Resonant royer design for low noise, isolated DC/DC 1W - Page 1
Forum section: electronics / projects

author: Martinn
author_rank: Frequent Contributor
timestamp: October 03, 2023, 11:22:19 am
content: In my search for low common mode noise isolated DC/DC modules I stumbled across a cheap Chinese one which gave spectacularly good results. Most likely it is a royer converter:https://recom-power.com/en/rec-n-introducing-cutting-edge-technology-into-low-power-dc!sdc-converters-283.html?2Fixed in/out, unregulated, cheap...So I'd like to design my own. In contrast to the original Royer design, which uses core saturation to switch (and which is shown in the Recom page), I'd prefer a current-fed resonant one, as it is most likely lower noise and seems to have been standard in CCFL converters (See Jim Williamshttps://www.analog.com/media/en/technical-documentation/application-notes/AN118fb.pdf)Royer converters seem to have gone out of fashion (at least the resonant ones with CCFLs going extinct, maybe with the exception of cheap B0505S DC/DC modules) and I could not find any hints on dimensioning.Does anyone know how to properly design those? I am looking for a (what seems to be classic) current-fed resonant design as shown in AN118 figure 1.

author: Warhawk
author_rank: Frequent Contributor
timestamp: October 03, 2023, 11:42:12 am
content: Consider also ucc25800

author: Martinn
author_rank: Frequent Contributor
timestamp: October 03, 2023, 11:46:58 am
content: Of course, that's on the list along with SN6507 and LT1533.UCC25800 won't run from 5 V USB, which is something I'd need right now.

author: Warhawk
author_rank: Frequent Contributor
timestamp: October 03, 2023, 01:16:40 pm
content: SN501/05/07 are trusty workhorses. Additionally, catalog transformers from Würth are available.You may also want to have a look at fully integrated transformers on-chip that TI offer. I forgot the GPN now.

author: Martinn
author_rank: Frequent Contributor
timestamp: October 03, 2023, 01:32:44 pm
content: Not sure what benefit a hard switching push-pull would give over buying just an off the shelf module.I am looking for low noise, soft switching (possibly resonant).So current-fed resonant royer would be on the list. UCC25800 also, although that won't run from 5 V and needs proper dimensioning of the LLC tank.LT1533 probably would also work, but this is a hard to source (right now in stock), almost $15 part.A royer converter on the other side might run with two $0.02 BJTs - that's what probably is inside countless cheap B0505S-1W modules.

author: Warhawk
author_rank: Frequent Contributor
timestamp: October 03, 2023, 02:29:55 pm
content: Sure, Honestly I like discrete solutions. Nevertheless, you should consider things like an over-current protection. The UCC25800 was designed to reduce the primary to secondary parasitic capacitance to minimum. Mainly for HEV/EV inverters. This is difficult with flybacks (less leakage inductance = more capacitance and vice versa). The hard switching regulators like SN650x have at least frequency dithering. The other regulators I meant are e.g. UCC12041-Q1. However, the 5V in can't do more than 0.5W.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 03, 2023, 02:49:17 pm
content: I don't think the cheap modules would use a royer type converter. These tend to have a not so stiff output voltage, so the voltage dropping quite a bit under load. It looks more like they use some hard switching push pull converters similar to the SN650x, but from a cheaper (e.g. Chinese VPSC ... ) source.No matter what driver chip, the common mode background is mainly a question of the transformer. So how much coupling capacity and how much voltage at the windings that make up the capacitance. With good symmetry one can get a low effective voltage, close to having an electrostatic shield. A maybe a bit counterintuitive point is that the smaller the transformer the smaller the capacitance can be. In the old times they were measuring capacitance in cm. So a smaller, higher frequency transformer can be a bonus. A side effect is getting away with a smaller common mode choke to suppress CM residue even more.The driver chip is more effecting the EMI and very high frequency interference, well higher than the switching frequency. This is where the royer converter can be really good at, though the turn off delay of BJTs can also produce sharp edges at the end where on does not want them.The SM650x have frequency ditherng, but this can also be a problem, though they could also use an external clock.

author: Martinn
author_rank: Frequent Contributor
timestamp: October 03, 2023, 02:54:04 pm
content: For powering an analog frontend, short circuit protection is not absolutely necessary. Even the load is more or less fixed. I need 5 V input (from USB) to +-5 V (with -5 V only for the first stage, so most power on +5 V).Flyback is totally out of the question. Hard switching probably as well.UCC25800 requires you to design the LLC tank - which means two Ls. In the eval kit IIRC they combined those into one transformer as leakage and coupled inductance. This is doable, but far from trivial.Both LLC and Royer seem to handle poor coupling (high leakage) well, in contrast to a flyback where the leakage inductance energy would be dumped into some snubber. This makes them additionally well suited (because I'd like to reduce primary/secondary coupling), but I think more important are soft switching and sinusoidal waveforms.

author: Martinn
author_rank: Frequent Contributor
timestamp: October 03, 2023, 03:04:40 pm
content: Quote from: Kleinstein on October 03, 2023, 02:49:17 pmI found three sources claiming the cheap DC/DC modules use a Royer converter:1 the Recom page I linked in my first post mentions this (including schematic) - and they should know, this is their competition!2 it was mentioned somewhere here in the forum that simple (fixed in/out unregulated) 1 W modules use those3 Traco even specifies in some datasheets (search for "traco royer") that it's a royer design.I found both matching push-pull drivers and transformers at LCSC (as you mentioned VPSChttps://www.lcsc.com/brand-detail/12739.html), but I guess for rock-bottom DC/DC (https://www.lcsc.com/search?q=b0505s-1w) two $0.0001 BJTs will have to do.In either case, push-pull is as asymmetric as a flyback - for true symmetric drive it would have to be a full bridge primary driver. Or, if primary is asymmetric, reduce switching transients by reducing the slew rate (LT1533) or resonant switching.

author: Warhawk
author_rank: Frequent Contributor
timestamp: October 03, 2023, 03:13:48 pm
content: Quote from: Martinn on October 03, 2023, 02:54:04 pmWhen you say you need +/-5V, does it need to be isolated? The reason why I am asking is the LM27762 device. This is my favorite supply for analog circuitry that lives out of USB. (e.g. a scope probe)

author: Martinn
author_rank: Frequent Contributor
timestamp: October 03, 2023, 03:14:05 pm
content: BTW really strange waveforms in simulation...Attached Simetrix file. This time I used Simetrix because it is free (within the node count limit) and far more user friendly as LTSpice. Would be interesting to see how QSpice handles it.

author: Martinn
author_rank: Frequent Contributor
timestamp: October 03, 2023, 03:16:50 pm
content: Quote from: Warhawk on October 03, 2023, 03:13:48 pmGreat find - thanks! Very nice.Unfortunately I need an isolated supply. And that's where all this common mode mess is coming into play.

author: floobydust
author_rank: Super Contributor
timestamp: October 03, 2023, 03:43:50 pm
content: I always add a common-mode (output) choke with these small DC-DC converters.The transformer winding capacitance primary-secondary couples a lot of switching noise at say 1MHz or 300kHz plus harmonics which causes failure for conducted EMI and also tends to cause strange behaviour for secondary circuits.It depends on your transformer construction. If the DC-DC is using a hard-driven flyback converter (larger Murata) is just terrible for primary leakage inductance spikes getting through.The secondary-ground termination or total capacitance to ground of the secondary-powered stuff matters as well.Most of these small 1W modules appear to be saturated-core or fixed (they have poor regulation) but with an IC oscillator. You can tell by bench testing them and looking at the waveforms.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 03, 2023, 04:46:22 pm
content: FYI, that's a Baxandall oscillator. Royer is the saturation-commutated (not resonant) version.Parts like this may be of interest,https://www.digikey.com/en/products/detail/pulse-electronics/PM2190-011NLT/13998599the leakage (P1-P2 and P-S) is extremely low. They say ballpark 1uH max. but IIRC, I measured more like 10s of nH. (I'm not entirely sure if that was a measurement error, but I wondered so at the time and double checked. Maybe I should triple-check today...) CMRR can therefore be very good, even for hard switching edges (and better still for controlled risetime or resonant). Inductance may be a bit high for resonant but that's fine, just connect an inductor in parallel as needed.Tim

author: Martinn
author_rank: Frequent Contributor
timestamp: October 03, 2023, 05:28:02 pm
content: Quote from: T3sl4co1l on October 03, 2023, 04:46:22 pmHi Tim, thanks for the correction! There's even a whole section in Wikipedia dedicated to the misnaming:https://en.wikipedia.org/wiki/Royer_oscillator#Naming_Confusion:_%22Baxandall%22_vs_%22Resonant_Royer%22As even Jim Williams (who I think spent four years optimizing CCFL backlights) called them resonant Royer, I hope I am excused.As for the isolation transformers you cited: These would be candidates for LT1533 or SN6507https://www.ti.com/lit/ds/symlink/sn6507.pdfwhich is also slew rate limited, I guess.For a BJT baxandall, the base drive winding would be missing.But then there's the current fed resonant MOSFET version (german:https://www.mikrocontroller.net/articles/Royer_Converter#MOSFET-Variante), however this one is called.Huge benefit would be it does need neither center tap nor feedback winding, but two feed chokes. I omitted that so far as I would expect the MOSFET variant to switch harder (just a gut feeling), thus creating more noise. Also it is often mentioned that it might not start up reliably, particularly with slow input voltage rise time.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 03, 2023, 05:51:03 pm
content: Yes, variously "ZVS oscillator", "Royer", "Baxandall", etc. in common use. Indeed a shame JW missed the distinction (despite citing the Royer paper in his (C)CFL appnote!), but we can do our own little part to correct such mistakes. So, I'm doing my part.MOSFETs switch softer, generally speaking, due to lower gm. But that's a free variable if we're talking base windings anyway, so it doesn't really mean anything.The resonant current-mode version has quite smooth voltage, and ~square current waveform. The dI/dt can be set by differential inductance between sources/emitters, of course. And the CT (or common mode between a virtual-CT primary) has voltage cusps. To the extent these changes manifest in overall system EMI, they might be of interest to control, whether by way of making the current loops tighter (place transistors close together, use a CMC to/after the transformer?) or filtering in general (the transformer primary or secondary can be filtered for harmonics, just do it cautiously so as to avoid oscillation locking onto a filter mode instead of the main desired LF resonance).A hard CT on the primary isn't required, it's just better/easier that way; note that a triangle of inductors has equivalent CM and DM inductance, same as a series inductor to CT plus winding inductance does.Note alsoat least one inductor must have DCRin order to simulate in SPICE -- otherwise the loop current is a free variable (insoluble) and a singular matrix error results.Indeed, we could cap-couple the primary, from an L-C-L drain-to-drain network, to the transformer (with CT), through a CMC (which will be highly effective due to the CT providing CM impedance to GND), and thus even avoid DC common-mode on the transformer (but skipping the coupling caps is probably advantageous by way of not needing to add more poles to the system and thus more liability for oscillation at unintended modes).We could further note that the series supply choke (assuming we're using a CT winding) has the same voltage drop as the CMC would, and thus make it a three-winding choke where we lead the drain connections back up alongside it, and then add another CMC (which now drops nominally zero voltage, or only transient / high-frequency imbalance) to really clean it up. Give or take if the added stray inductance (leakage) is a problem for output regulation, or oscillation/dynamics. Obviously not a very off-the-shelf approach, but maybe something to consider if we're interested in winding our own, for minimum noise purposes.That's of course equivalent to a hard-[AC-]grounded primary CT, and supply inductor in the GND return path, assuming we don't need the transistors to be GNDed-source (which may depend on how gate bias voltage can be sourced).Tim

author: jonpaul
author_rank: Super Contributor
timestamp: October 03, 2023, 06:16:28 pm
content: Royer is a saturating CT transformerThe osc freq depends on the flux density of the core thus the applied V.It is NOT resonant,Jon

author: Warhawk
author_rank: Frequent Contributor
timestamp: October 03, 2023, 06:27:09 pm
content: Tim, Martin, threads like this are joy to read.

author: langwadt
author_rank: Super Contributor
timestamp: October 03, 2023, 08:18:17 pm
content: the supply in this is cute,https://docs.m5stack.com/en/unit/can

author: moffy
author_rank: Super Contributor
timestamp: October 03, 2023, 09:37:40 pm
content: What about a semi resonant flyback supply like they used in the old CRT tvs? I was recently repairing an older scope and noticed that they had a semi resonant flyback supply for the intermediary 30V. It tends to be low noise.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 03, 2023, 11:02:05 pm
content: Quote from: Martinn on October 03, 2023, 03:14:05 pmParts of the sim do look strange.Those base-winding-feedback quasi resonant switchers were very simple, and surprisingly efficient.I recall working on an emergency lighting switcher (12V to fluorescent light) to see if we could save a winding,...Having a high peak current drive into the base, without wasting current in droppers, proved very hard to beat, with low cost NPN BJT's of the day.

author: Someone
author_rank: Super Contributor
timestamp: October 04, 2023, 12:09:56 am
content: Quote from: Martinn on October 03, 2023, 02:54:04 pmDon't discount hard switching until you've tested it. Modern high frequency synchronous buck converters can produce "low" noise for audio applications, flybuck for isolation.

author: Martinn
author_rank: Frequent Contributor
timestamp: October 04, 2023, 08:51:38 am
content: Tim, judging by your forum nickname, this is home territory for you!Quote from: T3sl4co1l on October 03, 2023, 05:51:03 pmSorry couldn't follow you here. Could you help me translating this into a schematic?

author: Martinn
author_rank: Frequent Contributor
timestamp: October 04, 2023, 08:57:06 am
content: Quote from: moffy on October 03, 2023, 09:37:40 pmI think that was mainly to recover the energy stored in the horizontal deflection coil, elegantly combined with HV generation.Explanation here:http://www.bdtic.com/datasheet/fairchild/AN-9009.pdf

author: moffy
author_rank: Super Contributor
timestamp: October 04, 2023, 01:12:03 pm
content: Quote from: Martinn on October 04, 2023, 08:57:06 amA very nice article but it is still a low noise configuration because of ZVS and slowed rise/fall times due to the resonance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 04, 2023, 03:50:55 pm
content: Quote from: Martinn on October 04, 2023, 08:51:38 amSure:https://www.seventransistorlabs.com/Images/Baxandall_Output_Coupling.pngHmm, C1 and C3 do still participate, they aren't purely coupling capacitors: L1 has the cuspy common-mode voltage but T1 does not, therefore they serve doubly as coupling capacitorsandshunt C to GND, which limits their value somewhat. Which seems to further prefer the right-side version. And again, the RHS can be transformed into a ground-return choke (instead of L4), and something to keep R3/R4 supplied (probably another pull-up resistor, then a coupling (bootstrap) cap to source).Quote from: Martinn on October 04, 2023, 08:57:06 amEven more to the point: the capacitance is unavoidable as a result of complex yoke windings and a long high-voltage winding; pushing the bandwidth of the transformer up above even a few MHz is a serious challenge, especially if you need 30kV in the process. The capacitor also reduces peak voltage at the plate/collector making transformer leakage tolerable. The overall system will be a multi-element combination of transformer leakage and resonance(s), discrete components, and strays/parasitics, but as long as they can be tuned together in such a way that the equivalent circuit is as given, it all works out.Tim

author: moffy
author_rank: Super Contributor
timestamp: October 06, 2023, 04:58:41 am
content: Just as an exercise I thought I would try to model a semi resonant supply using QSpice and a C++ block as the controller. After a few false starts it was relatively easy, and having C++ code as the controller makes it almost trivial to modify and add features. A great way to debug logic. I have added a capture of what it looks like. The green trace is the L1/C1 junction and the blue trace is the MOSFET drive.

author: uer166
author_rank: Super Contributor
timestamp: October 06, 2023, 06:59:12 am
content: Quote from: moffy on October 06, 2023, 04:58:41 amVery nice, I've been waiting for a nice SPICE with C code integration (to simulate DSMPS and controllers)

author: Martinn
author_rank: Frequent Contributor
timestamp: October 08, 2023, 01:12:45 pm
content: Quote from: floobydust on October 03, 2023, 03:43:50 pmDid you measure its effect? What sizes do you use?I just re-read AoE 9x.14 which covers exactly this topic. They used a self-wound CM choke tuned to the switching frequency (100 kHz sine drive in their example), which reduced CM noise by 20 dB. They also tried various off-the-shelf ones, but found no improvement at all.https://x.artofelectronics.net/wp-content/uploads/2019/11/9xp14_low-noise_isolated-pwr.pdfp. 434BTW interesting component: Amobeadshttps://www.toshiba-tmat.co.jp/en/product/am_parts_emc.htmWhitepaperhttps://file.elecfans.com/web1/M00/20/B0/oYYBAFmkyImAf7lOAAlPo4vZMgk049.pdf

author: Martinn
author_rank: Frequent Contributor
timestamp: November 04, 2023, 04:38:23 pm
content: Coming back to my original questionQuote from: Martinn on October 03, 2023, 11:22:19 amthe best source of information I could find is this articlehttps://www.mikrocontroller.net/articles/Royer_Converteron mikrocontroller.net including many links to more detailed discussions (all in German).I designed a test PCB and started with a dimensioning (BJT with feedback winding) from the above wiki page. Interesting fact... this circuit will oscillate, no matter what. Unless you invert the feedback winding of course.Interesting observation from the thermal camera image: Although the RM8 N87 core seems massively overdimensioned, it seems to dissipate a lot of energy. At 1 W output, max flux is 130 mT which leads to some temperature rise already.So this does not seem to be an efficient use of such a large core. I'd have to do some calculations with different air gaps and find out how to minimize core losses (and use much smaller cores). Imagine the microscopic cores that must be in the sugarcube-size 1W SIP DC/DC converters. I chose the RM8 mainly because it is large enough to wind comfortably.I also added an oscilloscope image. Red is collector voltage, blue is the current in the primary resonant tank. Circuit operating from 5 V, 1 W output, 77% efficiency. No load power consumption is 130 mW. Works OK so far, although output voltage depends heavily on load current.Next will be measurement of the CM injection - that's why I built this in the first place.