Toshiba CN27E90 Technical Training Manual page 75

4-1. Theory of Operation
4-1-1. Operation of Basic Circuit
(1) To perform the horizontal scanning, a 15.734 kHz
sawtooth wave current must be flown into the horizontal
deflection coil. Theoretically speaking, this operation
can be made with the circuit shown in Fig. 11-7 a and
b.
(2) As the switching operation of the circuit can be replaced
with switching operation of a transistor and a diode, the
basic circuit of the horizontal output can be expressed
by the circuit shown in Fig. 11-7 a. That is, the transistor
can be turned on or off by applying a pulse across the
base emitter. A forward switching current flows for on-
period, and a reverse switching current flows through
the diode for off-period. This switching is automatically
carried out. The diode used for this purpose is called a
damper diode.
a H output basic circuit
H output
transistor
b H output equivalent circuit
SW1
L
D Co
Damper
Resonant
diode
capacitor
Vcc
SW2
Co
L
Vcc
Fig. 11-7
Description of the basic circuit
1. t1~t2:
A positive pulse is applied to base of the output transistor
from the drive circuit, and a forward base current is flowing.
The output transistor is turned on in sufficient saturation
area. As a result, the collector voltage is almost equal to the
ground voltage and the deflection current increases from
zero to a value in proportionally. (The current reaches
maximum at t2, and a right half of picture is scanned up to
this period.)
2. t2:
The base drive voltage rapidly changes to negative at t2 and
the base current becomes zero. The output transistor turns
off, collector current reduces to zero, and the deflection
current stops to increase.
3. t2~t3:
The drive voltage turns off at t2, but the deflection current
can not reduce to zero immediately because of inherent
nature of the coil and continues to flow, gradually decreasing
by charging the resonant capacitor C0. At the same time, the
capacitor voltage or the collector voltage is gradually
increases, and reaches maximum voltage when the deflection
current reaches zero at t3. Under this condition, all electro-
magnetic energy in the deflection coil at t2 is transferred to
the resonant capacitor in a form of electrostatic energy.
4. t3~t4:
Deflection
Since the charged energy in the resonant capacitor discharges
yoke
through the deflection coil, the deflection current increases
in reverse direction, and voltage at the capacitor gradually
reduces. That is, the electrostatic energy in the resonant
capacitor is converted into a electromagnetic energy in this
process.
5. t4:
When the discharge is completed, the voltage reduces to
zero, and the deflection current reaches maximum value in
reverse direction. The t2~t4 is the horizontal flyback period,
and the electron beam is returned from right end to the left
end on the screen by the deflection current stated above. The
operation for this period is equivalent to a half cycle of the
resonant phenomenon with L and C0, and the flyback period
is determined by L and C0.
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