8-4
To decrease the collector loss of Q3, the power supply
voltage is decreased during scanning period as shown in
Fig. 7, and VCE1 decreases and the collector loss of Q3
also decreases.
Q3 Collector loss decrease
by amount of this area
Power supply for
flyback period (Vp)
Power supply for
scanning period
(Vcc)
Scanning period
Flyback period
Fig. 7
Output stage power supply voltage
In this way, the circuit which switches power supply
circuit during scanning period and flyback period is
called a pump-up circuit. The purpose of the pump-up
circuit is to return the deflection yoke current rapidly
for a short period (within the flyback period) by applying
a high voltage for the flyback period. The basic operation
is shown in Fig. 8.
(a) Scanning period
(b) Flyback period
Fig. 8
Since pin 7 of a transistor switch inside IC301 is
connected to the ground for the scanning period, the
power supply (pin 3) of the output stage shows a voltage
of (V
CC
– V
F
), and C308 is charged up to a voltage of
(V
CC
– V
F
– V
R
) for this period.
Last half of flyback period
Current flows into L462
D1
C308
D308
VCC (+27V)
GND
R305
C306
L462 in
this order, and the voltage across these is:
V
P
= V
CC
+ V
F
+ (V
CC
– V
F
– V
R
) + V
F
about 58V is
applied to pin 3. In this case, D301 is cut off.
First half of flyback period
Current flows into V
CC
switch
D309
D308
IC301 (pin 3)
Q3
L462
C306
R305 in this
order, and a voltage of
V
P
= V
CC
– V
CE (sat)
– V
F
+ (V
CC
– V
F
– V
R
) – V
CE (sat)
,
about 54V is applied to pin 3.
In this way, a power supply voltage of about 29V is
applied to the output stage for the scanning period and
about 54V for flyback period.
D301
C308
D308
D309
R309
Q301
Q3
Q4
D1
2
7
L462
C306
R305
Switch
6
3
D301
C308
D308
D309
R309
Q301
Q3
Q4
D1
2
7
L462
C306
R305
Switch
6
3
Last half
VR
First half
