Hitachi RAM-90QH5, Service Manual, Page: 54 / 83

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(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 C711
reaches the specified value, current flows to PC701
1
-
2
. This turns the secondary circuit on, sets IC1
pin
2
to “LO”, and turns IC1 off.
(4) During OFF
While IC1 is on, the following energy charges the primary windings of the transformer:
Energy=Ll 2 /2. Here, L : Primary inductance
I : Current when IC1 is off
This energy discharges to the secondary windings during power off. That is, C707-C711 is charged
according to the turn ratio of each winding.
At the start, an overcurrent flows to IC1 because of the charged current at C707-C711.
The drain current at IC1 generates a voltage across R703. If it exceeds the IC1 base voltage, it sets the
IC gate voltage to “Hi”.
R706 limits the gate voltage to prevent excessive collector current from flowing to IC1.
This SW power circuit uses a frequency as low as 20 kHz. especially at a low load (when both the indoor
and outdoor units stop): This reduces power loss in standby status.
< Reference >
If the power circuit for P.W.B. seems to be faulty:
(1) Make sure that 5V, 12V, 15V and 16V on the control P.W.B. power voltage are the specified values.
(2) When only the 5V output is low:
REG 1 (regulator) faulty, 5V-0V shorted, output is too high, or REG 1 is abnormal.
(3) When 12V and 5V are abnormal:
The following defects can be considered:
1
Fan, operation, power, rush prevention relay (shorting in relay, etc.)
2
REG 1 (regulator is abnormal), etc.
Shorting on primary circuits.
When shorting occurs in the secondary circuits, there is no abnormality in the primary circuits because
of overcurrent protection.
The voltage rises when an opening occurs in the primary circuits, or the feedback system is abnormal.
(4) When 15V and 16V power supply is abnormal:
D708, D707 or Drive circuit is abnormal.
(5) When all voltage are abnormal:
IC1, R703, may possibly be defective. Also D cable may possibly be reverse connected.
If IC1 is abnormal, be aware that other components, such as the power module, REG (regulator), etc. are
possibly defective.