# Series pass regulator

1. Jun 20, 2017

### Dextrine

Does anyone know of any good references for a detailed guide designing a series pass regulator along with compensation?

I'm having some trouble getting the plant transfer function and would really benefit from some book or video or application note on this topic.

Even an older thread on this topic would be useful (I've been searching and I will continue to search)

2. Jun 20, 2017

### davenn

can you clarify that please -- just trying to determine you definition
are you talking about a linear voltage regulator ?
a specific IC or a circuit with discrete components ?

3. Jun 20, 2017

### Dextrine

Yes, sorry for being vague. I would like to make a series pass linear regulator using discrete components.

4. Jun 20, 2017

### Staff: Mentor

What reading have you done so far? What regulation specs do you strive to achieve? It's a lot easier to achieve 10% regulation over a small temperature range than 5% regulation over the Industrial temperature range... What are your load characteristics? What is the application?

5. Jun 20, 2017

### Dextrine

Well, I don't have anything particular in mind, which is why I was looking for a book or something generalized. Maybe some sort of technique?

http://www.ti.com/lit/an/snva020b/snva020b.pdf

I completely understand what they're doing up until the point where one would need a full transfer function. They start talking about the frequency response characteristics, which all make sense to me, but the gain, how to calculate the gain, is specifically where I am stuck.

I'm pretty sure there should be a factor of Rload/(Rds+Rload) multiplied in there, but when I draw out my feedback loop, there's no place for it.

6. Jun 21, 2017

### jim hardy

In that paper they use assumed open loop gain of 80db throughout. See page 8, and every Bode plot.
Section 7 is a remarkably clear nuts-and-bolts level explanation of stability .

In a voltage regulator, loop gain directly determines regulation
so
the more gain the better will be its voltage accuracy under changing load . You'd like infinity if you could achieve it.

Do you see that fig 4 has for input to error amp = (k X Vout) - Vref ,
where k = (attenuation of divider R1 & R2) = R2 / (R1 + R2) and is a not very small number typically between 0.1 and 1 ?
So gain of the loop = whatever is ( gain of error amp ) X (gain of the two transistors ) X 1/k ?
They've assumed 80 db which is a good sized number. I get 10,000 what do you get ?

So, when opamps became available the approach became grab all the gain you can muster , tune the loop for stability over frequency range of your amplifier.

For a first design with discrete components , just grab a nice zener diode and tack an emitter follower behind it.
Here's a far simpler "How To " from TI : https://e2e.ti.com/blogs_/b/powerho...ies-linear-regulator-with-discrete-components

i hope above helps.

@Dextrine That TI application note you found is wonderful, i'd not seen it before.

Last edited: Jun 21, 2017
7. Jun 21, 2017

### Dextrine

Thanks for all the input Jim,

Yeah I went through and re-read line by line and when I saw that 80db assumed gain I thought "dang it I should have read closer from the beginning and saved a lot of work"

I think I got it now though, I tried a simulation and everything makes sense, I have a great phase margin, plenty of stability. All in all great success!

8. Jun 21, 2017

### jim hardy

That's what counts.

We love to see "the light click on" .Thanks for updating us !

old jim

9. Jun 25, 2017

### Dextrine

So, coming back to my original question, are there any books specifically for designing linear regulators (including compensation/control)? I keep finding plenty of books entirely on SMPS but none strictly for linear regulators. There are some white papers here and there but not any rigorous textbook. Are they just considered too simple to warrant a book?

10. Jun 25, 2017

### jim hardy

I only remember some paperbacks from 1960's. Linear Power Supply design is a subset of analog transistor circuit design.
This series was "The Bible" back then, should be still around in the used market.

https://www.amazon.com/Transistor-Circuit-Design-Instruments-Incorporated/dp/0070637377

Might be some old timers here who still have a book specific to power supply design. A good one will cover thermal mounting, heatsinks, shielding, ventilation, capacitors, inductors and transformers.

Good Luck. Look for older books from 1940's through 1960's. They tend to be more practical oriented.
You can build a nice reference library that way without spending a fortune.

old jim