Electronics > Beginners
how does blackdog's PSU work?
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Kleinstein:
There is a limited advantage of a super fast regulation for a lab supply: Once you add 1 m of cable the response at the other end of the cable will not be very fast anymore and one would likely see ringing from the cable. So if one has a critical load, there should be local decoupling caps as part of the test circuit. For the lab supply there is limited need to be faster than the cable - it may be even an advantage to show some damping to the cable instead of the virtual short.

This is a little different with the LT3080: this is a voltage regulator that can be placed close to a possibly sensitive load. On the down side the LT3080 will not show such good DC specs - in parts due to not having extra sense lines down to the terminals. This not so good DC performance helps with AC stability and thus allows the fast response. Also the LT3080 is missing the adjustable current limit and the very fast recovery is likely with a small output capacitance, while low drop on fast transients is with more output capacitance.

There are different requirements for a voltage regulator compared to a lab supply:
The voltage regulator needs to get good response with a well behaved load with suitable load capacitance. Performance with a poorly chosen cap can be poor, up to oscillating. Some DC drop is usually acceptable.

A lab supply should react reasonable well with most loads. And should not oscillate with any load (with less than 90 degree phase shift). This often requires the capacitance with ESR at the output that with a well behaved load mainly slows down the response. So optimization is not for the best cast tested with a voltage regulator, but more for the difficult cases. Usually DC load drop should be very low,  which may require some compromises in the medium frequency range.
Cliff Matthews:
I enjoyed reading that  :-+  What's a good view on the necessity of both sinking and sourcing on a bench supply vs lab-grade?
David Hess:

--- Quote from: exe on March 18, 2018, 08:54:29 pm ---1) The output voltage is defined by current flowing through resistor P2  (20k multi-turn pot). But why voltage drop on P2 should be constant? I don't see a current source to ensure this. I only see a voltage source which is +5V above the positive output.
--- End quote ---

The inverting input of operational amplifier IC1a is tied to the output.  The non-inverting input follows the inverting input due to negative feedback.  Usually this is described the other way but the result is the same.  So the top of P2 is equal to the output voltage.  Since the reference produces an output 5 volts higher than the output, R32 which produces the reference current has a constant voltage across it.


--- Quote ---2) what is return path for current flowing through P2?
--- End quote ---

The reference produces a voltage which is 5 volt higher than the output voltage so the return path for current through P2 is the same as the current through the output.

The above is just a repetition of what T3sl4co1l said but may be more understandable if said in a different way.


I like the floating design which allows the use of low voltage operational amplifiers and easy high side current sensing.  If you are going to build a power supply at this power level, then the cost of an extra but small transformer is well worth it.

I like that the output voltage does not rely on modifying the closed loop gain which compromises frequency compensation although not usually to a degree which matters.

The displayed load transient response is poor; power supplies should have a very tame transient response to handle difficult loads.  In production designs, I slightly overcompensate for the *worse* case of component and load variation.

The controlled ESR bulk output capacitance is just a bad idea and indicates a problem with the frequency compensation.  I have seen designs with comparable output voltage and current which were much faster and better behaved that used less than 1 microfarad of output capacitance in series with like 10 ohms although I question whether this level is performance is actually ever needed in such a high power supply unless it is part of a source meter.  Someone else can run the numbers but this looks like dominant pole compensation on the output which largely defeats the purpose of using fast transistors and operational amplifiers.

Why use a Sziklai pair instead of a Darlington pair when not required?  This is especially odd since the output power transistor is a PNP instead of NPN although the difference in price probably does not matter anymore; PNP power transistors used to be much more expensive than NPN power transistors.  I suspect the lack of local feedback around the Sziklai pair is causing problems.  Designs like this using much slower transistors often include local feedback.  Even fast Darlington designs often use local feedback.

I would unload the outputs of the operational amplifiers with emitter followers or even FETs.  Heavy loading compromises precision defeating the purpose of using a good reference and remote sensing.  The ultimate performance of the suggested ADA4077 precision operational amplifier is completely wasted here.

The not shown input clamp diodes on the operational amplifiers may be screwing up the performance during transitions between voltage and current mode.  Also, the ADA4077 datasheet is horrible and I would never recommend or use this part because of it.

I am suspicious that the ADA4077 and NE5532 displayed any difference in AC performance with everything else going on.

I think the output TVS should have been an SCR crowbar circuit.  I would expect it to behave like one once and only once if it was actually needed.  I hope it was not needed for handling overshoot.

The preregulator is clever but I am surprised switching artifacts do not show up in the output.  Old designs used phase controlled SCRs and a big inductor, SCRs and transformer taps, or a buck switching regulator.
blackdog:
Hi David,

Can you explane this?
The displayed load transient response is poor; power supplies should have a very tame transient response to handle difficult loads.  In production designs, I slightly overcompensate for the *worse* case of component and load variation.

David did you read everything?
Wat is bad about this test see the two pictures below, do you find this horrible?

This is the current puls, 0.5 to 2-Ampere.


This is the respons on the power supply connector, about +-3mV abberation, sorry for the noise, the scoop is only 2mV/Div.



Ooo...
Why use a Sziklai pair instead of a Darlington pair when not required?  This is especially odd since the output power transistor is a PNP instead of NPN although the difference in price probably does not matter anymore; PNP power transistors used to be much more expensive than NPN power transistors.  I suspect the lack of local feedback around the Sziklai pair is causing problems.  Designs like this using much slower transistors often include local feedback.  Even fast Darlington designs often use local feedback.

My measurements tell me different.
The rightmost line (F) with the text Double Compound transistor indicates the output impedance of this power supply.
And it is measured with the NE5532A as used opamp, can you point my to a powersupply that has a lower Ri over this frequency range?



Ooo again...
The ultimate performance of the suggested ADA4077 precision operational amplifier is completely wasted here...
Nop, The DC stability is fine with this opamp in use, but the hf performance of the ADA4077 is ofcoure less then with a faster opamp.
Some DC measurements: With the NE5532A at 15V output the DC drift was < 0.2mV, thats less than 0.0015% within a time of 6 hours.

I am suspicious that the ADA4077 and NE5532 displayed any difference in AC performance with everything else going on.
Your susspicion is not correct :-)

Switcher
I had already explained that I do not want a switcher in a low noise power supply.
And that I finally switched to a transformer with tabs and three relays to limit the power loss.

Some attention
The only thing I think about is the opamp of the U control loop, in difficult situations there is quite a lot of energy going to the + input of this opamp.
This can probably be solved with two diodes that are anti-parallel from the + output to the + input of the opamp.

Current limiting
Some pictures about the performance of this power supply and this time the I loop performance.
This is measured over the current sense resistor.
I used one of my dummy loads for a hi current peak.
And this is the result without the transistor for the peak limiting, Within 10usec back to 5-Ampere, fast enough for you?


But I was not satisfied with that yet, so I applied Q6 to further limit the peak current and now its fine by me.


And again, during the development I sometimes test with other conditions than for a 30V and 5-Ampere power supply, which was the starting point.

Some remarks
Let me make it clear again, this is a power supply that has very low noise, good DC stability, and good dynamic performance.
He must meet my requirements, for my purpose.

I do a lot of measurements on sensitive electronics, and sometimes it is necessary that my LAB power supply can deliver more than 2-Ampere with great stability and low noise.
As an example, I develop voltage references that are in small ovens.
And testing these ovens i need a high performance power supply.
I do not want to say that everyone should build this power supply.
I want to show that you can achieve very good specifications with a fairly simple set-up.
If you want, use part of this design for your application.

This design is not meant for production, but only for my LAB.
And if I want to use more expensive parts, I do that because I think this is necessary for my application.

Kind regards,
Bram

blackdog:
Hi,

Many comments are made about the output network of this power supply.
And yes, this is built differently than with most power supply's.
And Why?...
Perhaps it is good that those who make comments about this configuration read the following document.

www.bramcam.nl/NA/NA-01-PSU/Calex-Power_Impedance-Decoupling.pdf

Perhaps this document makes things more clear.

Kind regards,
Bram
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