SEPIC converter design next steps - Page 1
Forum section: electronics / general

author: vectorotter
author_rank: Contributor
timestamp: January 14, 2021, 07:05:23 am
content: To give a bit of background my design hopes to be a ~100W 100-250V AC - 48V DC power converter with excellent Power Factor and a minimal EMI emitter.I've been reading Erickson and Maksimovic 's Fundamentals of Power Electronics and have previously worked on a buck converter design in grad school.The way I understand a basic or typical off-line converter to operate is a cascade of stages; first rectification then buck or boost or whatever. I think I understand the Power Factor problem for doing rectification with a diode bridge and capacitor (peak detector) and have started reading around how modern rectification is done. I've read (and believe now!) that a boost stage following full-bridge rectification (not smoothed to DC), given a suitable control scheme, can present an almost resistive load to the line as duty cycle is varied across one line cycle.Since I need V_out << V_line,pk I rather would like to use buck or SEPIC for the second stage. Also, I'll isolate from the line at this point as well as stepping down so the D ratio is larger.I've simulated the SEPIC on its own without a control loop. Schematic is here:Waveforms after startup dynamics:My naive interpretation is that the converter is undergoing some sort of subharmonic oscillation. It also has too much ripple compared to my calculations which I'll take another look at. Is this expected given the two inductor, two capacitor configuration?Next steps? Do I work through and attempt to try to find the transfer function by some analytical means? By the switch averaging technique or some such? Using something like this technique would also allow me to simulate the small-signal response in SPICE, right?Eventually I'd like to simulate the converter with control loop driven from the rectified line.Real world engineers: What would you do next? :-)

author: ocset
author_rank: Super Contributor
timestamp: January 17, 2021, 01:14:18 am
content: Hi,You want PFC and isolation from a single stage?Then you should do a flyback converter...it can just about manage 100W from 100-250VAC.SEPIC is not isolated.But you could do a Boost PFC followed by a Flyback.This would give easier control....PFC stages have terribly slow feedback loops so not all applications can stand them as a single stage.Here is my SMPS course (below) which has loads of different PFC types in buckboost, flyback and boost.It has sepics too but not PFC'd sepics.PFC SEPIC is rare as the high voltage sepic cap is expensive.https://drive.google.com/file/d/0B7aRNbu3Fes4TU92Mkw3YlA3ams/view?usp=sharingI can send you a working sim in ltspice of a buckboost pfc led driver if you want? It can be modified to give 48V out...but really...PFC stages need very large Cout as they have 100Hz ripple.....if you want flat 48vdc output from a pfc stage, then you will need massive cout.

author: vectorotter
author_rank: Contributor
timestamp: January 17, 2021, 07:40:23 am
content: Re: SEPIC not isolated. It is possible to split L_2 into a transformer and hence get isolation and step-down.Other points accepted. Let me digest all of this and come back.

author: ocset
author_rank: Super Contributor
timestamp: January 17, 2021, 03:45:11 pm
content: The thing is that L2 may have a net DC level in it, so this wouldn't be favoured for use as in an SMPS transformer.It may be useful to provide a bit of bias power, but even then, there are other ways.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 17, 2021, 05:37:45 pm
content: Err, 1N5408??I don't see any control at all, it's just open loop, a PWL I guess instead of a PULSE source? Have you allowed for startup resonances to dampen out? (And, are there parasitic elements not shown in the L, C, etc.?)Quote from: vectorotter on January 17, 2021, 07:40:23 amOtherwise known as flyback.It could be done with the big fat coupling caps (C2, plus one joining ground to ground), but, well, isolation...I think you will soon find why it's just slightly preferable, to do it the usual way it's done, i.e. with an independent active PFC stage. The advantage is:- PFC doesn't need to worry about output regulation, fast load response etc.- PFC output necessarily has to have mains ripple (why?); the regulator is able to remove this.- The regulator can be optimized to a very narrow operating point (say, supply voltage +/-10%), over all input conditions (e.g. 85-250V, DC and 43-60Hz or more). This makes resonant stages very attractive, for instance.That said, two-switch forward and flyback are very popular, and this may be as much for EMI as anything else. The magnetics tend to be relatively large, but they have to be relatively large given the switching frequency anyway.Over say 100kHz, you can't operate near saturation anyway, due to core heating. This removes the point in favor of full-wave topologies that would otherwise utilize a wider flux density range. The classic ATX power supply, was two bipolar transistors in half-bridge (full-wave output), with a base drive transformer (including positive current feedback, so the transistors drive themselves approximately at hFE(sat) fixed by transformer ratio), and relatively low frequencies (30-60kHz say?).With a little help from improved core materials, and just generally better devices, faster operation and so on, you might see a transformer of identical size in a modern PC PSU, but it's doing four times the output power, even while carrying load current only half the time (2-switch forward), or it's a resonant design.There are still plenty of, somewhat more specialized, applications for basically PFC-only designs -- typically these are where the high output ripple is acceptable. A battery charger say, or LED lighting. Which, LEDs either just run straight on the output (in which case the light output may flicker or strobe, causing visible banding in videos, or even causing eye strain); or may use a regulator, including an old fashioned linear one, to regulate current and keep the light output steady. And in that case, the efficiency lost to a linear regulator, can be quite tolerable -- the LEDs already aren't terrifically efficient (50-75% lost as heat) so losing a few points more isn't a big deal overall, and the voltage lost to the regulator really only needs to be the few volts of peak-to-peak mains ripple in the output, a much more economical case than for a general purpose power supply, say.Tim

author: ocset
author_rank: Super Contributor
timestamp: January 17, 2021, 07:48:54 pm
content: By "L_2" i thought you meant "L2", and hence my answer in post #3.As T3sl4co1l implies , there is a "flyback similar" version of SEPIC which is gotten by coupling the sepic inductors and keeping the sepic capacitor........and basically its like a flyback, but doesnt need a primary clamp because the sepic cap takes care of it.In a "coupled inductor sepic", the fet and diode currents are the same as they would be in a 1:1 flyback with the same inductor values (and obviously same f(sw) etc.If you want isolation and pfc in a single stage, as well as step down, then the isolated, PFC'd CUK converter is an idea....but to be honest. just stick to "Boost PFC then isolated converter."Power integrations do some very low component count chips for this. Eg you could use a hiper-pfs boost pfc followed by an innoswitch flyback for 100w...very low component count....specially since one of the hiper-pfs's has both diode, fet and controller in the same package.

author: vectorotter
author_rank: Contributor
timestamp: January 18, 2021, 11:54:25 am
content: Quote from: T3sl4co1l on January 17, 2021, 05:37:45 pmYes, I know. Step-by-step. Updated to a Schottky now. After fixing the cap ESR I saw the reverse conduction.QuoteCorrect just open loop at this point. Yes PWL instead of PULSE, no particular reason as yet. The plot was from 800us and yes, after the startup dynamics had subsided. Turns out that the idealised Cs having no series resistance were the root of the (what is now obviously) numerical instability. That's my inexperience driving a simulator.QuoteMy application is a static load unless under failure conditions but point taken.QuoteIs this because near to the V_in minima (zeros), the conversion ratio tends to infinity which is physically unrealisable by the converter...?QuotePoint taken.Something I should make clear and that is I'm trying not to constrain myself economically - if I end up with something with a higher BOM count/more expensive BOM that's technically more pleasing and I learnt more in the process then that's just fine.I was working toward something like this:and decided I didn't understand it well enough to just take it and develop a design from it so I started from somewhere in the hope of ending up somewhere near to the above circuit.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 18, 2021, 03:15:06 pm
content: Quote from: vectorotter on January 18, 2021, 11:54:25 amConstant current, too? How boring.Still, it has to get up to nominal from zero, there is variation around the operating point (noise and ripple), and the input is varying, so still plenty to do.I suppose you could cheap out and use a fixed duty or something like that, but you still need current limiting for startup and fault. And at that point, it's only another op-amp to add voltage regulation.QuoteEh, we could leave the switch fully on near zero crossing, and just build up whatever inductor current we get during that period; it's not likely enough for nominal output. But why wouldn't we even want to do that -- what is the definition of PFC?Related: why does a three-phase PFC not require a bulk filter cap (though I expect you'll still find them in such, just that their value isn't so critical for this one reason)?QuoteAh, so a two stage design should be... advanced enough to meet your goals, while being simple enough (both can be approached with plain old square wave design, and independently) to be feasible?If you squint at it, you can kind of see that as a mashup of the two, you've got the boost inductor, but not a boost switch, it's going into a set of switches and a transformer; well, you can figure if the switches all operate in unison, it acts like a boost, at least in the charging phase; you wouldn't want all switches off after that (nothing to clamp flyback voltage), but leaving a diagonal pair on, you can dump that into the transformer. And the secondary side is just diodes, so they'll clamp the boosted voltage, and it all works out. You can ignore the ZVS part for now (and, I'm not sure offhand how you'd make that work; probably something to do with the transformer's inductance, and more switch timing). Tricky part is the dynamics of switching itself: that boost spike has to go through the transformer's leakage inductance as well, so it probably needs a snubber, which will cut into performance somewhat.Tim

author: ocset
author_rank: Super Contributor
timestamp: January 18, 2021, 04:33:49 pm
content: Your isolated converter kind of looks something like an isolated Totem Pole PFC, but resonant.(AKA Bridgelss PFC).Also, the attached is a Boost PFC non isolated battery charger, and has no ripple voltage on its output due to the battery clamping it flat.its in the free download LTspice