Need help with bench SMPS - Page 1
Forum section: electronics / projects

author: illusive
author_rank: Regular Contributor
timestamp: June 30, 2015, 11:52:47 am
content: Hello everyone, i need some advice building my first bench power supply.I decided to go with switch mode design because of the easier heat management and better current handling compared to the linear power supplies.I need somewhere around 25-30V output voltage and 2-3A output current.The voltage needs to be variable but doesn't have to go down to 0. Current limiting is preferable if it is easy to implement.I was looking into regulators like LM2576 and LM2596 because they come in TO-220 package and are easy to use on prototyping board. This way i can put better heatsink on them for better cooling and adding additional filtering compared with the modules from ebay. I'm not familiar with many regulators like those two, so if you know of some better chip for the job please let me know. I don't have problem with buying finished module form ebay if it is a good one.I would appreciate your help, thanks!

author: LDM
author_rank: Contributor
timestamp: June 30, 2015, 12:02:48 pm
content: Though there may be advantages to an SMPS, it may not be what you want to build for a bench supply project – especially a first bench supply project – as a linear supply would be much easier to design.If you're not really interested in actually building the supply, any off-the-shelf bench supply would be the way to go.

author: illusive
author_rank: Regular Contributor
timestamp: June 30, 2015, 12:58:06 pm
content: Well, i have built many lower power linear and switch mode power supplies so i have some experience with both sides. For the power supply that i need because of the heatsink i will need quite a large enclosure and probably a fan.

author: void_error
author_rank: Frequent Contributor
timestamp: June 30, 2015, 01:09:51 pm
content: You can always use a switchmode pre-regulator before the linear regulator to keep the dropout voltage low enough so you avoid using large heatsinks. 6-9W (2-3V @ 3A) is not that hard to get rid of.

author: funkathustra
author_rank: Regular Contributor
timestamp: June 30, 2015, 03:15:15 pm
content: 30V @ 3A = 90W, which is not a trivial thing to design. Since you're designing a mains-powered supply, you'll have to use an offline switcher design, not a simple buck converter like you mentioned.You'll need an offline switcher controller, some beefy external FETs, and a custom pulse transformer you'll have to wind yourself. Power Integrations makes some useful controllers and integrated switchers, and they have design software available for free. The big dogs all make controllers, too.The difficulty with designing bench supplies is the variable output and current-limiting. Wide output voltages at those currents may be difficult to achieve with a fixed transformer winding, since your only form of control is duty-cycle adjustment, which has limitations. Because of that, you may have to use multiple taps on your transformer, and switch between them with relays to get your 0-30V output.Most people expect their bench supplies to be two-quadrant, and that's a whole other thing you'd have to mess with to get implemented.I couldn't think of a more icky project to build. Can't you find anything more fun to work on?

author: illusive
author_rank: Regular Contributor
timestamp: June 30, 2015, 03:56:28 pm
content: I need some basic power supply so i can work more easily on other, more fun projectTo be honest i don't really need anything more than 24 volts and 2A. I have a 24VAC 3.33A transformer to spare and i thought i can put it to good use. After rectification it will be about 32V DC, and with a LM2596 step down converter it should give me what i need.I will mount the IC on a heat sink, but it is not supposed to get too hot anyway.I can live without current limiting for now.I have similar switching regulator and i tested it at 20V 2,5A, output, the wires get hot but the regulator barely warms up.What do you think? Will it be a bad design?

author: jlmoon
author_rank: Unknown
timestamp: June 30, 2015, 04:15:08 pm
content: Quote from: funkathustra on June 30, 2015, 03:15:15 pmI agree, can't you find something more fun to build/work on? I would look at the power supply market, they are pretty cheap and some are quite good too.

author: funkathustra
author_rank: Regular Contributor
timestamp: June 30, 2015, 05:09:15 pm
content: Quote from: illusive on June 30, 2015, 03:56:28 pmActually, this isn't a totally unreasonable way of building a hobbyist power supply. It's inefficient and bulky, but it'll do what you want it to do, and should be relatively easy to build.Post some schematics if you want us to look them over before you build it up!

author: illusive
author_rank: Regular Contributor
timestamp: July 01, 2015, 09:30:41 am
content: What will be the optimal inductor size for that voltage and amp range with LM2596? I'm thinking maybe like 100uH?

author: Yansi
author_rank: Super Contributor
timestamp: July 01, 2015, 09:57:03 am
content: Please start with something easier to get some valuable experience.The inductance value is not sucked out of your finger, you should calculate it.If you are wondering how: U = L*di/dt (this should help!)

author: void_error
author_rank: Frequent Contributor
timestamp: July 01, 2015, 09:59:26 am
content: Quote from: illusive on July 01, 2015, 09:30:41 amCoilCraft has a neatcalculator. You'll get the highest inductor value for min input voltage and max output voltage if I'm correct. Add +20% for the inductor tolerance and you should have your inductor value.

author: funkathustra
author_rank: Regular Contributor
timestamp: July 01, 2015, 03:54:10 pm
content: Quote from: Yansi on July 01, 2015, 09:57:03 amThat's not really a useful formula to use to derive the optimal inductor value for a buck regulator. It'd be like calculating LED series resistors using Maxwell's equations.Anyway, illusive's intuition is spot on. I'd use something around 100 uH, and bodge something else in if it's not working :-)

author: illusive
author_rank: Regular Contributor
timestamp: July 19, 2015, 11:00:32 am
content: Is there any good way of detecting the output current? I want to cut of the load in case of short circuit, i don't want to change fuses every time. I can't put a sense resistor on the input of the regulator, because the input current is not equal to the output current, and i kind of don't want to put the resistor in the output because of the voltage drop. Are there any reasonable priced hall current sensors IC's or something of that sort?

author: albert22
author_rank: Regular Contributor
timestamp: July 19, 2015, 01:42:12 pm
content: As you already have the transformer you can get a module like this:http://www.ebay.com/itm/LED-Voltage-and-current-display-LM2596-DC-DC-Step-Down-CC-CV-Adjust-Power-Supply-/150878659741The schematics are available somewhere on the web.Here is a thread which includes a review of a similar (cheaper) modulehttps://www.eevblog.com/forum/reviews/b3603-dcdc-buck-converter-mini-review-and-how-the-set-key-could-be-fatal/The datasheet for the LM2596 explains how to select the inductor.