Resonant Royer (Baxandall) power converter questions - Page 1
Forum section: electronics / projects

author: ilya
author_rank: Contributor
timestamp: June 05, 2022, 07:51:32 pm
content: I'm playing with this power inverter circuit (attached).This is basically a variation on low noise high voltage generation circuits proposed byJim Williams in this app note from LT. This circuit can provide a relatively clean sine at the output, and it's possible to use a step-up transformer to get rather high voltages. At low currents this can be used to power tube preamps from low voltage DC using basic and cheap through-hole components.I tried this circuit with a coupled inductor (which is basically 1:1 transformer) and a Sumida 1:5 step-up transformer. It works fine with the voltage supply range shown. However, when I increase the voltage supply, I see proportional increase in power consumption (no-load condition). With 20V input I get almost 4Wt dissipation (unloaded). Inspecting various components temperature, I see that the transformer is the major source of these losses, and a resonant capacitor (C7) is the second one. Transistors stay cold even under a rather high load.When I increase the resonance frequency (i. e. reduce the C7 value), I see a small improvement in efficiency, but I get proportionally larger losses down the line in the bridge rectifier (not shown on the schematic).Here're the questions that I'm facing right now:1. Is it possible to reduce the losses in the transformer and where to start looking for the solution? Any documents on suggested transformer cores and winding details?2. Is it possible to reduce losses in the C7 resonance cap and how?3. Would Schottky diodes in the bridge rectifier be a sufficient solution for high frequencies (100kHz and upward)?Any ideas are welcome.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 05, 2022, 08:06:56 pm
content: Schottky diodes are a good idea, especially at lower voltage, like < 30 V. They are both fast and relatively low forward drop.For the losses at C7 the choice of the capacitor can make a difference. A PP type capacitor should be good, especially one with extra foil. Mylar capacitors have higher losses, though overall the loss at the capacitor should be one of the smaller issues.For the transformer there are different choices for the coil. For a low frequency this could be classical iron (especially thinner laminates like in audio transformes good ring cores). For higher frequencies the likely best material of choice would be nanocrystalline matrial like nanoperm - though it can be a bit expensive, hard to get and limited in the form factors.For really high frequencies some ferrites would be an option., but not all ferrites are equal.For the transformer a smaller core could reduce the no load loss, but would increase the sereies resistance (need more turns) and thus the loss under load. To a certain degree one could use the same core with more turns and thus less magnetization to get a similar effect as a smaller core - just heavier and more expensive, but it could help at the small end. The higher the frequency the lower the number of turns actually needed.For the transformer details, some application notes or general information from core manufacturer could help. The converter should be somewhat similar to a forward converter.

author: ilya
author_rank: Contributor
timestamp: June 05, 2022, 09:36:46 pm
content: I was not that sure about Schottkys because I'm planning to have 300V output. And Schottky diodes for these voltages come with large current capacity (which I don't need) and my local suppliers prices are insane for high voltage Schottky diodes. I guess the fast and dirty way will be to use a pair of diodes instead of one. Hopefully this will give me some voltage margin.For a cap, my initial choice was a (probably) metallized PP type film. I tried a smaller ceramic Y-type cap (just because I didn't have such small value film caps), and it didn't behave well. Overall, I found that the larger the cap, the better the circuit behaves under load - output V doesn't sag so much compared to the smaller cap. But with lower frequency the idle current was higher than with higher frequency. As for now, I'm aiming at a range within 30-100 kHz.Actually, to me this looked more like a push-pull topology, so that was what I started searching for. Surprisingly, I couldn't find any useful (non-academic) data on transformer winding (exceptthis note from TI, but it doesn't mention push-pull topologies in the abstract). I have RM8 N87 core available locally (that's TDK/Epcos, MnZn power ferrite series) that I hope will work. But I'm not so sure - it's far from nanoperm.It's the information regarding number of turns and winding structure that I'm looking for. Will be doing some more google-fu and experiments...

author: Hiemal
author_rank: Regular Contributor
timestamp: June 07, 2022, 11:46:31 pm
content: A good breadth of information might be also gleamed into by looking at other similar resonant converters, like induction heaters. Most of the requirements for those also apply here, with the resonant cap needing to be a very good quality/low loss one, like Kleinstein suggested. Polypropylene, Mica, film, etc, are all good choices.Paralleling multiple small capacitors also helps reduce the overall ESR and might help too.Alternatively, this converter is also sometimes called the ZVS converter, and is/was used a lot by HV enthusiasts to drive flyback transformers at great power levels. If I remember right, the resonant rise typically seen from these is about pi * the input voltage. Add in some slight safety margin and use the lowest RDSon fets you can realistically get to further reduce losses in the transistors; this voltage rating also applies to the diodes.As for the transformer, take a look at the various ferrite cores available. Core types that have lower losses will be better... there's a lot of math involved with figuring out specific loss amounts.https://www.tdk-electronics.tdk.com/download/531568/419f8df165fa780842566ab55510a8ce/pdf-generaldefinitions.pdfmight help a little.For the output rectifier, schottky diodes might be somewhat difficult to come by at 300V ratings... but, they'll be lower voltage drop, AND also, critically, fast. Most pre-packaged (if not all?) bridge rectifiers are slow diodes, typically intended only for low frequency rectification. You'll get a lot of switching loss in the diodes if you try to use them too fast. Ultrafast diodes might be the cheapest and easier to find option for you if you're going for higher voltage outputs.

author: moffy
author_rank: Super Contributor
timestamp: June 08, 2022, 09:56:18 pm
content: I have been playing around with a similar design and found that at lower resonant frequencies the circulating current amongst the resonant components was very high and droped quickly as the frequency increased. So for the transformer you go from I2R losses to core losses as the frequency increases.

author: ilya
author_rank: Contributor
timestamp: June 09, 2022, 11:34:53 am
content: Quote from: moffy on June 08, 2022, 09:56:18 pmThis is exactly what I found as well. With a smallish resonant cap the voltages are higher, but the regulation is poorer, although efficiency is a bit better. As the cap increases, the voltage drops, but the regulation improves. However, I see a reduction in efficiency.After experimenting with the circuit I found that the main source of losses is the inductor feeding the transformer. I've reshuffled the circuit a bit so that now instead of two inductors feeding the primary I got a center tapped primary and an inductor feeding a center tap.

author: moffy
author_rank: Super Contributor
timestamp: June 10, 2022, 05:10:54 am
content: Yeah, its a matter of adjusting components and values to find that acceptable sweet spot.