QRP Labs QCX 5W CW, Руководство по сборке, Страница: 113 / 138

QCX assembly Rev 1.08
113
Some excellent background reading are two papers by Paul Harden NA5N:
http://www.aoc.nrao.edu/~pharden/hobby/_ClassDEF1.pdf and
http://www.aoc.nrao.edu/~pharden/hobby/_ClassDEF2.pdf
Paul NA5N describes two defining features of Class-E:
1) Use of a square-wave drive to reduce switching losses: the transistors are either on, or
off… no lossy region in between
2) Reducing the effects of the transistor capacitances. Class-E has a resonant tuned circuit.
The capacitance of the transistors, normally an unpleasant lossy aspect, is now a part of
the tuned circuit.
Class-E also has a reputation for being difficult to achieve. All those intense mathematics Google
will help you find, don’t help. In reality, once you realise the secret – it is not so difficult. Calculation
of the impedance of a resonant circuit is simple, and there are many online calculators which will
do the job for you. For example,
http://toroids.info/T50-2.php which allows you to type in the
operating frequency, and the desired resonant circuit impedance. Then the calculator computes
the required inductance, capacitance, and the number of turns required for a certain toroid (in our
case we use a T50-2).
The Class-E design process is simple. Choose the output impedance. We choose 50-ohms,
because this is the input impedance of the Low Pass Filter we will use. The online calculator will
tell you what inductance is needed, and how many turns to wind on the toroid. The online
calculator also tells you the required capacitance to bring it to resonance at the operating
frequency. Here we resort to experiment, because it is a little difficult to know what the output
capacitance of the transistor is. The device capacitance varies depending on supply voltage and
whether it is on or off. A simple experiment is required, adding different small capacitances to the
circuit, and measuring the efficiency (measure supply voltage and supply current to calculate
power input; then measure RF power output. Divide one by the other to get the efficiency). It is
easy to find what additional capacitance is required to peak the efficiency. The resonance is quite
broad and non-critical.
In this implementation, three BS170 transistors are used in parallel. The BS170 is inexpensive and
small, but is rated for 500mA drain current and up to 830mW of dissipation. Per device. Three in
parallel provides plenty of capability to achieve a 5W output on a single band.
There are always minor variations between device characteristics from one transistor to the next. If
these were bipolar NPN transistors, we would not be able to parallel them in this way. If one
transistor takes more of the load and starts to heat up, its resistance further decreases and this
causes it to get even hotter. This process is known as “thermal runaway” and results (quickly) in
destruction of the transistor. Emitter resistors are used to help balance the load. But with
MOSFETs, their resistance INCREASES as the temperature goes up
– so there is an inherent
self-balancing when multiple devices are used in parallel, without any need for additional
balancing resistors which would increase component count and waste some power.