Inductor specs and current limit for buck converter - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: March 18, 2022, 03:55:09 am
content: Hi,when looking at a buck configuration of the MC34063 (Figure 10 of this datasheethttps://www.sparkfun.com/datasheets/IC/MC34063A.pdf) the output current is shown as 500mA with a short circuit of 1.1A (and the switch is rated at 1.5A). What determines the short circuit current? Is it Rsc? If so what is the formula?Couple of other questions:- how do you choose the inductance value?- what happens if the inductance value is too high or too low?- what is the criteria for the inductor's current rating? Just higher than the short circuit current?Thank you

author: PartialDischarge
author_rank: Super Contributor
timestamp: March 18, 2022, 05:33:41 am
content: Yes it is Rsc determined by the Vipk voltage thresholdFor determination of L read one of the many documents explaining it, like this onehttps://www.mouser.de/pdfdocs/BuckConverterDesignNote.pdf

author: Berni
author_rank: Super Contributor
timestamp: March 18, 2022, 06:07:44 am
content: In practice i resort to Wurth Electronics online calculator tools to calculate my switchmode supplies:https://redexpert.we-online.com/redexpert/#/home/article/power-design-toolsNot trying to promote Wurth here, they obviously made this tool as marketing to sell the inductors they make, but it is one of the better tools out there since its simple to use and gives you quite a bit of nice information like calculating the power losses(and thus efficiency) and providing some nice comparison graphs.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 18, 2022, 06:27:39 am
content: Not much, because that thing fakes it. If you must, go with NCP3063, which has a real current limit. But current mode converters can have lower ripple and higher operating frequency than these hysteretic controllers.Tim

author: magic
author_rank: Super Contributor
timestamp: March 18, 2022, 06:33:11 am
content: I'm surprised T3sl4co1l didn't tell you to just smash it with a hammerRemember that MC34063 and all its offshoots with darlington pass transistors have horrible conduction loss in buck mode: 2×Vbe + Vce(sat), or about 2V. 500mA would be pushing it, particularly at low input voltage and/or if you use the SO package. And there is no thermal shutdown in this thing.So that could be one reason why they rate it for 0.5A, or maybe that's the maximum current their demo circuit can deliver without sagging.To some extent maximum current is regulated by the value of Rsc, but people say it doesn't work very reliably in the original MC34063, so maybe that's why short circuit current is twice the nominal load current. I dunno, I have never used that crap myself.1.5A is the maximum safe current trough the built-in transistor.

author: ricko_uk
author_rank: Super Contributor
timestamp: March 18, 2022, 07:13:42 pm
content: Thank you allTim and magic,that's very interesting and useful!@Tim,you mention "...If you must, go with NCP3063...". It sounds like even that one you suggest is not a favourite part of yours...What "common" parts/families would you then suggest/use if you had to implement asafedesign where the IC is generally available from multiple vendors and have:- thermal protection- current limiting- perhaps other safety features?? What would they be?The input is 24V the output 3V3 and 5V (two separate PSUs) with up to 1.5A o/p current.Thank you again

author: magic
author_rank: Super Contributor
timestamp: March 18, 2022, 07:31:33 pm
content: Sorry, SMPS design is not my thing.I just happen to be familiar with MC34063, having encountered it in the wildBTW, I got the voltage drop wrong: of course it's less than 2V, something like 1.5V at moderate currents. I remembered the 2V number because that is given in the datasheet, for full 1.5A current.Good news is that at 24V input power dissipation is lower due to duty cycle.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 18, 2022, 07:49:39 pm
content: The general recipe is trivial:1. Pick a controller which hasswitch current limitaka cycle-by-cycle current limit. Usually based on sensing the voltage drop over the integrated transistor; or, some chips allow you to use external shunt resistor.2. Look at the value of this current limit;worst caseaka "max". This is tabulated in the datasheet.3. Pick an inductor with saturation current equal or larger.That choice of controller fails on the first step already. Just say no. Actual current limit is pretty much mandatory to have, IMHO. I mean, you can definitely drive a car which lacks one of the four wheels if you have no choice and know how, but it will fail in certaincorner casespretty easily!After this, you don't need to worry about blowing up the controller. Now the value of inductance simply controls the rate of change of current (see formula of self-inductance: V = L di/dt; arrange as you wish). This means, smaller inductance causes larger current variation during the switching cycle - this is, larger ripple current.If you assume CCM, you can work it all out from just the basic inductance formula! Calculate duty cycle D = Vout/Vin, draw the triangle ramp of inductor current, assign on-time and off-time, and knowing average (output) current, work out how much higher and lower the current needs to go, given the time available. This depends on the inductance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 19, 2022, 03:40:33 am
content: Offhand, TPS54233 comes to mind, though a higher voltage limit would be preferable. Some in the MP24xx series also quite reasonable (though they're not well stocked right now).There are tons of parts in those families, and others (TI's SimpleSwitcher, D-CAP2, etc.) worth a look, and various from other MFGs. Diodes Inc., Alpha & Omega, onsemi, Renesas, Richtek even (though they don't have many in these ratings).Let's see...Looks like onsemi is mostly THT parts in these ratings, Microchip too.Oh, MP9842GL-P has stock at DK actually. Not too much else from MPS stocked there right now but this one looks useful.Huh AOZ128x were poorly stocked (or discontinuing?) some years ago, maybe it was the x=0 or 1 I'm thinking of, but the x=3 shows stock and is useful here, nice.And of course LT/AD, which I tend to skip over due to cost. Like, MAX20077ATCB/VY+ starts at $3.13/singles, and they quickly go up into the >$6 range from there. And Maxim is a recent acquisition, not even their core AD/LT families. Lots of fine parts, and lots more oddball/specialty/niche parts at that -- but unless you've painted yourself into a corner best served by one of those niche parts, you're probably better off sticking with the cheaper sources.Tim

author: ricko_uk
author_rank: Super Contributor
timestamp: March 19, 2022, 09:03:12 pm
content: Thank you Tim and Siwastaja!