I recently got a current mode (outp xfmr center tap not bypassed) Class
D amp going on 136khz, basically modeled after the W1VD / G0MRF
circuits. FETs are 59N25s, 4820p / 10ohm snubbers, 5t+5t / 21t 3C90
42mm output xfmr, 62uH (25t T157-26) choke. Output xfmr depends on the
power supply in use:
30V linear supply: 5t+5t / 21t 3C90 42mm, 300W out
48V switching supply: 5t + 5t / 16t 3C90 51mm, 500W out
I am also using the G0MRF keying circuit, ie. P-channel MOSFET (IRF9540)
in the driver (MCP1404) supply line.
If the driver is left active (key down), the beginning and end of the
input RF envelope produces a "clicking" noise... assume this is from
current spikes being developed and sunk by the FET body diodes? If the
driver is keyed with input RF already applied, thereby interrupting the
drive to the FETs, there is no such behavior. The keying is shaped,
resulting in a ramping of the driver supply voltage and resultant gate
waveforms to the FETs.
These current spikes do not bother my 30V linear power supply, however,
when I attempt to use a 48V switching supply (rated at 17.7A), it seems
to be OK with the current spike generated on the beginning of the RF
envelope, but not the spike at the end of the envelope and goes into
fault mode. If the driver is keyed with continuous RF applied, it's
happy and does not trip out. But keying the driver doesn't seem
practical for digi modes without some exotic circuitry or engaging
computer generated signaling to key down *after* RF drive is applied and
key up *before* RF drive is removed.... again, this phenomena only
happens at the beginning and end of the input RF envelope.
Is there way to mitigate these current spikes, especially the one
generated when RF is removed?
73 Eric NO3M
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