----- Original Message -----
Sent: Sunday, July 18, 2004 5:08 AM
Subject: LF: Re: clicking matching
transformer...
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...
