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Op amp Linear power supply
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Old 1st January 2018, 08:22 AM   #21
Mooly is offline Mooly  United Kingdom
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Op amp Linear power supply
Thanks, yes it is similar although it is not editable directly from the sim. Easy enough to add a new model though if needed.
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Old 1st January 2018, 02:06 PM   #22
MorbidFractal is offline MorbidFractal
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I have said it before and I will say it again...

You really really need to be concerned about loop stability.

I have lost the link but there was some chap on You Tube who spent what must have been years Vlogging about a regulated power supply that never worked because there was no concept about the loop gain/stability of the circuit.

The, lack of, solution was very similar to some of the things that have been posted to this thread and it spent all of its time oscillating.

I do not know anything like it all but I almost know this much. For the moment I'll just leave the schematic, not his.. mine, here and, perhaps, talk about it later.
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Old 1st January 2018, 03:18 PM   #23
MorbidFractal is offline MorbidFractal
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Just before I go off and buy some more Cider... One of the keys to this sort of stuff is that the feedback/stability is dependent on the load. In this case that would be C2 and its associated ESR, R9... and, of course, what you dangle on the end of your supply.

I will be bold and suggest that, unless some idiot comes along and starts slapping extra capacitance on the output coz it reduces something, the load is going to be resistive or a current sink and, as such, if you design your supply to be stable, taking into account its component variances, it will be stable.

All the fans of Three Terminal Voltage regulators will have read the data sheets, cough, and know that strange things happen when you ignore advice about input decoupling and, more importantly, output load capacitance... including ESR.

Anyway... Here is our 220uF capacitor with an ESR of 50mR being driven by an 500mA AC Current Source.

It is a DC Pole, XC = 1/2piFC, up to the ESR zero frequency, Fz = 1/2piRC. Phase transitions from -90 degrees, lag, to 0 degrees.
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Old 1st January 2018, 03:48 PM   #24
MorbidFractal is offline MorbidFractal
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The 500mA current source is from the 'Darlington'. The OP is using a 'Darlington'. Ignoring other stuff I am not confident about supposedly the current gain is Beta1 X Beta 2. Say 250 X 200 or 50,000. Otherwise it varies all over the shop depending on actual current and voltage across the devices plus day of the week.

So we tread on it and put an Emitter resistor in. Small signal 1V on the input base gets converted to 1V across the Emitter resistor and now I get called for my mistake. 500mR gives me 2Amps per Volt driving the output capacitance and its ESR.
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Old 1st January 2018, 09:59 PM   #25
MorbidFractal is offline MorbidFractal
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Now we throw in the rest of the circuit. I've gone and relabelled some things. It is almost as simple as you can get.

Q1 and Q2 form a long tailed pair degenerated by R2 and R3. Kittens. We are throwing away gain but if you are just going to pick a couple of the same transistors out of your parts bucket they are unlikely to be matched in terms of offset.

The 'load' is a current mirror, Q3 and Q4. R4 and R5 perform a similar function in respect of unmatched devices. The output, via R6 and C1, throwing away more gain, drives our Darlington.

The loop is broken by AC voltage source, V1. This is the standard trick used to measure 'loop gain'. You plot V(A)/V(B) assuming one of the nodes is low impedance, A. <- That may be wrong but it is near enough.

Once upon a time I would also randomly plot AC responses for various nodes in a circuit and gain no idea about the 'loop gain' or overall circuit stability. Run the AC analysis and we get the second picture.

Previously we had the CFILT/Pole RESR/Zero and now we have in effect a Single Pole up to the 0dB loop crossover. For frequencies of interest the circuit is always first order, constant -90 degrees phase shift, and we can claim 'unconditional stability'.

That was thanks to R6 and C1 which act to cancel the RESR/Zero. Loop gain crosses over at about 150KHz, 1A load current, and I guess we might go cry about the 'low' bandwidth. It is even more miserable at 1mA load, third picture...

However the 'thing' is the circuit is still 'unconditionally stable', assuming you do not dangle something nasty on it. Things might be 'improved' but it is really just chasing rainbows.
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Old 1st January 2018, 10:39 PM   #26
MorbidFractal is offline MorbidFractal
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I suppose I should mention that R11 is there to balance offsets due to input bias currents. Its, standard, value is chosen to, almost, match the parallel value of R9 and R10.

Previous pictures show that the circuit is, nominally, first order up to crossover and then 'something' goes wrong.

Part of my overall ignorance means that I would find it hard to give a complete analysis of the circuit but take a guess that the Darlington is running out of steam. Meaty stuff usually cops out before the small signal stuff.

Plot VB/Ib, input resistance of the Darlington, and... First picture.

The Darlington starts losing the plot at around the same time as the loop gain deviates from being 1st order.

Chasing a small rainbow we can gain a bit, not much, by connecting the collector of Q5 to ground. Second picture.

It is, possibly/probably, indicative of CB capacitance. In this case, unless you are going to exceed the voltage rating of the device it's for free so why not.
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Last edited by MorbidFractal; 1st January 2018 at 10:46 PM.
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Old 1st January 2018, 11:39 PM   #27
MorbidFractal is offline MorbidFractal
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And... What do you get for your pennies?

Picture 1) 100mA/1000mA Transient response. Output impedance is about 4mR.

That is a limitation of the DC gain.

Picture 2) Same but close up.

The excursion, 45mV, 900mA X 50mR, is entirely due to the ESR of the output capacitor. The nature of the recovery is down to the circuit being, nominally, entirely first order.

Recovery is about 10uS which is indicative of the loop bandwidth, about 100KHz.

Picture 3) 1000mA Load with 500mA 1KHz Excursion.

Output impedance is 5mR but it will tend to 50mR as the frequency goes up.

Picture 4) 1000mA Load with 100Hz 1V ripple on VCC.

Whatever the dB rejection is in respect of 1V vs 400uV.
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File Type: png Screenshot at 2018-01-01 23:24:59.png (17.2 KB, 30 views)
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Old 4th January 2018, 09:57 AM   #28
Jenyok is offline Jenyok  Russian Federation
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Super regulator.
See attached circuit.
.
One more circuit with protect I-load (1.2A - 1.5A protect).
.
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File Type: jpg begemot_power_draft2.jpg (834.7 KB, 122 views)

Last edited by Jenyok; 4th January 2018 at 10:00 AM.
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Old 4th January 2018, 02:21 PM   #29
MorbidFractal is offline MorbidFractal
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Quote:
Originally Posted by Jenyok View Post
Super regulator.
No it is not. It is an Oscillator!

Picture 1) Circuit.
Picture 2) Loop Gain.
Picture 3) Transient performance 100mA-1000mA @ 1KHz
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File Type: png Screenshot at 2018-01-04 14:17:10.png (30.5 KB, 65 views)
File Type: png Screenshot at 2018-01-04 14:18:43.png (32.5 KB, 62 views)
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Old 4th January 2018, 07:46 PM   #30
Jenyok is offline Jenyok  Russian Federation
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This is a test of a circuit.
See attached files.
.
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File Type: png Begemot_AC3.png (153.4 KB, 84 views)

Last edited by Jenyok; 4th January 2018 at 07:50 PM.
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