Spec did you ever cast an eye over this circuit again? I'm not in a hurry to get it up and running, it was just an idea. Perhaps it could be used as a linear post regulator along with some of my / our SMPS circuits.
TL431 data sheet:
https://www.ti.com/lit/ds/symlink/tl431.pdf
2N5458 data sheet:
https://www.fairchildsemi.com/datasheets/2N/2N5458.pdf
BD139 data sheet:
https://www.fairchildsemi.com/datasheets/BD/BD135.pdf
Gordon, the backing data for your #1 PSU is given above. I have redrawn your circuit so that it can be more conveniently referred to in this post, but the circuit is exactly as your original schematic, except FET type number corrected.
As I have said before, this is a clever design and I understand the objectives:
(1) Use minimum components
(2) use a JFET as a constant current generator to both isolate any ripple/noise on the input voltage from the voltage control functions and also to provide a high gain and therefore high accuracy for the voltage control feedback loop.
(3) Use a cheap but non-the-less accurate voltage reference and amplifier (TL431)
Unfortunately the circuit has a few practical issues:
(1) There is so much voltage gain that it would be very difficult to stabilise it in the frequency domain. This applies not only to the phase changes in the main amplifying path but also the various signal paths due mainly to parasitic capacitances.
(2) The constant current generated by the JFET will have a very wide range due to the wide Idss (vgs=0) of 2ma to 9ma.
(3) The JFET Id saturation current starts at 2V. At a Vds less than this the JFET acts, not as a constant current generator, but as a resistance.
(4) For some reason, this type of constant current generator is never very happy and is inclined to be temperamental. A far better performing , but less elegant, constant current generator can be made with a couple of bipolar junction transistors.
(5) The TL431 is a shunt regulator so it cannot source current to drive the base of the series pass power bipolar transistor. Thus the current for the base is the constant current passed by the JFET less 1ma required for the TL431 to operate. Depending on the particular JFET used this would give a base current of 1 to 8 mA. The Hfe (current gain) of the BD139 is 25 so the maximum current the BD139 could output would only be 25mA to 200mA. Because of other factors the output current would be less than this.
(6) Even if a resistor replaced the JFET, this is still a very high gain circuit. The TL431 needs a high gain to achieve its accuracy. This means that frequency tailoring will be required to ensure frequency stability. You can see evidence of this in the TL431 data sheet in the applications section which shows a PSU similar to your #1 PSU but without the JFET constant current generator. The capacitor shown shapes the overall frequency response to ensure stability. This is standard practice for high gain feedback amplifiers. Our old friend the uA741 op amp has a massive 30p capacitor fabricated into the circuit for this sole purpose.
(7) Finally a general point. There is no decoupling on PSU#1. Just to give you a clue, if you look at a well-designed piece of analogue commercial equipment you will often find capacitors splattered all over the place. This is especially so with high end audiophile amplifiers. The reason for this is that to the amplifier the supply lines and other critical points must look like OV (ground) to them, especially as the frequency increases. If that is not the case the physical circuit bears no resemblance to schematic that you work to. This is a big and complicated subject but for the average circuit you can get by with a few basic rules. There is a standard saying with analogue designers: ‘hundred nan to deck’. This means decouple all critical areas with a 100nF low loss capacitor (normally ceramic) to ground or OV.
That takes care of the high frequencies in most cases, but you also need to decouple at low frequencies too, normally with a 47uF upwards low loss aluminum electrolytic or tantalum capacitor.
Decoupling is not just a thing done by some pedantic analogue design engineers, it is essential. You are probably thinking that you have seen hundreds of circuits with no decoupling. And so you will. But my experience is that many home constructors come to grief because of frequency instability cause by insufficient loop frequency tailoring, no decoupling and poor layout. Decoupling is not always necessary, but it is always a wise precaution. Take the uA741 for example. It is a very friendly opamp and is slugged down to have a narrow frequency response so will tolerate almost anything, but it will still perform best with a good layout and decoupling.
So in summary Gordon you innovative design has been let down by the shortcomings of practical components- they are just not up to your high expectations. I’m sorry to say this but for the reasons already outlined and other reasons which I haven’t gone into, this circuit would be difficult to tame. But no worries, I will post a circuit that will make up for my negative view and give you something to really get your teeth into.
Hi Spec and Gordon. I've nothing technical to contribute here - but it's great to read this kind of interesting, in-depth, technical and polite conversation. It's general enough and sufficiently well-explained that the posts here will be of benefit to people working on all manner of circuits. In particular, Spec, that's a neat description of the Nyquist stability criterion (a lot of stuff fell into place for me the first time I saw that formally explained!).
Thanks all for spending the time on this.
That said, I do have a technical question, but I'll post that in a bit when I've some more time...
Yes, thanks very much Spec, that was indeed thorough, yet equally succinct and understandable. I expect it will take me a few repeat reads, to get this very clear in my head such that I may be able to put it into practice, which isn't by any means a criticism of Specs fine description, rather of my poor memory! Given that he has the flu, it is very impressive! I look forward to reading some more of Spec's wisdom.
You probably had $10.00 in you hand.
.....and you thought you would give her a good tip for cleaning the bed.Good thing you checked out before her boyfriend came back.
One of my staff was in San Paulo Brazil fixing some clean room HDD servo equipment when he joyfully signaled to the lady (also wearing an operating room mask& suit )that he fixed it perfectly **broken link removed**
She turned red beneath her mask and in Brazil that means he wanted...
to do her.
If Q16, Q19, Q20, Q23 do not have the exact same gate turn on voltage the current will not share equally.
It would help if each had its own source resistor.
The OPA192 error amp does have built in frequency compensation. It starts rolling off at 2Hz on a unit slope and bites the dust at 10Mhz. There is a voltage divider in the loop R18/R19 of 5:1 but that is roughly cancelled by the gain of R23 and R17. The dominant pole will be R17 and the total miller capacitance of the MOSFETS. So it is possible that at the closed loop intersection there may be a 180 deg phase change. To answer your question, I suspect nothing much will stop the PSU from oscillating apart from prayers, a not uncommon approach for many PSUs. I intend that the PSU will be adaptable to suit a wide range of opamps.The error amp has no frequency compensation. What keeps it from oscillating?
Thank you very much Spec, that was more than I was expecting. I will look at it in more depth in a bit.
I wasn't expecting you to turn that little circuit of mine into a bench PSU, but it will be interesting to investigate if not quite on the scale implied by your schematic.
I have started one of these jobs you wish you hadn't. I'd had a box of CMOS logic chips stuffed away in a cupboard as I hadn't been doing anything digital for awhile. I'am sorting through them, categorizing them and putting them in little drawers! I started playing with some of the chips to break the monotony. Need to get it finished today.
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