Lab PSU from a 75W Single Output Switching Power Supply - Page 1
Forum section: electronics / projects

author: mmFooD
author_rank: Contributor
timestamp: September 05, 2017, 09:37:41 am
content: I have Mean Well AC/DC converter that I am planning on using in a lab PSU design. I have drawn up some preliminary thoughts and was hoping to get some input. I am doing it primarily to have a decent power supply. But I would also like to learn, especially how to make use of rotary encoders and lcd displays and how to design regulator circuits.Some specs:Input:12VDC 6AOutputs:0-30V adjustable (~2A )12V fixed5V fixed3V fixedI have both 50k 10-turn pots and rotary encoders availible for setting current and voltage on the adjustable output. It would also be nice to have a 1x16 LCD showing the values of at least the adjustable.Some questions:Is it a good idea to first step up the 12V to 30V and then have the CC and CV circuits or is there a better way?What regulators do you recommend for the different blocks?anything else I should consider?

author: Yansi
author_rank: Super Contributor
timestamp: September 05, 2017, 09:48:46 am
content: You should definitely have the CV/CC regulation in one circuit. This will suck.

author: mmFooD
author_rank: Contributor
timestamp: September 05, 2017, 09:51:55 am
content: Quote from: Yansi on September 05, 2017, 09:48:46 amI'm not sure I understand what you mean by "one circuit".

author: Yansi
author_rank: Super Contributor
timestamp: September 05, 2017, 10:10:12 am
content: You have a constant current circuit followed by a switchmode voltage regulator (buck). That ain't gonna work right!You should use the buck to do both the CV and CC regulation.Or even maybe use a single buckboost convertor for the whole path, omitting the standalone 30V booster.

author: mmFooD
author_rank: Contributor
timestamp: September 05, 2017, 10:13:23 am
content: Quote from: Yansi on September 05, 2017, 10:10:12 amYes that would be better. Is it possible to have a "buckboost" converter to be set by a pot / encoder to 0 - 30V from a 12V input?

author: Yansi
author_rank: Super Contributor
timestamp: September 05, 2017, 09:30:55 pm
content: Why not? Depends on your design skills. This should be nothing complicated. Design steps for the buck-boost are well known, then there is a plenty of suitable current mode PWM controllers, the CV/CC can be done using two simple opamps. The whole regulated PSU could be possibly done using just two 8pin ICs.I can only draw you a simple principle schematic for inspiration and put some ideas around.

author: Yansi
author_rank: Super Contributor
timestamp: September 05, 2017, 09:59:27 pm
content: So here's one idea and very crude cruel drawing.A simple non-isolated buckboost converter with a 1:1 ratio transformer, using a shunt capacitor C2 to supress the leakage inductance. Rs1 being the current shunt for the currentmode control. (Don't forget to add slope compensation, you will be doing here over 50% duty cycle here most of the time (every time the output goes over approx 10V)). One opamp (the upper one in the diagram) does the current loop, the bottom one does the voltage control loop. X1 and X2 are the compensation networks. Rf2/Rf1 being the voltage feedback divider. Rs2 being the current shunt for the output.The circuit might look somewhat complicated at first glance, but really it is not. Just imho a somewhat clever way of doing it simple.I'd recommend using a bog standard UC3843 PWM control IC for the converter, running at maybe 40 up to 60kHz. The transformer bifilar wound on an iron powder core like T106-26. The two opamps need to be the "commonmode voltage includes ground" type, LM358 suffices here. The summed output may be forced directly into pin 1 of the UC3843, having the internal error amp blocked and used as a current source (pin 2 to GND).EDIT: Note there is a mistake, with this polarity of the feedback loop, the diodes should go the other way round.

author: mmFooD
author_rank: Contributor
timestamp: September 06, 2017, 09:14:17 am
content: Nice! Thanks, will take a look at this.What is the purpose of the transformer? Is it to replace the inductors? The T106-26 seems to be just the core, are you implying that I should wind them myself? In that case how many turns should I go for?Quote from: Yansi on September 05, 2017, 09:59:27 pmI have a bunch of lm324 which is declared to have commonmode voltage including ground as well. Will they work as well?

author: Yansi
author_rank: Super Contributor
timestamp: September 07, 2017, 12:41:50 pm
content: To replace what inductors? We haven't talked about replacing anything yet. The transformer is just how the buckboost works. There may be some two separate inductors topologies (that is the SEPIC converter), but the coupled inductors may have some advantages.LM324s are fine. They are almost the same characteristic-wise.

author: Yansi
author_rank: Super Contributor
timestamp: September 09, 2017, 01:21:58 pm
content: At 60kHz with that T106-26, for your converter, I'd wound like 35 turns (~100uH). That will yield a reasonable primary current ripple of under 2 amps p-p.Wind the inductor bifilar (two wires at once). The primary one I'd recommend 1mm diameter, secondary I'd use the same. Maybe, you could get away with 0.8 wire, the current density of the effective currents I calculated still seems reasonable. You need to try. Always you can switch the iron powder core for a proper ferrite gapped one, or to use the SEPIC topology with non-coupled inductors. But I think you will need larger inductance in the SEPIC topology and getting hundred(s) of uH with the saturation current well over 8A won't be easier either.Maybe I'll test the above suggested topology myself

author: Zero999
author_rank: Super Contributor
timestamp: September 09, 2017, 08:15:22 pm
content: 12V can be a straight pass through, with some passive filtering components: capacitors, an inductor and damping resistor, ferrite beads etc.The 5V and 3.3V fixed outputs, just use buck converters.Do all the outputs need constant current? The converter ICs will have some rough, possibly non-adjustable, current limit built-in. The current limit on the 5V and 3.3V sections, could be a latching cut-off, with a delay. The 0V to 30V will probably require a constant current mode too, which is challanging, especially suppressing glitches and oscillations, when changing between constant voltage and current modes.The 0 to 30V is more tricky. You could use a SEPIC, Cuk or buck-boost converter because the output can be greater, as well as lower, than the input. You may also want lower noise, because you may want to use it for powering more sensitive, analogue circuitry, than the 5V and 3.3V rails, which are more likely to be digital. A hybrid switched mode + linear regulator design might be the most appropriate choice.