DIY Scalable Bench Power Supply Design

[extide]

x2 I would love a full set of schematics. Seems like a fun project, and I could definitely use a bench PSU! I have some stuff laying around as well...

[akis]

Some random thoughts:

I have made 3 different versions of bench PSUs, both working on my desk, except the very first ones whose diodes melted everything around them. They are 0-30V/5A with 2 channels each (total 4 channels).

The second version uses pairs of LM2679s (5A) to produce 3-6V over the required voltage and then followed by a linear regulator using two FETs to determine current limiting and voltage (one for each). Because of the LM2679s it runs very cool.

I also have one wire (should have two) running all the way to the load to sense the voltage so that it stays good under large currents. The wire is soldered on the same crocodile clip as the power output so it is not messy at all.

I added an ammeter from ebay at the output which has its own sense resistor, but that is a bit of a waste because I already have my own sense resistor for the current limiting circuitry. It would be more efficient to use one resistor only.

There is also a linear version without the LM2679s and it gets very hot, so there are 5 fans in there controlled by another circuit, and thermal shutdown. The linear version produces very clean output especially at high currents (all the way over 5A) whereas the LM2679 introduces some noise spikes.

[void_error]

Some random thoughts:

I have made 3 different versions of bench PSUs, both working on my desk, except the very first ones whose diodes melted everything around them. They are 0-30V/5A with 2 channels each (total 4 channels).

The second version uses pairs of LM2679s (5A) to produce 3-6V over the required voltage and then followed by a linear regulator using two FETs to determine current limiting and voltage (one for each). Because of the LM2679s it runs very cool.

I also have one wire (should have two) running all the way to the load to sense the voltage so that it stays good under large currents. The wire is soldered on the same crocodile clip as the power output so it is not messy at all.

The linear version produces very clean output especially at high currents (all the way over 5A) whereas the LM2679 introduces some noise spikes.

I was kind of forced to go with 20V 3A output mainly because of budget constraints. I've done the math and had to stick with a rather cheap (don't read crap quality) and readily available 24V 100VA transformer. The higher voltage transformers (would need a 36V one for 30V output) were significantly more expensive for the same VA rating. On the other hand, I still have two transformers taken out of an old Epson printer, one is about 30V @ 1A min, the other one about 10V @ 2A and I will use them for a second lab PSU. For this one however, the higher output current was more important.

I was thinking about the feedback wires being separate to counter the voltage drop from the regulator output to the output plugs. PCB layout should be carefully done to avoid ground loops and other nasty stuff, especially since this design will probably use about half a dozen boards, the reason being my current lack of gear to make PCBs larger than about 5cm by 10cm at home, currently single layer only, and it's too expensive to have them made elsewhere (even locally) in one-off quantities.

As for the switching noise and the ripple rejection (the LM2596 runs at ~150kHz), I'll go brute force and use LC filters at the input of the linear regulator. It was cheaper for me to use a smaller heatsink and a switchmode pre-regulator than have to get rid of 60+ watts of heat.

x2 I would love a full set of schematics. Seems like a fun project, and I could definitely use a bench PSU! I have some stuff laying around as well...

I'll have to warn you though, if you want to build everything yourself, especially the digital control part, you'll need a bit of experience to get everything right.
The analog part, however, is pretty simple and it can work with analog controls (pots) and that was the initial idea, but then it all went south...
As for the fun part, every project attempted with a certain level of complexity will have its ups and downs.

To get an idea you could look at Ian Johnston's design, from which mine borrowed a few ideas  . Also, since I haven't prototyped anything yet, none of the schematics posted so far are guaranteed to work perfectly or even work at all without some small tweaks (no matter how well you do the math, real world physics can and somethimes will always be one step ahead of you). 

[dannyf]

If you don't need very high voltage, you can start with a laptop power supply: about 20v, a few amps, light weight, and compact.

[mariush]

      ---> +12V fan supply will be a 7812

suggestion.. use something adjustable, cheap and easy to vary voltage between something like 8v and 12v (add a thermistor in the adjust loop for example). Keep voltage above the 5v regulator + whatever voltage drop it has.

      ---> 78L05 fed from the 7812's output will power the  DACs and ADC

You're kidding. 78l05 is 100mA max and in a package that's not very good with heat. 12v in , 5v out.. you're looking at 20-30mA before it burns.
Use something like 1117 (as common as 7805) if you want lower voltage drop... 1117 can do 5v @ 0.75-0.8A with about 1.1v voltage drop.

[akis]

If you don't need very high voltage, you can start with a laptop power supply: about 20v, a few amps, light weight, and compact.

I have done that. Not very successfully. I went and bought about a dozen laptop adapters from ebay, 19V-20V, 75W, and discovered with horror that they are very nasty. I also posted a thread here on this board. I put 2 * 19.5V in series to make 38V, they cost around £8 each, much cheaper than transformer. The problem is what comes out the output, namely huge voltages due to the Y2 capacitor, earth wire connected to V- for almost all of them except a very few and other nasties.

[void_error]

To be honest, I have put a bit of thought into that, and I actually worked as a laptop technician so I know what's in them, but since I'm also an EE student and a future electronics engineer and I have a 'knack' for designing and building stuff I went with a homebrew design. Also, electronics has been a hobby of mine for the past decade so I think I know what I'm doing. I just thought I'd share a bit of my knowledge for the help I got on this forum and I dare say it's one of the finest I've come across so far.

      ---> +12V fan supply will be a 7812

suggestion.. use something adjustable, cheap and easy to vary voltage between something like 8v and 12v (add a thermistor in the adjust loop for example). Keep voltage above the 5v regulator + whatever voltage drop it has.

      ---> 78L05 fed from the 7812's output will power the  DACs and ADC

You're kidding. 78l05 is 100mA max and in a package that's not very good with heat. 12v in , 5v out.. you're looking at 20-30mA before it burns.
Use something like 1117 (as common as 7805) if you want lower voltage drop... 1117 can do 5v @ 0.75-0.8A with about 1.1v voltage drop.

The microcontroller will handle the fan control, it'll be either ON or OFF. Although it'll probably be overkill, I want an extremely reliable power supply.

The bulk of the current will be on the +5V digital rail, to power the MCU, the LED displays and the character LCD display, and as I stated, It'll be a step-down switchmode regulator capable of 3A continuous output, bolted onto the same heatsink as the switchmode pre-regulator and the linear regulator.

The ADC (MCP 3208, 500uA max @ 100 ksps) used at less that 100 sps and DACs (two MCP4921s, 700uA x2 + less than 2x500uA -> 2x10k output resistors to ground) which will have the data sent to them only when I push a button to apply the selected voltage and current limits (4V & 3V respectively) will be powered from a separate +5V rail and the total current draw will be tiny, less that 10mA average, so a 78L05 will be sufficient.

If I remember correctly I have a few dead PC motherboards with 1117s still working on them and all the necessary equipment to take them off and still have them in working condition. I also have a leftover brand new 78L05 in a SOT-89 package (I might have got that wrong), so I might use that.

I have done that. Not very successfully. I went and bought about a dozen laptop adapters from ebay, 19V-20V, 75W, and discovered with horror that they are very nasty. I also posted a thread here on this board. I put 2 * 19.5V in series to make 38V, they cost around £8 each, much cheaper than transformer. The problem is what comes out the output, namely huge voltages due to the Y2 capacitor, earth wire connected to V- for almost all of them except a very few and other nasties.

I'm absolutely disguisted by those cheap pieces of junk, had about half a dozen of those fail in the first few minutes after being taken out of the box and powered up.

[akis]

I do not know why you cannot make larger or two sided boards. I have little more than an old tile cutter and some plastic tubs and regularly make two sided boards. Today I am making a two sided board 210 x 135mm, it did not fit well in the small tubs so now it's etching in a bigger one in the oven at 50 C.

I could make a couple of boards for you, it's not that hard, as long as they are smaller than 20 x 15.

Drilling is the most laborious job however.

[void_error]

I do not know why you cannot make larger or two sided boards.

I could make a couple of boards for you, it's not that hard, as long as they are smaller than 20 x 15.

Drilling is the most laborious job however.

My photoresist drying box is made out of junk parts (free stuff) and I couldn't find a larger flat alu plate and adequate power resistors so I ended up having a 60mm x 120mm temperature controlled heating surface made out of two socket A copper CPU cooler baseplates soldered together using a heat gun. I'll probably build a larger one in the near future. So far I managed to make 0.4mm thick traces with less than 0.1mm spacing reliably. I'll mostly use SMD components because they're cheap and I don't have to drill so many holes.

If I'm going to make two sided boards I'll have to find a way to plate them and I've seen a few methods on youtube which seem to work and use readily available chemicals. I'd have to drill the holes perfectly perpendicular to the board though and I have yet to find a reasonably priced stand (or whatever it's called) for my dremel 

Sending stuff from Western Europe the where I live wouldn't be worth the money. Shipping is ridiculously expensive for no apparent reasons, even via snail mail.

[akis]

You're using much finer resolution and tolerances and different methods to me. I am at 0.7-0.8mm tracks and spacings and have not tried to go lower. I use the very old fashioned method : photo-resist, UV exposure, development, etching, drilling. The only "heat" required is during etching and I do that by boiling the kettle

[akis]

I bought this off ebay

http://www.ebay.co.uk/itm/UV-Exposure-Unit-for-Hot-Foil-Pad-Printing-PCB-etc-/290496322574?pt=LH_DefaultDomain_3&hash=item43a2ee1c0e

it arrived squashed, bent, broken glass, but it works (after I repaired it).

The UV tubes are not great, they have developed black patches neat the edges so if the PCB is too wide it will not expose properly near its edges.

I am now buying new UV tubes to replace the old ones.

[void_error]

You're using much finer resolution and tolerances and different methods to me. I am at 0.7-0.8mm tracks and spacings and have not tried to go lower. I use the very old fashioned method : photo-resist, UV exposure, development, etching, drilling. The only "heat" required is during etching and I do that by boiling the kettle

I use the exact same method. The photoresist is POSITIV20, and it says in the datasheet that for higher detail it has to be dried at 70C for 15 minutes.
Here's some stuff I made. The DIP14 IC is a LM324.

I got a 5m 12V UV LED strip from ebay for about $25, put 48 of them in a box, works like a charm. The whole thing cost me $30. The tiny PCB with a SOT23-6 and 0805 passives is my latest creation. 5V @ 50mA from a single AA alkaline cell 

[akis]

Do you have a close up picture of the UV box you have made? How far can you bend/twist those UV light strips, I presume you have made a serpentine shape?

[void_error]

Didn't bother bending it, used a PCB with some grooves cut for the power rails. Oh, and some black Duct Tape 

[akis]

So you bought a UV LED strip, cut it into strips of the right length and mounted it on a PCB with simply a V+, a V- and a polarity diode? Is that all?

Do you not have a glass panel to place the artwork and the PCB and something spongy to push everything down tight?

[Zepnat]

Hi, I've not read this thread but saw LM2596 mentioned and had to throw my two pence in. My experience is DO NOT buy them from china as they are weak and nothing like the genuine ICs.
I've bought 20 ICs and two regulator boards from ebay and aliexpress sellers and they have all exploded under test. Yes all 20 went bang.
So I bought an IC from CPC in the U.K. and it's indestructible.
In my experience if you've got a lm2596 from china any more than 1A out and 20v in, you are pushing your luck.
Plus they only run at 50Khz, the real lm2596 runs at 150khz I don't know how you can fake a IC but they're rubbish.

[akis]

I have bought a bagful of these http://www.ebay.co.uk/itm/5pcs-LM2596S-DC-DC-Buck-Converter-Adjustable-Power-Supply-Step-Down-Module-UK-/141210161794?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item20e0c81282

I have blown up 3-4 of them, unfortunately, and have pulled 2A out of them before they start to buzz, what's fair however is they do say they need a cooler for over 1A.

[void_error]

So you bought a UV LED strip, cut it into strips of the right length and mounted it on a PCB with simply a V+, a V- and a polarity diode? Is that all?

Do you not have a glass panel to place the artwork and the PCB and something spongy to push everything down tight?

That's pretty much it.

I'm using regular printer paper and make it transparent to UV with Transparent21 spray since I found out it works better than using overhead transparencies, especially because my printer sucks at printing on those (5 years old). I then put a piece of 2mm thick glass onto the coated PCB to keep the paper nice and flat, toner side down.
I'm going to need a flat surface inside the box to put the PCB on, but I'm too lazy to come up with something so I'm using an old HDD 

Hi, I've not read this thread but saw LM2596 mentioned and had to throw my two pence in. My experience is DO NOT buy them from china as they are weak and nothing like the genuine ICs.
I've bought 20 ICs and two regulator boards from ebay and aliexpress sellers and they have all exploded under test. Yes all 20 went bang.
So I bought an IC from CPC in the U.K. and it's indestructible.
In my experience if you've got a lm2596 from china any more than 1A out and 20v in, you are pushing your luck.
Plus they only run at 50Khz, the real lm2596 runs at 150khz I don't know how you can fake a IC but they're rubbish.

I'm getting most of my parts from here http://www.tme.eu/en/
Some of them are more expensive than elsewhere but I won't get ripped off.
I usually order them via my local parts shop, but only if I need small quantities and everything is less than $30.
For this project though, since shipping is $7 and parts arrive in 2-3 days, and all the components I need cost about $70 I'll go for a direct order as it'll be the cheapest way, the local shop will be more expensive.

[Zepnat]

Cooling them doesn't seem to make any difference, I tried soldering to a brass plate which was screwed to a heat sink but they still went bang.
While the genuine IC with 40v input and a 12v 55w bulb on the output and no heatsink, just cycled the output as the thermal protection cut in and out.

[void_error]

Just got to the part where I have all the main blocks figured out. Don't ask me why I have so many boards just yet.
    ---> Rectifier + Filter board
    ---> Regulator board - with all the power stuff + ADC & DAC
    ---> MCU board - with a PIC16F887 and some glue logic
    ---> Interface board - LCD display, 7 segment displays, controls

Now my question is regarding the feedback from the output plugs.

Should I separate the ground on the regulator board in two and tie them up at the output ground (negative terminal), and run a separate feedback wire from the regulated output terminal (positive) back to the board? 

I suppose it should improve regulation.

[akis]

What is in between the regulator board and the output jacks? Should not be very much.

Sampling the voltage right at the output socket is a good idea you can do it with a 100k resistor. Then you can also sample the voltage right on the load via say a 2 k resistor. When the two voltages differ due to power loss on the wires the 2k resistor wins. Ideally you would also have a ground wire yo sample voltage at the load and a differential amp.

[void_error]

I'll post a schematic in a short while, I'm currently working on it.

Edit: ...and here it is.

V_FB will be tied to V_OUT at the positive output terminal.
AGND will be tied to PGND at the negative output terminal.

[void_error]

I have finally breadboarded and tested the whole thing. 
No magic smoke has been released during testing. 

Unfortunately I couldn't get rid of the negative supply so I used a voltage inverting charge pump made up of a MC14584 hex schmitt trigger inverter, two 10uF caps and 2 1N4148s, running off the 5V rail. The output is only about -3.3V but that's enough because it only supplies is the base current for the two PNP transistors via the LM324 and 2 10k resistors.

Everything is really stable with only some 100nF decupling caps across the supply rails and output. I guess the breadboard's parasitic capacitances play a role here...

Haven't used the INA138 for current sensing, used a quarter of the LM324 as a differential amp and set the current limit to about 1A max. For the pre-regulator I used a LM317 with a TL431 as a temporary solution, everything bolted onto a rather beefy heatsink I took out of an old server power supply.

This is likely to be the final version and at the time of writing I was too lazy to redraw the schematic (It turned into a mess after I changed my mind about half a million times  ).

Now, for the good stuff: a pic of the test rig

[void_error]

I finally got to the part where I design the PCB and look for parts. All fine until I get to the inductor selection part.

Should I use a ferrite core inductor or a powdered iron core inductor? 

As far as I know from the LM2596 datasheet they should both work, although with a low loss core like ferrite ringing may occur when the regulator is operating in discontiuous mode, which will be most of the time, since I will rarely use it near it's nominal output current.

Also, the duty cycle will go up to 100% (at max output voltage and output current) which means the inductor will have to pass DC without saturating.

Damn it... I'm stuck. 

[dannyf]

-LM324-

it has a pnp input stage if I recall correctly. That means that it can swing to with 100 to 200 mv of the negative rail. Coupled with vbe or vgs of the regulator, it means that you don't need the negative voltage generator, unless you have to swing to 0v output.

it also works well as the current sensor here.

I would also use a pnp here in the current sense amp.