TL431 Problem in SMPS circuit - diyAudio
Go Back   Home > Forums > Amplifiers > Power Supplies

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 31st May 2012, 08:31 AM   #1
diyAudio Member
 
Join Date: May 2012
Default TL431 Problem in SMPS circuit

Hello
as you see this image,vout is equal :Click the image to open in full size.


Click the image to open in full size.

but in SMPS circuit i see something interesting.
Click the image to open in full size.

Vout=2.5(1+47.5/2.7)=46.48

another one
Click the image to open in full size.


Vout=2.5(1+12000/100)=302.5

if i change the R1 and R2 in this equation i have these result
Vout=2.5(1+2.7/47.5)=2.64
Vout=2.5(1+100/1200)=2.708

it means if output voltage is fix on the desire volt it has error !!!!

why?
what is my fault?

Please tell me how the Tl431 work in SMPS circuit?

and more question that i asking myself

1-it means we calculate the R1 and R2 with output voltage and find the output voltage of Tl431 with VTl431=2.5(1+R1/R2)?
2-it means if the Pin Reference=2.5 the output of Tl431 is equal 0 according below schematic?
Click the image to open in full size.
or
output Tl431 is equal to VTl431=2.5(1+R1/R2)


-------------------


i create this schematic circuit
Click the image to open in full size.

V Pin 3 to ground=1.91
Vref pin 1 to ground=2.54


what is my fault?is it not the similier circuit to SMPS feedback output?


but it works fine
Click the image to open in full size.
V Pin 3 to ground=5.2
Vref pin 1 to ground=2.54
  Reply With Quote
Old 31st May 2012, 12:29 PM   #2
diyAudio Member
 
Osvaldo de Banfield's Avatar
 
Join Date: Dec 2011
Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina
The wrong in your think is that in a secondary side regulation from a SMPS, the TL 413 is used not as voltage shunt regulator, but as voltage controlled current source for the led in a photoupler (Or photoisolator). You must calculate the current trough the led diode, nor the voltage.
__________________
LW1DSE
  Reply With Quote
Old 31st May 2012, 02:00 PM   #3
diyAudio Member
 
Osvaldo de Banfield's Avatar
 
Join Date: Dec 2011
Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina
See:
http://powerelectronics.com/mag/50107.pdf

http://switchingpowermagazine.com/do...he%20TL431.pdf

Regards.
__________________
LW1DSE
  Reply With Quote
Old 31st May 2012, 05:37 PM   #4
diyAudio Member
 
wrenchone's Avatar
 
Join Date: Feb 2004
Location: Silicon Valley
Your problem is that you are looking at multiple output curcuits using weighted feedback networks that have a dominant (main) output and include a limited amount of feedback from the auxiliary outputs to improve their cross regulation. The output you chose for your calculations was one of the aux outpuits with only a small contribution to the total regulation (read big feedback resistor), so you got a nonsense result. If you had chosen the master output for your calculation, you would have gotten a number much closer to the number for a single output case, but a litte higher. In the case of the first example, that possibly would have been the +5V output.

For a single output situation you can indeed predict the output voltage of the supply from the simple equation. It is true that the TL431 controls the output voltage via the current it pulls through the optocoupler, but since that is inside the loop and the open loop gain of the TL431 is very high, this does not enter into the basic equation for output voltage.

The value of resistor in series with the optocoupler is important in that it determines the mid-frequency gain of the control loop. Too small, and the supply might be difficult to stabilize. Too large, and the power supply may not have enough control range to regulate at light/zero load.
Also remember that a TL431 need 0.5-1 ma bias current to run properly (it's right there in the data sheet). A ~1k resistor should be placed across the optocoupler LED to provide that bias current without turning on the LED, otherwise you might see some funny results for regulation.
  Reply With Quote
Old 1st June 2012, 04:50 PM   #5
diyAudio Member
 
Join Date: May 2012
Hello
Thanks so for your replay
i found my problem and appreciate

(this forum doesn't have thanks button)?
  Reply With Quote
Old 1st June 2012, 04:52 PM   #6
Speakerholic
diyAudio Moderator
 
Cal Weldon's Avatar
 
Join Date: Jan 2004
Location: Near Vancouver
We tried that but it was overused and became a distraction.
__________________
Next stop: Margaritaville
Some of Cal's stuff | Cal Weldon Consulting
  Reply With Quote
Old 1st June 2012, 06:05 PM   #7
diyAudio Member
 
Join Date: May 2012
Dear wrenchone
could you tell and introduce me a good reference about SMPS circuit that i can design and realize how they work?

Thanks
  Reply With Quote
Old 3rd June 2012, 05:06 AM   #8
diyAudio Member
 
wrenchone's Avatar
 
Join Date: Feb 2004
Location: Silicon Valley
Try Pressman's SMPS book (2nd or 3rd ed.). I was nurtured on the 1st ed (I'm an old guy), but later editions have more to say about stuff that actually matters these days. There are other texts, but they may be overly pedantic/too much concerned with math rather than actual circuits - don't get me started on that topic. The Pressman book will be a start, at least.

Last edited by wrenchone; 3rd June 2012 at 05:11 AM.
  Reply With Quote
Old 3rd June 2012, 07:24 AM   #9
diyAudio Member
 
Join Date: May 2012
Thanks Dear friend
  Reply With Quote
Old 4th June 2012, 12:29 PM   #10
diyAudio Member
 
Osvaldo de Banfield's Avatar
 
Join Date: Dec 2011
Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina
I also have the Marty Brown (I donīt remember if the name is well written), and it is also o good Book. In act, I buy this first, and second the Pressmanīs one.
__________________
LW1DSE
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
TL431 and SMPS Question pftrvlr Power Supplies 1 23rd July 2008 09:38 PM
Problem with TL431 regulation, plse help me. commstech Digital Source 27 31st March 2008 12:53 PM
What is the purpose of TL431 in this circuit? Sunsun22 Digital Source 16 30th October 2006 09:36 AM
Problem with TL431 in a SMPS Ultima Thule Power Supplies 32 2nd November 2005 06:36 PM


New To Site? Need Help?

All times are GMT. The time now is 07:26 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright Đ1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2