0
\$\begingroup\$

When compensating switch mode power supplies we usually use IC that incorporates error amplifiers, but sometimes there are ICs that come up without internal error amplifiers and so we can either use TL431 or an external OP AMP for compensation.

My question is when TL431 is recommended over an OP AMP ?

As far as I know OP AMP are more flexible than TL431 for designing a compensation loop and this is due to the incorporated pole of TL431, but beside this drawback, TL431 is widely used specially in flyback topology.

\$\endgroup\$
3
\$\begingroup\$

The LM431/TL431 is an adjustable zener diode, with somewhat more stability with temperature (TL431 is better). The suffix identifies the temperature stability and static accuracy. It works very well in SMPS loops because it is often driving an opto-coupler which throttles the PWM to regulate the output voltage, often with better than 1% load regulation. In this case an op-amp would have to emulate what a TL431 already does out-of-the-box. The TL 431 is a combination precision Vref, op-amp and NPN current sink.

An op-amp is best suited for PID loops as integrators, with correction time in microseconds. This can be a bipolar powered circuit for constant RPM motors under CPU control, so a well tuned PID loop is very important. Professional grade power amplifiers often use an op-amp for excellent DC stability. Other than a master Vref, the TL431 cannot do much else in such circuits. However in linear power supplies you will find the TL431 in great use. My Chinese-made triple power supply uses several of these for a master Vref for each independent supply.

\$\endgroup\$
2
\$\begingroup\$

A TL431 acts pretty much like an op-amp plus a reference. You need a reference regardless of whether you use a discrete op-amp or not. The amplifier portion of the TL431 contains compensation much as a discrete op-amp.

The TL431 is handy in a typical galvanically isolated supply because it can directly drive a feedback optoisolator.

Image from the datasheet

enter image description here

\$\endgroup\$
2
\$\begingroup\$

The TL431 is (probably) the lowest cost and best value for money IC in existence.

A TL431 in even 500 quantity costs under $US0.01 in China (and probably not vastly less in large quantities).

For that you get 0.5% accuracy voltage reference, 50 ppm/C variation including all parameters across whole temperature range, an internal comparator, useful current open collector output (100 mA+), and more ... .
All in an eg 3 pin SOT23 package (or smaller).
So PCBA area and mounting costs are minimal.

IF you can meet your design requirements using a TL431, including functionality and all up cost, then any other solution is unlikely to match it. Designers will try quite hard to make the TL431 meet their requirement.

\$\endgroup\$
  • \$\begingroup\$ Good selling points, something I missed. \$\endgroup\$ – Sparky256 Jan 1 at 20:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.