Power Supply; Features; Standby Operation; Switching Operation - Hitachi 36SDX88B Service Manual

2. Power Supply
The MM1 chassis consists of the standby power supply (which is activated when the power cord is plugged in, to control the micro-
processor) and the main switching supply (which operates the deflection and signal functions). 1901 is a hybrid IC, with a built-in
MOSFET.
2.1
2.2
2.3
2.4
2.5
Features
Small SIP full mold package.
Small number of external components.
Soft switching and quasi resonant operation.
Designed for a light load mode.
Guaranteed avalanche energy.
Many protection functions.
Soft start function.
Standby Operation
When the power cord is plugged in, T951 (pin 16 and 17) will provide regulated +7V standby, and from this a regulated +5V
is supplied to IO01 from 1956. 1001 is now in standby mode until the front panel power button is pressed, or the remote con-
trot power code is received. At power on, pins 14 to 16 of 1001 wilt go high, switching Q912 which causes current to flow
through Q931, which starts the switching circuit.
Switching Operation
When the D901 rectified voltage appears at pin 5 of 1901 starts oscillating at 22.5V and continues until the voltage drops
below 15.lV. After start up, the voltage at the return winding of T901 is rectified by 0980 and pin 5 is supplied with a stable
voltage proportional +8. The oscillator C981 is connected to the Drain of the MOSFET (pin 1, 1901) and smoothes the rising
edge of the pulse. This creaies a pseudo resonance function in order to delay activation of the MOSFET, to reduce loss and
noise interference.
Voltage Regulation
During normal operation, the +B voltage is checked for stability at Pin 1 of Q931, and changes in voltages are sent to 1902,
and back to feedback pin 6 of I901 in order to change the pulse duration and stabilize the +B voltage.
1901 Protection
1) Current Limiter : pin 4 of 1901 senses an over current condition at 0.5V. The switching operation continues but the MOS-
FET will turn off each pulse when pin 4 is detecting an over-current condition. This pin is also used for pseudo resonance
mode action.
2) Over-Voltage Protection : If the input voitage is too high (pin 5 voltage higher than about 30.8v), and oscillation will stop.
3) Thermal Protection : Oscillation will stop if 1901 case temperature is above 150 degrees Celsius.
4) Soft Start : At initial start-up, the 4 volts present at pin 7 is input as a differential voltage to pin 4 by means of C987. This
modifies the pulse during start-up and allows a current limited soft start condition.
5) Secondary Circuit Protection : Protection action of secondary-circuit is made up of Q938 entirety. If Q938 turns on, it
makes Q939 turn on, then make pin 5 of 1901 tow to stop oscillation.
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