Dear LF Group,
Andy Wrote:
>The parasitic diode is no good in this configuration for catching the
>spikes from what I call the transient soak choke, I use separate clamp
>diodes up to the rails effectively 'crosing over' the centre tapped choke
>- see the circuit in the new LF Experimenters Book.
I think this is the main function of the "transient soak" chokes in the
Decca design - diverting the current into the fast diodes rather than
through the Mosfet diodes during the switching transient. There is
not much chance of cross conduction through the mosfets, since
the transformer coupled gate circuit inherently prevents the gate
drive from overlapping.
Johan wrote: ........Current switching and Voltage switching.
Full and half H-bridges (the Decca TX has this configuration I guess?) are
voltage switching amplifiers where the drain voltage is a square wave and
the fundamental frequency energy is sucked out by a series resonant
circuit to the load.
The Decca design is indeed a voltage-switching full bridge - but the
series resonant tank circuit has a high impedance everywhere
except the output frequency, so it won't see a capacitive load at
the harmonics - mine seems quite happy running into a 2 x pi
section low pass filter. There are some fairly nasty looking spikes
which seem to be produced by the drive circuit that reach the
output one way or another otherwise - although I'm not sure if they
are actually there, or just being coupled into the output monitor
wiring, which runs through one of the PA compartments.
Cheers, Jim Moritz
73 de M0BMU
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