Two smaller inductors in parallel instead of one larger inductor? - Page 1
Forum section: electronics / projects

author: timb
author_rank: Super Contributor
timestamp: February 15, 2014, 08:54:23 am
content: I'm working on a very space limited circuit (a microprocessor based boost-buck converter) where I need a 33uH inductor capable of handling at least 3.5A (we'll be charging various Lithium batteries at a maximum rate of 2A). I've found some inductors that meet this requirement but they're 8mm tall, which just quite fit. 20x15x5mm is what I've got to work with, so I'm wondering if I can use two 17uH inductors in parallel instead of the one bigger 33uH one?

author: AndyC_772
author_rank: Super Contributor
timestamp: February 15, 2014, 09:08:11 am
content: Inductors add like resistors. Put them in series.

author: peter.mitchell
author_rank: Super Contributor
timestamp: February 15, 2014, 09:41:52 am
content: As andy said, you need to put them in series - parallel increases their current handling, eg, sharing core saturation and resistive losses, series increases their inductance, eg adding more windings.Also, depending on the inductor, your planned configuration may not work as desired. If they are vertical toroidal inductors, and they are placed tightly, side by side, this may effect their performance - even though toroidal inductors have low flux leakage, the tight coupling can still cause issues.The dimensions given, is that solely for the inductor, or for the entire circuit? If it is for the inductor alone, two ofthesein parallel would be nice, they would fit the size requirements and they are shielded, so little issue of mutual inductance/coupling.

author: Niklas
author_rank: Frequent Contributor
timestamp: February 15, 2014, 11:00:37 am
content: Is it possible to adjust the inductance value to 22uH or 47uH? Digikey returns a couple of in stock alternatives that could be possible to use regarding size and current, but not inductance.http://www.digikey.com/product-search/en?FV=fff40003%2Cfff80013%2C4c000e%2C4c000f%2C4c0011%2C4c0038%2C1080006%2C1080007%2C1080008%2C1080019%2C108001a%2C108001f%2C1080031%2C108006e%2C1080072%2C1080074%2C1080083%2C1080085%2C1080086%2C1080089%2C108008c%2C108009d%2C10800a8%2C10800a9%2C10800ac%2C10800b0%2C10800b1%2C10800b7%2C10800bc%2C108011f%2C1080147%2C1080189%2C108018c%2C1080190%2C10801a1%2C108054a%2C108065c%2C108069d%2C1080853%2C1080891%2C1080893%2C10808da%2C1080911%2C1080914%2C1080943%2C1080964%2C108099d%2C108099f%2C10809ab%2C1080a41%2C1080a76%2C1080b00%2C1080b1f%2C1080b26%2C1080b85%2C1080bf9%2C1080bfe%2C1080c3f%2C1080c4f%2C1080c7e%2C1080c82%2C1080c8e%2C1080cc2%2C1080cca%2C1080ccf%2C1080cd0%2C1080d5e%2C1080d82%2C1080d91%2C1140003%2C1400008%2C1770004f%2C1770005e%2C1770009b%2C17700104%2C17700195%2C1770019f%2C177001a6%2C17700501%2C1770050a&k=inductor&mnonly=0&newproducts=0&ColumnSort=0&page=2&stock=1&pbfree=0&rohs=0&quantity=&ptm=0&fid=0&pageSize=25

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 15, 2014, 11:37:26 am
content: Quote47uH yes, 22uH MaybeFor switching regulator applications, increasing inductance is generally not a problem (and can reduce ripple), the only caveat being that for the same package size, a higher inductance will usually have higher DC resistance.If you stick to E3 (10/22/47) values, you will generally have a wider range of parts to choose from.

author: peter.mitchell
author_rank: Super Contributor
timestamp: February 15, 2014, 12:49:50 pm
content: Quote from: mikeselectricstuff on February 15, 2014, 11:37:26 amAlso, you get worse transient response because the inductor "carries" the load for longer. As you said, DC resistance will increase, this is a bit of an issue for Buck-boost, because in the change over between boost and buck you will essentially be using the inductor as a "jumper". Another thing with higher inductance in same size package, the saturation current will decrease.Quote from: AcHmed99 on February 15, 2014, 11:54:29 am6mm? I was under the impression his height limit was 5mm, from his prior dimensions.You are correct though, if you increase the switching frequency you can get smaller inductors and need less output filtering too.What kind of volume are you doing with these boards? Would there be few enough to hand wind toroids and lay them horizontally or enough to get custom wound ones from a supplier (or china)?

author: dannyf
author_rank: Super Contributor
timestamp: February 15, 2014, 01:09:17 pm
content: It depends in how the regulator works. S burst mode regulator will generate higher ripple under light load if a bigger inductor us used.

author: timb
author_rank: Super Contributor
timestamp: February 20, 2014, 01:45:02 am
content: Ah yes, of course, I'd put them in series!That's what I get for working so late.Thanks for all the advice guys, I always know I can count on you guys for good advice!In the end, I think using a faster switching speed is the answer. This is for an ultra-small battery charging board for the Spark Core microcontroller. The plan was to use an MSP430 to implement a software based switching regulator, but instead of directly controlling the switches I think I'm going to use an off the shelf boost/buck converter and just control the feedback input with a PWM output from the MSP430 (tied through an RC filter and comparator). It switches at around 2.7MHz, has integrated MOSFETs and uses a 1uH inductor.Ultimately that will afford me a much smaller footprintandallow a bit more safety. (Plus get the product out the door faster!)Hopefully that seems like a sane solution.