Re: LF: Re: Class D current spikes

[<gw8glo@aol.co.uk>]

Try using a synchronous keying circuit, only uses a few ics and is reliable. I have used it on a modified Decca 5501 on 136 and a similar tx on 475 with no blown fets even to a short circuit, guard circuit used as in the decca design.

73 Dave

-----Original Message-----
From: Eric NO3M <no3m@no3m.net>
To: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Sent: Sun, 24 Feb 2019 18:07
Subject: Re: LF: Re: Class D current spikes

Andy

This recommendation is all well and good, but seems applicable to only CW where either the operator is manually keying or using computer controlled keying with a constant carrier input.

With digi modes (WSPR, JT9, etc.) that do not present a constant RF input, the traditional PTT signaling is of no use as it keys active prior to the RF envelope start and goes inactive after the RF envelope ends.  The spikes would still be present and problematic.

Other than Rik's suggestion of killing the lingering active high driver output, it seems that only using a supply control scheme (whether driver or FET drain supply) would require sampling the original RF input to detect the end of the envelope and apply the key up (inactive) signal.  Prior to reaching the driver subsystem, the input RF would need to be delayed by some amount.

73 Eric NO3M

On 2/24/19 9:41 AM, Andy Talbot wrote:

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