Electronics > Beginners
how does blackdog's PSU work?
(1/14) > >>
exe:
Hi!

I'm trying to design my own PSU. I took @blackdog's power supply as a reference (schematic attached). But the more I study it, the less I understand how it works.

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.

2) what is return path for current flowing through P2?
T3sl4co1l:
1. Consider what the op-amp inputs see, as output voltage varies.

2. Through the output node.  Note that all the little bias currents from the reference section act to pull the output down.

The circuit in general:

Oh geez, those aren't fucking JFETs.  Who drew this...

Anyway: this is the only amateur linear power supply design I've seen that is actually reliable, and working, as shown.  (Compare to the 30V 3A mega-thread, or whatever it was, which is a living train wreck...)

It makes some strange decisions, like using an active rectifier (doesn't really matter; easily substituted with a cheaper, traditional, FWB diode), and schottky diodes alongside shitty old 1N4001s, which can be more coherent.

Interestingly, the V/I error amps have low integrator windup.  I can't decide if this was by cosmic coincidence, or full top-down intent.  It's quite possible that the decision to fix the reference above the output was a direct consequence of this step -- a lot of trouble to go to, but as long as you're going to go that extra kilometer, you get something nicely behaved indeed.

I think the only things I really object to are:
- The fuse is silly.  If fault protection is desired, a current limiting stage can be added, at the input or output.
- The "dropout protection" circuit doesn't make sense either; if you're going to specify ~200W worst case dissipation, don't hack it, just put in enough transistors and heatsink to handle it.
- A preload (besides the reference bias) on the output would help by biasing the output stage into conduction, improving transient response (the problem is all the nonlinear junctions -- the output transistors and LEDs, aren't fully conducting at the no-load condition).
- Get rid of the output capacitors, or use minimal amounts.  Also, why bother buying a low-ESR cap, just to put a resistor in series with it?  Is compensation really that critical that it needs a 10% stable ESR?

Overall, small complaints, and simple tweaks.  This is a good design.

Tim
exe:
Thanks for answering. I'm still digesting the circuit. I tried to "think as an opamp", still don't get one thing:

1) If output voltage goes up, the voltage on P2 also goes up?? Why doesn't swing up to maximum or oscillate? (Unless C24 smooths voltage good-enough so the circuit does not oscillate).

2) Can I drive current-setting and voltage-setting resistors with a DAC?

3) Is it really necessary to have TL317 and LM329 in front of LT1021? Looks a bit over-engineered to me. Can I just use a 6.2V zener and 2n3904  emitter follower as a pre-regulator?
blackdog:
Hi,

Que? => Oh geez, those aren't fucking JFETs.  Who drew this...
I have drawn this schematic and extensively tested! :-)

About the active rectifier, which is to limit the power loss, that should be clear...
If you want to throw away a extra 10-Watts, be my guest, no problemo to use diodes.

The pre regulator works, just adjust it for your transformer, and why did i drop this part of the schematic?, because 2 transformers (2x9V) and some relay, works without winding noise of the transformer.

I do not understand why over and over again people whine over the reference section, it is to much, overdesigned etc etc.
The Architect specifications were LOW NOISE! High stability, at full load, and i mean < 5uV rms, and Yes! i mean uV and not mV.
The reference section is not overdesigned, but well designed!
If you want to have an average power supply, build somting else  :-DD

The currentsource make the power supply extreemly well regulated.

Why Low ESR components on the output, because these are almost always, also good quality components.

The fuse is only for calamities, probably never blown up, if you want to kick it out, your problem  :)
 
Get rid of the output capacitors, or use minimal amounts NO, NO, NO!  |O
These capacitors with the series resistors and the loop time constant determine the dynamic behavior of this power supply.

 
Look at this picture...


A 9.5-Ampere Puls generated bij my Dynamic Load, the opamp at that moment was the NE5532A and the output network used is in the picture.
The ringing you see is NOT! the power supply, but my Dynamic Load connected to the powersupply under test via twisted 0.5M cables and a high quality 6.8uF capacitor over the Dummy Load connection.
If i remove the 6.8uF capacitor, then there is no ringing, just a short puls of about 5mV piek for every 5A change and totale gone in about 25uSec.
The residu of the twisted cable induction and the 6.8uF capacitor create's the ringing, this is a difficult test for power supplies and i use it a lot.

Kind regards
Bram
T3sl4co1l:

--- Quote from: blackdog on March 18, 2018, 11:51:04 pm ---I have drawn this schematic and extensively tested! :-)

--- End quote ---
Ok but why JFET symbols?  Those are unambiguously JFET symbols, and the devices are unambiguously not JFETs, but MOSFETs.  Was this drawn in a package that doesn't have a MOSFET symbol and doesn't allow symbol creation?  (If so, please let me know so I can advise everyone to avoid it like the plague...)

(Similarly for the schottky diodes, but that's a lesser sin.)



--- Quote ---About the active rectifier, which is to limit the power loss, that should be clear...
If you want to throw away a extra 10-Watts, be my guest, no problemo to use diodes.
--- End quote ---

Doesn't bother me, on top of a 10W, 100W or more loss in the pass reg.  If you needed efficiency, the whole thing would be a switcher (which, yes, can be as low noise, but -- it also takes more work to get there).

If you had a...


--- Quote ---The pre regulator works, just adjust it for your transformer, and why did i drop this part of the schematic?, because 2 transformers (2x9V) and some relay, works without winding noise of the transformer.
--- End quote ---

Ah, so there was a pre-reg or tap switcher or the like, that would improve efficiency notably, allowing one to save on heatsink area.

The question is then: what thermal protection is there?  Easily enough added (or omitted(!)), of course.



--- Quote ---I do not understand why over and over again people whine over the reference section, it is to much, overdesigned etc etc.
The Architect specifications were LOW NOISE! High stability, at full load, and i mean < 5uV rms, and Yes! i mean uV and not mV.
The reference section is not overdesigned, but well designed!
--- End quote ---

Well, let's see.   LT1021-5 claims about 60 dB PSRR at 15V and 10kHz; at 8V, it's down about 5dB, so let's say 55dB.  That means its supply can be about 35000 nV/rtHz, to only increase the output noise spectrum by 3dB.

That's pretty fuckin' noisy!

On analysis, this would seem to be overkill.

Unless you have measurements showing it both ways (e.g., LM317 alone, with resistor divider; versus with LM329).

I didn't notice this point originally, but it's a fair point, and I see no data or argument defeating it.

The LT1021 doesn't even seem to be very good; an array of TL431s can do better, and might be cheaper.  REF50xx do better, dunno if cheaper.  I don't regularly shop for references.



--- Quote ---The currentsource make the power supply extreemly well regulated.
--- End quote ---

??


--- Quote ---Get rid of the output capacitors, or use minimal amounts NO, NO, NO!  |O
These capacitors with the series resistors and the loop time constant determine the dynamic behavior of this power supply.

 
Look at this picture...


--- End quote ---

I'm looking.  It looks pretty damning...


--- Quote ---A 9.5-Ampere Puls generated bij my Dynamic Load, the opamp at that moment was the NE5532A and the output network used is in the picture.
The ringing you see is NOT! the power supply, but my Dynamic Load connected to the powersupply under test via twisted 0.5M cables and a high quality 6.8uF capacitor over the Dummy Load connection.
If i remove the 6.8uF capacitor, then there is no ringing, just a short puls of about 5mV piek for every 5A change and totale gone in about 25uSec.
The residu of the twisted cable induction and the 6.8uF capacitor create's the ringing, this is a difficult test for power supplies and i use it a lot.

--- End quote ---

...Oh.

So, wait.  You performed a test, on a power supply, that does not measure the power supply's characteristic, but some external network instead?

Can you please explain that rationale to me?  Because that can't possibly be right.

The network should be around 0.3uH + 6.8uF --> 110kHz oscillation.  I don't know why the measured pseudofrequency seems to be ~half that.  It probably has to do with why the damping is modest, too (maybe the load is not simply an on/off switch and resistor, but has 10uF + 0.05 ohm in parallel with it, perhaps).

The voltage ripple should be measured at the power supply output terminals, which is the only possible point the power supply can control, after all (at least without remote sense terminals included).  The load step should be nothing more than a resistor and switch, with minimal damping.  Its rise time will be limited by cable inductance to about 0.6us, which is fine.

...Double-wait.  Why is the test at 9.5A when the supply is rated for 5A!?  Why is it not dropping out during the pulse?

This data is inconsistent and thus wholly unusable.  Who knows what it was measured on!  Are you a liar?

You realize you aren't making much of a case for this, right?

I guess I might as well retract my claim that it's good.  It's still better than others as I said before, but this disregard for data and measurement bothers me much more than the otherwise minor inconsistencies in the design.  >:(

Tim
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod