Author Topic: How this linear PSU circuit works, and why it doesn't (Read 29891 times)

xavier60 · « **Reply #25 on:** July 27, 2020, 11:15:11 pm »

The opamp's GND pin 4 needs to go to GNDA.
With R8 to 5V, Q1 wont turn on, it needs to go to 12V but this might cause too much maximum drive for Q1.
Include a resistor from the ORing node to GND, 33K for now

bogdan2014 · « **Reply #26 on:** July 28, 2020, 04:48:24 am »

I'm aiming for 5A so I chose a 0.22 ohm resistor. 1V across it at maximum current and 5W power dissipation.
What is the ORing?

xavier60 · « **Reply #27 on:** July 28, 2020, 05:11:47 am »

Quote from: bogdan2014 on July 28, 2020, 04:48:24 am

I'm aiming for 5A so I chose a 0.22 ohm resistor. 1V across it at maximum current and 5W power dissipation.
What is the ORing?

For 5A, consider using 100mΩ or less. I use s 50mΩ CS resistor.
The CV or the CC opamp can pull down the Base voltage of Q1 via D5 or D6. I call the junction of D5 and C6 the ORing node.
Also, depending on what your unregulated voltage is, 5A might not be possible to do safely with one 2N3055.

bogdan2014 · « **Reply #28 on:** July 28, 2020, 07:02:28 am »

I can fit two of them on the heatsink, so that's not a problem. I chose 5A since it was easier on the calculations, but I doubt the transformer will be able to deliver it, and that I'll ever need that much current.
I added all the mods and an extra transistor footprint so I have everything on the PCB. I'm thinking, does the auxiliary power supply have to be referenced to GND or GNDA like the op-amp?

xavier60 · « **Reply #29 on:** July 28, 2020, 08:00:14 am »

Quote from: bogdan2014 on July 28, 2020, 07:02:28 am

I added all the mods and an extra transistor footprint so I have everything on the PCB. I'm thinking, does the auxiliary power supply have to be referenced to GND or GNDA like the op-amp?

Either way, some unwanted current will flow through the CS resistor.
With my bench supply based on this design, I derive the 8V control rail from the unregulated input so its current has to flow back to the unreg negative. This was fine until I added a digital panel meter, powered from the 8V which caused the complication of its current draw flowing through the CS.
So if you don't intend adding a digital panel meter, reference the AUX supply to GNDA. You might even make it selectable.

EDIT: You still need to take the CV compensation from the ORing node. It will greatly reduce voltage overshoot when recovering from an overload.

bogdan2014 · « **Reply #30 on:** July 28, 2020, 08:15:19 am »

Quote

You might even make it selectable.

That sounds good. I think I can accomodate a jumper.

ChenBH · « **Reply #31 on:** July 28, 2020, 11:29:36 am »

Newbie here:
Why is there a diode from E to B of Q4? guarding from returning current from a non-resistive load? Why use a combination of PNP and NPN in the darlington configuration?

bogdan2014 · « **Reply #32 on:** July 28, 2020, 12:06:57 pm »

The diode serves as a protection to return the current to the bulk capacitor when the transistor is closed.
The pnp/npn combination is used because it's a series pass topology, and it works by pulling the base of the pnp transistor to ground instead of applying a voltage to it.

xavier60 · « **Reply #33 on:** July 28, 2020, 12:08:23 pm »

Quote from: ChenBH on July 28, 2020, 11:29:36 am

Newbie here:
Why is there a diode from E to B of Q4? guarding from returning current from a non-resistive load? Why use a combination of PNP and NPN in the darlington configuration?

D9?, it's to prevent the B-E junctions of the 2N3055's being reversed biased if an external voltage is connected while the PSU is powered down and damaging the transistors.
I don't see why a PNP Darlington wouldn't work as well. I think that NPN silicon power transistors are still perceived to be better and cheaper than the PNP type. Google it.

bogdan2014 · « **Reply #34 on:** July 29, 2020, 05:16:51 am »

Updated schematic and PCB

xavier60 · « **Reply #35 on:** July 29, 2020, 10:42:06 am »

Quote from: bogdan2014 on July 29, 2020, 05:16:51 am

Updated schematic and PCB

The CV compensation needs to be taken from the ORing node. It will greatly reduce voltage overshoot when recovering from an overload.
Will Q3 and 4 be mount under the PCB on a heat sink?
5 amps through a bridge rectifier without a heat sink would be pushing the limits. Even if it doesn't fail, the high temperature could damage the PCB and nearby components as well as the solder joints.
I use a 15A bridge on the heat sink.

Some header pins for 12V out could be useful later.

xavier60 · « **Reply #36 on:** July 29, 2020, 07:35:00 pm »

R7 and R8 need to connect to 12V,

bogdan2014 · « **Reply #37 on:** July 30, 2020, 04:54:05 am »

Transistors are TO-3 style (metallic case) so I'll mount them on a heatsink placed outside of the enclosure and run wires to them. The rectifier has a hole for mounting on the heatsink, so I'll do that if necessary.
EDIT: fixed the negative pin of the LV bridge rectifier. Also added a copper plane to that return node.

xavier60 · « **Reply #38 on:** July 30, 2020, 10:05:32 am »

That looks like a very simple design. One more thing. If the CV pot develops intermittent wiper contact, the output voltage might go high. A simple precaution is the put a bleed resistor from the wiper pin to GND, a 470K. A bypass capacitor wouldn't hurt either.

Beware of knock off Bourns 10 turn pots. After some use, the resistance wire can break. This will cause the PSUs output to jump suddenly from zero to full voltage as the pot is rotated. There is no simple precaution for this.
Bochen 5 and 10 turn pots are proving to be more reliable for me.

bogdan2014 · « **Reply #39 on:** July 30, 2020, 10:10:52 am »

That's the disadvantage of this topology. I've had lab power supplies from the 80s that gave out 40V instead of 5V because of bad pots. For sensitive circuits I have a programmable power supply, but there are times where a good old analogue PSU is better since it allows you to vary the voltage gradually (not stepped). Plus I have loads of unused 2N3055 and three identical transformers that would be a shame not to put to good use.
If there aren't any other suggestions, I'd like to send the files and get the PCB done.

xavier60 · « **Reply #40 on:** July 30, 2020, 10:20:53 am »

Only one last possible problem. If you are really using the TL072, it wont work properly with it's inputs close to ground voltage.
I use the TLC072. Even an LM358 will work ok. The current limiting response will be a bit slower.

bogdan2014 · « **Reply #41 on:** September 08, 2020, 10:26:08 am »

Boards have arrived. On first sight, everything seems ok. I had to lower R4 to 100k to get the full current range.

xavier60 · « **Reply #42 on:** September 08, 2020, 10:32:11 am »

Will you be testing it?
A load transient recovery test will show how well the voltage regulation responds to sudden changes in load current, also how stable it is.

bogdan2014 · « **Reply #43 on:** September 08, 2020, 12:09:57 pm »

I only did a quick test with a 12V/20W halogen light bulb. I'll be doing some more tests with different loads.

bogdan2014 · « **Reply #44 on:** January 07, 2021, 08:40:50 am »

I finally managed to assemble the entire thing in an enclosure and did some tests. The main issue is the heatsink, which gets crazy hot. I might have to add a fan or find a bigger one.
Some turn-on turn-off measurements below.

xavier60 · « **Reply #45 on:** January 07, 2021, 09:41:58 am »

The turn on looks unusually slow.
Depending on how your transformer's secondary windings are, it might be possible to reduce the unregulated voltage for when the output is set to low voltages.
With mine, the two 15Vac secondary windings are relay switched to parallel or series to best suit the set output voltage.
I also have a large fan cooled heatsink. It originally belonged to an aircon inverter, and cut in half.
https://www.eevblog.com/forum/projects/linear-lab-power-supply/?action=dlattach;attach=726186;image

bogdan2014 · « **Reply #46 on:** January 17, 2021, 04:54:50 pm »

The project is available on gitlab for those interested: https://gitlab.com/bogdan-kicad/eevblog-linear-psu

Quote

Depending on how your transformer's secondary windings are, it might be possible to reduce the unregulated voltage for when the output is set to low voltages.

I could do that, yes. The transformer has a 12V tap off of the 24V secondary winding.

bogdan2014 · « **Reply #47 on:** October 06, 2021, 05:13:35 am »

I'm in the process of building the second PSU of this kind. I'm happy to say that the first one has proven to be very reliable. However there's one thing that made me curious. In this topology if the control circuit fails, the output goes to the highest possible voltage. I've seen this in most linear power supplies, but is there a way to build it so that the op-amp biases the output transistor instead of pulling the base down?

xavier60 · « **Reply #48 on:** October 06, 2021, 07:06:14 am »

Nice job!
I don't think it's possible to design a control circuit that can be trusted to inherently fail to safe.
Assuming that the power transistors are kept well within their SOA and safe temperature, the greatest risk I found was the resistance wire in the 10 turn pots breaking, suddenly sending the PSU to its maximum voltage setting.
This is the design being currently used in my own PSU.
https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/msg3582664/#msg3582664
Later in the thread there is a tap changer circuit.
With this topology, a load response test needs to be done to check the stability.
The compensation can change with different parts used.

bogdan2014 · « **Reply #49 on:** October 06, 2021, 05:44:25 pm »

The first one has a 10-turn pot for voltage and a standard pot for current. This time I'll use standard pots with fine/coarse adjustment. The 10 turn pots are chinese "bourns" and not once I had them fail.
I'm not happy about the panel meter, these are the only digital ones that I could find and that display both voltage and current, but don't like them. I'm thinking of building one myself with an MCU.


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Author Topic: How this linear PSU circuit works, and why it doesn't (Read 29891 times)

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