UCC28911: UCC28911 schematics and layout optimisation

Hi Zaid,

I have checked the schematic and it looks good.

Only couple of concerns, but nothing great. At 8W the peak current is close to 0.5A. So required Lm is 2*P/(Ipp^2*Fsw) which gives close to 750uH. Choosing 825uH is fine but might slight reduce Fsw when operating at full load.

The Cout choosen is 1.2mF. I suggest increasing it to 1.6 to 2mF based on transient requirement for a step load of 0.5A. If you need bigget step transients, say from 0 to 1.5A the capacitor will probably have to be 2.5x this value.

Ensure that R6 & R7 comes close to 29.35k, Maybe very slight tuning might be needed depending on performance at the time of testing.

I would suggest adding a resistor in parallel with snubber capacitor, close to 100k as this will help with the leakage reset quickly. Also have provision to connect a resistor and capacitor across the secondary diode as this will help cleaning out the aux winding waveform spikes.

For your question on isolated sensing, you will need isolated amplifier. The following app note will be helpful with the portfolio of isolated amplifiers and their application in isolated sensing.

https://www.ti.com/lit/an/slyt827/slyt827.pdf?ts=1695222572665&ref_url=https%253A%252F%252Fwww.google.com%252F#:~:text=Isolated%20voltage%20measurements%20help%20optimize,data%20across%20an%20isolation%20barrier.

In case you require any help with isolated sensing, please feel free to raise a post on this topic as it will be forwarded to the correct experts.

Please let me know if you have followup questions on the power supply.

Thank you

Regards,

Harish

Hi Ziad,

Thank you for sending the updates to the schematic.

I think all the updates look good. The isolated voltage measurement should work.

I was referring to putting a 100k resistor across C2 if needed.

Please let us know if you have further questions.

Regards,

Harish

Hi Ziad,

Thank you for the reply.

The following are the list of reference designs with UCC28911. You can check the layout to see which might suit you

https://www.ti.com/reference-designs/index.html#search?keyword=UCC28911&tlogy=Flyback,Flyback-%20DCM,Flyback-%20Quasi%20Resonant,Flyback-%20Non%20Sync,Flyback-%20Active%20Clamp,Flyback-%20CCM,Flyback-%20Integrated%20Switch,Flyback-%20Transition%20Mode,Flyback-%20Synchronous,Flyback-%20Multiphase,Flyback-%20PFC

TI might not have an equivalent product but these reference designs should be good place to start optimization.

Regards,

Harish

Hi Ziad,
Thank you for the reply.
The input voltage spec which you shared earlier is 230Vac with 15% tolerance. So assuming minimum close to 200Vac and minimum bulk voltage is close to 175V during operation, we can calculate Nps and Npa as follows:
Tr=2us, assuming Fsw=100khz, Dmax = 0.49 (approx)
Nps = 0.49*175/(0.413*(5+0.5)) = 37
Npa = Nps * (Vocc(min)+Vf)/(Vddoff+Vd_aux) = 37*(2+0.35)/(7+0.5)=12

So might want to double check on these. I see the Wurth transformer is specified for minimum 260Vdc. But bulk voltage could be much lower than that as per your specs. 

Regards,

Harish