Toshiba MM20E45, Техническое учебное пособие

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9-14
1-6 Hor Drive Circuit
Because difference of hor scanning frequency varies
remarkably the base current flowing in hor output transistor,
it is necessary to change operational condition of drive
circuit.
First, it is required to change duty ratio of drive pulse.
Comparing waveforms (a) and (b) in Fig.17, it is understood
that the rate of storage time t
stg
of hor output transistor in one
cycle becomes very large when frequency is high, even if t
stg
is the same. Accordingly, to avoid damage of transistor due
to supply of base current to hor output transistor in period that
flyback pulse is generated, it is necessary to make narrow the
period that base current is supplied (that is; period that drive
transistor QH33 is turned off.), when frequency is high.
Reversely, when frequency is low, it is necessary to shorten
the period that base current flows, to prevent failure in
operation of high voltage circuit. This term is explained later.
From these reasons, duty ratio is switched over so that in low
frequency off-period of drive transistor QH33 becomes
wide, and in high frequency off-period becomes narrow.
This switching-over, as explained in the section HOR OSC
CIRCUIT, is done at frequency of arround 28kHz, and QH15
is used for the element. Not only duty ratio is switched over,
but also amount of current is changed. This is caused by that
the base current gradually decreases from the initial value in
the rate of slope which is decided by inductance of secondary
winding of drive trans. Since period of flowing current is
long when frequency is low, the decreasing rate of current is
large, and I
B1end
becomes remarkably small comparing with
the case that frequency is high. Then when frequency is low,
voltage applied to primary of drive transformer is switched
over to raise it.
The voltage which is supplied to primary of trans, is adjusted
by resistor which is connected to +15V source, and resistor
RH26 is shorted by turning on of QH34 when frequency is
low. By this operation, the voltage of approx. 15V in low
frequency, or approx. 6V in high frequency is switched over
to be applied to pin1 of transformer.
The operation of drive circuit itself is the same as that of
ordinary TV set excepting that FET is used for drive transistor
QH33. The explanation is omitted here.
tstg
I
B1
end
0
a
fH=15.7kHz
tstg
I
B1
end
0
b
fH=31.5kHz
Fig. 17 Base current of hor output transistor
Fig. 18
+15V
RK26
RK25
QH34
QH35
TH03
1
3
6
4
QH30
QH33