Good resource for learning about building high output SMPSes - Page 1
Forum section: electronics / beginners

author: Red Squirrel
author_rank: Super Contributor
timestamp: August 20, 2016, 06:00:27 am
content: I'm interested in learning more about power supplies, AC-DC converters, and DC-AC converters. (inverters)Whenever I read stuff I always get hung up on the math, as it can get complicated and math is just not my thing. Is there a resource out there that explains the concepts in plain english, like, "the bigger you size this component, the more more current it can support, but nnn happens so you need to do mmm to componsate" etc. Typically if you just throw a formula at my face I have no idea what to do with it, what numbers go where etc. Do I put the value as miliamps, as amps, as milihenries, as henries? etc I imagine it's impossible to get away from the math, but just looking for a resource that might explain it better.I want to start with smaller capacity PSUs, probably a bench one as a start (though SMPS is probably not best bet for that but would make a fun project anyway) then move up to big ones, like say, make a 48vdc one that can handle 100 amps or even more crazy stuff. My guess is it just involves adding more mosfets, and sizing all the other components appropriately, and/or paralleling several smaller ones and using current limiting so they self balance, etc. Eventually I want to get to a point where I can make them reliable and safe for every day use in some setups, but for now, just want to do it for fun.

author: Simon
author_rank: Unknown
timestamp: August 20, 2016, 06:36:15 am
content: Anyone promissing you electronics without maths is lying, especially with SMPS, SMPS rely heavily on raw phisics. Physics = maths

author: Ammar
author_rank: Regular Contributor
timestamp: August 20, 2016, 06:38:09 am
content: Maybe try a simulation software and play around with values. LTspice should work fine and is free. However, I have heard of simulators that are especially for SMPS.With regards to the units you use in formulas, you generally convert them to Henrys, Farads, Amps, Volts etc. So 61uH becomes 0.000061 H and 22 mA becomes 0.022 A. I suppose it may take some getting use to.To practice maybe try choosing a topology, calculating with the formulas then simulating. You can typically find ICs that are specially built for these purposes too and the data sheets will contain typical application circuits. Linear technology are particularly good at this. Good luck!

author: bitseeker
author_rank: Super Contributor
timestamp: August 20, 2016, 11:19:19 pm
content: Red Squirrel, there's a short series on power conversion and SMPS by The Post Apocalyptic Inventor that's a good intro.https://www.youtube.com/playlist?list=PLxaGnte1Dq0lgNtfA9H5Hd_YDyjylThJJ

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 21, 2016, 03:36:01 am
content: How much is "maths"? 'Cuz, "adding a few MOSFETs" is doing arithmetic, so you're certainly capable ofsomelevel of math...Tim

author: Red Squirrel
author_rank: Super Contributor
timestamp: August 21, 2016, 06:17:31 am
content: Quote from: T3sl4co1l on August 21, 2016, 03:36:01 amI was thinking more the ones where they give a huge formula and it's like "here you go" to say, figure out how big to make an inductor, transformer etc. Often I have no idea where to even start, so just need more resources that either explain that part better or explain in a way that is ELI5 like "if you make this part bigger, you can put more current through it".Thanks all for the various resources it will give me some good starting points. Obviously I'll want to learn the math too, you can't really get away from that.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 21, 2016, 02:32:13 pm
content: First of all, realize that those equations are almost all ratios. Absolutely nothing worse than a cooking recipe. When you have two cups frequency and a teaspoon of capacitance, you need so-and-so inductance...When you see a big long equation, force yourself to read it -- don't let your eyes glaze over and skip it! The first thing you will probably notice is, it's some quantity divided by some other quantity. A ratio. Next, look for the units. We're using real quantities here, and the units have to work out right.Dimensional analysis itself has a simple algebra, because you have to know the definitions of conventional units. I'll grant you that -- it's annoying, and you have to memorize them all if you want to check the equation yourself.FWIW, using circuit quantities (volts, ohms, seconds..), 1F == 1 second/ohm, and 1H == 1 second*ohm. 1 ohm = 1 volt/amp.Physically speaking, capacitors respond to charge. Charge is 1 coulomb == 1 amp*second. The equation of a capacitor is I = C * dV/dt, where dV/dt is the rate of change of voltage. Or if we switch things around:I dt = C dVCurrent, applied for some time dt, causes a change dV in capacitor voltage.Physically speaking, inductors respond to flux. Flux is 1 weber == 1 volt*second. The equation of an inductor is V = L * dI/dt, where dI/dt is the rate of change of current. Or if we switch things around:V dt = L dIVoltage, applied for some time dt, causes a change dI in inductor current.And because of these, you can see why capacitance has units of amp*sec/volt, and inductance has units of volt*sec/amp.Nice thing about switching supplies: as the waveforms are all square, you don't need to use any calculus. dt is simply the time difference (on or off), V is the applied voltage on the inductor (and I is the current in the capacitor to figure out supply input/output ripple, but that's secondary), dI is the amount the inductor's current changes by, and so on.Note that the inductor doesn't care what the absolute value of 'I' is, only how it changes over time. This is why current mode control is paramount: you need a controller monitoring inductor current, to see that it doesn't just keep ratcheting up forever or something. Many, many controller designs fail to provide this basic functionality, so beware!As for waveforms and getting a feel for that, check this out:http://schmidt-walter-schaltnetzteile.de/smps_e/smps_e.htmlPop into a basic (say buck or boost) design and poke some numbers. See that the voltage, inductance and pulse width are always simple ratios of each other. You'll usually end up with a "1+ratio", or a "(1+qty1)/qty2", or something like that, because of how things average out during a cycle, versus what happens during a single pulse. (The formulas are listed on the help page.)With this background, you should be able to get a more intuitive feel for what numbers and units matter.Tim