Re: LF: Re: Class D current spikes

[Andy Talbot <andy.g4jnt@gmail.com>]

For on-off keying of a switch mode PA you really should be using supply line control.  Switching the drive then trying to reduce rise /fall times for keyclick elimination by controlling drive amplitude(something that seems to be done in some designs) is really asking for trouble.  Even hard-switching drive gives its problems , as we see here repeatedly

Instead, why not a simple emitter follower in the positive supply, then you can apply waveform shaping  to the base drive at low current, where you've got full control.   Even a 20A supply could be controlled this way with a suitably rated transistor.  Quite likely have to be a Darlington.   There will be a slight fixed drop across it, say 1V at saturation / full power, so a few watts of dissipation, but it's only operating non saturated for a short period during the rise / fall times.

A power FET could be used, but you'd need a drive voltage Vgs higher than the supply, albeit only a few mA capability; nothing a small switcher module couldn't manage.

 

Andy

www.g4jnt.com

On Sun, 24 Feb 2019 at 14:26, Rik Strobbe <rik.strobbe@kuleuven.be> wrote:

Hello Paul, Eric, Andy,

AC coupling with a large C will not solve the problem.
At "key up" one of the driver outputs will remain high thus the gate of theFET connected to that output will remain high (and the FET conducting) during a time depending on  RC constant of the driver / gate network. For 0.22uF and 4.7kOhm the RC constant is 1 millisecond. So for a driver running on 12V iit will take about that time for the gate voltage to drop below the treshold. During that 1ms the FET is more or less shortcircuiting the powersupply. One of them will die sooner ot later ;-)

73, Rik  ON7YD - OR7T
________________________________________
Van: owner-rsgb_lf_group@blacksheep.org <owner-rsgb_lf_group@blacksheep.org> namens N1BUG <paul@n1bug.com>
Verzonden: zondag 24 februari 2019 15:03
Aan: rsgb_lf_group@blacksheep.org
Onderwerp: Re: LF: Re: Class D current spikes

I will be watching this discussion closely. Mine is a W1VD kilowatt
deck:

http://w1vd.com/137-500-KWRev3.0.pdf

It has blown FETs several times. The problem always occurs at the
end of the RF envelope. To further confuse this issue, I have lost
only one FET while running with my U3S exciter in hundreds of hours
of operation. I have lost many FETs when driving it with HF
transceiver and down converter. I have also lost FETs driving it
with a phasing exciter. It seems using it with either of the latter
exciters almost guarantees one or more dead FETs within a few
minutes. I was running at about 300 watts output during all of these
failures.

73,
Paul


On 2/24/19 8:42 AM, Eric NO3M wrote:
> The driver and FET gates in mine are AC coupled (0.22u, SB560
> schottky shunt, 10 ohm series, 4.7k shunt) and the problem still
> exists.  Apparently same thing Paul N1BUG was having problems with.
> The spike on the end of the RF envelope is what seems most ominous
> and results in my 48v switcher going into fault mode.