Evaluation of a 40V/5A DIY lab power supply - Page 1
Forum section: electronics / projects

author: TheJC
author_rank: Newbie
timestamp: August 20, 2019, 09:00:03 pm
content: Hi everybody,I'm currently trying to build a lab bench power supply for myself.It should reach 40V at 5A. I will also try to control it digitally with a µC.So far I only designed the analog-power output stage of the supply I wanted to later add a Buck pre-regulator before the linear regulator.I just want to know if this design is viable or should I just scrap the idea and begin new.Thank you in advance for all your suggestions.

author: mikerj
author_rank: Super Contributor
timestamp: August 20, 2019, 09:25:32 pm
content: The INA169 has a minimum common range of 2.7v, so current sensing won't work at low output voltages. Better to sense on the unregulated side of the pass transistor.To get 40V out, the gate of the NMOS will need to be driven at 40v + Vgs at whatever current you are drawing (5 volts or so). The LT1013's maximum supply voltage is 30v so that isn't going to work.T1 introduces a lot of extra loop gain into the current regulator, so stability issues are likely. Additionally you are loading the U1B output with a capacitor to ground, something else likely to cause stability problems unless an op-amp is rated for this (and I suspect the LT1013 isn't).The 22uF cap on the gate looks pretty hefty, at best this is going to hurt transient response.

author: TheJC
author_rank: Newbie
timestamp: August 20, 2019, 10:41:46 pm
content: QuoteThe datasheet says it's okay up to a supply of 44V. But is only specified up to 30V. Anyway, I thought I try this opamp because I couldn't find a good alternative that is in stock and not as expensive. I originally had an eye on the ADA4522 that is better suited, as it is fully specified up to 55V.QuoteDo you believe it would be better if I would replace T1 with a diode? To only pull the gate voltage down when current maxes out.QuoteOk, might be too big. I will shrink it, it will oscillate with no cap there

author: David Hess
author_rank: Super Contributor
timestamp: August 20, 2019, 11:41:47 pm
content: Quote from: TheJC on August 20, 2019, 09:00:03 pmI suggest starting at a lower voltage and current to get a better idea of what works and what does not work.QuoteThe design is not viable. Scrap the idea and begin new.1. T1 adds uncontrolled voltage gain within the current control loop which will make frequency compensation near to impossible. Replace T1 with a diode and PNP emitter follower or use a different way to combine the voltage and current loops.2. Cascading current sense amplifier U2 with current error amplifier U1B makes frequency compensation more difficult and is not necessary in any case. Both functions can be performed in the same stage. (1)3. Output capacitor C3 is unusually small for a general purpose 5 amp supply but not for a higher performance one with faster current limiting. But a low output capacitance per amp requires careful attention to frequency compensation. Placing the current shunt in series with the output as shown actually makes this easier if AC feedback is taken from before the current shunt because it adds a zero (phase lead) to the frequency compensation just like the ESR of the output capacitor does.4. C1 is useless and will just cause tears.5. The LT1013 (and most operational amplifiers) will have difficulty driving the input capacitance of a power MOSFET output stage. Consider adding a power buffer to improve performance.6. C2 and C4 are attempts to meet outrageous frequency compensation requirements and should not be required.(1) Instead of using current sense amplifier U2 to create a ground referenced voltage representing the output current, do it in reverse and create an output referenced voltage representing the maximum current. In other words, level shift the current control signal to the output voltage level. Then the error amplifier for the current control loop just follows the output voltage and there is only one stage. (2)(2) Of course this does not allow easy read-back of the output current level in a digitally controlled design. I might use a separate current shunt amplifier as shown to make a ground referenced signal just for that although this makes calibration more complex.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 21, 2019, 07:49:20 am
content: The design Idea is no viable for several reasons. A point not yet mentioned: Using a high side shunt and instrumentation amplifier is very tricky, as the common voltage can be quite high compared to the small drop at the shunt. This is easy to cause problems at the high frequency end when the CMRR of even the good amplifiers get worse.A MOSFET as a source follower is tricky, as the trans-conductance goes up with current - so in case of a short the current can spike to very high values. As a follower a BJT is more forgiving, as current gain tends to saturate at very high currents, kind of limiting with worst case current.The concept with a transistor as a follower is easy for low voltage, but no longer simple at more than some 30 V, as the error amplifier has to drive such a high voltage.For the beginning I would start with a much lower power (e.g. 0.5 A) version, to limit the amount of smoke to escape.

author: TheJC
author_rank: Newbie
timestamp: August 21, 2019, 12:39:18 pm
content: Ok, first of all, thank you very much for your advice. I decided I'll try to design a smaller supply for like 20V and 1 amp and later step up to 40V/5A.The DC POWER SUPPLY HANDBOOK looks very interesting and I'll definitely take a look at it.Do you guys have any more good papers about linear PSU design or at least some general rules?

author: David Hess
author_rank: Super Contributor
timestamp: August 21, 2019, 02:47:41 pm
content: Quote from: Kleinstein on August 21, 2019, 07:49:20 amHP was fond of floating the entire control circuit to the power supply's output allowing operation on +/-15 volts or less even on their lower voltage supplies. This presents problems however for a digitally controlled supply unless the whole digital section is floated as well.

author: David Hess
author_rank: Super Contributor
timestamp: August 21, 2019, 02:49:31 pm
content: Quote from: TheJC on August 21, 2019, 12:39:18 pmTake a look at power supplies produced up until the 1990s by HP, Tektronix, Power Designs, Lambda, etc. They are fully documented and reverse engineering them from their service manuals can teach a lot.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 21, 2019, 02:58:58 pm
content: The follower version is kind of limited by the voltage range of the OP.The floating regulator that HP and many other current supplies use is more flexible. It can be used both for low and high voltages. It still is a little more complicated with the compensation. The main downside is that it needs a second supply for the regulator part. The other point is that the more current controlling output stage tends to need a little larger capacitance at the output.Schematics form older commercial supplies are a good starting point. Not just the expensive ones, but also the newer cheap Chinese ones - they tend to use the same principle, but simplified.To check the design it is a good idea to use a simulation, before actually solder.

author: tszaboo
author_rank: Super Contributor
timestamp: August 21, 2019, 03:02:46 pm
content: To add more problems: These shunt monitors exhibit large offset voltages, so CC mode wont work with smal lcurrents.The IRFP150 has about 1 K/W Rthjc, meaning that with a 25 degrees heatsink, you have a 175 degrees junction with 150W. So 40V/5A is pushing the limit a bit.The oring of the two error voltages probably wouldn't work the way you want it. The FET should be driven directly by a opamp, doing some kind of integration function (cap in the feedback) because 1K on the gate will slow it down too much.Try 25V/1A first, see if that works. You can do plenty with a supply like that.

author: David Hess
author_rank: Super Contributor
timestamp: August 21, 2019, 03:55:35 pm
content: Quote from: NANDBlog on August 21, 2019, 03:02:46 pmEven if you can get a single transistor to handle the power dissipation, it can be advantageous to instead use several smaller ones just to spread the heat load out over a larger surface area rather than concentrating it in one spot.

author: xavier60
author_rank: Super Contributor
timestamp: August 22, 2019, 12:28:49 am
content: The floating type design makes possible the best performance such as very low regulation dropout voltage and fast control of the output stage.Having everything referenced to the + output can be a little confusing I have found.Although it did result in easy off-line SMPS pre-regulation control with one of my projects.The link shows a - output referenced design. The BJT output stage is controlled by a voltage to current level shifter transistor. It has very stable control and is reasonably fast. It has 1V dropout.https://www.eevblog.com/forum/projects/linear-lab-power-supply/?action=dlattach;attach=751023

author: tszaboo
author_rank: Super Contributor
timestamp: August 22, 2019, 01:56:02 pm
content: Quote from: David Hess on August 21, 2019, 03:55:35 pmWhat I wrote there, you have to read it this way: Not possible, not gonna work.

author: TheJC
author_rank: Newbie
timestamp: August 22, 2019, 02:06:45 pm
content: Quote from: David Hess on August 21, 2019, 02:49:31 pmVery good idea, I already took a look at some old HP/Agilent ones. Keysight has a good page with a lot of old HP/Agilent supplies and also their Service manuals. Unfortunately, the easy ones are only for lower voltages. But still, they are very good to learn.Quote from: Kleinstein on August 21, 2019, 02:58:58 pmOh yeah I do that all the time, but sometimes you can't go around soldering or at least building it up on a breadboard.

author: TheJC
author_rank: Newbie
timestamp: August 22, 2019, 02:17:24 pm
content: Quote from: xavier60 on August 22, 2019, 12:28:49 amWould be definitely a better idea. Also, it would be then possible to implement David Hess idea of having the current feedback at the output level.I just tried to simulate the above circuit (only the CV part) and the output ripple seems to be massive. Idk if I build up wrong or its just the LTSpice simulation, I will have to take look tonight.

author: profdc9
author_rank: Frequent Contributor
timestamp: August 22, 2019, 04:10:14 pm
content: The MC34071 can have up to 44 volt supply. This may be useful if you want to push the output up to 40 volts. For example, I have these schematic in my archive I got off of another forum:You could probably push it up to 40 V and 5 A with the right component subsitutions.

author: David Hess
author_rank: Super Contributor
timestamp: August 22, 2019, 06:21:32 pm
content: Quote from: xavier60 on August 22, 2019, 12:28:49 amEven when they make more sense, amateurs often avoid floating designs because they require an extra low voltage transformer or secondary winding. In practice however, the extra complexity makes the rest of the design much easier.Quote from: TheJC on August 22, 2019, 02:06:45 pmI tend to simulate it *after* I get it functionally working. I have had too many problems with the SPICE modeling power pass elements accurately and at least for me, working out the frequency compensation for a first cut is faster with graph paper and pencil if the control loop is kept simple for good performance and ease of design. If I cannot work it out manually, then it is likely to have stability problems anyway.Quote from: profdc9 on August 22, 2019, 04:10:14 pmThere are a bunch of old but still available non-specialist operational amplifiers which support 44 volt operation including the LF355/6/7, LF412, LF441, LM201A, some 741 varieties, LT1001 series, LT1006 series, OP-07 series, etc.The supply voltages can be bootstrapped to allow the use of a lower voltage operational amplifier however this is tricky.

author: rstofer
author_rank: Super Contributor
timestamp: August 22, 2019, 06:49:53 pm
content: The problem with these DIY power supplies isn't with delivering 5A at 40V - as long as the input voltage is on the order of, say, 45V. No, the problem is with delivering 1V at 5A and dropping 44V*5A or 220W. It's the low output voltage that eats them alive.Almost all commercial supplies will have transformers with tapped secondaries and relays to select the tap based on the voltage setpoint. It's easy for manufacturers to get transformers made any way they want. Not so for the one-off hobby project.There are plenty of high side current sense amplifiers that are design for this kind of thing. It's probably better to buy one than try to recreate it.

author: David Hess
author_rank: Super Contributor
timestamp: August 22, 2019, 07:05:42 pm
content: Quote from: rstofer on August 22, 2019, 06:49:53 pmThe power issue is real but can be handled with brute force if necessary rather than design finesse. I am a believer in designing big linear power supplies by starting with what big inexpensive power transistors, heat sinks, and transformers are available and working backwards from there.

author: TheJC
author_rank: Newbie
timestamp: August 22, 2019, 08:29:02 pm
content: Quote from: rstofer on August 22, 2019, 06:49:53 pmI know this is why I later want to add a pre-regulator to keep the power loss manageable. I first wanted to get the series-regulator part up and running and afterward the pre-regulator.It's definitely a better idea to use a premade high side current sense amp to avoid the problem of the higher voltages and also the problem of matching resistors when using a standard opamp.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 22, 2019, 08:45:49 pm
content: AT least center taped transformers are easy to get. So switching between 2 taps is also a possibility. However it is tricky in the way it is done in the normal cheap supplies, as the step is quite large.One can use electronic switch over, a little like a class H audio amplifier - this is faster and thus needs less spare room. So 2 tap electronic switching may be as effective as 3 or 4 taps with relays.For something like 4 A at 40 V one would definitely like some ways to reduce the heat. For a small 20 V 1 A version one can get away without and do it brute force with just enough heat sink and an over-sized transistor.I would avoid high side current sensing - the extra amplifier adds delay and makes stability more tricky.

author: David Hess
author_rank: Super Contributor
timestamp: August 23, 2019, 09:59:42 pm
content: Quote from: Kleinstein on August 22, 2019, 08:45:49 pmA lot of old HP supplies change the rectifier configuration so the high range doubles the output voltage and the low range doubles the output current.QuoteAs I pointed out earlier, the high side sensing and error amplifier can be combined so only one operational amplifier is required. A lot of supplies do this for high side current sensing.

author: dom0
author_rank: Super Contributor
timestamp: August 24, 2019, 12:19:04 pm
content: Quote from: David Hess on August 21, 2019, 02:47:41 pmI think the obverse is true. Floating the control loop is annoying if you want analogue programmability, but is not much of an issue with digital control, since digital signals are easily isolated.

author: David Hess
author_rank: Super Contributor
timestamp: August 24, 2019, 09:08:15 pm
content: Quote from: dom0 on August 24, 2019, 12:19:04 pmIt applies to both but few people consider analog programmability today. Digital isolation is definitely easier.My Tektronix power supplies only allow analog voltage programming so no isolation is required for the current error amplifier which is floating.

author: TheJC
author_rank: Newbie
timestamp: August 26, 2019, 04:12:50 pm
content: Okay, I looked into quite a lot old schematics from Hp and others in the last couple days. And adopted some ideas into my new design.I referenced everything on the output instead of at the input, which made CC circuitry simpler.The only problem would be the generation of the +-15V rail. I thought about using a second smaller transformer for this.