I am currently working with few 150 W SMPS designs for industial applications with heavy transient load condition. Current design works well by mean of line/load regulation and device reliability but facing some issues regarding transient loads, say 500 Hz to 1 kHz 50% to 100% load transients. It can't maintain output voltage and faces very large voltage dip.
I had tested some reputed brand SMPS, they have very good response in such load conditions without stability issues. Only change I notice was, they use op-amps in feedback loop, while I use TL431.
My questions are:
Is TL431 have any limitation over op-amps?
If I want to replace TL431 with op-amp, on which characteristics of op-amp should I focus while selecting?
Best Answer
The TL431 is well suited for stabilizing switching converters. It is difficult to beat this 3-leg component in terms of simplicity and costs. However, you have to know some of its limitations and needs to obtain the best of its performance:
Some more recent devices require less bias like the TLV431 (100 µA) but are limited in maximum voltage. The NCP431 might be a good option but watch the pinout in SOT-23. As you can see in the above picture, you can add bias by adding resistance in parallel with the LED which creates a free current source (\$\frac{V_f}{R_{bias}}\$)
As a final note, make sure the optocoupler is well characterized, in particular its low-frequency pole \$C_{opto}\$ which will work against the theoretical phase margin you want. You have all the needed details in the APEC seminar I taught in 2018.