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Date: Mon, 25 Feb 2019 15:15:07 +0100
From: DK7FC <selberdenken@posteo.de>
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To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: Class D current spikes
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Hi Eric,

What happens to the supply behaviour if you just switch some Cs parallel 
to the output, say 4x 4700 uF (low ESR) rated at 63V ?
Maybe it solves the problem of the supply going into fault mode, 
although it is no the perfect solution.

73, Stefan

Am 24.02.2019 13:08, schrieb Eric NO3M:
> 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
>
>