Acer AC701 AC701 Monitor Service Guide - Page 12

4. POWER SUPPLY A. RECTIFIER AND FILTER CKT: The 90Vac-265Vac input line voltage is rectified by BD901, and smoothed by C906 to provide DC operating voltage for the circuit. B. 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 Q903 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 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 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 C946 is fed through a voltage divider,R970, R971, R973, C975, R976, R977 and VR901 to the error amplifier U904 compared to an internal 2.5V reference, then determine the feed back level of U901 pin 1. C. 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. D. SYNCHRONIZED CKT: The oscillator frequency is defined by R919, C916. The equation is approximately: Fosc=1.8/ (R919*C916). The external signal "SYN+ & SYN−" E. POWER SAVING CKT: The monitor has ON and OFF states. The monitor enters OFF mode if H-sync and/or V-sync are removed. When in the OFF State, PS1 should below simultaneously to turn off +13.5Vs outputs. PS2 should enter PWM Pulse to pre- CRT- heater. In this state, input power should be less than 5W and LED-1 LED illuminate yellow. 12