Linear Voltage Regulator Design

[audioguru2]

Hi Alun,
Now that you have a negative supply, you can use a TL071 or TL081 opamp for U3 because their inputs operate up to the positive supply. Their supply voltage is only 36V max so will probably be exceeded when the load is low.

TI probably has an opamp with inputs that operate at the positive supply and a 44V max supply, but I couldn't find one.

 

[Alun]

audioguru,
You could use the MC34071, the inputs to U3 are held at the output voltage not the supply so as long as the ouput's 2.2V lower it'll be fine.

You could even use the MC34072 (dual) for U1 and U2 to save space but U3 needs to be separate as it needs the extra -3.6V supply.

 

[audioguru2]

Hi Alun,
Good point! ;D
The MC34071 is rated for a 44V supply so the extra input voltage is fine.

 

[audioguru2]

And you also save money if you use a dual MC34072 for U1 and U2. A dual costs just a few cents more than a single opamp. ;D

 

[Alun]

We could get round the negitive supply problem and save an op-amp by putting the current sencing on the negitive rail as long as the output floating doesn't cause a problem. ;D

 

[cyw1984]

Hi nearly old man Staigen,
You are absolutely correct! I was way off with my selection of high-value "turn-off" resistors for the output transistors in my "fixed" circuit. I have now chosen lower value resistors based on what is inside a TIP120 darlington.

I used a 1k resistor in series with the collector of the 2N3906 to limit its collector current and dissipation if the project's output is shorted. It will still provide enough drive to the output transistors to blow a fuse if the output is shorted.

I think the circuit can deliver 30V at 1.5A with a 34V supply to protect the 36V max-rated opamp.

It would be fun to try to stop this extremely high gain circuit from oscillating. ;D


Hi Alun and Cyw,
I don't think a single 2N3055 or power Mosfet can dissipate more than about 60W even on a pretty big heatsink. Therefore the max current is only 2A at low output voltages. That's why I proposed using 2 output transistors in our 0V-30V modified project for 3A output, and using 3 output transistors on our modified for 5A project.

I am sorry..i have try this circuit...but i don't know it 's not work..
the output is not linear and the output is disorder

how to shoot it?

 

[Alun]

I am sorry..i have try this circuit...but i don't know it 's not work..
the output is not linear and the output is disorder

how to shoot it?

Revolver, air pistol, shotgun - take your pick. ;D

I'm joking of course, I know English isn't your first language, I'll try to correct your post for you, I think you must've meant:

I am sorry.. I have tried this circuit... but it's not working, the output isn't linear and is the voltage is wrong.

How can I troubleshoot it?

It might be oscillating, you should attach a 100uf capacitor followed by a 100nf ceramic disc capacitor on the output of the regulator, if this doesn't work then please answer the following questions:

What are you powering this circuit from?
Have you looked at the output on an oscilloscope?
What output voltage are you getting at what input voltage?
Have you checked the output from the DAC?

 

[audioguru2]

You are darn right it's oscillating. It has way too much open-loop gain.
An LM317 voltage regulator IC has a very tight layout and frequency compensation capacitors inside, and nowhere near as much open-loop gain. 

View attachment 37658

 

[cyw1984]

Revolver, air pistol, shotgun - take your pick. ;D

I'm joking of course, I know English isn't your first language, I'll try to correct your post for you, I think you must've meant:



It might be oscillating, you should attach a 100uf capacitor followed by a 100nf ceramic disc capacitor on the output of the regulator, if this doesn't work then please answer the following questions:

What are you powering this circuit from?
Have you looked at the output on an oscilloscope?
What output voltage are you getting at what input voltage?
Have you checked the output from the DAC?

Vin=40V
I used the power supply to represent DAC