To: rsgb_lf_group@blacksheep.org
From: Eric NO3M <no3m@no3m.net>
Message-ID: <043bec62-8144-5ff3-98a6-9f04699f6a4c@no3m.net>
Date: Sun, 24 Feb 2019 07:08:01 -0500
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Subject: LF: Class D current spikes
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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