7. The binocular core formed from two unbranded ferrite tubes actually gave slightly higher
power output than the four BN43-202 stacked cores, and this higher power output was even
more noticeable above 50W. Efficiency of the tests using the unbranded ferrite tubes was
slightly worse than the four BN43-202 stacked cores at lower power levels, but was better
at higher power levels.
This final conclusion is the most surprising. Basically, it is not necessary to use expensive Amidon
ferrite and ship it around the world. The results with locally available unbranded ferrite are just as
good, and at 10-20x lower price! A very pleasing result indeed.
These conclusions led to the final design of T3 which uses the two side-by-side unbranded ferrite
tubes to create a binocular core, and the 2:3 turns ratio which is apparently definitely optimum for
IRF510-based power amplifiers in this application.
Further reading on the power amplifier section of the design:
•
My starting experiments on a single-ended 35W IRF510 40m CW PA and then a 50W
IRF510 push-pull successor are documented here:
•
Mike WA2EBY’s two articles in QST’s March and April 1999 issues are very instructive.
Mike builds a broadband HF amplifier stating “with only 1W of drive, you’ll get over 40W out
– from 160 through 10 meters!”. He uses two IRF510 transistors with 2:3 turns output
transformer, a very similar arrangement to the QRP Labs 50W amplifier. See
and scroll to the bottom of the page.
The similarity of Mike’s design and the fact he is aiming for linear operation, could inspire you to
try this 50W amplifier as a linear. Mike does not present any linearity measurement (two tone
IMD3 for example). But I feel a lot of further experiment would be possible with this amplifier, for
linear applications.
6.4 Thermal considerations
Before going any further, and on the topic of the main PA section, the heart of this project – it is
necessary to discuss HEAT.
Heat is my old enemy, for as long as I can remember. No less so on a POWER AMPLIFIER!
A problem with the IRF510 is the relatively high thermal resistance of the junction-to-case,
specified as 3.5 C/W. This means that for every watt of dissipated heat, the junction temperature
relative to the case will rise by 3.5 C.
Additionally the datasheet quotes a case-to-sink value for a flat greased surface, of a minimum of
0.5 C/W and a maximum junction-to-ambient thermal resistance of 62 C/W. The latter assumes no
heatsinking which is clearly not relevant here unless our hobby is rapid explosions of inexpensive
MOSFETs.
Furthermore the operating junction temperature range is specified as -55C to +175C absolute
maximum rating. Practically we should try to stay substantially below 175C to assure long-term
reliability of the transistors. The failure rate of most silicon semiconductors decreases
approximately by half, for a reduction in junction temperature from 160C to 135C. Military
standards are that junction temperature does not exceed 110C. (Source: ON Semiconductor, App
note AN1040-D).
So now let’s calculate. Suppose we are producing 50W, at an efficiency of 50% (to keep it round
numbers). That means for every 50W of RF we produce, we are also producing 50W of unwanted
50W QCX PA kit assembly
1.00q
50
