Am 30.05.2017 15:04, schrieb Graham:
...to question the choice of a half bridge , over that of the full
version?
A half bridge is easier to build because both sources of the FET are at
GND level. So you are not forced to use a decoupling transformer for
driving the FETs. I would recommend to use one anyway, and it works
quite well and it is an advantage to pull the gate of the non conducting
FET to a negative voltage.
The H bridge has an advantage when mains voltage is used directly, i.e.
320 V DC switching voltage.
There are 1000 V FETs however, meanwhile.
Oh i just see the point, a half bridge has two FETs in series and the
other half are capacitors. I was talking about the other circuit which
uses the lower two FETs not the left two FETs of the H bridge :-)
I would not recommend a hlaf bridge at all.
Its tempting to link the harmonic content , with the high loss rate
of devices , In your 136 vk experiments, with the H , did you
loose fets , other than by loss of tune ?
I only lost FETs in the beginning, until i discovered that i made an
error when building up the driver. After correcting this i didn't loose
any FETs. I passed several kWh through one pair of FETs. No problems in
WSPR, OP32, CW, even slow hell (which actually is for linear PAs only!).
I have a fan blowing on the heat sink whos speed is controlled by the
temperature of the heat sink. So i always get an impression of the
temperature (when listening to the speed :-) ). When the antenna was not
spot on the resonance, the fan blew a bit faster, so the losses must
have been somewhat higher. But it never cost the FETs (IRFP460A).
Is there a ferrite , that is comfortable in 136 and 477 service ?
Several, almost any.
but worth consideration for a driver, having no transformer
The standard FET drivers (ICL7667, TC4422, IXDD614CI...) are switching
between Vss and GND. It is however saver to pull the FETs gate voltage
to a negative value when OFF. This is given by the transformer. Since
the voltage of the opposite winding must be negative when the other one
is positive, it is impossible that both FETs are conducting, what ever
the driver and transients does.
effectively all band from audio up ?
From DC up :-) Yes, but not with some high/low side drivers like
IR2110. I built a second bridge for VLF some years ago and used it down
to 100 Hz...
73, Stefan
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