"Richard Clark" wrote
On Fri, 14 May 2004 13:57:58 +0200, "Marc Battyani"
wrote:

Yes, this is exactly what I want to do : Inductive coupling.
Maybe you didn't see my first posts, what I want to do is transmit 3W of
power by induction over a 12mm distance.

Then this returns us to the native Z of the source, the FET, and the
sink Z of the load AFTER the link. Your link shows a 1:1 coupling (as
best I can tell) and it follows that the load should be on the order
of half an Ohm. It also follows that the characteristic Z of the FET
load should also exhibit this value (it does not) as well as the
coupling circuitry to the link.

Move the primary loop back into the Drain lead to the positive rail
path, and connect the 2200pF cap (which may be too much) from the
Drain lead to ground path. The other circuitry is superfluous. The
characteristic Z of this load is roughly equal to the FET; and as the
FET on time is roughly 120°; and depending upon coupling, then you
might find 60 - 80% efficiency. It will require some tuning as the
Bandwidth will be 1 or 2 MHz around resonance. If you want some other
actual load resistance other than half an Ohm, then you need to boost
the transform ratio (it works by the square of the windings ratio).

Give this a try and report your findings.

Much better!
A get 70% efficiency now
(with a 330pF cap)

Thanks

Marc