity. Most probably it could be shutting down due to OVP.

2. 240khz suggest the system is unstable. This mostly happens because of noise affecting the Vs pin which drives it OVP. You will have to ensure layout recommendations of the datasheet are properly implemented with single point grounding star connection.

3. Also aux winding signal should be clean, following leakage reset time recommendations stated. 

Please share a copy of the calculator and schematic. There are lot of reference designs to check your layout.

https://www.ti.com/reference-designs/index.html#search?keyword=UCC28730

Thank you

Regards,

Harish

Hi

Just FYI, I recently used a Webench design and it turned out to be full of serious errors in all different parts of the design, startup didn't work due to design error, soft start timing was all wrong and would have taken several minutes to get the controller going, slope compensation component values was off by several powers of ten and so was the closed loop control components.  The design "included" basically all the errors that TI themselves have made an article about called "Common mistakes in Flyback power supplies", and more.

Webench caused A LOT of extra work. It would have saved a lot of time not using Webench at all and design everything manually from the start based on the datasheet info, which is what I had to do anyway in the end since not a single component value in the webench design could be trusted.

So in my experience - Be extremely careful with using webench and double check the whole design and all component values against the datasheet design rules.

Hi,

I agree with your comments. Webench is not optimized for all conditions, hence would request to follow the datasheet and calculator.

Regards,

Harish