Hi Tom, |
Good luck with your PA.
Tell us the component values when you're finished, as well as the
actual efficiency. From my LF PA i remember the coil, which was quite
large already ('air-cored').
Regarding current peaks at the first switching: I would simply ignore
the problem and see if the FET will survive (and how old it becomes).
If it's lifetime is actually to short, then i would start thinking
about a solution for the problem. One of them just costs 2 EUR or so,
so it is worth to try...
Am 14.10.2018 00:16, schrieb DK1IS:
Just to close this thread:
Concerning the problem of the first drive pulse with DC power already
on: Bob, W1XP, made an important contribution in a private mail. He
pointed out that the current spike I was worried about would be limited
by the FETs output characteristic anyway. Looking to the data sheet of
my intended IRFB 4227 this seems to be true although there could
persist a certain limitation by the save operation area. Perhaps I was
a little bit too naive estimating the peak current as Vdc / Rdson ...
Be that as it may, using the newly discovered DC soft start feature of
my power supply should overcome this challenge. Controlled by a logic
level input it runs up the output voltage with a time constant of about
100 ms. This will protect the FET, avoid any key clicks and won´t
affect the readability of the long lasting vlf signals. To keep the
signal phase constant (essential on vlf) oscillator and gate drive of
the FET won´t be keyed at all.
I´ve just modified the antenna / loading coil system to resonate at
8270 Hz without transformers, - 3dB band width now being 168 Hz.
Including 25 m RG213U from the arbour to the shack (being later on part
of the matching network) the feeding impedance is now 480 Ohms pure
resistive which will be matched to the rest of the class E PA by its
output network. Thus I`m able to omit the two matching transformers
being used up to now, see www.qrz.com/db/dk1is.
Now I´m waiting for the driver ICs coming from China - in Germany up to
now without additional tax ;-) !
Thanks for all the feedback, 73,