Power Supply; Rectifier And Filter Circuit; Flyback And Pwm Circuit; Subber Circuit - Hitachi CM621FET Service Manual

4. POWER SUPPLY

A. RECTIFIER AND FILTER CIRCUIT:
The 90Vac-265Vac input line voltage is rectified by D901- D904, and smoothed by C906 to
provide DC operating voltage for the circuit.
B. FLYBACK AND PWM CIRCUIT :
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 Q903 and turning
off Q902 to complete start-up action.
During the On-time of Q901, energy is stored in T901. 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 R907 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 T901
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 C946 is fed through a voltage divider R970, R971, R972, R975 and VR901
to the error amplifier U904 and compared to an internal 2.5V reference ,then determine the
feed back level of U901 pin 1.
C. SNUBBER CIRCUIT:
Energy stored in the leakage inductance of T901 cause a voltage spike which be added to the
normal reset voltage across T901 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.
D. SYNCHRONISED CIRCUIT:
The oscillator frequency is defined by R912, C912. The equation is approximately :
Fosc=1.8/ (R912*C912). The external signal "SYN+ & SYN−"
that comes from horizontal circuit FBT can synchronise the working frequency of power
circuit Via R919, C916 and R918.
E. POWER SAVING CIRCUIT:
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 +13.5Vs. In this state, input power should be
less than 15W and LED-1 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 +13.5Vs and +7.1Vs outputs. In this state, input power should be
less than 5W and LED-1 LED illuminate amber.
11