First SMPS build, questions - Page 1
Forum section: electronics / beginners

author: JPortici
author_rank: Super Contributor
timestamp: June 30, 2016, 10:16:10 pm
content: Good evening/morning/day (depending on local timezone)I decided to get into SMPS, as i wanted to include a cheaper alternative for power in a project.so, my goal is to design and build a power supply which hasInput: Anything from 10 to 24 volts (a battery pack, laptop power supply, ...)Output: dual voltage, +12V@2.5A max, -12V@1.5A max. max current is not critical but please notice that it is a fairly unbalanced load. it could very well be +12@2A and -12@500mAneedless to say, the obvious choice was a flyback topology so i proceeded to build one around aLM2588i used one of the suggested examples with the addition of ferrite beads between sections and a pi CLC filter in the output, like soPower -> bead -> VIN -//-> VOUT -> Bead -> C-L-C -> Bead = 12Vthe LM2588 was used to getonlythe +12V rail. Correct me if i'm wrong but if i use a center tapped transformer (or one with multiple secondary windings) for a dual power supply i should keep the load as balanced as possible, otherwise only one of the voltages will be fixed, the other will vary with its load (relative to the regulated load).For the negative rail i wanted to use a buck regulator configured as inverting, powered from the +12V Rail. I tried to build one around a LM2576 but it doesn't work... the circuit should be correct (it follow the example in the datasheet and it is theoretically correct) but the power supply i use to test it goes into protection. from what i gathered in the datasheet it may need a crapload of current to first start... i will have to redo that for start but whatever, no problem i guess.The construction:I build one on a single sided copper board -really ugly board -> attached image-EHRR... I actually built three of them. First two were on a double sided board but the first time i got some component patterns wrong (confusing datasheet drawings) the second time i inverted the secondary winding polarity.. third time, i didn't have a double sided board laying around.Anyway, IT WORKS!! (the +12V converter)i had two 82 ohm power resistors at hand to test it, with a 300mA load and correct probing i am getting about 5mV peak-to-peak ripple -> attached image. wonderful!! will test with higher load in another momentthis has been a very educational project so far. it really teaches you about ground current path and component placing. The output capacitor for example: at first it was located next to the output filter inductor but the ground current path was a dead end alley. it introduced a 2Vpk ripple with a lot of oscillation. as soon as i moved it so that the negative pad was connected to another point in the ground plane, the oscillation disappeared, the ripple went down to what's in the image.The signal was measured with a tek 7704, 7A26 Vertical amplifier plugin. Probe set to 1x.Questions:- In the end what sets the maximum working current? Provided it's lower than the switching element maximum current (in this case, 5A) i would say it would be the lowest of the saturating current of the various inductors. am i correct?- The controller/inductor tends to get really warm, but it shouldn't (i have yet to test efficiency though). I probet the switch pin and this is what i get: oscillation then the switch goes off, at about 1.67 MHz... but i have no idea of why it is happening (layout related?) and i'm sure it shouldn't happen. I think this is why the switcher and the inductor get warmthank you for your patience

author: JPortici
author_rank: Super Contributor
timestamp: July 01, 2016, 06:09:02 am
content: Quote from: AcHmed99 on June 30, 2016, 11:18:23 pmI'm not sure of what you mean hereThe coupled inductor used was a coilcraft MSD1583 MEDhttp://www.mouser.com/ds/2/597/msd1583-220521.pdfI understand why you wouldn't use beads, however i noticed that the ripple was greatly attenuated after it, much more than using a inductor, even at higher currents

author: Zero999
author_rank: Super Contributor
timestamp: July 01, 2016, 12:25:51 pm
content: Quote from: AcHmed99 on July 01, 2016, 11:16:54 amI'm not seeing that either. Check your firewall settings.See the attached text file for the text obscured by the [email protected] tags.

author: mariush
author_rank: Super Contributor
timestamp: July 01, 2016, 01:03:51 pm
content: There were some mail to: (without space) tags in front of those, one is still there making the -12v@1.5A max a bogus linkThe antivirus or something detects those mail to : tags and tries to protect email addresses.

author: mariush
author_rank: Super Contributor
timestamp: July 01, 2016, 01:08:10 pm
content: Yeah, the forum adds a mail to : tag in front of whatever it thinks it's an email .. i didn't add the code in front of that -12v @ 1.5A max in my above post.

author: madires
author_rank: Super Contributor
timestamp: July 01, 2016, 01:09:59 pm
content: Quote from: JPortici on June 30, 2016, 10:16:10 pmI've build an inverting DC-DC converter based on the LM2575 (12V in / -12V out) and it needs a high inrush current when powering up. That's explained in the datasheet. In my case I've needed just 100-200mA and the LM2575 was a simple solution, besides having a few in my drawer. For the LM2576 and 12V input you get about max. 700mA output with an inrush current of over 5A. I'd suggest to look for another solution. You could add a winding for -12V to the flyback inductor.