Date: Mon, 25 Feb 2019 15:48:08 +0000
From: Chris Wilson <dead.fets@gmail.com>
Message-ID: <1516612920.20190225154808@gmail.com>
To: Rik Strobbe <rsgb_lf_group@blacksheep.org>
In-Reply-To: <1551107358099.3307@kuleuven.be>
References: <043bec62-8144-5ff3-98a6-9f04699f6a4c@no3m.net>,<5C73F86B.8080201@posteo.de> <1551107358099.3307@kuleuven.be>
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Subject: Re: LF: Class D current spikes
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Hello Rik,

Monday, February 25, 2019, 3:09:18 PM, you wrote:

> Hi Stefan, Eric,

> this might solve the PS tripping problem, but on the other hand it
> might also provide the MOSFET the draincurrent it needs to die ;-)

> 73, Rik  ON7YD - OR7T

> ________________________________________
> Van: owner-rsgb_lf_group@blacksheep.org
> <owner-rsgb_lf_group@blacksheep.org> namens DK7FC <selberdenken@posteo.de>
> Verzonden: maandag 25 februari 2019 15:15
> Aan: rsgb_lf_group@blacksheep.org
> Onderwerp: Re: LF: Class D current spikes

> 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
>>
>>


I have a series of scope captures from my W1VD amp showing huge
voltage spikes on the FET drains at the end of WSPR and especially
OPERA TX sequences. I suspected inductance was the cause. In fact I
try not use OPERA at high power as i am pretty sure to kill a FET
after a few sessions... Oddly (although I hope msomeone more
knowlegeable has the answer), using WSPR from my U3S direct from CLK0
of  th  Si535a  or  whatever it is never causes these spikes...  Or at
least they are well contained. 

-- 
Best regards,
 Chris                            mailto:dead.fets@gmail.com