Diode & inductor selection for MT3608L - Page 1
Forum section: electronics / beginners

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: August 02, 2022, 11:10:03 am
content: I'm interested in using the Aerosemi MT3608L boost converter IC. Data sheet is here:https://datasheet.lcsc.com/lcsc/2201121530_XI-AN-Aerosemi-Tech-MT3608L_C2932326.pdfThis is my first time making my own boost module so I have some basic questions I would appreciate help with.Diode section states:QuoteIs there any harm in using an over-rated component, something like a SS54 Schottky diode?Inductor section states:QuoteI kind of want to buy some toroids for experimentation and some SMD inductors for compactness in the finished product. Is it possible to use toroids on something this fast and if so, what ballpark toroid material and wire gauge (I'm aware of skin effect) should I be looking at? With respect to SMD inductors, what is considered "low DCR" and what is typical for saturation current safety factor?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 02, 2022, 12:11:20 pm
content: Yes: a bigger diode has more capacitance, which will cost more switching loss, particularly in hard switching (CCM).Hand wound inductors aren't really interesting here, at least not that I would think, offhand. These are small parts: 2.5A peak and 4.7-22uH easily fits in a 6mm cube. Taiyo Yuden NRS6045 and related, for example, would be fine here. Can just get a range of values and try each?The whole thing will be SMT (or, most of it, anyway? the regulator, and diode by the sounds of it, at least..), so this should be fine. If you're not comfortable with SMT yet, do consider picking up a hot air machine, paste flux, tweezers, and anything you might need to support your hands or vision if applicable.Just a note, if you're doing your own protoboard, pay very close attention to the switching loop, i.e. SW and GND pins, diode, and output cap. This loop cannot be over even 1cm in length. Not for regulators of this speed. Use ground plane design, ground stitching vias, and it should be fine.Anyway, if you must, #8 powdered iron from Amidon is probably the one to choose. A_L is quite small so I hope you're patient with winding.Example:T37-8/90A_L = 12nH/t^2 means sqrt(4.7 / 0.012) = 20 turns, not too bad at least. And 2x turns gives 4x inductance, or 40 turns needed for 22uH, etc. If you find losses seem high, you can try #2 mix as well; but A_L is even lower still(!).You might also find a better time just doing air core: free standing, or on a paper tube or whatever. As a means of experimentation that is, of course. An excellent calculator here:https://hamwaves.com/inductance/en/index.htmlAlso, for point of reference: 26 AWG should be sufficient for this current. 24 is a bit on the big side but will fit alright; anything bigger, you won't get enough turns on that core. Anything finer (28+) you can always use multiple in parallel, twisted together (ala litz cable -- note the strands are individually enameled!), which will get lower AC losses than solid wire of the same cross section.Note that compensation depends on inductor and output capacitor values (internal compensation is fixed!), so when you're varying them,check the output load step responseto see the effect. Remember to check up to rated output current, if it should cause it to enter CCM, as response changes significantly in CCM.Tim

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: August 02, 2022, 05:23:26 pm
content: Quote from: T3sl4co1l on August 02, 2022, 12:11:20 pmThanks for the quick and detailed response. Will be sure to pick out an appropriately sized diode, something a bit smaller I guess. Given the high switching speed, is it even worth trying to perfboard/dead bug just as a proof of concept before I try my hand at a PCB layout? And when you say step response, do you mean anything more sophisticated than applying various resistive loads and seeing what the voltage regulation looks like? I'm somewhat limited by my test bench (still a beginner): voltmeters, DS1054Z, and some wire wound resistors.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 02, 2022, 06:46:49 pm
content: Deadbug is okay, if you can connect to the poor thing well enough...I've done it before, not terribly tricky to make two cuts close together to connect to the middle pins -- but it takes some practice, and a lot of care.For reference, I usually do modest one-offs (up to say 50 parts) as hand-carved copper clad:This doesn't use SOT23-6s but does have regular SOT23s with between-pads traces of the same width required.I use a nice sharp utility blade (get the coated bimetal type) and make a V-groove by angling the blade back and forth in subsequent cuts. A small burr is left; flatten out by burnishing, scraping or sanding.Vias can be made by drilling thru holes and clinching, then soldering, a wire link through.I never understood the, I guess traditional?, "deadbug" approach, like actually flipping and gluing parts---what a confusing mess. Maybe it was better back in the days of multi-pin TO-39s and such, but especially what a flimsy mess that is for SMTs, with such thin and tightly spaced leads.When I use DIPs this way, I prefer to cut a small rectangle of copper clad, scribe 0.1" centers on it, and use a hacksaw blade to carve slots between pads. Then bend the leads out and gull-wing SMT the DIP:Or another, more topical example:TPS54233 with aux winding, 12V in, +/-5V output I think it was? Works fine. Same current, though a bit slower switching.Regarding step loading: it's the step that matters. Load current was X, suddenly it steps to Y. What's the voltage/current doing in the microseconds surrounding the event? Can be as simple as fiddling with the resistors by hand (manually actuated, you might say), or an actually-good excuse to use a 555 timer, say to drive a MOSFET switching a load resistor.Tim