Hi Jay, LF,
Thanks very much again!
Well, if the PA efficiency is 90% with standard parts, what can you
achieve with the speial core material? I can obtain the 77 ferrite
material here but never heard about the 79 material. I can hardly
imagine that there is much more room for significant improvements. More
infos apprechiated.
For now, as a result of this day i want to show some photos i've just
taken. BTW the PA is now driven from the Elecraft K2 which i built in
2003. Just 1W RF is needed (at 1900 kHz => 475 kHz) to drive the PA input!
Gate voltage waveform from the ICL7667:
http://dl.dropbox.com/u/19882028/LF/Gate%20voltage%20475%20kHz%205VperDIV%20ICL7667%2BIRFZ48N.JPG
Drain, unloaded:
http://dl.dropbox.com/u/19882028/LF/Drain%20Source%20voltage%20without%20a%20load.JPG
Drain at 40W RF:
http://dl.dropbox.com/u/19882028/LF/Drain%20Source%20voltage%20%40%2040W%20RF%20out.JPG
Comments?
No, i don't have a Hello Kitty scope, the pink colour comes from the
auto adjustment of the image...
73, Stefan/DK7FC
Am 31.05.2012 20:27, schrieb [email protected]:
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
|
