Buying Power Inductors (~10-20A, ~100uH) - Page 1
Forum section: electronics / projects

author: rev0
author_rank: Contributor
timestamp: November 03, 2019, 01:25:34 am
content: Where do folks go to buy medium power/medium inductance power inductors? I've searched around the usual suspects (Digi-Key, Mouser, LCSC, Aliexpress) but they're either egregiously expensive, or just nowhere to be found. I'm looking for the approximate range of inductor that one would use in a 10-20A switching regulator, for switching frequency 100-200kHz for a DIY power supply. And I really don't want to be hand-winding in case I want to produce it in larger volume, like 10-100 units. So far I've found a few, for example I'm using this one in my custom 18650 tester for up to 6A, 180kHz switching frequency:https://www.aliexpress.com/item/32812527721.htmlFor larger sizes, I'm only seeing stuff like this, which I just can't imagine is priced reasonably (that suggests that the inductor on the common DPS5020 power supply is 18% of the total BOM, assuming no volume discounts of course):https://www.aliexpress.com/item/4000196600714.htmlThanks in advance for any help!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 03, 2019, 01:39:38 am
content: What the heck do you need so much inductance for, at such frequencies? Typical would be more like a tenth of that, or even less at those voltages.There is a paucity of products for high power levels; for that, you either need to stack up a bunch of COTS parts, or get custom parts made. Winding shops are modestly distributed, you can probably find one relatively local -- shop around; if nothing else, Coilcraft, XFMRS and others have national reach and can do it (if maybe not at the best price?).Tim

author: rev0
author_rank: Contributor
timestamp: November 03, 2019, 01:44:58 am
content: Maybe my switching converter design skills aren't up to task then.. I basically just sized it to make decent looking constant current for charging batteries in my tester.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 03, 2019, 07:22:22 am
content: I buy directly from Wurth Elektronik, especially prominent in Europe. Their products are available at Digikey & similar., but I get about half to 1/3 of the price when I buy directly - even just a few pieces. I.e., a $6 at Digikey part may be $3 in single pieces and $2 in production amounts (thousands). And the samples are free. I guess they won't sell to individuals, but are very helpful with small scale enterpreneurs, even if the orders are very small. Also, their design tools that automatically calculate for AC losses, are great. Yes, I'm a happy customer and will recommend them.Do note that with the COTS power inductors, there is a limit in size after which the availability of the parts suddenly stops, and you need to roll your own.100uH 20A sounds like you might be just over that limit. Consider higher fsw, allowing higher Iripple, or so to reduce the inductance. If still to no avail, consider multi-phase design to divide the current. With multiphase, you can allow higher Iripple per inductor and still have similar Vripple.Silicon is cheaper than ferrite (inductors) or aluminium (heatsinks). I did exactly this; had to design a 22->75V, 750W boost converter. Doing it in 4 phases allowed using very cheap COTS components (including $2 Wurth inductors x 4), and at the same time, spread the heat from MOSFETs and diodes to four distinct areas on PCB, making for easier heatsinking with less critical thermal resistance. The total board area isn't much more than it would with 1- or 2-phase solution (heatsinking area dominates, anyway), and no custom magnetics required. (Also, for such high conversion voltage ratio, the duty cycle is far enough from 50% so that ripple reduction by multiphase is a real advantage.)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 03, 2019, 08:22:45 am
content: Don't worry about ripple, batteries can handle it. Do worry about providing clean power; unfiltered outputs (and inputs -- it goes in both directions!) can do wacky things to attached and nearby hardware (including the circuit itself).Usual circuit is to put a big cap at the output, absorbing the ripple. The battery may have a low impedance still (so that the ripple current is shared in parallel between them), who cares. Or add another L (and maybe a C) to filter it completely.I guess you already have a current mode controller, which is the important part. You'll wrap that with a voltage error amp (so the current setpoint is driven by the voltage error), and that's basically all you need for a CC-CV battery charger. 4.2V and a few amperes is really all you need per Li ion cell. Very easily charged chemistry!In CC mode, the V amp saturates, delivering maximum current -- without blowing up anything, it just sits there happily at design-maximum output, no more, no less. In CV mode, the output voltage settles down to the setpoint and current is gradually throttled down (gradually due to the compensation R+C across the error amp).Bonus, you can change the setpoints/maximums and use it for general bench use.At this power level, it shouldn't really be necessary to use multiple phases as mentioned above, but if you're charging multi-cell packs, it gets attractive. Consider designing a PCB module for the constant current stage (current error amp, PWM modulator*, output switch, filter), building a few, then wiring them in parallel, driven by a common volt amp so they all track the same current.*Bonus if the clock signals are phase shifted, so you get the benefit of multiphase, but this isn't strictly necessary. There's still some advantage from using modest size modules in parallel, versus one large ungainly module.Tim

author: penfold
author_rank: Frequent Contributor
timestamp: November 03, 2019, 01:13:41 pm
content: I agree with the previous, sounds like you've just designed a bit conservatively, exactly what I can end up doing until I get a little wakeup-call like not being able to find the correct components ;-)Lead acid batteries will take a fair whack when it comes to ripple current a brief overloads, but to say that it an optimum way to treat them (especially for more modern battery geometries and plate chemistries) for a long life is debatable.If you do actually require such an inductor, there are plenty of sources of commercial inductors that have been pointed out, but you may find getting the exact one, especially when derating inductance for temperature and current etc, a bit tricky, so custom is often the way to go, if you're not too familiar with the design procedure for a HF inductor then perhaps pass on this one, unless the guys at Wurth Coil Craft etc will calculate and build you a one off sample, they can be pretty loose with their samples... especially if you hint that there may be more in the pipeline.Cheers,Dan

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 03, 2019, 02:22:54 pm
content: Quote from: penfold on November 03, 2019, 01:13:41 pmIf anything, at low to medium charging currents (as much as C/8 or so), ripple actually helps to break down sulfation.

author: rev0
author_rank: Contributor
timestamp: November 05, 2019, 01:00:56 am
content: Quote from: T3sl4co1l on November 03, 2019, 08:22:45 amJeez dunno why I didn't think of trying that, you've put me to shame. I went from a 47uH 10A inductor and 440uF of output caps giving me 6.1% ripple current to a smaller, cheaper combo of 10uH/22uF + 1.5uH/22uF giving me 1.4% ripple current. Still gotta build it to confirm, but I'm quite happy with these simulation results!Also, true that plain old battery chargers don't need to treat the batteries too nice, but I'm basically building a battery analyzer that outputs pretty graphs, so I appreciate having a low current rippleExample (processed) output of my current version below:

author: rev0
author_rank: Contributor
timestamp: November 20, 2019, 04:31:45 am
content: In case anyone is curious, I have built and tested the low inductance/capacitance filtered version and compared it to the original. It has no improvement at the very slow timebase level (2kHz sampling rate) which is dominated by PWM resolution, but at fast sampling rates (2.5ms and 50us timebase) it shows significant improvement, up to 3x less ripple current.