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Variable Output voltage by changing feedback resistor for TOP271

Posted by: PST on Sat, 07/03/2021

1. We need to design a 65W laptop adapter with different output depending on feedback resistor by keeping rest of circuit same. Output voltage requried are: 15.27V , 19 , 19.8V, 20V or 20.67V .

2. We selected this IC: https://www.mouser.com/datasheet/2/328/topjx_family_datasheet-11851.pdf

3. On its page 15, a reference design for 19V/65W is given.

4. What we are planning is to keep R18 as 13K fixed & connect a parallel resistor across(call it Rp) it to change the output voltage & keep rest of circuit as it is.. There are few cases for it:

a) R18 = 13K & Rp=open, then output voltage will be 15.38V
b) R18 = 13K & Rp=51.1K, then output voltage will be 18.98V
c) R18 = 13K & Rp=41.2K, then output voltage will be 19.84V
d) R18 = 13K & Rp=40.2K, then output voltage will be 19.95V
e) R18 = 13K & Rp=34.8K, then output voltage will be 20.66V

5. Will this work? We want to keep circuit common and change output voltage as required?

6. If this dont work, then can you please suggest correct method to do it.

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Submitted by PI-Wrench on Tue, 07/06/2021

This should work, but keep in mind that when you alter the resistor values in the voltage divider chain, you also change the loop characteristics of the circuit, because the altered resistance value interacts with the compensation capacitor on the TL431. You will need to verify loop stability at the operating extremes (lowest and highest voltage).

Hi @PI-Wrench,

Thanks for reply.

1. Now we are using design DER243 as attached. On its page7 it has very much similiar circuit.

2. Hope same logic works in this also, can you please confirm in this DER243 also?

What we are planning is to keep R18 as 13K fixed & connect a parallel resistor across(call it Rp) it to change the output voltage & keep rest of circuit as it is.. There are few cases for it:

a) R18 = 13K & Rp=open, then output voltage will be 15.38V
b) R18 = 13K & Rp=51.1K, then output voltage will be 18.98V
c) R18 = 13K & Rp=41.2K, then output voltage will be 19.84V
d) R18 = 13K & Rp=40.2K, then output voltage will be 19.95V
e) R18 = 13K & Rp=34.8K, then output voltage will be 20.66V

 

3. Can we buy this DER243 board from somewhere? We will buy and test on it directly.?

 

 

첨부 파일 파일 크기
der243_2.pdf 4.63 MB
Submitted by PI-Wrench on Mon, 07/12/2021

The DER-243 is an old design from 2010. It does not appear available from any of our normal distributors.  As a last resort, you may want to try  contacting customer service at PI. Paralleling a resistor with the bottom half of the voltage divider chain for the DER-243 design will vary the output voltage - the same cautions on verifying loop stability would apply in this case as well.

Submitted by PST on Mon, 07/12/2021

Hi @PI-Wrench,

Thanks for replying

Can we change the output voltage on the fly also?
1. We have a arrangement where output feedback parallel resistor on the fly and we want that output change accordingly.
2. Universal AC input(95V-265Vac) will be connected all the time so that when different parallel feedback resistors are connected the output voltage changes accordingly? Can this happen on the fly also?

3. We will verify the output on DSO when changes output on the fly on different load currents. Any other other parameter you suggest to verify for loop stability?

 

Submitted by PI-Wrench on Tue, 07/13/2021

When you change the voltage on the fly, expect some overshoot. If there is also ringing along with the overshoot, the loop is marginally stable.An optimal response would be a small amount of critically damped overshoot.

Submitted by PST on Sat, 07/17/2021

1. We are using Page 7 circuit 19V/65W of DER-243, REv1.4 , dated 19Jan2010

2. Any comments/suggestions you suggest to have optimal performance for on the fly voltage changes to have

a) Loop stability

b) less overshoot?

3. Any other comments for better results?

Submitted by PST on Mon, 07/19/2021

Hi Wrench,

We made schemtaic and pcb design on DER243 based. Can you please share your email address or any method by which we can send you for certain queries?

My email id: info@mecdevices.com 

Submitted by PI-Wrench on Tue, 07/20/2021

For performance optimization, I would suggest looking at loop stability for the corner conditions - ie, lowest and highest adjusted voltage. Optimally, you would need a gain phase analyzer for loop analysis, but in practice you can gain a lot of insight into the circuit by examining the behavior for a 25% load step, also be observing the output as you change the output voltage via the programming resistors. Components C16, C19, R19, and R22 are the critical components for loop compensation. Any further queries can be submitted here on the forum.

Submitted by PST on Wed, 07/21/2021

Hi Wrench,

We made the design schematic and pcb based on your inputs. Can we get it reviewed. 

Dont want to post here as it have all the gerber files also, we have certain changes according to our requirement.

Submitted by PI-Wrench on Thu, 07/22/2021

Can you send some images of the schematic and layout? I see no reason to post the Gerber files.