Choosing an inductor for a boost converter - Page 1
Forum section: electronics / projects

author: aiq25
author_rank: Regular Contributor
timestamp: September 15, 2019, 02:54:56 am
content: I’m designing a 5V/2.4A output boost converter, to work with a li-ion battery.In choosing an inductor, I can go with a approx 7x7 mm size inductor, with a DCR of 15-20 mOhm, versus 10x10 mm size inductor with a DCR of < 10 mOhm of DCR.I’m not sure what the trade off is in physical size of an inductor and lowering the DCR versus using a smaller inductor. The smaller inductor has more DCR but I’m worried a larger inductor might have more magnetic/AC losses. I would like to improve DCR but not if it means increasing AC losses. Can someone help explain if there is a con to using a larger size inductor in order to help lower DCR?

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 15, 2019, 09:37:33 am
content: Do both inductors have the same inductance and saturation current rating? If not (and likely they don't) then this isn't an apples-to-apples comparison.Note that off-the-shelf inductors need to be generously derated with respect to claimed saturation current if you don't want to accidentally blow up switches and scorch the board from overheating. I'm talking about maybe allowing a peak current of 5-6A in a part claiming a 10A saturation current.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 15, 2019, 11:03:51 am
content: No, physical size is not a good hint for AC losses.You need actual numbers for AC losses - some manufacturers give them, most won't.Otherwise, your only way is to test and measure.If the "typical applications" on the datasheet lists DC/DC conversion, and it's ferrite based, it'slikelythe AC losses are notcatastrophicallybad - but you still need to measure and verify.Say Inductor #1 may have 1W of DC loss and 0.5W of AC loss, while inductor #2 may have 0.5W of DC loss and 10W of AC loss, and inductor #3 may have 0.25W of DC loss and 0.25W of AC loss.A classical initial assumption that you double the total losses from the DC loss number (so that it ends up being 50%/50%) is something, but not sufficient.I almost exclusively use Wurth electronics inductors because they give real AC loss numbers through their web-based selector. Some others have similar tools as well.

author: splin
author_rank: Frequent Contributor
timestamp: September 15, 2019, 04:26:50 pm
content: Coilcraft have a really good tool that estimates the DC and AC coil and core losses:https://www.coilcraft.com/apps/power_tools/dc-dcPut your circuit parameters in and it will list suitable inductors which you order by price, losses, size etc. Click on the 'Total losses' entry in the results list to see a breakdown of DC and AC losses. .

author: aiq25
author_rank: Regular Contributor
timestamp: September 16, 2019, 01:27:53 pm
content: Quote from: MagicSmoker on September 15, 2019, 09:37:33 amNo, one inductor rated current was ~6A and the other 10A. You are correct it's not a fair comparison but I was trying to get an idea. Thank you for pointing out the peak current handling capability, I didn't notice how much I would have to derate them by. In my design I noticed for 15% peak-to-peak current + 10% margin, I also added 30% margin to the inductance to account for DC current bias, frequency, initial tolerance, temperature, etc... Know I'm thinking I need to add maybe 40-50% margin after looking at some datasheets.Thanks for the suggestions Siwastaja and splin. I was considering Coilcraft and Wurth initially because I have used them for other projects and really like their products but for this project the assembly process is a big factor for me. Seems like a lot of overseas PCB assemblers charge a lot more for Coilcraft and Wurth versus TDK/Panasonic, etc... (this is with the PCB assembler buying all the components). But I'm looking for the best performance, so cost will have to take a back seat.

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 16, 2019, 02:08:18 pm
content: As Siwastaja already hinted, (gapped) ferrite will likely have fairly low AC losses in this application even up to a couple hundred kHz, and Coilcraft heavily favors that material for their high current inductors. Coilcraft also gives realistic specs for their products and I can recommend them wholeheartedly. Wurth also makes good products, but they are much less popular in the US due to their relatively higher cost (they are imported from Germany).In contrast, the off-the-shelf inductors from TDK and Panasonic tend to be a bit more cavalier with their ratings, and note that any inductor with a powdered iron core (typically evidenced by being painted one or two colors) will likely exhibit considerable core loss at frequencies above 40-50kHz if flux swing is more than 50-100mT. Also, don't wantonly increase the inductance to counteract the effect of saturation; it is altogether better for loop stability, if nothing else, to instead choose an inductor with a higher saturation current rating. Note that most commercial off-the-shelf inductors define saturation as when inductance has fallen to 70% of the unbiased value.

author: aiq25
author_rank: Regular Contributor
timestamp: September 16, 2019, 02:13:41 pm
content: Quote from: MagicSmoker on September 16, 2019, 02:08:18 pmThanks. Yeah I will consider the ferrite core. My current rating is not based on the inductor current but the IC I’m using (which has a limitation of 6.5A).

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 16, 2019, 02:17:59 pm
content: Quote from: aiq25 on September 16, 2019, 02:13:41 pmYou are boosting a single Li-ion cell to 5V/2.5A max, right? So minimum input voltage is around 3.2V? Then a good solution would be 8uH * 100kHz, and the peak current will be around 5.4A, so a converter IC with aminimum peak current ratingof 6.5A (don't get sucked in by the "typical" values here) should work fine.

author: aiq25
author_rank: Regular Contributor
timestamp: September 16, 2019, 02:32:53 pm
content: Quote from: MagicSmoker on September 16, 2019, 02:17:59 pmI was considering minimum input voltage of 2.7V. I’m trying to use a TPS1236P, which is minimum current of 6.5A (nominal 8A).

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 16, 2019, 02:41:00 pm
content: Quote from: aiq25 on September 16, 2019, 02:32:53 pmYou're still okay as far as peak current goes at 2.7V minimum, but the only commonly available lithium chemistry that will tolerate being drained down to that voltage is LiFePO4. Doing this to most other lithium chemistries can cause permanent damage.

author: aiq25
author_rank: Regular Contributor
timestamp: September 16, 2019, 06:43:44 pm
content: Quote from: MagicSmoker on September 16, 2019, 02:41:00 pmYeah from my calculations I should have enough margin with the IC limit being at 6.5A.Can you explain a bit more on how you came to the 8uH number? I was thinking 3.3uH would be good.

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 16, 2019, 07:40:51 pm
content: Quote from: aiq25 on September 16, 2019, 06:43:44 pmI was assuming a current ripple ratio of 0.4 (that is, peak to peak current is 40% of average input current) and gave you inductance * switching frequency. You didn't specify switching frequency (indeed, you didn't even specify one in your reply above!) so I had to assume one. If you change the switching frequency then the inductance changes in inverse proportion, all else being equal.Note that operating in DCM (current ripple ration >2)is perfectly reasonable at this power level, but since that requires a peak current >2x the average input current, your chosen IC with a 6.5A limit won't suffice anymore.

author: aiq25
author_rank: Regular Contributor
timestamp: September 16, 2019, 08:06:18 pm
content: Quote from: MagicSmoker on September 16, 2019, 07:40:51 pmAh ok. Yes I missed that. Switching frequency is 750kHz. 15% current ripple on the input.

author: splin
author_rank: Frequent Contributor
timestamp: September 16, 2019, 08:16:44 pm
content: Quote from: aiq25 on September 16, 2019, 02:32:53 pmQuoteAssuming you mean the TPS61236P (google can't find a TPS1236P), you could take a radical approach to the problem - such as reading the datasheet:QuoteLooks like a good reason not to use a 3.3uH inductor. I used the coilcraft DC-DC inductor selection tool I mentioned earlier:https://www.coilcraft.com/apps/power_tools/dc-dcThe TPS61236P switches at 1MHz so I entered that and other parameters as shown in the attached. I limited the ripple current to 30% to get the tool to find inductors around 1uH, and used the filter to limit the size to 11mm x 11mm. The attached pdf contains the results.The cheapest inductor @0.32 is the SD43-102, 29.7mohms, 4.7 x 4.2mm, 549mW total losses of which the AC losses are 34mW. 549mW is 4.5% of the O/P.The MSS1038-102 @$0.62 is 5.4mohms, 10.5 x 10.2mm, 137mW losses of which 49mW are AC.All the inductors shown in the coilcraft results have DC losses at least double the DC losses; I guess most similar ferrite inductors won't be very different so I would use DC resistance as the primary selection parameter. Obviously you will have to measure the actual losses of your short listed parts.The TPS61236P switching frequency varies between 750kHz and 1250kHz so you would need to check the chosen inductor is suitable over the whole frequency range.

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 16, 2019, 09:58:03 pm
content: Quote from: splin on September 16, 2019, 08:16:44 pmHmm... OP definitely should have mentioned the part number and, ideally, that it wasn't a bog-standard switcher IC to give us some forewarning to check the datasheet ourselves; mentioning that sort of critical detail on post 8 is poor form, but sadly all too common around here.

author: aiq25
author_rank: Regular Contributor
timestamp: September 17, 2019, 02:33:58 am
content: Quote from: MagicSmoker on September 16, 2019, 09:58:03 pmI should’ve mentioned it, next time I’ll give more detailed. It’s just that I was asking this as a general question, applicable to all boost applications. I’m really not locked into a particular IC yet. I only found boost regulators from TI and Monolithic Power Systems that should work for my application, this is searching through Digi-Key. I’ll search again, thinking about Analog or Linear should have something.

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 17, 2019, 11:20:27 am
content: Right, well the trend among switcher ICs is definitely towards higher and higher switching frequencies and while that can minimize size, it often doesn't because AC losses in windings and core materials increase exponentially with frequency. For example, skin depth in copper at 1MHz is 65um, which is the same as the thickness of1oz2oz Cu plating on a PCB and limits the wire diameter to 35 AWG or smaller (if a significant fraction of the current is AC, as in a DCM choke or a transformer). The specific core material used is a lot more important at 1MHz, too - it has to be one of the advanced ferrites such as Ferroxcube 3F4 or TDK/EPCOS PC200, and even then you'll need to limit AC flux swing to 50mT or so to keep losses under control.And I'm not even mentioning all the knock-on effects you'll have to deal with as a result of even tiny stray Ls and Cs being meaningful at this frequency.So if you have a choice and you aren't too experienced at smps design - the latter seems to be the case based on the questions you are asking (hey, we all started off at zero knowledge) - then I'd aim for a switching frequency in the range of 100kHz to 400kHz, especially if you want to stick to using off-the-shelf inductors where you might not know the specific ferrite used, and which are invariably wound with a single wire or flat ribbon so there is no accommodation for skin effect.That said, the IC you had tentatively chosen doesn't require you to design the frequency compensation and feedback network, which I know can appear to be an overwhelming advantage to the neophyte smps designer; resist the temptation! I'm not a huge fan of TI's switcher ICs these days - they seem to be shitting out new TPS part numbers like a refugee with cholera - but an alternate part to consider that is altogether more flexible is TPS61089/610891. It's about $1 more but has adjustable frequency of 200kHz-2.2MHz, an adjustable switch current limit with a guaranteed minimum value of 7.3A, and a minimum input voltage of 2.7V.Of course, you could also use a much more common controller IC and an external switch, but that might not cost any less overall and be harder to get working so perhaps not the best choice, especially for those new to smps design.Oh, and a handy site I always recommend is:http://schmidt-walter-schaltnetzteile.de/smps_e/smps_e.htmlEDIT - removed unused asterisk; fixed plating weight vs. skin depth

author: aiq25
author_rank: Regular Contributor
timestamp: September 17, 2019, 02:51:58 pm
content: Quote from: MagicSmoker on September 17, 2019, 11:20:27 amI have done one SMPS design before (using Push-Pull topology) but all of the magnetic components were designed by someone else, so this is a good learning experience for me. I was aware of the issues with AC losses at the higher switching frequencies but you're right, I don't have all the knowledge yet.QuoteI have looked at this IC and was going to use it for another application. This design is my first where I'm trying to design everything myself, so I was looking to use something easy but that doesn't seem to be the case anymore. lolQuoteCost is not a major factor for me, at this point. Efficiency is more of a concern and the TPS61236P datasheet shows very high efficiencies, one of the reason I chose it.QuoteThanks, this is a great website!

author: aiq25
author_rank: Regular Contributor
timestamp: September 17, 2019, 03:00:52 pm
content: Quote from: splin on September 16, 2019, 08:16:44 pmThanks, I did not catch this.QuoteThanks. I will definitely check out this tool.