Hi Roger,
40 Vpp is just normal if the power supply has about 12V! You can assume
about 3,5...4 times the supply voltage for u(DS), if well matched!
Most programs will give you just an imagination on how the values must
be, since you oftendon't know the Q of the coils and Cs. You always
have to tune the PA for optimal efficiency, not for maximum output
power! Thus it is essential to match the antenna to 50 Ohm, if this is
your designed output impedance, not to a maximum antenna current. A
square wave gate voltage is essential too. Use a tiny MOSFET driver
such as ICL7667. And use impulse Cs, no ceramic Cs! I am using WIMA
FKP-1 2kV types (not sure if available in UK. There are surely others
such as the popular "silver mica")
I do not have a ready to use design for 12V/20W (do you need one for 1
kW?), just some recommendations. Instead of a IRF510 i would recommend
a IRF540N. If your antenna matching is reasonably OK you can choose
e.g. a IRFZ48N. These FETs have a smaller ON resistance and will
produce lower losses. Switching times are uncritical at 137 kHz. A
TO220 device is totally OK for 20W. In my Class E PAs i am using no
output transformer at all! There is a L between Drain and +, a C
parallel to D-S and a L at Drain which is in series to a C is connected
to ground. Between L and C you will find 50 Ohm when correctly
dimensioned. Then you have to decouple the DC part e.g. by a 1 uF
series C and pass the signal through a standard 5 pole low passfilter
(12,2nF/53µH/24,7nF/53µH/12,2nF). That's all. You should get an
efficiency of at least 90 % !
Best was to optimise the PA is to watch the U(DS) wave form. You will
find infos how it should look. Ah, it should look as on the homepage of
http://classe.monkeypuppet.com/ :-)
Wish you good luck und fun in optimising!
73, Stefan
PS: It is helpful to create an excel file where you type input voltage
and input current and output power and let you calculate the efficiency
to see if it has increased or not. You can start with an adjustable
voltage source if the driver has its own supply! :-)
Am 07.09.2010 21:08, schrieb Roger Lapthorn:
Stefan/Mal,
Clearly my design was not working in class E properly as the PA was
dissipating a fair amount of heat and it was taking about 0.8A to
produce 40V p-p output from a 13.8V supply which is about half as
efficient as it should be. I took a PA design start from a design
program I found on the net by Tonne Software called "Class E" and the
values seemed to be close to optimum when I adjusted caps and L up and
down a small amount, but the dissipation seems miles off what a good
class E design should take. The IRF510 gate voltage is set just below
3V with a resistive pot. The drive is not a square wave though, so I
suppose this could be part of the issue?
Stefan, if you have a better class E PA circuit for 137kHz suitable for
around the 10-20W out region I'd be interested in seeing it. I would
prefer to use toroid inductors because of size. I have several T106-2
cores to hand as well as several 16mm diameter 3C90 cores.
I'm still learning..... and not too proud to ask for help from those
with more experience.
73s
Roger G3XBM
2010/9/7 Stefan Schäfer
<[email protected]>
Hello Roger,
Maybe you can send us a circuit with its values. I have built class E
PA for 2200/160/80/40/30/17/10m and have some experience. If you are
using a IRF510 and stay in the QRP range (< 10 W) at 137 in a class
E stage you wouldn't even need a heat sink at all if well dimensioned!
See the infos at
http://www.classeradio.com/
This is most useful if
you are starting with that design. On 137 kHz problems are much smaller
than on HF :-)
Best 73, Stefan/DK7FC
Am 07.09.2010 20:17, schrieb Roger Lapthorn:
Just hit a bit of a snag and blew up my
(last) IRF510 PA
because of totally inadequate heatsink. So, I'm on 137.5kHz WSPR but
only with about 1W from the simpler PA at present, so ERP low uWs. I
need to get some heatsink and more FETs!
Any reports (now less likely) still appreciated.
[Mal - it started out as a Class E design attempt but hadn't been
optimised, so I guess this counts as class D. ]
73s
Roger G3XBM
On 7 September 2010 17:34, Roger
Lapthorn
<[email protected]>
wrote:
My
137.5kHz
WSPR TX transverter is now working fine, so I'll be on-air
with WSPR tonight (Sept 7th) from 5.30pm - 11pm UK time.
After my success on QRSS3 with G3XIZ I'm hopeful he'll now decode my
WSPR signal tonight, assuming Chris can take a listen. If anyone else
within, say, 100km of Burwell, Cambs JO02dg cares to take a look for
the WSPR beacon (running around 35% TX time) I'd be most
grateful.
Not sure if anyone else has had this problem, but I was using
the small 16mm 3C90 cores (cream colour) as the PA bifilar output
transformer and I had all sorts of issues with the darn thing
overheating and even one cracking. I replaced this with a choke wound
on a T106-2 core and made the output network work in class D/E and now
everything is working very predictably. The larger diameter 3C90 cores
seem fine in the step-down transformer matching the "in the air" loop.
The WSPR beacon is now going ON.....
73s
Roger G3XBM
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G3XBM GQRP 1678 ISWL G11088
--
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http://www.g3xbm.co.uk
http://www.youtube.com/user/g3xbm
G3XBM GQRP 1678 ISWL G11088
--
http://g3xbm-qrp.blogspot.com/
http://www.g3xbm.co.uk
http://www.youtube.com/user/g3xbm
G3XBM GQRP 1678 ISWL G11088
