Using multiple Inductors in Series / Parallel in a buck regulator - Page 1
Forum section: electronics / projects

author: mrpackethead
author_rank: Super Contributor
timestamp: December 27, 2014, 08:59:09 am
content: I have a design issue that i'm working through. I have a buck regulator that needs a 22uH inductor that is passing up to 18A. Power efficency is a big issue, as well as thermal considerations.. So, i need to use an inductor with a low DC resistance. That ends up being quite a sizeable component ( 22 x 20 x 18mm or so ), which doe'snt fit my packaging very well.I was thinking that potentially i could use multiple smaller inductors. ( which have a lower height profile - which is the main issue ) and create an equivalent inductance out of four or so parts. However i'm wondering what coupling effects might occur and how best to lay things out..Anyone out there have any experience with this.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 27, 2014, 06:16:07 pm
content: Coupling between inductors might be a problem if you choose unshielded (rod or bobbin) type coils, but will be almost negligible with shielded (molded powder, enclosed ferrite, toroid) types.Seems to me, more like 12 x 20 x 10mm should be achievable. Have you searched Digikey? Coilcraft?You might also consider a higher switching frequency; I'm guessing that's around 100-200kHz, which isn't too bad, but going up to 500kHz isn't unreasonable at that current if you're careful.Incidentally, if you simulate with multiple parallel inductors, remember to specify a realistic DCR. SPICE can't resolve parallel inductors with zero loop resistance (nor capacitors in series and no branch resistance -- beware RSHUNT generally hides this effect).Tim

author: mrpackethead
author_rank: Super Contributor
timestamp: December 27, 2014, 07:27:24 pm
content: Quote from: T3sl4co1l on December 27, 2014, 06:16:07 pmThats really helpful to know, i'm going to look at that, I would have almost certainly picked enclosed ones, anyway.QuoteThere are plenty of inductors at that size, but the trade off is the Rds. Efficiency is king in this application!Quoteits about 70khz. Certainly could run at 500khz, and reduce the inductance (and be able to make it physically smaller), but thats a trade off too, as you increase the loss's in the FETs when they turn on/off, and the driver as it needs turn more often.Incidentally, if you simulate with multiple parallel inductors, remember to specify a realistic DCR. SPICE can't resolve parallel inductors with zero loop resistance (nor capacitors in series and no branch resistance -- beware RSHUNT generally hides this effect).Tim

author: mrpackethead
author_rank: Super Contributor
timestamp: December 27, 2014, 09:22:02 pm
content: Winding my own, isn't any option.. this isn't a once off build!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 27, 2014, 11:52:45 pm
content: Wow, that's REALLY slow. I think you'll find it's more than worthwhile (for efficiency and cost) to attempt a higher frequency, even if only 200-300kHz.Tim

author: Neilm
author_rank: Super Contributor
timestamp: December 28, 2014, 11:45:40 am
content: Quote from: mrpackethead on December 27, 2014, 07:27:24 pmThere are some very good FETs that work at over 500kHz. Have a look at some of the main manufacturers. I have used one that happily switched 30A at these sorts of frequencies.

author: nctnico
author_rank: Super Contributor
timestamp: December 28, 2014, 12:12:59 pm
content: Quote from: mrpackethead on December 27, 2014, 08:59:09 amNo problem with shielded inductors. I have used several inductors in series and parallel in a few designs.

author: owiecc
author_rank: Frequent Contributor
timestamp: December 28, 2014, 12:16:45 pm
content: If you are brave you can go to really high frequencies with GaN.

author: mrpackethead
author_rank: Super Contributor
timestamp: December 29, 2014, 02:22:30 am
content: Quote from: 3roomlab on December 28, 2014, 03:45:18 amWurth.

author: mrpackethead
author_rank: Super Contributor
timestamp: December 29, 2014, 06:37:00 am
content: Quote from: T3sl4co1l on December 27, 2014, 11:52:45 pmThe gain in efficiency by running at a higher frequency is only that you can use a smaller inductance, which should mean a lower resistance. But you increase loss's, as you have increased switching loss and increased drive loss. In this application, i need to keep loss's to an absolute minimum as thermal issues are at play, so, i am reducing both the resistance of the inductors ( by using physically big ones ) and running at relatively low frequency. The reduction from say 300hz to 70khz, is a factor of 4.5, and the loss's in the fets, reduce significantly. I'm even running two FETS in parallel, to reduce the effective resistance ( and although i increase switching losses the overall effect is a net reduction in power ). Careful selection of the Fets, is also important.

author: mrpackethead
author_rank: Super Contributor
timestamp: December 29, 2014, 06:37:39 am
content: Quote from: nctnico on December 28, 2014, 12:12:59 pmthanks, i'm getting the same advise from several sources. :-)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 29, 2014, 07:43:39 am
content: Quote from: mrpackethead on December 29, 2014, 06:37:00 amNo, it's a knock-on effect:In an ideal Steinmetz core loss model, loss is proportional to both Bmax^2 and F^2, so increasing frequency while holding L and V constant reduces ripple current but does not reduce core losses.In a generalized Steinmetz core loss model, the exponents are variable. Real materials have different responses, but for almost all of them, it increases more sharply with Bmax than with F, so that a higher frequency (still keeping voltage and inductance constant) leads to lower core losses.If you have occasion to play with just pumping square waves across a ferrite core transformer, or any other kind of inductive component, this is pretty easy to see. I've seen ferrites with exponents something like 3 and 2 respectively, so the savings with F (V constant) goes something like f^(2/3). Adjusting the frequency on such a tester, you can really feel the increased core losses at low frequency, especially near saturation.The reduced losses at higher frequency mean you can get away with lower inductance (higher Bmax) for the same losses, with a smaller core and winding. (Of course, the smaller core will reach a higher temperature rise.)Copper losses are usually a significant part of the total, even if not indicated by traditional models (after all.. trying to reach any useful analytical conclusion about coil losses is difficult, and rarely generally applicable). The Q factor of a winding (in and of itself, not counting the core and losses) generally goes as sqrt(F), so that coil losses are lower at a much higher frequency, too.Making any sort of analytically reasoned argument is really pretty crappy though. You have to check out parts yourself to know for sure. I'd suggest trying out some numbers on Coilcraft's website, because they have a loss calculator that operates directly on their database search, very swanky. The numbers from some of their newer powder composite type coils are pretty slick, and very compact besides.Tim