Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

Changing voltage of smps

Status
Not open for further replies.

techie

Advanced Member level 3
Joined
Feb 5, 2002
Messages
839
Helped
58
Reputation
116
Reaction score
9
Trophy points
1,298
Location
Pakistan
Activity points
7,799
I have a lot of switch mode adapters rated 15v-1A. I want to reduce the voltage to 12Vdc with. Is that possible without degrading the performance of the unit. The design uses a TL431, and optocoupler feedback for regulation. It also has some zener diode near the output circiutry.

Any lead would be appreciated
 

Cluricaun

Advanced Member level 2
Joined
Sep 28, 2001
Messages
627
Helped
17
Reputation
34
Reaction score
7
Trophy points
1,298
Activity points
3,315
SMPS

Hi

There are normally a comparator which sets the voltage you want by lowering this level you can change the output. But regarding TIs datasheet the normal (1+R1/R2)Vo is valid in some applications so perhaps this is the easiest way. Check here:

By changing the Vref that the comparator uses to determine the switching instances you can lower the Vo.

123-s.ti.com/sc/ds/tl431.pdf


Good luck
 

techie

Advanced Member level 3
Joined
Feb 5, 2002
Messages
839
Helped
58
Reputation
116
Reaction score
9
Trophy points
1,298
Location
Pakistan
Activity points
7,799
Would doing that (reducing the output voltage from 15 to 12v) degrade the performance of the design. I mean to say that would this change put any extra burden on any semiconductors in the ciruit.
 

S

sick_man

Guest
haha

hum

you cant

add a 12 v regulator is my advice

it isnt so easy
as the switching transformer will still supply the top voltage
even when the stages drive is reduced

so the lt will always remain the same

the regulation stage in one of these switching psu is purly to give the current

the transformer dictates the output the tl chip is purly there to "regulate" these rails {keep them steady} so either add a regulator
or give up
otherwise youll get wierd results .....UNLESS it already has adjustments of each rail and post switching transformer regulation to enable change
 

Sebi

Member level 3
Joined
Jan 2, 2002
Messages
63
Helped
4
Reputation
8
Reaction score
4
Trophy points
1,288
Location
Hungary
Activity points
444
Just search a resistor what coming from 15V out to TL431. Identify the value and decrease it 10...12% with paralel resistor.
 

E-design

Advanced Member level 5
Joined
Jun 1, 2002
Messages
2,014
Helped
506
Reputation
1,014
Reaction score
407
Trophy points
1,363
Activity points
14,195
Hi

I don't think that by fiddling the values in the feedback comparator's circuit will have much of a affect on the performance. 12v is close enough to 15v

All that will happen now is that the switching duty cycle will be slightly reduced (That is how a smps normally regulates). Some smps's do exactly that, they have a pot that you can adjust to set the output voltage.

You can add a 12v regulator but it is easier just to change the resistor value.

If the voltage diff was much more, say 5v from 15v I will remove some of the windings on the XFMR as well as changing the resistor value.

The TL431 is just an adjustable zener and the opto provides isolation between the Prim and Sec side. The extra zener is for crowbar protection should the regulation cct fail and the volts shoot up.

E
 

Status
Not open for further replies.
Toggle Sidebar

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top