Two Questions - SMPS Buck Regulator and Inductor - Page 1
Forum section: electronics / beginners

author: Docara
author_rank: Regular Contributor
timestamp: November 22, 2019, 10:01:26 am
content: Hello everyone,Making 30+ boards for a basic (on/off) daisy chained lighting control system (RS485).I'm trying to find a very low cost, low part count and very low quiescent current Buck regulator that would be suitable (low ripple) for directly driving an MCU, differential driver and an automotive high side switch without the need for a secondary LDO linear Regulator or problems with noise.The application is a marine (automotive system) supplied from 12Vdc and the components supply will be 3V with an absolutely maximum calculated load of 250mA so I'm guessing a 500mA to 1A would be a good design value.Could someone suggest possible candidates.And secondly, other than the principle of inductance and inductors, I've not had any dealings with using inductors in anger. Other than basic current and voltage values, how do you go about choosing an inductor for, say, a noisy supply line application or similar or when to use a common mode device.I'm interested in line transients and supply filtering applications.Many thanks

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: November 22, 2019, 12:36:53 pm
content: Perhaps you could give a board with a MP1584 a shot. (about $0.25)For a quick test you could buy a ready build board for a few bucks on Ali or even find a local supplier when you want to have it tomorrow.There are plenty online conductor calculators, go for low ripple current through the inductor:http://schmidt-walter-schaltnetzteile.de/smps_e/abw_smps_e.html

author: Docara
author_rank: Regular Contributor
timestamp: November 22, 2019, 05:21:35 pm
content: Thanks for the reply JohnnybergI did see MP1584 on our favourite auction site the best board I found was £23 for 30Jeees Ali is cheap.Further to my question what sort of values would be acceptable for ripple? I knew ripple is an issue which is why I was going to put a linear after it

author: Siwastaja
author_rank: Super Contributor
timestamp: November 22, 2019, 08:41:50 pm
content: For powering MCU, differential driver (assuming digital communication), and general purpose power switches, any decent switch mode supply will do just fine. I see no requirements for low ripple. Just pick the cheapest, most integrated, most modern part and follow the appnote example circuit and layout.Input filtration and protection is the hardest part, especially if you need to care about load dumps. It may be easiest to pick a regulator that can handle up to about 100V input.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: November 22, 2019, 09:08:10 pm
content: I would use the proposed value of the online calculator.If you worried about noise, a extra LC filter at the output could help. And, of course, low ESR caps.

author: Docara
author_rank: Regular Contributor
timestamp: November 23, 2019, 10:02:51 am
content: All good advice thank you bothThe inductor part was more to do with my general learning. I know that inductors limit change and can 'store' energy but I don't know how to choose them in practice.What would be or how could I find out a desirable/suitable low ripple value to plug into the calculator?

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: November 23, 2019, 01:24:29 pm
content: if you are inexperienced in designing SMPS, just use the reference design from the datasheet:Or look how others done it:The reference design uses 10uH, the cheap board on Amazon 4,7 uH

author: Siwastaja
author_rank: Super Contributor
timestamp: November 23, 2019, 01:38:15 pm
content: You can calculate the inductance using some basic formulae and math, it's simple enough to do in Excel in 5 minutes, no complex modeling needed.First you need to decide thecurrentripple. This is how much the current through the inductor (and the output capacitor) changes during each cycle. Note that if your output capacitors are ideal, have infinite capacitance and zero ESR, current ripple is meaningless for output ripple voltage, which is always zero.From the definition of inductance, V(t) = L * di/dt, solve for L.We often decide the current ripple to be something like 25 to 50% of nominal or maximum output current. Say, for 1A output, di could be 0.3A.You know the frequency, so you know dt. Say for example, your input is 10V and the output is 2V, so in a buck the duty cycle is 20% on, 80% off. At 1MHz, t_on is 0.2 us, and t_off is 0.8 us. When the switch is on, the inductor sees 8V over it; when the switch is off, the inductor sees 2V over it as the current goes through the freewheeling switch (active or passive). All of this makes much more sense if you draw the switch control signal, and the expected triangle waveform of the inductor current, on paper with the circuit diagram.The hardest part for off-the-shelf inductor selection is the AC loss estimation or calculation. Data for this is never provided in any inductor datasheet, yet the range is orders of magnitude, some inductors are completely unsuitable for switch mode power supplies (say, results in 5% efficiency or an inoperational converter), some have nearly zero AC loss. The process is either:1) Some manufacturers give you a calculator tool (nowadays web-based). You input your converter parameters and get a result out.2) Luck and experimentation. You just buy an inductor and prototype it. Yes, really.#2 isn't too bad for a small converter, just pick aferrite-based,physically small, definitely shielded SMD inductor with enough saturation and thermal current rating, and it's likely good. Those look like small enclosed cubes of ferrite. I have never had bad luck with these.Saturation current of the inductor must be at least the maximum short-circuit current the controller will provide before entering protection (see the MAX column in the datasheet!), plus half of the current ripple. People often overlook this and make marginal designs using expected output current.And, modern converters are fine with ceramic output capacitance. For your low voltage ripple, just add enough ceramic output capacitance. With near-zero ESR, the voltage ripple, theoretically, reaches zero as you increase the capacitance. Paralleling multiple in small packages like 0603 or 0805 is good for decreasing stray inductance (which would cause part of the ripple to go past the cap).At some point, adding a second LC stage may become cheaper than increasing C. Additionally, using very small values of L,C (of lossy types) as a secondary filter may help you filter very high frequency noise, but this isn't called "ripple".The caveat is that most regulators enter pulse-skipping modes at low loads, effectively (from the output ripple viewpoint) meaning they work at much lower frequencies.

author: thinkfat
author_rank: Unknown
timestamp: November 23, 2019, 02:57:36 pm
content: I've had good success with TI Simple Switcher based converters, using their Webbench designer to lock down the components.

author: Docara
author_rank: Regular Contributor
timestamp: November 23, 2019, 05:04:43 pm
content: Quick UpdateHave stumbled on and probably decided on the TPS62172. They are 3.3V fixed output @500mA (max). Fromwww.LCSC.comthey are coming in at £0.38 and only need 4 components. 2x decoupling caps, 1 inductor and a 100K resistor.Via TI's Workbench they have given a schematic, a full BOM and a suggested layout design. They stated that ripple would is 6.6mV P-P at 3.3V @500mA output -is this a reasonable value especially as my absolutely worse case scenario is 230mA?I think the deal breaker was that if I have dropped a massive bollock I can just use the 5V version TPS62173 which is pin for pin compatible. There are others in the range which have different current and voltage options but needing an additional resistor, I will include a provision for this in the PCB design.Via Aliexpress I was finding I could get 30pcs of the MP1584 populated board (the same ones that JohnnyBerg has since posted here) for £0.45 + postage but had no control over anything obviously. I found a number of (albeit anecdotal) reports that these boards didn't follow the applications notes or component values causing instability).My thanks to Siwastaja this was the sort of info I was after.

author: thinkfat
author_rank: Unknown
timestamp: November 23, 2019, 08:55:34 pm
content: Quote from: Docara on November 23, 2019, 05:04:43 pmThe boards you get from Aliexpress usually don't even have a genuine switcher IC. For example, practically all boards with the LM2596 that are all over Aliexpress and Ebay have a different, relabeled chip that uses a different, lower switching frequency. Of course these don't meet the spec for the LM2596, but you only notice if you drive them hard.

author: Docara
author_rank: Regular Contributor
timestamp: November 24, 2019, 06:59:19 pm
content: yeah thanks for the heads up, I was beginning to suspect something was a bit suspect.