Hitachi RAC-50YH7, Service Manual

Page: 65 / 103

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4. Power Supply Circuit
●
Fig. 4-1 shows the power circuit.
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There are two switching power supply in Power PWB.
●
Switching power supply 1 is generating the secondary power for control circuits and DC35V indoor unit.
●
Switching power supply performs voltage conversion effectively by switching transistor IC1 to convert DC330V
to high frequency of approximately 20kHz to 200kHz.
●
Transistor IC1 operates as follows:
(1) Shifting from OFF to ON
●
DC about 330V is applied from smoothing capacitors C020
䊝
and
䊞
in the control power circuit. With this
power, current flows to pin 4 of IC1 via R701 and IC1 starts to turn ON. Since voltage in the direction of
arrow generates at point
c
at the same time, current passing through R708 and D702 is positive-fed
back to IC1.
(2) During ON
●
The drain current at IC1 increases linearly. During this period, the gate voltage and current become
constant because of the saturation characteristics of the transformer.
(3) Shifting from ON to OFF
●
This circuit applies a negative feedback signal from the 12V output. When the voltage across C708
reaches the specified value, REG2 turns on and current flows to PQ8
1
-
2
. This turns the secondary
circuits on, sets IC1 pin 1 to “Hi”, and turns IC1 off.
Fig. 4-1 Power circuit for P.W.B.