TL431 Feedback Question

Thread Starter

Galacta

Joined Sep 8, 2020
5
Hello Everyone! I'm very new to the world of power electronics, and have been teaching myself the basics recently as I've been reading through Christophe basso's Spice Simulation book. I'm also new to the site, so I hope this is the proper form for this question. In Basso's book I just got to the section concerning feedback using the TL431. While I understand the derivation for the equations just fine, I've been unable to get the proper response in LTspice for a type 2 compensator. I've tried various different models for the TL431, and for some reason I can't seem to get the response to match the op amp based circuit, or the theoretical TL431 circuit. I've also tried different optocouplers as well. Any ideas as to what is wrong?
 

Attachments

Dodgydave

Joined Jun 22, 2012
9,936
Hi there , welcome to AAC, Don't know about Lt spice, but the Tl431 is set by the two resistors R7, R8, if Vout is 5V then both resistors should be the same value.

There are several members on here who use Ltspice and can give you an answer.

This Zener is used extensively in switchmode PSUs.
 
Last edited:

Papabravo

Joined Feb 24, 2006
15,705
Hello Everyone! I'm very new to the world of power electronics, and have been teaching myself the basics recently as I've been reading through Christophe basso's Spice Simulation book. I'm also new to the site, so I hope this is the proper form for this question. In Basso's book I just got to the section concerning feedback using the TL431. While I understand the derivation for the equations just fine, I've been unable to get the proper response in LTspice for a type 2 compensator. I've tried various different models for the TL431, and for some reason I can't seem to get the response to match the op amp based circuit, or the theoretical TL431 circuit. I've also tried different optocouplers as well. Any ideas as to what is wrong?
Basso has written more than one book. Which one are you talking about? If it is Switch-Mode Power Supplies, is it the 1st edition © 2008, or the 2nd edition © 2013

Here is a working TL431 Design, a negative 12 Volt regulator with a working symbol and sub-circuit file. Enjoy

TL431.png
 

Attachments

Last edited:

Thread Starter

Galacta

Joined Sep 8, 2020
5
Basso has written more than one book. Which one are you talking about? If it is Switch-Mode Power Supplies, is it the 1st edition © 2008, or the 2nd edition © 2013
Switch Mode Power Supplies Spice Simulations and Practical Designs 2nd Edition.
 

Papabravo

Joined Feb 24, 2006
15,705
Switch Mode Power Supplies Spice Simulations and Practical Designs 2nd Edition.
Thanks for clearing that up. That is a pretty tough read for someone new to the world of electronics. The dialects of spice that are used as examples in the book do not always correlate well with LTspice. They rely on the creation of text files as you would do in writing a piece of software and running the simulation in "batch" mode like we used to do on "big iron". LTspice has a graphical user interface. The underlying simulation has similarities, but also differences.
 

Ian0

Joined Aug 7, 2020
1,859
Texas has a very nice application note on the subject. I found it very helpful.
https://www.ti.com/seclit/ml/slup340/slup340.pdf
It's like you have drawn, with a series RC in parallel with C5.

Seems like the previous replies haven't realised that you're doing the feedback through an optoisolator for a switched mode and the whole thing needs frequency compensation to keep it stable.
 

Thread Starter

Galacta

Joined Sep 8, 2020
5
Thanks for clearing that up. That is a pretty tough read for someone new to the world of electronics. The dialects of spice that are used as examples in the book do not always correlate well with LTspice. They rely on the creation of text files as you would do in writing a piece of software and running the simulation in "batch" mode like we used to do on "big iron". LTspice has a graphical user interface. The underlying simulation has similarities, but also differences.
It's been a bit confusing at times, but nothing I haven't been able to understand. The only real problems I've had are this one with the TL431, and getting the averaged PWM switches to work in transient simulations. Considering how fast the cycle by cycle is anyway, that one isn't as much of an issue.
 

Papabravo

Joined Feb 24, 2006
15,705
It's been a bit confusing at times, but nothing I haven't been able to understand. The only real problems I've had are this one with the TL431, and getting the averaged PWM switches to work in transient simulations. Considering how fast the cycle by cycle is anyway, that one isn't as much of an issue.
It is true there is more than one model out there for the TL431. I don't have a 2nd edition so I don't know if there is a model in the book that you can use. Do you have any idea why they are not working as you expect. I would try the models you have in a known working circuit, like the one I gave you to see if you can identify the differences. It is from the 1st edition of Switch-Mode Power Supplies
 

Thread Starter

Galacta

Joined Sep 8, 2020
5

Attachments

Top