Resonant Royer/Baxandall power converter questions

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Well-known member
Feb 25, 2005
I'm playing with this power inverter circuit:
which is basically a variation on low noise high voltage generation circuits proposed by Jim 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?
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.

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