Toshiba CN27E90 Technical Training Manual page 67

(4) To decrease the collector loss of Q3, the power supply
voltage is decreased during scanning period as shown
in Fig. 10-7, and VCE1 decreases and the collector loss
of Q3 also decreases.
Scanning period
Flyback period
Fig. 10-7 Output stage power supply voltage
(5) 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. 10-8.
D301
Q301
6 3
Q3
D1
Q4
(a) Scanning period
Q3 Collector loss decreases
by amount of this area
Power supply
for flyback period (Vp)
Power supply
for scanning period
(Vcc)
C308
D308
D309
Switch
7
L462
2
(6) Since pin 7 of a transistor switch inside Q301 is connected
to the ground for the scanning period, the power supply
(pin 3) of the output stage shows a voltage of (VCC-
VF), and C308 is charged up to a voltage of (VCC-VF-
-VR) for this period.
(7) First half of flyback period
Current flows into L462
(+27V)
and the voltage across these is:
VP=VCC+VF+(VCC-VF-VR)+VF about 50V is
applied to pin 3. In this case, D301 is cut off.
(8) Last half of flyback period
Current flows into VCC
Q301 (pin 3)
and a voltage of
VP=VCC-VCE (sat)-VF+(VCC-VF-VR)-VCE (sat),
about 40V is applied to pin 3.
(9) In this way, a power supply voltage of about 27V is
applied to the output stage for the scanning period and
about 50V for flyback period.
Q301
6
R309
Q3
Q4
C306
R305
Fig. 10-8
67
D1 C308
GND R305 C306
switch
Q3 L462 C306
D301 C308
3
D309
Switch
7
D1
2
Last half
(b) Flyback period
D308 VCC
L462 in this order,
D309 C308
R305 in this order,
R309
VR
First half
L462
C306
R305