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Dear Scott, LF Group,
It is fairly normal for ferrite-cored transformers
to make funny clicks and squeals due to magnetostriction of the ferrite. This is
the magnetic version of the piezo-electric effect; as the flux density changes,
the dimensions of the ferrite change, leading to vibration of the core. It
doesn't seem to do much harm.
The voltage experienced by the FETs depends on the
circuit configuration - assuming we are talking about an "MRF" type of push-pull
circuit, in an ideal circuit with a purely resistive load, the peak drain
voltage will be twice the supply voltage. It will be slightly higher if the
circuit is a "current fed" configuration with a capacitive-input low pass
filter. In practice, the presence of HF ringing potentially leads to much higher
voltages; the best way to tell is to connect an oscilloscope to the drain,
which may reveal frightening results! As was discussed on the reflector some
weeks back, this can be minimised by reducing the output transformer leakage
inductance to the minimum, keeping the primary inductance low, and connecting
series RC damping from each drain to ground. If you are using a low-pass filter
on the output with a shunt input capacitor, you will also need a suitable
un-bypassed choke feeding the DC into the centre tap of the transformer primary.
However, these measures will reduce but not entirely eliminate the ringing, and
the peak drain voltage in my amplifier is about 3 - 3.5 times the supply
voltage. So 48V supply should be OK with 200V devices, provided suitable
attention is paid to the ringing. Certainly, there are several TXs around
operating at similar levels. If there is excessive ringing, the MOSFETS behave
as big zener diodes, clipping the waveform and dissipating a lot of heat in the
process. The TO220 devices in the pictures on your web site will not be able to
dissipate much more power than they are already at 300W output, so this is
certainly something to look for.
The class E design seems to give cleaner waveforms,
but results in higher peak voltage in the first place - the component values are
probably more critical too. Also, as Alan points out, you have to think about
what can happen when the antenna goes off-tune and gives a highly reactive load.
I reckon the push-pull design has the virtue of simlicity for supply voltages
less than about 50V. Another possibility suitable for higher supply voltages is
the 'Decca' bridge configuration (or the "half-bridge" as used by G4JNT in his
off-line design). This circuit topology is inherently self-limiting, a feature
that effectively limits the peak MOSFET voltage to a little over the DC supply
voltage.
Cheers, Jim Moritz
73 de M0BMU
----- Original Message -----
Sent: Saturday, July 17, 2004 9:31
PM
Subject: LF: clicking matching
transformer...
Any thoughts on if a matching transformer should
'click' when the key is released... Obviously it is likely an inductive
reaction to the rapid change in current supplied...
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