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Designing Programmable Power Supplies

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shaljdav

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Hi everyone, witch is the best method of designing Programmable Power Supplies with linearly adjusting output voltage?
 

Thank you for your suggestions. But my goal is to design Programmable Power Supply with the output voltage range from 1V to 130V (with 0.1V step unit). In my circuit I am using LTC7138 Step-Down Regulator (here bellow you can find the picture of it). Firstly I thought to use 10 bit digital rheostat instead of R2, but so far as Vout = 0.8(1+R1/R2), my output voltage cannot be changed linearly. Then I decided to use N-channel mosfet instead of R2 (Ron of transistor is reversely proportional to Vgs voltage, and hence Vout of regulator will be directly proportional to Vgs). But that linear range of mosfet is very poor,and it is not arrange m. So now I do not know how to solve this problem. LTC7138.png
 

Hi,

eventually some specifications.

1..130V.. a wide range.

For output voltage setup: I recommend to use a voltage output DAC - a resistor - to the existing V_FB node.
How to calculate the three resistors has been discussed several times here in the forum.

Linear setup, but inverted: A high DAC value gives a low output voltage.

Klaus
 
Thank you for your suggestions. But my goal is to design Programmable Power Supply with the output voltage range from 1V to 130V (with 0.1V step unit). In my circuit I am using LTC7138 Step-Down Regulator (here bellow you can find the picture of it). Firstly I thought to use 10 bit digital rheostat instead of R2, but so far as Vout = 0.8(1+R1/R2), my output voltage cannot be changed linearly. Then I decided to use N-channel mosfet instead of R2 (Ron of transistor is reversely proportional to Vgs voltage, and hence Vout of regulator will be directly proportional to Vgs). But that linear range of mosfet is very poor,and it is not arrange m. So now I do not know how to solve this problem.View attachment 141330

First, you have never mentioned that your intention is to realize a 1-to-130VDC Linear Adjustable Regulator
Also, Vout=0.8*(1+R1/R2) is a linear equation and if you fix R1 and use a DAC Rheostat instead of R2, the Output Voltage will be quite linear.
Rds(on) of a MOS is-yes..- linear but in a small region.I will post a principal schematic after a while..
 

Hi,

* it´s not a linear regulator, it´s a swtich mode regulator

* V_out is proportional to 1/R2. Vout is not linear/proportional to R2.

(It is proportional to R1, but R1 needs to withstand 130V and therefore a DAC rheostat is not usable)

Klaus
 

hi,
KlausST answered instead of me :D :D
yes typical maximum voltage values of DAC rheostats is about 5V. It seems like the best method here is to use what KlausST has recommended.
 

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