Current Doubler transformer for Full Bridge SMPS? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: December 23, 2022, 12:11:34 am
content: Hi,We have been sent an isolated Full Bridge, 12-36vin, 32vout, 300Wout, 240kHz, Planar transformer, synchronous. We should test it up.We compare this split secondary version (which this is) with a Current Doubler version, which we recommend instead.There is utterly no comparison, and it should be done with Current Doubler output. We are shocked that the customer wants split-sec output.The split sec version has some 200% more current in the transformer secondary…..compared to the current doubler version….and since the primary (in both versions) is 33Arms…..then reducing the overall current in this transformer is crucial. Not only that, but the split sec version has worse overall coupling between primary and secondary, because two separate sec’s are harder to couple to the pri than a single sec (even though they both have the same overall turns)….its blatantly obvious that its easier to “interleave wind” a transformer with one primary and one secondary….(as opposed to a single primary and a split secondary) and this applies even more with a planar transformer.Also, the current doubler version is much more amenable to use with secondary synchronous rectifiers….you can see this by just looking at the schematics (attached)…the current doubler version offers a more overall reduced impedance pathway to current that suddenly gets it path broken by an off-going synch FET, when that FET turns off when the current in it has reversed…..(this always happens in light load).The sec diode off state voltages, and sec side snubber losses are the same in each case.But it is a known fact that with a Current Doubler transformer, the leakage inductance as seen from the primary can be made smaller.___--___---__---___--__---___--____--___Thankyou for reading thus far.......And the real problem for the split-sec transformer version of the full bridge is its totally disastrous behaviour when used with synchronous rectifiers........compare here with the Current Doubler version of the Full Bridge........as soon as the Full Bridge goes from full to no load, the split sec version goes haywire...whereas the Current Doubler Full Bridge shows superb performance...without any synchronous rectifier current limiting it shows no high overcurrents or overly high FET voltages.Will you support us in telling our customer to change to the Current Doubler version?LTspice and PDF attached (showing synchronous rectifier performance)__---___--____---____--____---___---___Overall no brainer…Current Doubler transformer wins…wouldn’t you agree?The Current Doubler version was "made in heaven" for synchronous rectifiers....just see it on the attached LTspice.LTspice and pdf attached

author: jonpaul
author_rank: Super Contributor
timestamp: December 26, 2022, 01:05:18 pm
content: Farrington Never heard of a "current doubler", use a common term.please post images, not files like asc or pdf that wast more of our time.Enjoy your holidayj

author: Stray Electron
author_rank: Super Contributor
timestamp: December 26, 2022, 03:54:31 pm
content: Are there even any transformers in those circuits? It looks like there are only chokes in both of them. I've never heard of a current doubling transformer either but I have made what amounted to one but replacing the transformer with a single secondary and a half wave rectifier with a transformer with twice the output voltage and a center tapped secondary and wiring it as a full wave rectifier. Basically it gave two output pulses per cycle instead of one so it delivered twice the power at the same voltage. Other than that I have no idea.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 26, 2022, 06:27:45 pm
content: Quote from: jonpaul on December 26, 2022, 01:05:18 pmHuh, surprised, guess you just never encountered it. Maybe it's a newer thing, I don't know?e.g.http://www.how2power.com/pdf_view.php?url=/newsletters/1602/articles/H2PToday1602_design_Zwicker.pdfIs as it says on the tin; the current mode complement (swap parallel/series, L/C) of the voltage doubler.Granted, what he's doing with it, who knows.Regarding the rating, I don't feel strongly about it. 300W isn't that big a deal for most any full wave topology. CD helps with planar as you don't need a tapped winding, but does need more inductors.Tim

author: jonpaul
author_rank: Super Contributor
timestamp: December 26, 2022, 07:48:50 pm
content: never heard of Bob Zwicker, How2Power or current doubler. Sees lke extra parts and trouble for zero benefit.converters we made under 1kW were handled by double forward or foward.I fail to see the reason for the double current nor the questionj.

author: 2N2222A
author_rank: Regular Contributor
timestamp: December 27, 2022, 04:38:31 am
content: There is nothing wrong with .ASC files. Just install LTSpice it is free. Then you can actually simulate the circuit operating and change it.

author: Thunderer
author_rank: Regular Contributor
timestamp: December 27, 2022, 04:58:53 am
content: Quote from: Faringdon on December 23, 2022, 12:11:34 amSo, both draw 33A from the line and at an indentical output voltage the "current doubler" provides double current / double power? Is that transformer an over-unity design? Finally, we got a working Perpetuum Mobile, it just doubles the applied power.Plus, the gibberish from the How2power PDF where -0.5A and +0.5A result in 1A beats me dead.Last thing, you say Full Bridge, where is this Full Bridge in your schematics? On each branch, your loads are to ground, so where in the Full Bridge?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 27, 2022, 06:04:23 am
content: Quote from: Thunderer on December 27, 2022, 04:58:53 amIf you like, a simple model is to imagine the full winding has a CT. Ground the CT (or, well, Vout or whatever in this case, but AC ground I mean) and you have a FWCT rectifier, fine. Cut the CT, and put chokes from either end to ground (which can be a tapped choke or two independent), and you've got a virtual CT if you will; you're right back where you started.So, whereas a FWCT rectifier needs double the winding for given voltage, CD uses a double voltage winding and no CT for the single voltage and double current.Hence, "current doubler".QuoteI can't speak for them, also that was just the first link off Google, further reading is suggested as always.Anyway, how is that any more unrealistic than the voltage doubler that takes in +/-1V and gives 2V? It's literally theoretically identical to the C-W multiplier, nothing but a V-I transformation of the circuit. Does... do you believe the C-W multiplier doesn't work--?Tim

author: Faringdon
author_rank: Super Contributor
timestamp: December 27, 2022, 02:28:34 pm
content: QuoteThanks, this is one of the mains reasons we are considering the Current Doubler version.I once interviewed at a Planar transformer company, and not knowing what a "Current Doubler" was meant the interview would be terminated with immediate effect.

author: Thunderer
author_rank: Regular Contributor
timestamp: December 27, 2022, 06:48:40 pm
content: Quote from: T3sl4co1l on December 27, 2022, 06:04:23 amCould not follow you, because I am more of a visual person. Can you sketch it, please?Quote from: T3sl4co1l on December 27, 2022, 06:04:23 amYes, +/-1V and gives 2V. But the current doubles because you doubled the voltage. Which the current doubler in the whitepaper does not, the 5V are the same at both outputs. In the buck configuration, at 5V with a 10R load, you have 0.5A. If load is 5R, then you have 1A. Same for the current doubler.Anyway, my point is thatno current doubling appears magically while the winding wire section stays the same. Either case, a simple buck, or this current doubler wonder, both rely on an incresed copper wire section in the transformer.

author: Faringdon
author_rank: Super Contributor
timestamp: December 27, 2022, 07:07:47 pm
content: QuoteThanks, but thats why we picked Currnt Doubler...because its less wire losses in the transformer...compared to a Full Bridge with Split secondary, AYK.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 27, 2022, 07:27:47 pm
content: Quote from: Thunderer on December 27, 2022, 06:48:40 pmThus:Of course for one final transformation, the diodes and chokes can be swapped; this is merely a convenience for using synchronous rectifiers, or perhaps for EMI purposes.See also for flavor, the full CW version:Which I've built. It's...weird. The voltage waveform, as you go along the stack, consists of shorter and shorter duration pulses (square input), while the current adds up to the final output total. It can take quite a large (peak) voltage to get the desired output.But that is exactly as we should expect -- as that is precisely the description of the capacitor C-W multiplier, where large peak currents are required to get a given voltage output (at some average current).And the output is current mode; the input should be current mode as well, which relaxes the voltage requirement obviously (a current source creates just whatever voltage it does), but is a little harder to adapt to traditional voltage-source generators.It is precisely the V-I dual of the same circuit. Or node-mesh dual. If you don't believe me, I strongly encourage you to investigate the transform yourself -- it is quite tricky to solve a circuit with this many nodes and loops, until you've done it a few times; it's good practice!QuoteActually the wire section will be reduced, because the single winding is fully in use, not half the time as for FWCT (which is only a 41% disadvantage, not huge). Compared to FWB or FWD it's the same, but FWB is less desirable at high currents (at least, I haven't seen any fast switching sync rects for FWB), and FWD even less so (but more so at high voltages, for obvious reasons; assuming C-input rect. of course -- flyback, resonant).Tim

author: JohnG
author_rank: Frequent Contributor
timestamp: December 28, 2022, 04:05:56 pm
content: As discussed, a single secondary winding gives better transformer utilization versus a CT. A simple way to see the root cause is that each winding in a CT does not see half the RMS current. Although each winding of the secondary sees an AC current of half the amplitude, they also see a DC current of half the inductor current.However, nothing is for free and what you gain in the transformer, you lose in the rectifier. You can have a FB rectifier, but then you always have two switches in series. They might be lower voltage devices, but now you need a high-side gate driver, there's more undesired power loop inductance, etc.If you go to a current doubler, you often punish the inductors, because each of the two inductors now sees half the duty cycle for a max of 50% (<50% in a real converter), and the ripple usually goes up. That can be a lot worse than the hit on the transformer from going to a CT, especially if you normally run at a high effective duty cycle. Sure, you could maybe mitigate this some with a fancy coupled output inductor, but this is difficult in practice and is rarely done.Incidentally, here are some links to a current doubler circuit from 1908. You need to look at both pages, the second is the schematic, but the first discusses the rectifier in the schematic, for those unfamiliar with mercury vapor rectifiers.https://www.biodiversitylibrary.org/page/17500516https://www.biodiversitylibrary.org/page/17500517I credit the above links to Rudy Severns and his paper "Circuit reinvention in power electronics and identification of prior work".John

author: Thunderer
author_rank: Regular Contributor
timestamp: December 29, 2022, 09:27:39 pm
content: Quote from: T3sl4co1l on December 27, 2022, 07:27:47 pmThank you for taking the time to explain, it is very appreciated. I have to ruminate now.Well, still have not yet figured it out how to get notifications when someone quotes my messages, so I would not miss them. Going now to the settings.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 29, 2022, 09:34:54 pm
content: Try "Show new replies to your posts", which may also need to be expanded by clicking the "A Free & Open Forum For Electronics Enthusiasts & Professionals ^" heading.Tim