Author Topic: why a series pass darlington pair and a error amplifier doesnt work as a PSU?  (Read 600 times)

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Offline Efe_114

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i know they say this is very unstable but i dont see any stability issues when im experimenting. when does these stability problems appear? and why? maybe this is a very simple and boring question but forgive me im a beginner. :-//
EDIT: i meant DONT work in the title sorry for that.
« Last Edit: December 09, 2018, 05:51:14 am by Efe_114 »
Test gear: Aneng8009, 30V 5A chinese PSU( 1.5V peak-peak noise)
 

Offline T3sl4co1l

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You'll have to be more specific.  Do you have an example circuit?

Tim
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Bringing a project to life?  Send me a message!
 
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Online Wolfgang

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Did you forget about frequency compesation ?

Some literature here:

https://electronicprojectsforfun.wordpress.com/power-supplies/

Do you have Horowith & Hill Art of Electronics ? Also worth a look, the Power Supply Chapter is available free on the net.
 
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Offline Efe_114

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https://easyeda.com/editor#id=3a44946aa21b4c47a35e7593c23f8c9c
 here is the schematic i dont know if you can open it or not
edit:the voltage fed into lm358 vcc is vcc not 5 v i forgot to rename it
« Last Edit: December 09, 2018, 06:39:57 am by Efe_114 »
Test gear: Aneng8009, 30V 5A chinese PSU( 1.5V peak-peak noise)
 

Offline Efe_114

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Did you forget about frequency compesation ?

Some literature here:

https://electronicprojectsforfun.wordpress.com/power-supplies/

Do you have Horowith & Hill Art of Electronics ? Also worth a look, the Power Supply Chapter is available free on the net.
i cant find any of the art of electronics books in my country also its forbidden to buy books online customs doesnt allow it
thank you for the link i will check it out
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Online bd139

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Offline Efe_114

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thank you very much  ;D is art of electronics availableas a e-book? i couldnt find.
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Online bd139

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It isn’t officially as far as I know. If you don’t have a local supplier it can be found on library genesis.
 
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Offline Efe_114

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but why everyone told me links and no one just answered my question :(
Test gear: Aneng8009, 30V 5A chinese PSU( 1.5V peak-peak noise)
 

Online Rerouter

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It works if your load is a fixed resistance, the issue comes when you have inductive wires leading from its output, and capacitances in what your powering,

This is what a previous poster hinted at with frequecy compensation, Its not uncommon for larger PSU's to have a small network of passives to make sure "the gain falls below 1 before the phase shift reaches 180 degrees

Essentially your op amp and darlinton have some propegation delays, and response slew rates that mean the device does not react immidiatly to a change in the measured output voltage (e.g. your powering a flashing led, every time it changes state, the amount of current drawn changes, and the control loop needs to react)

At some point a change in the measured input is so fast that by the time the PSU begins reacting, the signal has already started moving in the other direction (e.g. the inductance of the wiring ringing), if the output has any amplification above the frequency of this change, it will build and build until it oscillates,

So this is why you will find filters or similar on the sense point, e.g. a 1nF capacitor, after the divider, this reduces the amplitude of higher fequency signals so that the power supply behaves to ringing or similar, at the trade off of slower responses, (There is a lot of math if you want to fully understand optimising this),

There are other quirks aswell like limiting the op-amp slew rate by fitting a capacitor between the output and the negative input, e.g. you have a big beefy power darlington, Its incapable of reacting as fast as the op amp input, so you might fit that capacitance, to slow how fast the op amp changes its output to be closer to what the transistor is capable of, again to reduce the risk of oscillations, or ringing on step changes.

There are other methods aswell, but those are starting points, confining your device to 1. not try and react faster than your power element can, and 2. behave under "reactive" loads and under any frequency of step response.
 
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Online bd139

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Read the PDF that I posted carefully, at least in the first few sections about evolving a power supply from scratch.

Three hints:

1. Do not rely on simulation here. Real life is likely to be different and far more problematic. Build the circuit and watch it. Even the most perfect simulations and basic test rigs don't always do what is expected. Try sticking a big capacitor on the output, or connect the load on the end of a few meters of wire and see what happens, or switch the load on and off quickly and see what happens.
2. Look at 9.1.1 "adding feedback" section C goes into stability and what the issues are.
3. Work through this on a pencil and paper, keeping it 1:1 with the schematics in Art of Electronics, not EasyEDA. It produces much ugly and too much mapping between brain and theory.
 
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Offline Efe_114

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It works if your load is a fixed resistance, the issue comes when you have inductive wires leading from its output, and capacitances in what your powering,

This is what a previous poster hinted at with frequecy compensation, Its not uncommon for larger PSU's to have a small network of passives to make sure "the gain falls below 1 before the phase shift reaches 180 degrees

Essentially your op amp and darlinton have some propegation delays, and response slew rates that mean the device does not react immidiatly to a change in the measured output voltage (e.g. your powering a flashing led, every time it changes state, the amount of current drawn changes, and the control loop needs to react)

At some point a change in the measured input is so fast that by the time the PSU begins reacting, the signal has already started moving in the other direction (e.g. the inductance of the wiring ringing), if the output has any amplification above the frequency of this change, it will build and build until it oscillates,

So this is why you will find filters or similar on the sense point, e.g. a 1nF capacitor, after the divider, this reduces the amplitude of higher fequency signals so that the power supply behaves to ringing or similar, at the trade off of slower responses, (There is a lot of math if you want to fully understand optimising this),

There are other quirks aswell like limiting the op-amp slew rate by fitting a capacitor between the output and the negative input, e.g. you have a big beefy power darlington, Its incapable of reacting as fast as the op amp input, so you might fit that capacitance, to slow how fast the op amp changes its output to be closer to what the transistor is capable of, again to reduce the risk of oscillations, or ringing on step changes.

There are other methods aswell, but those are starting points, confining your device to 1. not try and react faster than your power element can, and 2. behave under "reactive" loads and under any frequency of step response.
thank you for the explanation
Read the PDF that I posted carefully, at least in the first few sections about evolving a power supply from scratch.

Three hints:

1. Do not rely on simulation here. Real life is likely to be different and far more problematic. Build the circuit and watch it. Even the most perfect simulations and basic test rigs don't always do what is expected. Try sticking a big capacitor on the output, or connect the load on the end of a few meters of wire and see what happens, or switch the load on and off quickly and see what happens.
2. Look at 9.1.1 "adding feedback" section C goes into stability and what the issues are.
3. Work through this on a pencil and paper, keeping it 1:1 with the schematics in Art of Electronics, not EasyEDA. It produces much ugly and too much mapping between brain and theory.
i already built it then i made a schematic with easyeda also i didnt use simulation i only tested with my equipment. Maybe my multimeter is very bad because only gear i have is aneng8009 i tried to use ac true rms function to measure ripple when i connected to a chunky capacitor through a inductor. is there a way of measuring ripple without an oscilloscope(beacuse i dont own one)?
i tested this with 2n2222 as a power transistor so maybe it was fast enough cuz iit isnt a power transistor?
if im am all wrong dont blame me , blame my multimeter.
Test gear: Aneng8009, 30V 5A chinese PSU( 1.5V peak-peak noise)
 

Online bd139

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You'd probably need a scope to analyse that properly. The problem is even if the DMM indicates the correct voltage it has two problems with it:

1. The average DC voltage might be 5V but that doesn't mean the voltage is 5V.
2. The AC frequency might be considerably higher than the AC converter can work out on your DMM.

Your DMM is totally fine for this stuff. A Fluke 87V won't give you any more insight into this than the Aneng as an example.

A good way to watch this with a DMM is to rather weirdly watch the input current. If the input current jumps around something is probably oscillating. Also sometimes using an AM radio next to it works pretty well listening out for harmonics.

2N2222 has a transition frequency of around 250MHz. That shouldn't be a major problem here as it's the pass transistor and not the feedback element. If it is you can slow it down by connecting a ceramic capacitor of around 470pF across base-collector junction. The feedback loop is controlled by the LM358. This has unity gain bandwidth of 1MHz so all sorts of weird can happen there potentially. You really need to go and read about phase margin / stability before approaching this stuff I think then carefully read chapter 9's stability comments. Even the simplest of opamp circuits can be quite poorly behaved without frequency compensation.

Also this whole thing depends on how you assembled it as well. Solderless breadboards are crap for power supplies for example. They have all sorts of little feedback loops in them that don't cause problems until they do!
« Last Edit: December 09, 2018, 11:41:53 pm by bd139 »
 

Offline Efe_114

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i placed a 100nf capacitor between opamp output and negative like its told in art of electronics thats what i have as a capacitor so i just placed what i have in hand and i measured input current and i dont know why but thiis worked suprisingly well ! i dont know how did this work this well so i think im measuring something wrong i dont want to go offtopic but would you reccomend me DSO shell oscilloscope? customs limit is 22 euro here so i cant buy a decent siglent scope
Test gear: Aneng8009, 30V 5A chinese PSU( 1.5V peak-peak noise)
 

Online Rerouter

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What that capacitor does is slow the output of the op amp down, e.g. from 40MHz that a normal op amp can react to, down to say 50KHz
 


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