Dear Scott, LF Group,
To save long-winded explanations, I have drawn a
circuit diagram of my modified version of the G0MRF PA output circuit, with
some details of the output transformer. It is too big to send via the
reflector, but if anyone wants it, let me know and I will e-mail it
direct.
My unit gives 400W out with 37V supply - for 300W
out with a 48V supply, I reckon the primary windings of the output transformer
should be changed from 4 to 6 turns. The value of the choke in the DC supply
is not critical - the iron dust cores can often be found in old PC SMPSUs. The
most significant change to the original circuit was to remove the decoupling
capacitor from the centre tap of the transformer primary, and increase the
inductance of the choke, converting the circuit into a "current feed" class D
configuration. This means the DC supply has a high impedance at 137kHz,
which prevents high transient currents into the low pass filter capacitors
when the MOSFETs switch. The ideal drain voltage waveform would be a half sine
wave, allthough in practice there is still considerable ringing at the
switching transitions, allthough much reduced. The value of the damping
resistors and capacitors is a compromise; reducing R and increasing C reduces
the amount of HF ringing, but increases dissipation in the
resistors.
Cheers, Jim Moritz
73 de M0BMU
