maximum junction temperature”. Since the drain current in this application is NOT in fact
continuous (it is an RF cycle, and each transistor conducts for half the RF cycle), one might argue
that the permissible rating is higher than 5.6A (25C) / 4.0A (100C). In any event, we can see that
higher current will eventually kill the transistor so increasing the impedance ratio has limitations.
Furthermore, the higher current will also cause a higher magnetic flux density in the ferrite
transformer core. As the core approaches saturation it will become lossy, increasing its
temperature, and this will lower efficiency (and power output). In the end it would be necessary to
increase the physical amount of iron in the core.
The conclusion is that at higher impedance ratios, we can obtain more output power for a given
supply voltage; but the penalty is higher current which places more strain on the transistors and
the ferrite of the output transformer.
The question of materials to use is an important one from a kit production standpoint. A driving
factor is cost. Availability is another. A beautiful binocular core typically used is the BN43-202
made by Amidon or an equivalent by Fair-rite. But these are expensive, and they must be shipped
from US and imported. They are also rather small so the choice is to stack multiple binocular cores
together, or to buy larger more expensive cores. It would be much more cost-effective to be able
to use locally available unbranded ferrite, which is lower cost and doesn’t carry the shipping and
import costs. But ONLY if the local ferrite provides equivalent performance! Therefore ideally we
want low cost ferrite, only IF it works well enough, and in the smallest possible size, available
locally. The binocular format is convenient and efficient, and is usually used in power amplifiers.
I undertook a large number of tests of various materials and turns ratios, some of the more useful
ones are plotted in the following graphs. Materials tested:
•
Large unbranded ferrite toroid
•
Large unbranded ferrite binocular core
•
Amidon BN43-202 ferrite binocular core, four stacked together, or two stacked together; the
size of the four stacked together was similar to the size of the large unbranded core or the 2
unbranded ferrite tubes side-by-side.
•
Two unbranded ferrite tubes laid side-by-side
50W QCX PA kit assembly
1.00q
48
