TL431: The TL431 in a feedback network

Shalev Ben Harush

Part Number: TL431

Tool/software:

Hey,

I’m currently working on my project as an undergraduate EE student, and would like to ask a question regarding the TL431.

My project is about a FLYBACK SMPS capable of providing a wide output voltage range (5-50V), controlled by a MCU.

For output voltage feedback, I’m using the configuration of the TL431, and an optocoupler, as you can see in the attached image.

My desire in this configuration, is to keep the voltage at the TL431 cathode, stable on 2.5V, therefore, a change in output voltage will translate to more - less current in the optocoupler - which I can sense and compensate in the MCU.

My question is, is the desired TL431 functionality I’ve mentioned possible in this case? I’ve already determined the values of all resistors.

Please advise if there are any more adjustments I should make.

Thanks in advance.

5 months ago

0 Andrew Li 5 months ago

TI__Expert 3970 points

Hi Shalev,

Thanks for your question!

Please see the articles below for some design suggestions for TL431 flyback compensation:

https://www.ti.com/lit/an/slla618/slla618.pdf

https://www.ti.com/lit/ta/ssztcw5/ssztcw5.pdf

I hope this helps!

Best Regards,

Andrew Li

0 Shalev Ben Harush 5 months ago in reply to Andrew Li

Prodigy 10 points

Hi Andrew,

Thanks for the comment.

The first article suggest compensation for a specific output voltage of a flyback, where you set the values for the resistor divider in advance so that in steady state it'll reach the TL431 2.5V internal reference. Like in a typical way with Op-Amps.

Notice im working on something a bit different, my output range is wide and this is not an "off the shelf" flyback.

Is there a way to handle this?

0 Andrew Li 5 months ago in reply to Shalev Ben Harush

TI__Expert 3970 points

Hi Shalev,

R1 and R2 are unnecessary as the behavior you are looking for is a 2.5V Zener diode. The Vref pin will clamp Vka = 2.5V when shorted together.

Make sure to keep Ika within the device limitations (1mA to 100mA) for all output voltage ranges.

Additionally, a compensation network may be required in order to avoid output oscillation.

Best Regards,

Andrew Li

0 Shalev Ben Harush 5 months ago in reply to Andrew Li

Prodigy 10 points

Is the behavior described - a 2.5V zener diode is going to stay the same when a compensation network is connected?

0 Andrew Li 5 months ago in reply to Shalev Ben Harush

TI__Expert 3970 points

Hi Shalev,

Yes, the 2.5V Zener diode behavior will stay the same when a compensation network is connected.

When used as part of a compensation network, the current shunted through this device will change.

Best Regards,

Andrew Li

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TL431: The TL431 in a feedback network