design of load resonant converter with variable load

If you aren't too worried about frequecy stability and can tolerate some third harmonic distortion, look at the Baxanadall class-D oscillator - Baxandall, P.J, Proc I.E.E 106, B, 748 (1959)

The reference comes from Tony Williams, who has built quite a few over the years, and posted quite a bit on the subject.

The circuit is probably best known from Jim Williams' series of application notes for Linear Technology, on high frequency inverters for driving cold cathode back-lights as used in laptop computers (application notes AN-45, AN-51, AN-61, AN-65) though Jim Williams describes the inverter as a Royer inverter, referring back to the non-resonant inverter described by Bright, Pittman and George H. Royer in 1954 in a paper "Transistors as on-off switches in saturable core circuits" in Electrical Manufacturing. AN-65 does include a reference to Peter Baxandall, but to his 1960 paper "Transistor Sine-Wave LC Oscillators" in the British Journal of the IEEE paper number 2978E which is cited in a discussion of root-mean-square power measurements. Because there is no British Journal of the IEEE, one can wonder if he ever saw the original.

If the Q of the tank circuit remains in the range 5 to 10 recommended by Tony Williams, the third harmonic distortion is around 1%, which doesn't matter in many applications. For lower-Q conditions I've been brooding for years about a scheme to suppress the thrid harmonic content - see my posting here Mar 9 2003.

I've been poking around the circuit with LTSpice recently, but I've yet to get any of the "improved" simulations to run. Here's the net-list for the classic circuit

• C:\Bill\work\Baxandall\Classic_Baxandall_Cass_D.asc L1 N003 N005 0.00025 Rser=0.022 L2 N005 N008 0.00025 Rser=0.022 M1 N008 N006 0 0 FDS6680A M2 N003 N004 0 0 FDS6680A C1 N008 N003 100n L3 N001 N005 0.001 Rser=0.044 Cpar=100p V1 N001 0 5 Rser=0.001 R1 N006 N003 10 R2 N008 N004 10 L4 N002 0 0.0000253 Rser=0.004 Cpar=100pF L5 0 N007 0.0000253 Rser=0.004 R3 N007 N002 100 .model NMOS NMOS .model PMOS PMOS .lib C:\Program Files\LTC\SwCADIII\lib\cmp\standard.mos .tran 0 14m 4m 10n K1 L1 L2 0.99 K2 L1 L4 0.99 K3 L1 L5 0.99 K4 L2 L4 0.99 K5 L2 L5 0.99 K6 L4 L5 0.99 .ic V(N003)=0 V(N005)=7.854 V(N008)=15.708 V(N007)=2.5 V(N002)=-2.5 I(L5)=-0.5 I(L4)=-0.5 I(L2)=0.17 I(L1)=0.17 I(L3)=0.3 .backanno .end

If you cut and paste it and put a .cir suffix on the file you should be able to run it under LTSpice. It is a 15.9kHz oscillator, so 10msec of data is more than you need.

Bill, you've often referenced that interesting paper, is there a copy available on the web? Do you have one you can scan?

How about that one? :-)

Are you referring to in this interesting 12-message s.e.d. thread,

Subject: Peter Baxandall's class-D oscillator revisited. Newsgroups: sci.electronics.design Date: 2003-03-03 14:41:29 PST

I haven't seen a copy of the paper since 1966. Tony Williams might have one, but I think he's quoting the reference from an old text-book he owns. Any U.K. academic could probably get hold of the ppaer, but won't be allowed to put it on the web - it would be a breach of copyright.

You'd need a U.S. academic for that, and one with access to a long established library, because I suspect that =CBlectrical Manufacturing" is long dead. Sod's Law being what it is, copyright will nevertheless be vested in some rapacious offspring ...

I think I saw the paper in 1970, and it really wasn't worth reading.

--=20 Bill Sloman, Nijmegen

Yes. I was never able to source a copy of Baxandall's original paper, so the quotes in that thread were from Thomas Roddam's 1963 book on Inverters and Converters.

The first time I saw the circuit was in an old Wireless World article, being used to generate the hf bias for a tape recorder. This may have been drawn from a Ferranti Semiconductors App Note. FS is now Zetex.