Hitachi RAM-40QH5 Service Manual page 45

(2) During ON
The drain current at IC1 increase linearly. During this period, the gate voltage and current become
G
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
G
reaches the specified value, REG2 turns on and current flows to PQ8
circuits on, sets IC1 pin
(4) During OFF
While IC1 is on, the following energy charges the primary windings of the transformer:
G
Energy = LI
This energy discharges to the secondary windings during power off. That is, C707-C710, C718 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-C710, C718.
G
The drain current at IC1 generates a voltage across R716. If it exceeds the IC1 base voltage, it sets
G
the IC gate voltage to "HI".
R716 limits the gate voltage to prevent excessive collector current from flowing to IC1.
G
This SW power circuit uses a frequency as low as 20kHz, especially at a low load (when both the
G
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, 17V and -5V 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:
Fan, operation, power, rush prevention relay (shorting in relay, etc.)
1
REG 1 (regulator is abnormal), etc.
2
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/17V power supply is abnormal:
D710, D713 or drive circuit is abnormal.
(5) When all voltage are abnormal:
IC1, R716, 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.
[When the switching power supply seems to be abnormal, the voltage between IC1 pin
the leads of R202 and R201) and IC1 pin
This is because the protection circuit of IC1 is operating.]
to "Hi", and turns IC1 off.
1
2
/2. Here, L : Primary inductance
I : Current when IC1 is off
(to be measured at R216 lead) may be between 11 and 16V.
5
– 51 –
- . This turns the secondary
1 2
(to be measured at
4