Hitachi RAC-25NH5, Service Manual

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(2) During ON
●
The drain current at IC901 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 C919
reaches the specified value, REG2 turns on and current flows to PQ2
1 - 2 . This turns the secondary
circuits on, sets IC901 pin 1 to “Hi”, and turns IC901 off.
(4) During OFF
●
While IC901 is on, the following energy charges the primary windings of the transformer:
Energy=LI 2 /2. Here, L : Primary inductance
I : Current when IC1 is off
This energy discharges to the secondary windings during power off. That is, C910, C911, C912, C914 is
charged according to the turn ratio of each winding.
●
At the start, an overcurrent flows to IC901 because of the charged current at C910, C911, C912, C914.
●
The drain current at IC901 generates a voltage across R906. If it exceeds the IC901 base voltage, it sets
the IC901 gate voltage to “HI”.
●
R906 limits the gate voltage to prevent excessive collector current from flowing to IC901.
<Reference>
If the power circuit for P.W.B. seems to be faulty:
(1) Make sure that 5V and 12V on the control P.W.B., upper arm U, V and W, and the lower arm 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 Microcomputer is abnormal.
3 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 17V are abnormal:
D908, D909 or drive circuit is abnormal.
(5) When all voltage are abnormal:
IC901, R906, etc. are possibly abnormal.
* If IC901 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 IC901 pin 4 (to be measured
at the leads of R904 and R903) and IC901 pin 5 (to be measured at R906 lead) may be between 11 and
16V. This is because the protection circuit of IC901 is operating.]