Panasonic EURO 4 Chassis, Техническое руководство

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3DQDVRQLF
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9.2. Horizontal Output Stage
Control of the horizontal output stage, or to be more
precise, the horizontal switching transistor, is
achieved as explained in the previous section.
Essentially, the base resistance of the horizontal
switching transistor consists
of the very low
impedance resistor R506 and the series connected
secondary winding of driver transformer T501.
In practice, the parallel connected east/west
modulator diodes D551, D552 perceptibly reduce
the load on the switching transistor.
This type of transistor additionally possesses an
integrated
diode section in parallel to the
collector-emitter section which conducts in inverse
mode i.e. negative applied to the collector.
This inverse mode of the transistor takes place
during the first half of the sweep, up to about line
centre. In the subsequent second half of sweep, the
transistor operates in normal mode again with
conductive base emitter junction.
Only during the relatively brief flyback time is the
switching transistor blocked.
By virtue of the low impedance circuitry already
described, it is possible for base peak currents of
b 0.9A and - 0.9 A to flow sufficiently
quickly in both directions.
It can be imagined that the significantly higher
emitter currents (i.e. max. = 4.5 A) cause enormous
"flooding" of charge carriers in the N-P and P-N
junctions of the semiconductor.
To ensure that the rapid switching action takes place
and that the charge carriers can discharge rapidly in
the base zone, base driving has been made
correspondingly low-impedance.
In connection with this, it should also be mentioned
that the control pulses have been formed specially
for dual utilisation of
Q551 in normal and inverse
mode. In principle, the pulse duty factor of the base
drive has been altered from 12
m
S flyback time and
52
m
S sweep time to 20
m
S flyback time and 44
m
S
sweep time. Thus, it is also possible to process any
unavoidable delays in the driver transformer. As a
result of prompt driving, the switching transistor has
adequate preparation for the subsequent operation
phase.
The supply voltage of the horizontal output stage is
taken from the switched mode power supply which is
approximately 147v for CRT sizes of 66cm (28 ins)
and 59cm (25 ins). The exact control and switching
waveform during a line period corresponds to that of
the switched resonant circuit.
Here, the line switching transistor operates by virtue
of its three operating states. Conductive. Inversely
Conductive, and Off, in such a way that the charge
and discharge processes of collector capacitor C551
and inductor L552 produce the sweep and flyback
intervals exactly.
Only during the horizontal flyback time, is Q551
blocked for 20
m
S. The capacitor (C551) in the
parallel resonant circuit forms, (by virtue of its being
charged by the supply voltage), the positive half of a
sine wave oscillation.
The natural tendency of a parallel resonant circuit to
reverse
the charge stored in the capacitor
subsequently into the inductor as magnetic energy,
leads to polarity reversal of the current.
From this, the negative component of the sine wave
oscillation normally forms at the collector of
Q551 .
This tendency is however prevented by the
Collector/Base diode section of Q551 . After the
collector/base start up voltage has been reached,
the transistor becomes conductive and clips the
negative component. The east/west modulator
diodes would also assist this function and thus
reduce the power dissipation in the switching
transistor.