DIY Scalable Bench Power Supply Design

[prasimix]

Rest of the circuit here is mainly supplied by separate bias power supply. So it is effectively "before the regulator" as you pointed out. Idea about current measurement before the regulator will ensure staying within working range of LTC6102 (over 2.7V) when Vout approach 0V.

[dannyf]

working range of LTC6102 (over 2.7V) when Vout approach 0V.

Not sure what "2.7v" is. But if it is the minimum supply voltage, you don't need to put the sensing in front of the regulator.

[prasimix]

Sorry, it's 4V is the case of LTC6102 used in example. So if I understood correctly you cannot measure let say a current of 100mA when Vout is 1.2V. Therefore current monitor need to be before serial regulator with min. 4V drop.

EDIT: Hm, I'm little bit confused with this one. LTC6102 has indeed separate V+ and it seems that it can be anything between 4 and 60V. But in all their examples V+ is connected to positive rail. Is it just because they presume that current monitoring will be performed on higher voltage then 4V?

[dannyf]

One is supply voltage range, another is common mode voltage range.

Apples and oranges.

[prasimix]

Yes, just edited my previous post

[Marco]

If it's not before the pass FET the high side current sensor becomes more complex or you'll need an extra negative supply.

I don't see the problem with a FET really, yeah the gate displacement currents will cause errors in the dynamic case ... but the current sensor has it's own internal FET which does the exact same thing and they are heavily bandwidth limited any way. The dynamic case isn't that interesting.

[prasimix]

I probably need to get a couple of LTC6102 (or maybe better -1 version) and see what is possible. According to absolute maximum ratings table -INF and -INS can go down to V+ – 4V. In another table (electrical characteristics) for V+ min. is 4V. So to be able to make measurement for Vout close to 0V (or let say below 1V) one have to use min. supply voltage.

[David Hess]

That's exactly the circuit I was thinking about, having done a lot of research on high-side current sensing. The last version I've been working on allows easier substraction of the base drive current from the output current which means I can move the current sensing circuitry before the pass transistor, at least 5V above ground.

I think one of the more recent Agilent power supplies did exactly this by mirroring the base drive to the power transistor and adding it to the measured current.

Does something like this make sense? Current monitor is located before serial BJT/FET so main current + serial regulator current are measured and available on the LTC6102 output. To remove serial regulator current from the measurement, two additional op amps is used: one to measure voltage drop on R11 that is delivered inverted to the IC1A and subtracted from LTC6102 output voltage. Resulting value is then delivered to the current control "logic".

I think that is basically what they were doing.  I remember studying the base drive circuits wondering why they were so complicated and concluding that they were compensating for the base current so the current limit measurement could be made on the other side of the pass transistor for the reason you mention in your other post.

As Marco mentions if you use a power MOSFET, then this would not be an issue.

[void_error]

As Marco mentions if you use a power MOSFET, then this would not be an issue.

There's still the gate-source capacitance which will matter with fast-changing load currents, I think, although I'm not sure how much it'll impact the current measurement precision if you measure current before the series pass element.

[David Hess]

As Marco mentions if you use a power MOSFET, then this would not be an issue.

There's still the gate-source capacitance which will matter with fast-changing load currents, I think, although I'm not sure how much it'll impact the current measurement precision if you measure current before the series pass element.

Dynamic current as far as the load is concerned is already poorly controlled because of the relatively high value output capacitor typically present.

If high precision dynamic output current is desired, then a different topology would be appropriate like a transconductance output with a minimum of output capacitance.  It would be a precision current regulator with a secondary voltage control loop.

[void_error]

Had a look at the LTC6101 and I'm wondering how well it's going to work with a variable supply voltage as it's going to be after the pre-regulator which tracks roughly 5V above V_OUT although I'm more comfortable AD's discrete solution with a few modifications.



Thinking of replacing the zener diode with a LMV431 and R_BIAS with a current sink using an N-channel MOSFET...  That way 5V could be used to power the opamp driving the MOSFET with the best thing being the avoidance of specialized parts like integrated current shunt monitors. I'll also include some input protection diodes.
If there's anything wrong with this idea please let me know.

[void_error]

After a truckload of simulations I've settled on the final design, I hope (I always find something to simplify/tweak).

Used a 3.3V zener for the floating opamp and two PNP transistors in a darlington configuration instead of the P-channel MOSFET. They will be slower but at an overall gain of (literally) over 9000 the base current can be safely ignored.

Linearity seems to be better than 0.5% with a current measurement range of 30mA to 3A. The reason it can't measure lower than is the output voltage swing of the MCP6021 (why this one? because it's quite cheap for its performance), at 1V/1A that means 30mV for 30mA after the difference amplifier used to remove the current drawn by the series pass transistors drive circuirty, which has been substantially simplified (does this even make sense?) in the last version.

I'll most likely order the parts somewhere in January next year as I still have a few other smaller projects to deal with as well as designing a PCB I can make at home for this one - using solder paste and a hot air rework station for SMD components, without soldermask/silkscreen on the board - done that before and it works well, trace orientation relative to the component pads is critical or surface tension will screw things up.

[mikerj]

Low voltage zeners have pretty horrible performance.  If you need a low cost but reasonably stable reference the TL431 is hard to beat.

[void_error]

If you need a low cost but reasonably stable reference the TL431 is hard to beat.

The zener is used only as a voltage regulator (fed by a current sink) so stability is not that important. Might replace it with the LMV431 as it requires a lower current to operate (TL431 needs 2mA minimum) so I don't waste power on the current sink's transistor (33V @ 3mA worst case is a bit too much for a SOT-23 device).

[void_error]

  Red stuff is there to make it easier to read.

[holko]

Red stuff is there to make it easier to read.

[]https://www.eevblog.com/forum/projects/bench-power-supply-design/?action=dlattach;attach=121894;image[/]

(Removed picture in the reply for obvious reasons...)


I don't believe it is a a big issue, but I would add an resistor in the >1k range on the output to tame the output capacitor.

Other wise it looks as like a really nice and robust design!
To really keep it robust It might be an idea to but a bidirectional TVS at the output.

-Holko

[void_error]

I don't believe it is a a big issue, but I would add an resistor in the >1k range on the output to tame the output capacitor.

Or I can move pin 2 of R16 to the other side of the diode...

To really keep it robust It might be an idea to but a bidirectional TVS at the output.

The TVS isn't on the schematic but there's going to be one on the output jack board.

Otherwise it looks as like a really nice and robust design!

Thanks! I'm glad I didn't get stuck at the first version that worked on the breadboard. One thing about this design is that it's easily modifiable for different input/output voltages and currents (just change some resistor values) and doesn't use expensive / hard to get / specialized parts. I suppose it can even be scaled up to something ridiculous like 70V/5A (or more?) if the pre-regulator and LM317 are replaced with something suitable and more paralleled series pass transistors are added for that kind of voltage/current.

[hugo]

"That also means I didn't get the chance to play with the LM2596 either, using a LM317/TL431 temporarily in place of the pre-regulator set to track the same way as the LM2596 would do."

Could you elaborate/share the schematic of a such pre-regulator using LM317 and TL431 ?

[void_error]

Page 22, Figure 20, LM317 instead of 7805, R1 replaced with Q1/D1/R1/R2/R3 (like in the last schematic), resistor values chosen to work with the TL431's 2.5V reference. Used the LM317 mainly for current limiting so the magic smoke is confined to the inside of the semiconductor packages in case something goes wrong .

Oh, and I forgot to include the 'tracking pre-regulator disabled below 300mA (or whatever, TBD after thermal calculations are done) current limit' feature in the last schematic but that's pretty easy to implement - only a comparator, a bunch of resistors and a PNP transistor are needed.

[hugo]

How did you come up with those feedback resistor values R1/R2/R6 ? R6 should be between 1k and 5k according to the data sheet.  Are you using a circuit simulator ?

[void_error]

How did you come up with those feedback resistor values R1/R2/R6 ? R6 should be between 1k and 5k according to the data sheet.

I first did the math for minimum output voltage (approx 4.7V with the chosen values) which ended up being 680R & 1k8, then selected R2 to have an output voltage higher than the minimum input voltage I needed for the linear regulator part (about 25V for 20V out). With the 15k resistor in place that'll be 33.4V assuming V_FB is exactly 1.23V which will most likely not be the case as it could be anywhere between 1.18V & 1.28V. R6 was chosen to be high enough to reduce the current that would flow through the BE junction of Q1 (working as an emitter follower) into the output. If you reduce the value of R6 the minimum output voltage will increase. If the output needs to go lower then a load resistor is needed between R16 pin 2 & R22 pin 1. R22 may require tweaking .

Are you using a circuit simulator ?

I simulate some parts I'm not sure about before I prototype it on a breadboard.

[void_error]

Small update: I found a better chip for the pre-regulator - the LTC3824. It can also go up to 100% duty cycle, uses an external P-channel MOSFET which means there's more headroom for the linear regulator as the LM2596 will drop roughly 2V at full current with the switch permanently on while the MOSFET will drop a lot less, depending on the actual part used, but overall I'll end up with increased efficiency.

Two more nice features are the higher input voltage and current limit set resistor which means that now I can build other bench PSUs with different specs based on the same schematic . Maybe I'll need a small linear pre-regulator for the LM317 for higher input voltages... details, details.

I might have reached my goal of designing an easily scalable PSU.

[prasimix]

Do you have any information to share about your experience with LM2596? How it was works and does tracking is doing well with different voltage and load?

[void_error]

Do you have any information to share about your experience with LM2596? How it was works and does tracking is doing well with different voltage and load?

No, I don't. Found the LTC3824 and I'm planning to use that as it's more flexible. A switchmode regulator will always be slower than a linear one so I suppose if there's enough bulk capacitance at its output I shouldn't have any problems. So far I don't have a constant current DC load or a good scope so I'll have to improvise a circuit to measure the output ripple under various loads after I build it. I still have a PCB to design though...

[prasimix]

I understand. Did you already find an appropriate P-ch mosfet for LTC3824?