Good quality LM723 LAB Power Supply

[blackdog]

Hi, 

I found in my bin with transformers a type that is 20V at 3.5-Ampere.
And I thought this transformer could become the basis of some explanation of what is all important for a good LAB Power Supply.
A lot of information that I find important in Power Supply's is often not found in designs on the Internet or only partially.
And I would like to share this information with you.
There are many similarities with other power supply's, even from a totaly different setup, such as the Harrison and HP schematics that I prefer.
So this schematic / project is mainly meant to explain what is involved in developing a good LAB Power Supply.

If i'am wrong about my explanation or i have to do it better, please tel me!

First I want to say this, this is a project in development, and meanwhile I have done already a lot of measurements on the schematic below.
This Power Supply will not be a "One Size Fits All" Power Supply.
So don't ask me if is is posible to change it to 30V 5-Ampere, or 60V at 2-Ampere... you are on your own then. 
Processor controlled, please don't ask, just two 10 turn Model 534 potmeters for setting U and I. and a switch for enable.

The basis of this Power Supply is a standard LM723 circuit with parts around it, that make's it a good Power Supply. (my opinion)

Preliminary Specifications
The Output current wil be 2-Ampere Max.
Max output voltage 20V.
Current control from say 50mA to peak 2.2-Ampere, no sharp current limiting knee! this is not posible with one base emitor voltage.
DC Ri < 0.002 Ohm, at the moment around 0.001 Ohm.
Small output capacitor 100uF preferred or 150uF, will depend on the final measurement and the capacitor quality.
Dynamic properties, at the moment between 55 and 65 phase margin, have tot do more testing on that later.
Picture of the dyamic behaviour will come later.

KIND POWER SUPPLY!
What this means? there are so many professional Power Supply's that have Power On and Power Off abbarations and i downt like that!

I have done my best so that the output of this Power Supply never gives more than 0.2V abberation at power on/off or the enable switch.
This only happens just before the power supply is coming on and when the Power Supply is already off for about five seconds.
I will do my best to make these abberations even smaller, that's where most of the work is done.
It is a combination of the lowest possible noise behaviour on the output and the timing to keep the abberations as clean as possible.
At the moment it is almost perfect, and i mean enable/off is perfect for small and large output currents.
Switching to power supply off with al kind of loads, is perfect.
There is a very small abberation when powering on, about 0.25V, that is no problem, but i will do work some more on that part.

The explanation of the schematic below
Keep in mind that the schematic is not yet finished and will certainly change.



Let's start with the transformer, so this is sucandair 20V at 3.5-Ampere.
This gives at double-phase rectification a little more than 2-Ampere to output current without overloading the transformer.
Usually you take about 60 to 65% of the current that the transformer can supply to a resistor or lamp.
By rectifying, together with the buffer capacitor, the maximum current without overloading the transformer in continuous operation is about 60 to 65%.
This is because the transformer has to supply large peak currents to charge the capacitor as well as the current that draws the load.
These peak currents make the i2r losses in the transformer higher than with a load such as a lamp or an ordinary resistor without bridge rectifier and capacitor.

R1 and C2 is a Snubber circuit, these components remove the ringing of the transformer when the diodes come out of conductance below say 0.5V.
The transformer sees suddenly no load anymore and will ring, (The induction and the winding capacity makes a small resonator circuit)
When you wil pull the power plug or switch the power supply off, the transformer will also "ring" R1 and C2 wik kill this.
I used a standard component of the brand WIMA.

The transformer also is used to make aditional voltages.
D4 and C7 makes a "clean" Power supply for the LM723 and the current source around Q4 that is used for the current limiting.
The ripple on C7 is far smaller than the ripple on C4 and also the DC level is higer as say 2-Ampere load,
so if de mains voltage is not to low, i can still have a clean 20V output at 2-Ampere current.
R5 help's with keeping fast pulses from the mains out of this power rail, Ri of the tranformer + R5 and C7 makes a 30Hz Low Pass filter. 

The bridgerectifier is a 15Amp Low VF Glass Passivated type.
At 7,5-Ampere peak current the Vf is 0.9V, and this is low, it kan even be better with Schottky types, but for me this is good eneugh.

C1, C3, D1 and D2 are making a negative supply rail, also R2 is here used for some filtering, the real value of it comes later, for now it is 1 Ohm.

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End 6-1-2019
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The time is up for today, tomorrow I will make some mistakes in the text, English is not my mother tongue.
I will keep updating this first post as I continue to explain this schematic.

I would like to hear your comments.

Kind regards,
Bram

[BravoV]

... English is not my mother tongue.

Bram, as non native English speaker, don't worry, your's is just fine and fully understandable  , I've seen much-much worst from others.

Subscribed, as this thread reminds me of a similar sized step-down transformer that is gathering dust, really tempting, I might join this project.

Thanks for sharing.


Edit : What does LED1 do ? As constant current mode indicator ?

[iMo]

I like 723 PSUs What needs some elaboration is the more precise current protection setting, afaik.

[blackdog]

Hi,

BravoV
Later i will explane the LED in the current source, but yes you can use it as a Power.

imo
I know the "problem" with current limiting, but most of the time you don't need precision curent limiting. (sharp knee)
What I mentioned before, I find an impeccable behavior when switching on and off, fast current limitation and relatively small output capacitor more important.

It is the same with Power Supply's as with DMM's, often you need three to four with different specifications.
This Power Supply has one more good feature and that is that the hum and noise is less than 50uV at full load.
This of course depends on good wiring technique and the placement of the transformer.
So this is not the best Power Supply I can make, but made with readily available parts and kind for its load.
And of course not to forget, a single secundary transformer winding, everybody is loving this! 

And I like to hear from readers, why in the setup of this Power Supply that I have chosen, it is not possible to upgrade it to say 30V output voltage.

If my headache has dropped a bit today, I will post a modified schematic with some measurement data.

Kind regards,
Bram

[glarsson]

The comment on R19 "Direct on Base Q4" should probably refer to Q5.

[blackdog]

Hi glarsson,

Good find!
Thanks.

Bram

[blackdog]

Hi,

It's time for eye candy!

These are the voltage levels on the important power supply lines.
Yellow is the output of the power supply.
Pink is the voltage on the buffer capacitor C4.
Blue is the voltage on the buffer capacitor C3 and it has a negative value.
This value is measured at a value of C4 of 5200uF and a low mains voltage of 225V, which should normally be around 230V.

At the very left side you can see where the "0" level of the trace is located.
In the blue trace this is 1 division from above.
On the yellow and pink trace this is 1 division from the bottom.

These are the levels at 3.3V output voltage and a light load of around 15mA.



These are the levels at 3.3V output voltage and a load of 1.9-Ampere.



And for this post the last picture.
These are the levels at 20V output voltage and a load of 1.9-Ampere.


Kind regards,
Bram

[Kleinstein]

There are few point's I don't like very much about the circuit:

The obvious point is the constant current sink with an LM317 at the output - this is somewhat mixed up, but easy to fix. It may also reach the voltage limit of the 317.

The 723 gets quite a high supply. The good point at this is the extra filtering to avoid the ripple drop. 
So the circuit is limited to a relatively low maximum output voltage. I don't see a reason to give it so much (-5 V) negative voltage. 2.5 V should be plenty, as the inputs on the 723 can work down to about 1-1.5 V. I would guess one could reduce C1 quite a bit in size and this way reduce the power loss at the negative voltage regulation.  It's better to drop the voltage over the capacitor than resistors.

The reference use is a combination of the 723 internal and the TL431.

The power off protection looks a little odd, especially if one tries to charge a battery.

I know it is relatively common, especially in older power supply circuits, but I don't like the voltage adjustment with the feedback divider: this changes the loop gain with the set voltage and thus the compensation has to be a compromise between too much ringing at low voltage and sluggish regulation at high voltages.

The current source around Q4 is relatively temperature stable, but the voltage to compare too depends on the chip temperature. This makes the current setting at low currents rather temperature dependent. It would be slightly better with a PAT current source. Anyway the current limit is not really stable, as it's relative to the VBE voltage.

The 723 is a good voltage regulator, but like most other voltage regulators a poor choice for building a good lab supply. Voltage regulators are different from in lab supply in that they only need crude current limit and the compensation is made for a reasonably well defined load, while a lab supply should be stable with any reasonable passive (could be realized with an RLC combination) load.

[blackdog]

Hi,

Some more pictures.

These pictures are about the abberations of the output voltage of this power supply.
The Enable switch is at the moment fine.
But the power switch or pulling the plug, need some extra work, I already have some ideas to change this.

The schematic is changed, it has now version number 0.2

OK lets look at the abberations or the absence thereof.
I start wih the Enable switch, I put the scope on single shot and move the tumbler switch quickly back and forth to be able to display a few periods.

3.3V 15ma Enable switch used.



3.3V 15ma Enable switch used.



20V-15mA-Enable-On-Off.



20V-15mA-Enable-On-Off



20V-1.9A-Power-On.



3.3V-1.9A-Power On-a.



3.3V-1.9A-Power On-b.



3.3V-15mA-Power On-a.



3.3V-15mA-Power On-b.


So i have stil to do some work on these functions, but first build a new circuitboard, because the one i have now is not so nice anymore 

Kleinstein
Thank for your remarks!

I wil do some testing with the TL431 at 2.495V setting to see whether this voltage is enough for the LM723 to function properly.
This save's a capacitor and two resistors and less noise that I have to filter out...

With my transformer I don't get above 36V as total power supply for the LM723 at about 235V mains voltage.

I already indicated in the beginning that this is a power supply project for me and I hope others can learn something from it if they want to.
I show here how I approach the parts of the power supply and what all influence each other.
This is a project to use the parts I had for a "nice" LAB power supply.
And the second reason is to use this project to explain what's involved in designing a power supply.
For the beginners: A Lab power supply is not simple to design!

Battery charging
The battery charging must be done via a diode, just like testing an LED ALWAYS via a series resistor!

I think everyone should know how their measuring instruments work, so what is and isn't possible.
But I know that a lot of people who work with electronics have no idea what the specifications of their measuring equipment is.
I have more than 50 years of experience in electronics, and have seen several colleagues do stupid things, and occasionally I also contribute to that.
It becomes quite complex to design a power supply(and other instruments) that can deal with all the laziness and abberations of humanity. 

Setting the output voltage.
You are right, the stability/phase margin is easier to set if the gain of the power supply is at a fixed value.
I will  test a TL431 and an LM723 on a breadboard to see if I can adjust the schematic so that I can vary the reference voltage on the +input of the LM723.
And a fixed divider for the -input.

Current limiting
I am myself aware of the temperature drift of the current limitation.
This too I have indicated several times that this is not optimal.
I do not see it as a big problem, this is not a "precision" current controlled power supply.
If I want something like that, I build one according to the Harrison concept.

Let's look at it from the other side, e.g. the Rigol DP832 Power Supply.
This power supply has 470uF over its outputs.
You can set the current to 5mA, but if something goes wrong, the energy from the 470uF capacitor will be dumped into the D.U.T..
It is not much use then to set the current limit to 5mA.
The DP823 set to 12V and say 5mA will always break or damage your LED if no series resistor is applied.
It can help by using the enable button on this instrument, but most people are just lazy. 

Kind regard,
Bram

[schmitt trigger]

Bram;
I have followed many of your posts in this forum. You take the time to properly document and explain your imaginative designs.
And you don't become upset when people asks you questions.

So...I am going to ask mine: I see what you are attempting to do with C1, a capacitive divider.

But would C1 actually see the proper polarity voltage? I know there ill be significant AC ripple. But my question is whether the voltage across C1 *could* become negative

[Kleinstein]

The voltage at C1 is Ok. Under normal operation there is always a significant DC bias. Only during turn on there might be a slight chance to get a small (e.g. 0.5 V) negative voltage. C1 is more like part of a capacitor supply or charge pump circuit.


For the tests it might be interesting to note that the critical case for voltage regulation is with a relatively large low ESR capacitor at the output.
As this makes fast regulation difficult, one does not need to get fast response and low overshoots in this case - but it should be stable.

For the regulator circuit there are mainly two types of circuit, with different properties:
1) The regulators similar to the one shown here, with a low output impedance output stage (e.g. darlington as emitter-follower). These usually give easy voltage regulation and can get away with a small capacitor at the output. However it gets tricky with a voltage higher than some 20-30 V and the current regulation is more tricky and sometimes rather slow. If one does not need very fast regulation the voltage loop can be rather simple and slow, as the output stage already gives a kind of baseline performance also for the fast part.

2) A regulator circuit with an output stage that primarily sets the current (e.g. a transistor in Emitter circuit) and thus high output impedance. A common implementation is the classical circuit with a floating supply for the regulator. These regulators need an capacitor at the output and in more slow implementations it may need to be quite large. The advantage, especially of the floating regulator circuit is that it's very flexible to work to higher voltage if needed and the current regulation it easier and thus often better (except for the output capacitor). However the voltage regulation is more tricky and needs a more involved compensation to be fast and thus alow a relatively small output capacitor. This type of circuit is usually found in lab supplies.

[IconicPCB]

As drawn, LM317 and associated 47ohm resistor will NOT act as a constant current sin/source. The resistor must be inline with the current flow.

[iMo]

LM317 and 337 always keep 1.25V between Out and ADJ. So for const current source you feed current into INP and the output is the ADJ, the const current is 1.25/R + few uAmps of ADJ.
Mind the 317 and 337 need a MIN input voltage to work properly.
PS: people usually filter the 723's Vref with 10-100uF (from mid of the divider to gnd).

[IconicPCB]

Please re asses Your understanding of LM317 operation.

The way the schematic is drawn the voltage difference of 1.25 volts does not come into the reckoning.

Edit:  the resistor should be connected between the OUT and ADJUST lines and the current should be taken through the resistor at ADJUST pin anyways LM317 is arse about with respect to circuit polarities.

[blackdog]

Hi,

Schematic version
The schematic has been given a new version number and I'm going to try today if controlling at the positive input of the LM723 goes as I want.

Current Source.
Concerning the LM317 current source, don't bite my head off, it's just a schematic error, it is working fine in my test setup. 

Reference voltage.
I tested yesterday with an LM336-2.5V reference, but when testing the DC Ri it turned out to be worse.
Maybe the open loop gain will be lower as the inputs of the LM723 go to ground level.
Maybe because the current source in the emitters of the difference amplifier in the LM723 gets more difficult, because of the low voltage it has to work on.

To find it out exactly that, I should do more measurements on this.
But for now I will leave the reference voltage at 4.95V with the TL431b.
There is a good chance that R3, R4 and D3 will be replaced by a LM317 current source, and I will try to do it right the first time. *grin*

Later today some more information.

Kind regards,
Bram

[not1xor1]

Bram;
I have followed many of your posts in this forum. You take the time to properly document and explain your imaginative designs.
And you don't become upset when people asks you questions.

So...I am going to ask mine: I see what you are attempting to do with C1, a capacitive divider.

But would C1 actually see the proper polarity voltage? I know there ill be significant AC ripple. But my question is whether the voltage across C1 *could* become negative

I doubt it would be correct to call it a capacitive divider. It is just a classical charge pump.
When the transformer output connected to C1 is positive, C1 is charged via D1. When the transformer output gets negative C1 charge is shared with C3 via D2.
After a few cycles both capacitors share approximately the same voltage, i.e. about the same of the rectified positive voltage.
Of course in this case the frequency is just 50Hz not 100.

[blackdog]

Hi,

Here is an extra picture related to the uncertainty regarding the negative power line and the ripple frequencies.
The frequency counter in the lower left corner of the image is linked to C4 (channel-3) and indicates 100Hz as I am in Amsterdam, the Netherlands.
A large part of Europe has 230V and 50Hz as mains frequency properties.
The C3 ripple is built on from half periods, and that is also clearly visible here.




There's a chance I'll make this rectifier as simple as the extra rectifier for the + power supply of the LM723.
I do this together with a power source for the TL431b, so I can be sure that the 4.95V is clean and does not contain any power frequency components.
The high impedance of the LM317 current source together with the low Ri of the TL431b ensure a very high suppression of these signals.

We will see what my tests show, there are so many options to choose from...   

Kind regards,
Bram

[IconicPCB]

Hey Blackdog,

I qualified my comment with an "as drawn " statement.

My second response was in response to the counter claim,

There was no intent of malice in either of my posts

[blackdog]

Hi IconicPCB,

No problemo, i ask for remarks about the schematic or design. 

Sometimes an error can be in a schematic for days and the test circuit works just fine, this because I only drew it wrong.
I regularly ask on forums to look at my schematic for errors, I am not mister knows it all.

I also like humour, it is only quite difficult to show this on forums.
Furthermore, there are also quite large cultural differences between the different countries, so it can be tricky.  :

At the moment, my business administration takes up a lot of time, I don't know if it's possible to do another post here today.

But I am open for suggestions.

Kind regards
Bram

[IconicPCB]

No worries, i think all is good.

[blackdog]

Hi,

I am still very busy with work, so not much time to post here.

Negative power rail
I have already done several measurements on other parts of the design and as a result of this I changed the reference section.
This one now consists of a LM337 that is noise reduced and the accompanying diodes for protection are also added.

One of the reasons the TL431b has been replaced is that it was difficult to find a good setpoint that works sufficiently well with varying mains voltage.
After some measurements, an LM337 (preferably one of the ST brand) appeared to have a good noise behaviour which in the setup as shown here in the schematic has a lower noise than the TL431b.
The DC stability of the ST LM337 that I am testing now, can be called good after more than 12 hours.

Power section
I have selected two transistors from my stock, and I reached an Hfe of around 32000.
Because of this very high Hfe I'm going to try if the DC stability remains sufficient if I don't use the normal output(10) but the compensation output(13).
The main reason is to see if the phase margin gets better and the compensation capacitor can get smaller.

The second reason is that I have a BE level less and the dropout voltage will be lower.
I will have to test the above assumptions to see if they work out positively in practice.

Schematic version
Keep in mind that also this schematic is not yet finished and will certainly change at a later moment.
This topic is meant to describe the design process of a LM723 power supply and not a complete power supply kit.


The time is now up...

Please shoot at it 

Kind regards,
Bram

[Cliff Matthews]

Subscribed    Black Dog clean specs.. 20v @ 2 ish amps.. I'm in!

I have 6 candidates showing gains from 78 to 97 to sacrifice.. can these 2SA1695's join the fun?

[Kleinstein]

For the loop stability the Hfe of the output stage should not be that important. This would be more about voltage gain, which is close to 1 and the stability of the inner loop formed by Q4 and Q6. The inner loop stability could be adjusted with R25.  I don't thing it is a good idea to drive the output from pin 13 of the 723: usually a Sizlaki output stage wants to be drive from a low impedance source. So already R22 might not really help. A base to emitter resistor at Q4 could help to give some miniumum load to the 723.

It looks like there is something wrong with the negative supply generation: somehow C1  and D1 from the old circuit got lost, but they are needed, though C1 could likely be much smaller.  With the new circuit It looks even worse to have a high negative reference, as the effective reference now is the difference from the 7 V inside the 723 and the external 4.9 V reference. External 1.5 V should be better.

My preferred way would be not not use a negative auxiliary supply for the voltage regulation and instead add a fraction of the 7 V reference to the -input of the 723. So an extra resistor between pins 6 and 4 of the 723. The negative supply would still be needed for the constant current minimum load. This does not need to be that stable, so a simple 2 transistor or similar circuit should be sufficient.

For the current setting, I think the simple 2 transistor current source with it's TC could be better than the more stable LED + transistor version.

[Cliff Matthews]

25ma current source: Any idea why the 317HV was specified?
Could a a jelly-bean LM317 in series with a 6.2v zener do the same job?

[SK_Caterpilar_SK]

I have different schematic. Its a old lab powersupply for school purposes so basically idiot proof. No voltage spikes on startups no anything unwanted. Tested because I have the original unit which is performing great. The whole thing is nearly flawless. In its original config it uses the metal can variant of the 723 but I rewrote the shematic so that the pinout is compatible with the DIP version. Also uses some obsoltete transistors and zenners and diodes. I made a PCB layout for it so I can have another one to my desk. The original goes from true 0 to 20V. The only issue with it so far is that it requires a dual winding tranformer. I got the voltages and components so I can give you a list if youre interested to test it at all. It does 0 to 1A but I modified mine so it can do 30V max and 3 amps..

Soo if  youre just interested at all in testing it. I will get you the infos so you can do science on it if you want. I will send you the full image if you want because the max filezise is tiny for this to work properly.