Stefan
Well ... guess you have already answered your own question ;~)
I've stuck with transformer coupled designs ... VMCD for 137 / 500 kHz and CMCD for 160 / 75 / 40
meters. I don't have much experience with the bridge circuits which I believe is what you normally
use.
Without a load at the transmitter output both types behave well - they act as though they aren't
even keyed. I've mistakenly keyed them into incorrect loads (antennas for other bands) with no ill
effects. Operating into impedances other than 50 ohms cause a shift in the drain voltage/current
ratio as expected ... so there are limits. From experience a 1.5:1 VSWR during normal operation
isn't a concern.
The ringing on the off FET drain is an issue with VMCD and is mainly due to leakage reactance of the
transformer. In my experience, this is best minimized by choosing a core material that provides
enough winding reactance for the frequency in use without going overboard. For my 137 kHz KW
transmitter (yet to be written up) I used an FT-240-78 core. 78 material was specifically developed
for the frequency range between 100 and 200 kHz and works excellent. These cores are standard parts.
For my 500 kHz KW transmitter (yet to be written up) I used an FT-240-79 core. 79 material is even
better than 78 material at 500 kHz. Unfortunately, these are not readily available - 79 is a newer
material and available in only a few sizes that aren't conducive to what we're doing. I parted with
some NRE money last year and got Fair Rite to make some FT-240-79 cores for me. Last I checked they
hadn't added it to their catalog ;~(
Even with the correct core material and winding technique (interleaved primary and secondary
windings) some form of snubber from each drain to source is required. Choosing the correct R and C
is an interesting exercise. Rather than go over it here check out this paper by Rudy Severn:
http://www.ee.bgu.ac.il/~pel/links/DESIGN_OF_SNUBBERS_FOR_POWER_CIRCUITS.pdf
BTW, Rudy is one of the WD2XSH 500 kHz group. He also has a You Tube videos on the subject ... and a
book as well.
In my experience the ringing is not a problem as long as it is controlled and well under the voltage
limits of the FET. In my KW VMCD the drain square wave is 100 V (2 X supply voltage (50VDC)) and the
first peak of the ringing voltage brings this up to about 140 V. The FETs in use here are 240 volt
parts.
Jay
----- Original Message -----
From: "Stefan Schäfer" <[email protected]>
To: <[email protected]>
Sent: Thursday, May 31, 2012 12:37 PM
Subject: Re: LF: Re: Current mode class D PA for 475 kHz - success
Jay, MF,
I just tried the CMCD and immediately achieved 89% efficiency at 12VDC and 36W RF at 475 kHz
(dummy load). Just a first result. I can answer my own question now: Without a load the PA behaves
not critical! The current consumption is 0.2A only (including the current of the VFO and exciter)
!! :-) I'm going to improve the design now and will publish some photos soon...
73, Stefan/DK7FC
Am 31.05.2012 15:59, schrieb [email protected]:
Stefan
Thanks for the positive comments.
Have to admit I haven't been following what you're up to ... curious what problems you're running
into with VMCD at 500 kHz?
Jay
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