TL431 as a comparator?

Question

One of the things I've noticed in a few power supplies is the use of a TL431, a voltage reference, as a comparator of sorts.

It seems to be set up in a configuration where when the voltage across it exceeds some point, it conducts, and this turns on an optocoupler. But I thought, almost like a zener, the 431 would only drop a few millivolts across the optocoupler - not enough to turn it on - until it exceeded the set point plus some volts for the optocoupler's LED, but that would be too indeterminate to produce a precise set point.

Below is one example. Page 28 of the datasheet shows exactly what I'm talking about. It's quite useful in this configuration, because it turns out if you put a potentiometer across one of the resistors, configured as a rheostat, you can adjust the feedback and output voltage.

So how can it work or am I missing something?

Answer 1

Note that TL431 have three pins. U1 "measures" the voltage across R7 which is proportional to the output voltage because R6 & R7 is configured as a voltage divider. If the voltage across R7 is above 2.495V (see TL431 datasheet) it will "open" and current will flow trough R4, the opto coupler and U1 to ground. The voltage drop over the opto coupler doesn't matter because unlike a zener diode, U1 have a separate voltage sense pin.

Answer 2

TL431 is a cheap op-amp with integrated reference voltage fed to the non-inverting input. It works as a comparator when in open-loop configuration. But here it is operating closed-loop as the error amplifier. It is the key to the transient response and stability of the entire supply.

The circuit you show is a type-1 compensator controlling the output voltage. The bandwidth is much smaller than the switching frequency, and thus the switching aspect of the power converter is “out of the picture”. You could replace the switcher with a stiff linear pass-element amplifier and it would not change the response of the power supply.

Of course it would be rather undesirable if power supplies had “bang-bang” control with a comparator. Instead, we need a “smooth” control loop. TL431 happens to be a convenient part of it. You could replace it with a discrete op-amp and a discrete Zener reference, and get same performance at a higher price.

The feedback loop response is determined by the feedback around TL431 and by the pole added by the optocoupler. The -3dB bandwidth of such regulators is in the 1-10kHz range. The slow optocoupler plays a big role in limiting the response speed.

For further reference, see e.g. https://www.onsemi.com/pub/Collateral/TND381-D.PDF