UCC25600EVM-How to calculate values of resistor and capacitor in the feedback section through optocoupler

Shashank Bheemanna Shetty

Other Parts Discussed in Thread: UCC25600EVM, UCC25600, TL431

Hi everyone,

I would like to use UCC25600 and referring to UCC25600EVM for my low power LLC design with Vin=12V, Vout=5V, Iout=4A, fr=100kHz. I have completed and verified by simulating my resonant component values. Now, I need to design the feedback through optocoupler. I'm not able to find suitable references for the same. I would appreciate any input for finding the solution.

The section which I am referring to is highlighted in the attached file.

Thanks and regards,

Shashank

over 7 years ago

0 Colin Gillmor over 7 years ago

TI__Mastermind 25570 points

Hello Shashank

A good place to start is the article by Ridley at http://switchingpowermagazine.com/downloads/15%20Designing%20with%20the%20TL431.pdf

The article by Basso at http://cbasso.pagesperso-orange.fr/Downloads/Papers/The%20TL431%20in%20loop%20control.pdf is also interesting.

Note, that with a 5V output you probably don't have enough headroom to connect the optocoupler LED to the Vout.

You will need at least 2.5V for the TL431 cathode and 1.5V across the optocoupler LED leaving only 1V linear operating range for the TL431 cathode.

If you use a 1.25V TL431, you still have only 2.25V linear operating range although with very careful design this might be ok. An easier solution is to

generate a separate VCC rail for the optocoupler -

The control loop cookbook by Dixon at https://www.ti.com/seclit/ml/slup113a/slup113a.pdf is a good starting point for general control loop design.

Hong's article at http://www.ti.com/lit/an/slua582a/slua582a.pdf deals specifically with control of the LLC converter.

Regards
Colin

Power management

Power management forum

UCC25600EVM-How to calculate values of resistor and capacitor in the feedback section through optocoupler