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DESCRIPTION
OF NEW
CIRCUIT
DC-DC Inverter Circuit
This model is provided with a DC-DC inverter circuit so
that it can be used outdoors operating on the automobile
battery (12V DC).
AC operation and DC operation are changed from each
other automatically by using the AC or DC power cord to
connect it to the set.
,
The DC-DC inverter circuit ensures not only to amplify the
output voltage but also to regulate the input voltage against
its fluctuation.
Fig. 1 is a block diagram of the DC-DC inverter circuit.
The square-wave signal which has been caused by the hori-
zontal oscillator circuit (IC501) is first applied to the integ-
ration circuit where it is converted to a sawtooth-wave
signal to be fed to the pulse-width converter circuit. In the
pulse-width converter circuit, the sawtooth-wave signal is
again converted into a square-wave signal then entering the
inverter drive circuit where it is amplified enough to drive
the inverter output circuit.
The output voltage from the inverter output circuit is set to
+115V DC, and it is applied to the error detection circuit
where it is compared with the reference voltage to control
the pulse-width converter circuit.
Fig. 2 shows what parts the inverter circuit is composed of
and Fig. 3 reveals the waveforms of the voltages which are
available at the points @ to ® in Fig. 2.
Coming from the horizontal oscillator circuit, the square-
wave voltage appears at the point @) and is integrated by
R779 and C775 so that it is converted into a triangular-
wave voltage as shown in Fig. 3-(B).
The triangular-wave voltage is applied to the base of Q2:
this transistor Q2 is biased to turn on only with the voltage
which is lower than the level indicated by the chained line
in Fig. 3-(B). Now that the triangular-wave voltage shown -
by the solid line in Fig. 3-(C) is applied to the base of Q2,
the transistor Q2 turns on only when it is given the voltage
Integra-
tion
circuit
indicated by the shaded area in Fig. 3-(B): as the result,
at the point © in Fig. 2 there appears the voltage having
the waveform
indicated by the solid line in Fig. 3-(C).
Then the voltage is sent to Q3 where its waveform is in-
verted to be the one shown in Fig. 3-(D). The transistor
Q3 is to activate the drive transformer T770. Actually when
Q3 turns off, T771 is allowed to turn on to get the output
transistor
Q770
in operation; the voltage caused at the
point © in Fig. 2 (at the base of Q770) has the waveform
as shown in Fig. 3-(E).
- at T771
increases so that the output voltage from D775
also increases.
The rectified output voltage is then sent to the succeeding
load circuit. However, a part of it is fed back to Q1 and
then to Q2 via ZD1 to control biasing of Q2.
Now suppose that there is a rise of the output voltage of
the transformer due to some reasons (e.g. due to an increase
of input DC voltage or an alleviation of the output load).
Then the current at the base of Q1 increases with its
collector's current also increasing. Therefore the voltage at
the base of Q2 decreases and this means that the turn-on
time of Q2 becomes longer than before: the output wave-
form then caused refers to that indicated by the brokin line
in Fig. 3B). The output waveforms available thereafter
are those indicated by the broken lines in Fig. 3-(C), -(D),
4E), -(F) and (G): this fact is to shorten the turn- on time
of Q770 so as to decrease the magnetic energy charged in
T771. As the result, the output voltage from T771 which
has been higher than specified is now decreased to its
normal level.
Pulse width
Inverter
Inverter
converter
drive
output
circuit
circuit
circuit
Error
detector
circuit
'
OUT
DC 115V
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