Linear lab power supply

[not1xor1]

Because I don't have the IC for high-side current sensing. I have added low-side sensing.
I know it's not ideal because control circuit current flows through the shunt. It's only a few milliamps.
As expected, there is a massive current spike when the output is short circuited.
I do have a solution for this that is currently working in a bench supply that I had previously completed.
I'm trying to adapt it to the present design.
I have made small changes to the CV loop which is working very well.

I think you are looking for troubles with that design.
If you do not want to use an aux. supply voltage for the regulator circuit (Harrison design) the circuit below is much simpler, much easier to compensate (may even work with just a small cap across the feedback resistor) and might work up to 30-35 Vout.



So far I've just simulated lot of variations of this circuit in all possible conditions, but failed to find the time to test it (my lab is a mess because the walls are covered by mould and I had to move most of stuff elsewhere).

[xavier60]

Q2 as a Common Base amplifier causes a possible complication.
Because it's non-inverting, the op-amp inputs had to be swapped.

Thank you very much. I'll try to prototype it myself as soon as I can.

You might already know this. For the best results, the components shown below need to be wired directly together at the heat sink.
What part number Darlington have you decided to use?

[perieanuo]

After you do your first highside current sensing, you never come back to low side


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[JuanGg]

. Because I don't have the IC for high-side current sensing. I have added low-side sensing.
I know it's not ideal because control circuit current flows through the shunt. It's only a few milliamps.
As expected, there is a massive current spike when the output is short circuited.
I do have a solution for this that is currently working in a bench supply that I had previously completed.
I'm trying to adapt it to the present design.
I have made small changes to the CV loop which is working very well.
 

Looking good. Thanks again. I will see if I can order those current sense ICs. Not aiming for a precision PSU here and I can calibrate current error in software, so we may get away with low side current sense for starters.

Q2 as a Common Base amplifier causes a possible complication.
Because it's non-inverting, the op-amp inputs had to be swapped.

Thank you very much. I'll try to prototype it myself as soon as I can.

You might already know this. For the best results, the components shown below need to be wired directly together at the heat sink.
What part number Darlington have you decided to use?

I'll try to make it in a perfboard as compact as possible, right next to the heatsink. I don't have any darlingtongs at hand, only some 2n3055s and tip42s. I will have to order some, so whatever you guys recommend.

I think you are looking for troubles with that design.
If you do not want to use an aux. supply voltage for the regulator circuit (Harrison design) the circuit below is much simpler, much easier to compensate (may even work with just a small cap across the feedback resistor) and might work up to 30-35 Vout.

Thanks for the input. I'm going to need an 8-12V rail anyway to power the micro, displays, relays and fans.

    Juan

[xavier60]

I'll try to make it in a perfboard as compact as possible, right next to the heatsink. I don't have any darlingtongs at hand, only some 2n3055s and tip42s. I will have to order some, so whatever you guys recommend.

It's not necessarily for the whole control circuit to be made compact and close to the heat sink. Mainly the high current rails, the input and output capacitors and those other components shown in the previous diagram that should be kept close together. I have about 15cm  wires between the heat sink mounted components and the control circuit on the breadboard.
 I'm using a TIP35C and BD137 for the Darlington at the moment. Using different transistors might alter the compensation.
I'm not familiar with common Darling types. What output voltage and  current are you aiming for?
I'm worried that although that my mockup is working well, it might not work the same with different parts, mainly the transistors and the op-amp. I'm using the TLC072 which is very well suited.

What equipment do you have to test with?

I have included the current state of the circuit.
If you go ahead, don't bother with the CC loop until the CV loop is working properly.
I really like the topology of this power supply. I hope that it is successful. There doesn't seem to be much other viable alternatives with single  control rail referenced to the - output terminal.

[JuanGg]

It's not necessarily for the whole control circuit to be made compact and close to the heat sink. Mainly the high current rails, the input and output capacitors and those other components shown in the previous diagram that should be kept close together. I have about 15cm  wires between the heat sink mounted components and the control circuit on the breadboard.

All right. I was thinking of doing one board with the rectifier, filter cap, relay, and 8V reg, then another one with the control circuit, close to the heatsink. I may do a separate small one with the components shown in #126.  The microcontroller would be closer to the front panel, as it has deal with displays, encoders and such.

I'm using a TIP35C and BD137 for the Darlington at the moment. Using different transistors might alter the compensation.
I'm not familiar with common Darling types. What output voltage and  current are you aiming for?
I'm worried that although that my mockup is working well, it might not work the same with different parts, mainly the transistors and the op-amp. I'm using the TLC072 which is very well suited.

I may order those same ones, they are not expensive and if I don't use them they will go to the parts bin, same for the op amps. I'll see if I have some with similar characteristics as the TL072. I'll try to prototype the thing with what I have anyway.

I am aiming at 20-25 V at 0.6 A. The thing is my transformer arrangement 10Vac in series with 15 Vac gives me about 38 V wich drops to 36 V whend drawing 1 A. Would that be too much input voltage? I just want to make sure.

What equipment do you have to test with?

-Rigol 1054z scope
-Cheap arbitrary function generator
-24V 4A electronic load (https://www.eevblog.com/forum/projects/arduino-based-electronic-load/)
-Two multimeters (a $5 one and a UT-61E), DPS3005 power supply module running of a wall wart.

I have included the current state of the circuit.
If you go ahead, don't bother with the CC loop until the CV loop is working properly.
I really like the topology of this power supply. I hope that it is successful. There doesn't seem to be much other viable alternatives with single  control rail referenced to the - output terminal.

Agreed. Thank you for taking the time.

Juan

[xavier60]

It would be good to find a suitable op-amp that's cheaper than the TLC072. It needs to have at least 15v/us slew rate and 10Mhz, and most important, the input common mode range has to include ground. Odd thing is the data sheet shows that the common mode includes ground only when the IC is powered by 12V. When I tested mine, I found no problem even down to 5V.

[JuanGg]

It would be good to find a suitable op-amp that's cheaper than the TLC072. It needs to have at least 15v/us slew rate and 10Mhz, and most important, the input common mode range has to include ground. Odd thing is the data sheet shows that the common mode includes ground only when the IC is powered by 12V. When I tested mine, I found no problem even down to 5V.

I have "breadboarded" the circuit shown in #129 (only the CV loop), but using a 2n3055 as the series pass transistor, 2n3904s and 2n3906s for the remaining transistors and a lm358 op-amp. (which only has 1 MHz bandwidth, but its common mode range does include ground). I used whatever I had at hand, so no big expectations. Not the best conditions either with intermittent contacts and long wires. I powered the thing from a 14 V supply and used a 9V battery for the 8V rail (voltage was near 8V). A 100K pot served as the CV ref.

With no load appart from the feedback divider, it regulates without problems from 0 to about 12V.  Using a 1K resistor as load made it oscillate, see scope screenshot.
Haven't had time for further testing. Also tried out the 6N137 opto couplers that I plan to use to comunicate both chanels of the psu.They seem to work just fine.

    Juan

[wasyoungonce]

It would be good to find a suitable op-amp that's cheaper than the TLC072. It needs to have at least 15v/us slew rate and 10Mhz, and most important, the input common mode range has to include ground. Odd thing is the data sheet shows that the common mode includes ground only when the IC is powered by 12V. When I tested mine, I found no problem even down to 5V.

I have "breadboarded" the circuit shown in #129 (only the CV loop), but using a 2n3055 as the series pass transistor, 2n3904s and 2n3906s for the remaining transistors and a lm358 op-amp. (which only has 1 MHz bandwidth, but its common mode range does include ground). I used whatever I had at hand, so no big expectations. Not the best conditions either with intermittent contacts and long wires. I powered the thing from a 14 V supply and used a 9V battery for the 8V rail (voltage was near 8V). A 100K pot served as the CV ref.

With no load appart from the feedback divider, it regulates without problems from 0 to about 12V.  Using a 1K resistor as load made it oscillate, see scope screenshot.
Haven't had time for further testing. Also tried out the 6N137 opto couplers that I plan to use to comunicate both chanels of the psu.They seem to work just fine.

    Juan

The oscillation could be due to the breadboard introducing capacitance in connections.  This is known especially if trying to build buck/boost PSUs.... as well as linear


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[xavier60]

The oscillation could be caused by layout or the slow op-amp or both.
I didn't expect to see the power transistor and main current paths on the breadboard.
I have the components shown in post #126 physically soldered together at the heat sink and separate from the  breadboard. The component connections are no longer than their own leads allow. The idea is to keep the physical loop formed by C1, Q3, C2 and R9, fairly tight.
No "load related" current should be allowed to flow through any parts for the control circuit. These unwanted current paths can also be caused by connecting test  equipment. I had trouble caused by a HF ground loop between my DSO and the bench supply that I used to power the mockup.

[xavier60]

I am aiming at 20-25 V at 0.6 A. The thing is my transformer arrangement 10Vac in series with 15 Vac gives me about 38 V wich drops to 36 V whend drawing 1 A. Would that be too much input voltage? I just want to make sure.
Juan

That voltage drop indicates a light load for that arrangement. You need about that voltage for 30V max out of the regulator output.
Need to keep the SOA rating of the transistor in mind. The transistor stress is difficult to determine  because temperature and time are also factors.
http://www.hammondmfg.com/pdf/5c007.pdf

[xavier60]

Dominant pole compensation can be temporally applied by adding 1uF between the op-amp's output and inverting input.
The 47uF output capacitor should be a low ESR type.

[JuanGg]

. The oscillation could be due to the breadboard introducing capacitance in connections.  This is known especially if trying to build buck/boost PSUs.... as well as linear
   

.    The oscillation could be caused by layout or the slow op-amp or both

Most likely. I will look for a better op-amp and make it in a perfboard.

.  I didn't expect to see the power transistor and main current paths on the breadboard.
I have the components shown in post #126 physically soldered together at the heat sink and separate from the  breadboard. The component connections are no longer than their own leads allow. The idea is to keep the physical loop formed by C1, Q3, C2 and R9, fairly tight.
No "load related" current should be allowed to flow through any parts for the control circuit. These unwanted current paths can also be caused by connecting test  equipment. I had trouble caused by a HF ground loop between my DSO and the bench supply that I used to power the mockup. 

There is no way I am going to draw more than some milliamps from that setup. Just wanted to test if the thing even worked with the components I used. If all looks ok, I'll solder the thing as proposed and socket the op- amp so it can be changed easily.I'll put the power transistor on the heatsink I have already mounted on the case.

[JuanGg]

.     That voltage drop indicates a light load for that arrangement. You need about that voltage for 30V max out of the regulator output.
Need to keep the SOA rating of the transistor in mind. The transistor stress is difficult to determine  because temperature and time are also factors.
http://www.hammondmfg.com/pdf/5c007.pdf 

The 10Vac transformer is 2A rated, while the 15 Vac is 1 A.
As per the datasheet, ≈40V c-e and 0.6 A is well within 2n3055's SOA.

Another thing to keep in mind is the current return for the fan, relay, Arduino and displays. That current has to go somewhere.

[xavier60]

.     That voltage drop indicates a light load for that arrangement. You need about that voltage for 30V max out of the regulator output.
Need to keep the SOA rating of the transistor in mind. The transistor stress is difficult to determine  because temperature and time are also factors.
http://www.hammondmfg.com/pdf/5c007.pdf 

The 10Vac transformer is 2A rated, while the 15 Vac is 1 A.
As per the datasheet, ≈40V c-e and 0.6 A is well within 2n3055's SOA.

Another thing to keep in mind is the current return for the fan, relay, Arduino and displays. That current has to go somewhere.

How are you deriving the control rail?
For simple analog control, there will be only a few milliamps of unwanted current through the shunt.
I'm also adding a 10ma load to the output which will return back to the left side of the shunt, so no problem.
Fan and relay current can be returned to the left side of the shunt also. But a micro-controller needs to be referenced to the right side.
A separate control supply will be needed.

Extra: High side sensing will solve some problems if it can be made to work properly.

[xavier60]

I have made a PCB for the regulator circuit from post #129.
Only the CV loop is working for now. It's performing the same as it did on the breadboard, very well.
I found some LM358 op-amps here, so I fitted one in place of the TLC072. It worked just fine.
I have the compensation set so that load transient recovery is 5us.

[JuanGg]

.     That voltage drop indicates a light load for that arrangement. You need about that voltage for 30V max out of the regulator output.
Need to keep the SOA rating of the transistor in mind. The transistor stress is difficult to determine  because temperature and time are also factors.
http://www.hammondmfg.com/pdf/5c007.pdf 

The 10Vac transformer is 2A rated, while the 15 Vac is 1 A.
As per the datasheet, ≈40V c-e and 0.6 A is well within 2n3055's SOA.

Another thing to keep in mind is the current return for the fan, relay, Arduino and displays. That current has to go somewhere.

How are you deriving the control rail?
For simple analog control, there will be only a few milliamps of unwanted current through the shunt.
I'm also adding a 10ma load to the output which will return back to the left side of the shunt, so no problem.
Fan and relay current can be returned to the left side of the shunt also. But a micro-controller needs to be referenced to the right side.
A separate control supply will be needed.

Extra: High side sensing will solve some problems if it can be made to work properly.

As we need an 8 V rail, it makes sense to derive it from the 10 Vac transformer, doing so from the full 25 Vac will be wasting power. The 10 Vac transformer is 2A rated as well, so there is more current available from that rail without limiting the power supply output current. I thought of doing it as attached. There is probably a simpler way, but that's what I came up with. In addition, taps can be switched depending on the output voltage to reduce power disipation. The relay can be controlled from the micro as that does not have to be particularly quick. I was thinking of using just an lm317 for the control rail. All current will be returned to the left of the shunt but from the arduino, which can take tens of mA, not drawing current from its pins. High side sensing would be definitely better, do I go ahead and buy some INA196 ICs? Any other solution?

I have made a PCB for the regulator circuit from post #129.
Only the CV loop is working for now. It's performing the same as it did on the breadboard, very well.
I found some LM358 op-amps here, so I fitted one in place of the TLC072. It worked just fine.
I have the compensation set so that load transient recovery is 5us.

I can't thank you enough for taking the time.  Nice to know that the LM358 does the job.

Juan

[perieanuo]

Put tap switching before bridge


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[JuanGg]

Put tap switching before bridge

How would I get the 12V rail then?

[perieanuo]

Simply.
You series the 2 secondaries, 1 bridge, and in bridge insert one end and the middle or the other end of your series.
You have 10 or 25V on your bridge then
You want 2 loads?
If you want 12V and 35 V from same gnd you need 1 bridge only.and the from your 12 or 35 get whatever voltages you want, fixed or variable.
I don't get why 2 bridges help and the current return with 35v is a bit strange


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[JuanGg]

Simply.
You series the 2 secondaries, 1 bridge, and in bridge insert one end and the middle or the other end of your series.
You have 10 or 25V on your bridge then
You want 2 loads?
If you want 12V and 35 V from same gnd you need 1 bridge only.and the from your 12 or 35 get whatever voltages you want, fixed or variable.
I don't get why 2 bridges help and the current return with 35v is a bit strange


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I don't know if I understood you correctly. I don't want to have fans, relay and displays drawing current from the 15V transformer at any time. Plus, 35V (theoretically, in reality it goes up to 38-9V) is too close or even over max input voltage of 780x series regulators or similar.

[perieanuo]

Simply.
You series the 2 secondaries, 1 bridge, and in bridge insert one end and the middle or the other end of your series.
You have 10 or 25V on your bridge then
You want 2 loads?
If you want 12V and 35 V from same gnd you need 1 bridge only.and the from your 12 or 35 get whatever voltages you want, fixed or variable.
I don't get why 2 bridges help and the current return with 35v is a bit strange


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I don't know if I understood you correctly. I don't want to have fans, relay and displays drawing current from the 15V transformer at any time. Plus, 35V (theoretically, in reality it goes up to 38-9V) is too close or even over max input voltage of 780x series regulators or similar.

You're free to choose what you like.
You find regulators for 40 V, I never saw one burned at this voltage.the 4 diodes for 2-nd unnecessary bridge, use them to lower voltage or any other mean to do this.
Put only one diode on 35v and you got lower V for your 7812 for ventilator...


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[JuanGg]

Simply.
You series the 2 secondaries, 1 bridge, and in bridge insert one end and the middle or the other end of your series.
You have 10 or 25V on your bridge then
You want 2 loads?
If you want 12V and 35 V from same gnd you need 1 bridge only.and the from your 12 or 35 get whatever voltages you want, fixed or variable.
I don't get why 2 bridges help and the current return with 35v is a bit strange


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I don't know if I understood you correctly. I don't want to have fans, relay and displays drawing current from the 15V transformer at any time. Plus, 35V (theoretically, in reality it goes up to 38-9V) is too close or even over max input voltage of 780x series regulators or similar.

You're free to choose what you like.
You find regulators for 40 V, I never saw one burned at this voltage.the 4 diodes for 2-nd unnecessary bridge, use them to lower voltage or any other mean to do this.
Put only one diode on 35v and you got lower V for your 7812 for ventilator...


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I wouldn't mind doing it that way, the thing is:
80 mA (fan) + 2*40 mA (led displays) + 40 mA (Arduino) + 30 mA (relay) makes for a 230 mA load, plus regulating circuitry. (It'll be lower on average, specially the fan, just doing a worst case calculation here).
The 15 Vac transformer is 1A rated, resulting in practical ≈620 mA max current as per the chart in #135. I would be limited to ≈ 390 mA output current, and disipating ≈5W on the regulator. Just a thought. Thanks for the input.
    Juan

[perieanuo]

I wouldn't mind doing it that way, the thing is:
80 mA (fan) + 2*40 mA (led displays) + 40 mA (Arduino) + 30 mA (relay) makes for a 230 mA load, plus regulating circuitry. (It'll be lower on average, specially the fan, just doing a worst case calculation here).
The 15 Vac transformer is 1A rated, resulting in practical ≈620 mA max current as per the chart in #135. I would be limited to ≈ 390 mA output current, and disipating ≈5W on the regulator. Just a thought. Thanks for the input.
    Juan

hi,
in real life you don't need tap switching.I don't do it even for 5 amps/0-27Vdc (limits of my 2'nd power supply).So I review my own theory, just cut off your budget and pain and series the secondaries, do the regulation and you're done.don't forget measuring current on low side will put problems for a beginner more than adding 4 $ with high side sensing, the right way to do it in your case.
If you do tap switch for future developement, that's another thing.
Anyway with your current/voltage specs you are well within SOA, you can do tap switch only for echology reasons, but buying relay+diodes+... isn't echological no more, it's just like the big lie buying electrical car will do good to the ozone
I just tried to simplify your design and get you focused on important stuff like keep it simple and get the voltage and current measurement simply and the PS stable.tap switch implies further decisional voltage comparator, watching what happens when commuting if you forgot to remove load you'll have to think what you do with the spike when you switch.imho too complicate for almost zero result.
tap switching method is for amps not for mAmps.But what the hell, I'm talking too much, as I said it before, do as you like
pierre

[xavier60]

The power transistor is mounted in direct contact with the heat sink bracket to reduce thermal resistance.
Although this power supply circuit responds to output short circuits much faster than most others, the LM358 takes 20us to take control of output current which peaks to 20 amps.  The TLC072 responds in 2us.
The 3 BD137 transistors from left to right are 8V series pass, Darlington 1st transistor and constant current  load.
Ill post an updated schematic after I do more testing. At this stage I have made only minor changes.