DIY precision lab power supply design, matches or beats Rigol DP832 (hopefully) - Page 1
Forum section: electronics / projects

author: TopQuark
author_rank: Frequent Contributor
timestamp: February 03, 2022, 07:47:18 am
content: As I have mentioned in my previous thread building a high performance dynamic load (https://www.eevblog.com/forum/projects/gt1mhz-bandwidth-electronic-load/), I am in the process of developing a precision lab power supply. From my testing so far, I think the final design can match or beat every spec in the RIGOL DP832 spec sheet, and I have the waveforms to show I can meet some of the specs with the current prototype.My design is based on the Analog Devices DC2132A dev board (https://www.analog.com/en/technical-articles/high-performance-portable-dc-bench-power-supply.html), I also took inspiration from Frex's excellent work linked here (https://www.diyaudio.com/community/threads/triple-outputs-160w-lab-psu-epsux3-version-2.263471/) and Analog Devices circuit note CN0508 (https://www.mouser.com/datasheet/2/609/cn0508-1877375.pdf).The DC2132A served as the basis of the design (although I blew up everything on the board and had to replace the LT3081 chip, use a Rigol DP832 to generate the -5v rail, used cheap chinese buck board for the switchmode pre-regulator)I chose a LT3081 based design as it can regulate down to 0V, stable and low noise with minimal output capacitance (very important, we don't want 1000uF at the output like the DP832), and has excellent voltage regulation. I also really liked the buck pre regulator in the DC2132A design.Based on that, I added digital voltage programming with the method described in CN0508and added a PI post filter for the buck pre regulator from Frex's designSo far, I have just borrowed ideas from existing designs, but there's still one issue that none of the existing designs has tackled adequately, which is precision, digitally programmable and fast current limiting. CN0508 and DC2132A gave up on digital control and just used a potentiometer connected between the LT3081 Iset pin and output, Frex's design uses a current mirror to inject a programmable current into the LT3081 Iset pin. The issue of relying on the LT3081 Iset pin is it has a +-15% tolerance over temperature, which means our design would not be able to match the DP832 current limit accuracy.My design relies on a external shunt, which generates a signal that is compared to the programming voltage signal by an opamp, which then drives the LT3081 Vset pin. This takes the current regulation loop out of the LT3081, and so we must minimize the number of opamps used in this loop. This means a relatively high value shunt (100mR), eliminating a gain stage, and have the shunt signal be directly compared to the programming signal. The power supply is floating, so we might as well do low side sensing to simplify the circuit. Some compensation is still needed to stabilize the loop, but the resulting control loop speed is still quite good with only one opamp in the loop.By introducing a high-ish value shunt into the circuit, together with the load sharing resistors connected to the LT3081, means we have no hope at matching the load regulation spec in the DP832 of 0.01%+-2mV. One could kelvin sense the voltage at the output terminal and close loop drive the Vset pin to compensate for that, but that would take the voltage control loop out of the LT3081 (which the LT3081 does extremely well), and at that point we might as well just build the power supply out of discrete power transistors.My solution to this problem is to utilize the Imon current source pin of the LT3081, and use it as a feed forward to raise the voltage control signal generated by the DAC. By calculating the expected voltage drop per amp of current draw at the output, we can add a resistor between the voltage setting opamp and the DAC, inject the Imon signal in between the resistor and the opamp, such that the control voltage input raises by (expected voltage drop at output terminal / gain of amplifier) V, compensating for drop across the output resistors. There's some delay between the Imon signal and the current draw, and there's some delay in the voltage setting opamp, so some compensation is needed there as well, but after figuring that out, this mechanism works really well as you will see in the waveforms.Currently I am still at the mess of wires, breadboarding stage, but I'll end up designing a proper PCB for it, and compare it against my Rigol DP832, and it's spec sheet. Also I am a noob when it comes to taming analog control loops and precision circuitry, so I'd be more than happy to hear your thoughts on the project.[ Specified attachment is not available ]

author: TopQuark
author_rank: Frequent Contributor
timestamp: February 03, 2022, 07:53:11 am
content: WaveformsCurrently there's only one LT3081 on the board after I blew up the original ones. There's 10uF at the output of the LT3081, and another 10uF at the output terminal after the load sharing resistors.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 03, 2022, 08:07:30 am
content: I don't like the idea of using voltage regulator chips for a lab supply. This is more a way for low performace as there are a few priciple drawbacks with this way:1) Voltage sensing is generally internally to the chip and not directly from the terminals. This adds to the output resistance.2) With the power stage and reference part in the same case the reference and amplifier part will get hot and thus adds to the drift / loading effects.3) The power handling capability is usually quite limited, especially with the more modern chips. Parallel connection is very tricky.4) The control loop for voltage regulators is made to work with a reasonably well haved load. A lab supply want stablitiy with essentially any resonably load. Those regulator chips are not magic and they can still oscillate with a difficult load.5) Most voltage regulator chips are not made for precision current regulation. Adding variable and precise current regulation as an after-thought is a real pain, often more tricky than designing a proper lab supply from scratch.

author: TopQuark
author_rank: Frequent Contributor
timestamp: February 03, 2022, 08:21:39 am
content: 1. Output resistance is fine, even with >100mR of output resistance, the load transient waveform shows the circuit can compensate for the drop in output voltage2. The voltage reference in this scheme is moved off the LT3081, the error amplifier offset drift in the LT3081 is acceptable (+-0.5 mV from -50 to 125C), also the LT3081 is only dropping 3-4W max with the pre regulator3. The LT3081 is designed and meant for paralleling, the datasheet shows multiple ways of doing it properly4. The step load in the transient load waveforms is not a easy load to handle, I can torture it even more with my dynamic load, but I do believe it will be fine5. I agree the current regulation is a pain, but it is manageable and the results are satisfactory so far, compensating for the current control loop was not as bad as I'd thought.

author: 2N3055
author_rank: Super Contributor
timestamp: February 03, 2022, 09:32:51 am
content: I'm gonna be that guy and ask: what exact parameter "matches or beats Rigol DP832"?Like Kleinstein said, the choices you made are not going into that direction, frankly...I respect enthusiasm, don't get me wrong, but it helps to have clear goals and path that takes you there.You seem to have taken manufacturers app note, and are trying to see what can be squeezed from certain regulator chip if you mix and match and tweak things here and there. If you want to get familiar with that chip and figure out things and edge cases for it that are not in datasheet, that is good approach.If you want to make a device that has to meet some target specifications, you start with target specification. Write it down. Then make a little research of different known art how you could go about accomplishing target specs. That will probably result in few different potential solutions, each with it's own set of PROS/CONS. You decide which set of PROS/CONS work for you and start with that design. At this moment you should already have a block diagram of device you want to make, and start doing implementation. This is choice of components and micro architecture time: shall I implement current sense amplifier this or that way. But you already decided that it will be low side, for instance... etc etc...Your topic would be better called "Musings and experimentations with LT3081 voltage regulator: how far can it go?"Nothing wrong with that, mind you. You will probably learn tons of useful stuff.

author: TopQuark
author_rank: Frequent Contributor
timestamp: February 03, 2022, 10:20:43 am
content: Quote from: 2N3055 on February 03, 2022, 09:32:51 amI mean my target is to match or beat all of the listed specs of the DP832. If you look at the wave forms, I have already matched voltage noise, transient response and load regulation. I did not choose my solutions half hearty, my first post is a condensed summary of what I have been doing thus far, and a lot of things that didn't work out was not mentioned, so it may seem like I arrived at conclusions out of no where, but I can tell you everything is considered carefully within my knowledge.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 03, 2022, 10:57:18 am
content: It looks like much of the regulation is from the extra 2 OPs with diode OR. So the LT3081 is more or less used as a protected expensive transistor.The performance so far looks good, but there are a few more points to check:A first point is the question if the output is really stable with a nasty load, like a large PP cap (e.g motor run capacitor) or a low ESR electrolytic cap (e.g. 2200 µF). There is a good chance that this does work, as the control loop in the LT3081 is separated from the load with the resistors for current sharing and the outer loop seems to be slow.For the current regulation part an inductive load (e.g. transformer, electromagnet, low loss inductor - make sure the protection diodes are in place) is the hard case.A 100 mOhms shunt is still relatively small and may thus limit the current regulation at smaller currents.For the performance one has to see that the specs for the rigol are upper limits, most units would be considerably better.For the load regulation on the LT3081 based version, the fast test does not include much of the thermal effects and the compensation of the drop may be nonlinear / temperature dependent. There is no surprise it works well under the condictions it is trimmed for. The real test would be with a different current / at a higher temperature.

author: 2N3055
author_rank: Super Contributor
timestamp: February 03, 2022, 12:11:22 pm
content: Quote from: TopQuark on February 03, 2022, 10:20:43 amCurrent limit shows 60A peak..... Or am I reading it wrong?

author: TopQuark
author_rank: Frequent Contributor
timestamp: February 03, 2022, 12:46:08 pm
content: Quote from: 2N3055 on February 03, 2022, 12:11:22 pmYea that's expected, the current is the readout from the load. Any single quadrant power supply with output capacitance will show that spike when shorted, it is just a matter of duration and what happens afterwards. Here's a zoomed in waveform of the current limiting in action. (Set to 1A)

author: TopQuark
author_rank: Frequent Contributor
timestamp: February 03, 2022, 12:51:39 pm
content: Quote from: Kleinstein on February 03, 2022, 10:57:18 amThanks for the feedback, adding 5 1000uF low ESR polymer caps to the output did reveal some nastiness. After tuning the circuit a bit more, here's the step response with and without the capacitor bank at the output. (Note the difference in time scale)

author: TopQuark
author_rank: Frequent Contributor
timestamp: February 03, 2022, 04:41:05 pm
content: Testing the current limiting function with an inductor between the power supply and dynamic load.Yellow trace is control signal going into the dynamic load, setting the current draw of the load.Blue trace is the current waveform as measured at the load.Green trace is the output voltage of the power supply.The two screen shots had the power supply current limit set to different levels, and the current limit does work properly, cropping the top off the sinusoidal load current waveform, despite the load current oscillating when not being limited.