5volts @ 5amps, 12volts @ 2amps, 10-20volt input? - Page 1
Forum section: electronics / beginners

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: May 03, 2014, 01:15:27 pm
content: I have been pondering this for some time now.Basically, the idea is to build a switchmode power supply, inside an Amiga 600, to then run off an external laptop power supply.Now, I'd like to have a buck boost regulator for the 12volt, so it can accept a lower input voltage (such as a battery) but am considering scrapping this idea due to space limitations...Yes, it needs to be quite small, and quite efficient... and I've just gone around in circles, and ended up back where I started, with no idea what to use...Any suggestions for reasonably priced, easy to find, efficient, and simple to implement switchmode regulators would be fantastic!Thanks!

author: madires
author_rank: Super Contributor
timestamp: May 03, 2014, 01:24:15 pm
content: Maybe LM2678 for 5A and LM 2676 for 2A. Both are buck converters and easy to use (just follow the datasheet). If you prefer a buck-boost converter LTC1871 might be interesting, but it requires an external MOSFET and two inductors.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: May 03, 2014, 01:44:46 pm
content: Quote from: madires on May 03, 2014, 01:24:15 pmLM2678 was on one of my "maybe" listspossibly for both the 5volt and 12volt (just for simplicity in ordering parts, I'd get the adjustable version)I'm not sure how much heat these things produce though... I was playing around with a MAX787 earlier today, and with a 3amp load, on the size heatsink I can probably fit (not very big) I got the regulator/heatsink to over 100 degrees Celsius..

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: May 03, 2014, 02:23:05 pm
content: I ordered some LM2678's, both 5volt and 12volt...Guess I'll see how it goes!Is there any issue with running inductors in parallel?and.. what inductor should I be using for the 12volt regulator?its bedtime now!

author: madires
author_rank: Super Contributor
timestamp: May 03, 2014, 02:28:27 pm
content: Just checked TI's LM2678 datasheet and they claim that the output switch is a MOSFET with a R_DS_on of 0.12 Ohms. With a current limit of around 6A and a duty cycle of 50% it would be about 2W. With a laptop brick (19V) I'd assume 1W power loss for 5V@5A.

author: madires
author_rank: Super Contributor
timestamp: May 03, 2014, 02:56:06 pm
content: Quote from: AmmoJammo on May 03, 2014, 02:23:05 pmParalleling inductors is like paralleling resistors, i.e. L = (L1 * L2)/(L1 + L2). It's better to use a single one. Basically you follow the datasheet and calculate the values of the components. Besides the required inductance make sure that the inductor supports the current. I make my inductors myself using ironpowder toroids. Ferrite toroids with an air gap are fine for high switching frequencies (would make sense for the LM2678). Some say that you can use standard ferrite toroids without an air gap too, if you don't exceed the magnetic saturation. Maybe someone else can give us more details about that.QuoteHad a cup of coffee and a piece of peaches-marakuja cake 5 minutes ago. And the garden needs some work, harvesting weeds ;-)

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: May 04, 2014, 01:10:46 am
content: Quote from: madires on May 03, 2014, 02:28:27 pmThanks for that! sounds quite alright to me!I'll try to design a board to fit into the space I have available, could be interesting!Quote from: madires on May 03, 2014, 02:56:06 pmI was really only looking to parallel inductors to use what I already had lying around, but I reckon I'll go with some through hole parts instead, and buy the correct values15uH for the 5volts, and 22uH for the 12volt is what I've decided on. Now to find/wind some suitable ones!Thanks for the info

author: Psi
author_rank: Super Contributor
timestamp: May 04, 2014, 02:19:25 am
content: You might be able to find something here that's all ready to go.Some of they're stuff is quite small as its designed to plug directly into the ATX socket.http://store.mp3car.com/Power_Products_s/2.htmThey're intended for running computer motherboards in cars.Some have regulated sepic 12V out from 6-24V Others just do 12V passthrough and buck down the input to 3.3V and 5V at high current.You should be able to find something that can output 5V@5V and 12V@2A without much problem. I see this one does 12-32V input, probably not sepic though.You would be better designing your device to run from a battery bank above 12V when flat.eg, 4S lipo or something.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: May 19, 2014, 09:50:05 pm
content: Quote from: Psi on May 04, 2014, 02:19:25 amI had a look there, but have rather limited, and specific dimensions, so can't fit any of those unfortunatelyDoes anyone have any recommendations where to get inductors from?I've been looking at Coilcraft, specifically at the RFS1113-153ME (15uH, ~5amp) and RFS1113-223ME (22uH, ~4amp)However, they want about $30 for postage...

author: peter.mitchell
author_rank: Super Contributor
timestamp: May 19, 2014, 10:06:02 pm
content: Quote from: AmmoJammo on May 19, 2014, 09:50:05 pmCheck Aliexpress for generic LTC3870 boards, ones like this are pretty decent;http://www.aliexpress.com/item/LTC3780-buck-module-5-32V-input-2-13V-output-on-board-computer-automatically-ITPS-Power-Module/828359612.html

author: senso
author_rank: Frequent Contributor
timestamp: May 19, 2014, 10:37:52 pm
content: Quote from: AmmoJammo on May 19, 2014, 09:50:05 pmAsk for samples?They sent me a couple inductors free of charge last year, you can also order them from Mouser.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: May 20, 2014, 09:58:39 pm
content: Mouser want $39 postage.Why does it seem to be so difficult to buy 4-5amp inductors without having to pay more for postage than the items cost!

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: May 20, 2014, 09:59:43 pm
content: Quote from: peter.mitchell on May 19, 2014, 10:06:02 pmThat won't fit into my limited dimensionsIt appears to only be 12volt output anyway?

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: May 21, 2014, 09:53:44 pm
content: RS Components, free shipping, done.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: May 28, 2014, 09:58:50 pm
content: So I ended up with these inductors, but due to my space limitations, the one on the right won't fit....I assume there's no issues with removing the plastic base, and soldering the leads directly to the board?

author: SeanB
author_rank: Super Contributor
timestamp: May 29, 2014, 07:04:50 am
content: No, provided it is the same inductance and the same material. Just use a drop of epoxy or superglue to hold it to the board instead of relying on the wire leads to provide support for vibration.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: May 29, 2014, 07:57:30 am
content: Inductance will be close enough, even if it does change a bitDoes anyone happen to know how this base is glued on? Im going to try heating it tonight to see if the glue will soften.

author: SeanB
author_rank: Super Contributor
timestamp: May 29, 2014, 10:03:28 am
content: I have seen many and they use a thermoset adhesive similar to superglue to hold it on. You need to heat the bakelite for a while with a hot soldering iron until the ferrite gets hot then it will pop off.