James Moritz <[email protected]> wrote:
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
Please sende me a circuit diagram.
Thanks in advance!
73 OZ8NJ Niels+
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