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
