Power Supply - Hitachi CM715ET Service Manual

F. Power Supply
F-1. Rectifier And Filter Ckt
The 90Vac-265Vac input line voltage is rectified by D901- D904, and smoothed by C906 to provide DC
operating voltage for the circuit.
F-2. Flyback And PWM Ckt
When power is initially applied to the circuit, C909 charges through R903, Q902.When the voltage across
C909 reaches a level of 16V, U901 is enabled, turning on Q901 and turning off Q902 to complete start-up
action.
During the On-time of Q901, energy is stored in T900. At this time the polarity of the output windings is such
that all output rectifiers are reverse biased and no energy is transferred. Primary current is sensed by R914 and
compared to a DC level that is determined by voltage feedback. When this level is reached, Q901 is turned off
and the polarity of all T900 windings reverses , forward biasing the output rectifiers. All the energy stored is
now transferred to the output capacitors. Many cycles of this store/release action are needed to charge the
outputs to their respective voltages.
The voltage across C949 is fed through a voltage divider R944, R945, R946, R947 and VR901 to the error
amplifier U900 and compared to an internal 2.5V reference, then determine the feed back level of U901 PIN1.
F-3. Snubber Ckt
Energy stored in the leakage inductance of T900 cause a voltage spike which be added to the normal reset
voltage across T900 when Q901 turn off. The clamp consisting of D905, R904 and C907 limits this voltage
excursion from exceeding the Vds rating of Q901. In addition, a turn off snubber made up of D906, R905 and
C908 keeps power dissipation in Q901 low by delaying the voltage rise until drain current has decreased from
its peak value. This snubber also damps out any ringing which may occur due to parasitic.
F-4. Synchronized Ckt
The oscillator frequency is defined by R912 , C920. The equation is approximately : Fosc=1.8/ (R912*C920).
The external signal "SYN+ & SYN -"
that comes from horizontal circuit T802 can synchronize the working frequency of power circuit Via
ZD903,R918,R919 and R916.
F-5. Power Saving Ckt
The monitor has ON, Suspend/Standby and OFF states.The monitor enters Suspend/Standby mode if either
V-sync or H-sync signals are removed. When the machine in the Suspend/Standby State, PS1 should be low to
turn off +15Vs. In this state, input power should be less than 15W and D960 LED illuminate yellow. The
monitor enters OFF mode if both H-sync and V-sync are removed. When in the OFF State, PS1 and PS2
should below simultaneously to turn off +15Vs and +8.3Vs outputs. In this state, input power should be less
than 5W and D960 LED illuminate amber.
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