Hitachi CM721FET, Service Manual

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C. Power Supply Circuit 

C-1 RECTIFIER AND FILTER CIRCUIT 

The 90Vac 260Vac input line voltage is rectified by DB901, and smoothed by C906 via the PFC circuit to 
provide DC operating voltage for the circuit. 

C-2 FLYBACK AND PWM CIRCUIT 

When power is initially applied to the circuit, C909 charges through R903 and Q901. When the voltage across 
C909 reaches a level of 16V, U901 is enabled, turning on Q903 and turning off Q901 to complete start-up 
action. 

During the On-time of Q902, energy is stored in T902. 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, Q902 is turned off and the polarity of all T901A 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 main voltage (202V) across R944, R945, R947, C956, R946, VR901 is smoothed by C945 and fed via 
the optp-coupler U900 to the primary regulator to the error amplifier (U901 pin1). 

C-3 SNUBBER CIRCUIT 

Energy stored in the leakage inductance of T902 cause a voltage spike which be added to the normal reset 
voltage across T902 when Q902 turn off. The clamp consisting of D905, R904 and C907 limits this voltage 
excursion from exceeding the Vds rating of Q902. In addition, a turn off snubber made up of D906, R905 and 
C908 keeps power dissipation in Q902 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. 

C-3 SYNCHRONIZED CIRCUIT 

The oscillator frequency is defined by R912, C920. The equation is approximately: 

Fosc=1.8/(R912*C920). The external signal that comes from horizontal circuit synchronises the working 
frequency of power circuit Via U980 to U906 (PIN 2). For divide frequency in over 50KHz by U906 
(74LS221) 

C-4 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 +15Vs. In this state, input power should be less than 15W and 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 +7.8Vs outputs. In this state, input power should by less than 5W and 
LED illuminate amber. 

C-5 DEGAUSS CIRCUIT 

The DEGU signal which comes from micro-p to control degauss coil act or not. 

When DEGU is high, Q946 will be saturated to make RL901 working, AC line provides a degaussing current 
to degauss coil. 

C-6 OVERLOAD ROTECTION CIRCUIT 

The primary current flows through the current sensing resistor R914. The voltage drop on R914 will be 
rectified by D921 and keep a dc level in C921 that is dependent on the output power. When this DC level 
reach the trigger voltage of Q912, the pin1 of U901 goes low and the power stage is immediately switched 
off. 

C-7 BROWN-OUT CIRCUIT 

When the input voltage down below the setting level (85Vac-70Vac), Q914 will be supplied with a trigger 
voltage, pin 1 of U901 goes low and the power stage will be shut down.