Linear lab power supply

[xavier60]

Gents-

Following along diligently, but finding it a little difficult to keep up. I've attached an annotated schematic highlighting my main questions, but I'll attempt to summarize below:

• Does PS2 represent a rectified mains transformer input? If so, what is the voltage? I saw a number of transformers discussed earlier on
• Does the uppermost line leading off to the right represent the positive output terminal?
• Does the lowermost line represent the negative output terminal?
• You've discussed using a microcontroller. Do the CV/CC Ref's correspond to control inputs, or are the related to the TL431 reference?
• VCC1 just represents the regulated control voltage created by the TL431 circuit, right? It is NOT an isolated supply?

I've built a few '317 and '338 style power supplies, blew 'em up, and I'm working on finding a suitable topology for my next project. I've been simulating various circuits involving throwing a current control loop around various voltage regulator ICs, but have so far been frustrated trying to compensate the darn things. Anyway, just wanted to say that this is a neat project, and I'm keeping an eye on it. Thanks to everyone who has contributed thus far!

Yes, the schematic follows conventional layout. Unregulated input on the left and regulated output on the right.
To reduce clutter, I have omitted the obviously needed bits, such as the usual transformer, rectifiers and large capacitors, or just about any DC supply such as a SMPS.
 The references can be from a micro-controller  via Digital Pots, DAC or filtered PWM. Or simply from mechanical Pots powered from VCC1 8V.
When Pots are used, make certain that the circuit is fail safe to the possibility of loss of wiper contact which is a real probability with cheap 10 turn Pots. Oiling(PAO) has been fixing them. Read my posts for more details.
 The recommended absolute input voltage is limited  by the transistor spec, 60V.
The real world limits are determined by difficult to measure and calculate interaction of factors such as operating current, power transistor Safe Operating Area rating and the heat sink's thermal resistance to ambient. Transformer tap switching helps a lot.
VCC1 may need to be from an isolated supply if more is to be powered than currently shown.
Q8's dissipation will be rather  high at higher loads because of its large voltage drop. And also, this current will be measured by the shunt resistor.

I have been testing the regulator circuit a lot with good results.
It's yet to be seen if the results can be reproduced.
Transistors can have a large difference in current gain from one device to another of the same part type.
Well designed circuits tolerate this variation.

[xavier60]

I have done just about enough tweaking for now.
I have deleted a transistor from the preload circuit.
I used the method in the linked PDF to do Phase Margin tests.
My measurements indicate a PM of 90° at the 0db crossing of 60Khz.
The regulator remains stable with no output capacitor.
The PM is 90° again at the much higher frequency of 300Khz.

http://www.ti.com/lit/an/snva364a/snva364a.pdf

[Kleinstein]

The phase margin test is good for the given load the output. The case without the capacitor at the output is not really relevant, as the difficult case is normally a load with low ESR capacitance combined with a kind of current sink.  This would be the hard case to judge stability. Under these difficult condition the phase margin will usually go down quite a bit - that is OK unless the phase margin turns negative.
A good phase margin with an easy load not always guaranties stability with a difficult load.

Looking at the circuit, I am not sure the regulator will be stable for all reasonable loads. Something like 100 µF with 10 mOhms ESR and some current sink could cause instability.

The output impedance can help to judge stability at different loads.

[xavier60]

The phase margin test is good for the given load the output. The case without the capacitor at the output is not really relevant, as the difficult case is normally a load with low ESR capacitance combined with a kind of current sink.  This would be the hard case to judge stability. Under these difficult condition the phase margin will usually go down quite a bit - that is OK unless the phase margin turns negative.
A good phase margin with an easy load not always guaranties stability with a difficult load.

Looking at the circuit, I am not sure the regulator will be stable for all reasonable loads. Something like 100 µF with 10 mOhms ESR and some current sink could cause instability.

The output impedance can help to judge stability at different loads.

The capacitor(NCC KME 47-100) on the output that I thought was low ESR, isn't. I measured about 500mΩ ESR.
When I replaced it with a 100µF MLCC, the worst case PM dropped to 15°. It also causes the load transient response to have a 50% overshoot.
I guess this is why I see regulator designs that have multiple series RC's across the output.
Testing with a constant current load didn't seem to cause any problem.

[Kleinstein]

The capacitors at the output have different purpose. A low ESR capacitor at the output can help to buffer fast transients, where the active regulator can no longer react. However it also is a difficult load to the regulator and make regulation more tricky with more overshoot. A capacitor with defined ESR (e.g. in the 1 Ohms range) does not help much to buffer the output and deliver much power. The purpose is mainly to improve stability.  Still it makes regulation at other frequencies slower. So ideally the capacitors should be relatively small, just enough to get good response.

50 % overshoot for a tricky load like the 100 µF capacitor is quite good.  The 100 µF are more like not a good idea to have at the output though. The 100 µF with 0.5 Ohms ESR might be suitable for the output, to provide some damping. Chances are one could use a smaller capacitor.

[xavier60]

I temporarily fitted the 100µF MLCC just for testing the Phase Margin.
I can eliminate the overshoot by peaking Q1, but the regulator becomes unstable with no output capacitor although this does  not  have to be a requirement.
If this poor tolerance to low ESR capacitors is generally expected of regulators, the design that I have posted should not be regarded as a failure.
Because it's quite happy with a standard 47µF electrolytic across the output.
I want this design to be successful in having good performance without being overly complicated.
The transient response is currently 15µs.

[Kleinstein]

Having a low ESR capacitor at the output is a difficult situation for any voltage regulator. So a low phase margin for this case is kind of normal -  not so good designs have a chance to oscillate under such conditions. I would consider only 50 % overshoot as still good for the difficult conditions. It can actually get better with the 47 µF electrolytic in parallel.

For this type of regulator with a relative high impedance output stage it is normal to require some kind of output capacitor.

A 47 µF electrolytic cap with some ESR sounds like a good solution.  It is still relatively small to what other lab supplies have.

[JuanGg]

I am getting a bit lost with all this capacitors and phase margins. I'll have a careful read.
Anyway, i have done some work on the case, mainly on the front panel, see photos and schematic below. I think it's rather neat as it allows the arduino to drive the indicator leds and read the pushbuttons from the same pin, alternating it as an input our output. 7 segment displays are just modules from e-bay.

I have gotten hold of some more transformers, so this is all I have (see last photo).
From left to right,
    - two 12 Vac 500 mA transformers (to power the arduino, fans and displays),
    - 22 Vac 2.2 A,
    - 2 taps @ 33 Vac 2A each . (Measured with no load, 2 A because wire gauge is the same as in other 2 A transformer)
    - and on the right what I had before, 2 taps@ 10 Vac 2A each, two 15 Vac 1 A.

Here is the options I see, all using the 12 Vac transformers for the arduino and displays:
    -Use what I have currently installed, 10 Vac tap + 15 Vac transformer per channel.
    -Use two 10Vac taps in series  (so 20 Vac) for one channel and the 22 Vac transformer for the other.
    -Use one 33 Vac tap of the big transformer per channel.  This will result on a rather high ≈ 45 Vdc input voltage, perhaps too close to the
     max 50 V of the filter caps.

That's it. Sorry for the nonsense. Thank you.

[xavier60]

A lot of options there. I think that he 33Vac transformer is just too high for most uses. The power supply would have to be derated to 500ma or less to keep the power transistors safe.
You could use the 22Vac transformer to make one medium voltage channel at about 1.5 amps.
And the 2x10Vac+2X15Vac transformer for a switchable low voltage high current  /  high voltage low current channel.
The safe minimum output voltage for each range at full current will be determined by the heat sinks.
Keep in mind that the thermal washers will add about 0.8° per watt of thermal resistance. You could easily add over temperature protection anyway.

[xavier60]

If I decide to go further with this project, this transformer looks nice.
 https://www.jaycar.com.au/12v-30v-100va-6a-multi-tapped-dual-type-2170-transformer/p/MM2015
Switches or relays could be used to select the 12/15 taps as well as series or parallel for higher current on the low voltage ranges.
Relays would allow auto switching to be added which would greatly lower heat dissipation  if the output is shorted on the high voltage ranges.

[JuanGg]

I think I'll leave it as-is for simplicity's shake. I can make changes later on.
    Juan

[xavier60]

I think I'll leave it as-is for simplicity's shake. I can make changes later on.
    Juan

Yes, because you are going for lower output current, the secondary switching isn't really needed.

[JuanGg]

All right. I'll make a perfboard with the main rectifier and filter cap, plus the auxiliary PSU: another small rectifier taking 12 Vac from the small transformers and regulating it to 12 V with a LM7812 regulator (for the fan) and to 5 V with an LM317 (arduino and displays).
Or maybe I should put the auxiliary PSU on the main board?(second photo, on the right, mounted vertically).
Regulators will be disipatting 0.5 W and 1.5 W respectively. As per the datasheet, LM7812 has a Juction to Ambient thermal resistance of 20ºC/W, so it should heat up to 35º assuming an ambient temperature of 25ºC (fine withought heatsink). LM317 has a Junction to Ambient thermal resistance of 38ºC/W, so it would heat up to 80ºC, I suppose it'll be fine without heatsink, but I can bend it and solder the tab to the perfboard just in case.

    Juan

[xavier60]

I also prefer having a separate board for the AUX rails along with the main rectifier. Keep ground return current paths for the 12V and 5V rails separate. They should meet only at a common point on the regulator board.
IF you power the op-amp with the 12V rail, don't run a ground wire from the AUX supply board to the regulator board. The only ground connection to the regulator board should be the 5V ground from the control board.
The fan might impose some ripple onto the 12V rail.

Extra: Im not certain how the LEDs should be powered. What voltage do the displays need?

[xavier60]

I see it's 5V for the displays. I need to think about that.
Does the  micro-controller have its own accessible internal reference?

[JuanGg]

5 V would be for the micro and displays. I don't know if the displays would make the 5 V rail noisy and if that would impact performance. The arduino itself has a 5 V reg, but its an smd package and I don't think that it can handle 1 W. It can be used for control circuitry though.

What are the op-amps powered from? I thought they would be from the 8V rail. Fans, being PWM driven, would definitely add some ripple to the 12 V rail.

The micro has an internal 1.1 V reference for the ADC's that can be enabled or disbled.

    Juan

[xavier60]

5 V would be for the micro and displays. I don't know if the displays would make the 5 V rail noisy and if that would impact performance. The arduino itself has a 5 V reg, but its an smd package and I don't think that it can handle 1 W. It can be used for control circuitry though.

What are the op-amps powered from? I thought they would be from the 8V rail. Fans, being PWM driven, would definitely add some ripple to the 12 rail.

The micro has an internal 1.1 V reference for the ADC's that can be enabled or disbled.

    Juan

You can stay with the 8V regulator as in my schematic to power the op-amps. Using the 12V instead would have eliminated many parts but might cause complications.
If the internal reference is used, there sould be no need to worry about fluctuations on the 5V

[JuanGg]

I have like ten LM7812 regulators, so I could use one for the fans and another one for the regulating circuitry. I have ordered all parts, so any option is feasible. I also have some LM317's.
    Juan

[xavier60]

I have like ten LM7812 regulators, so I could use one for the fans and another one for the regulating circuitry. I have ordered all parts, so any option is feasible. I also have some LM317's.
    Juan

Just occurred to me that increasing the supply voltage to the op-amps will affect the response to overloads as well as some other component value changes. You would need to use an LM317 set to 8V.

[JuanGg]

Just occurred to me that increasing the supply voltage to the op-amps will affect the response to overloads as well as some other component value changes. You would need to use an LM317 set to 8V.   

Ok. So I would have a 7812 regulator for the fan, an LM317 for the 8 V control rail, and another LM317 / 7805 for the displays. The Arduino can be powered from either 12 or 8 V and it'll generate it's own 5 V.
Would it be adequate to daysy-chain all regulators: Unreg. V > 12 V > 8 V > 5 V?
    Juan

[xavier60]

Just occurred to me that increasing the supply voltage to the op-amps will affect the response to overloads as well as some other component value changes. You would need to use an LM317 set to 8V.   

Ok. So I would have a 7812 regulator for the fan, an LM317 for the 8 V control rail, and another LM317 / 7805 for the displays. The Arduino can be powered from either 12 or 8 V and it'll generate it's own 5 V.
Would it be adequate to daysy-chain all regulators: Unreg. V > 12 V > 8 V > 5 V?
    Juan

I would use the 12V for the fans only and use the 8V for the Arduino as well as for the regulator control rail. At the moment, I can't see any big problem with daysy-chaining so long as it helps to share the dissipation.

[soldar]

Would it be adequate to daysy-chain all regulators: Unreg. V > 12 V > 8 V > 5 V?

I would rather have them be independent as it makes troubleshooting and repair easier. OTOH it could be a case where staggering them would balance power dissipation better. Your call.

[xavier60]

Also, the regulator mentioned earlier that is expected to reach 80° bothers me.  It's difficult to know what temperatures components really reach after the case is closed unless there is some continuous air flow. 

[JuanGg]

Also, the regulator mentioned earlier that is expected to reach 80° bothers me.  It's difficult to know what temperatures components really reach after the case is closed unless there is some continuous air flow. 

I'll build the thing up on a headboard, test it and measure temperatures just in case. I am thinking of deriving both the 8V and the 5 V rail from the 12 V one. Another thing to keep in mind is return current paths. Fan and displays wouy go before the shunt. I think the displays will be fine being driven from the other side of the shunt. I'll just put in a couple resistors in series with the clk and data lines, just in case.
    Juan

[xavier60]

Also, the regulator mentioned earlier that is expected to reach 80° bothers me.  It's difficult to know what temperatures components really reach after the case is closed unless there is some continuous air flow. 

I'll build the thing up on a headboard, test it and measure temperatures just in case. I am thinking of deriving both the 8V and the 5 V rail from the 12 V one. Another thing to keep in mind is return current paths. Fan and displays wouy go before the shunt. I think the displays will be fine being driven from the other side of the shunt. I'll just put in a couple resistors in series with the clk and data lines, just in case.
    Juan

Because AUX current will be from an independent supply rather than from the main unregulated rail, you should be able to reference everything to the top side of the shunt. Do a sketch to confirm.