Acer AC501 AC501 Monitor Service Guide - Page 14

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 t he 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 t his time the polarity of the out put 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 feedback voltage across R931, D909 is smoothed by C911 and through cross t he divider which assembled by R914, R916, R917 and VR901 to the error amplifier (U901 pin2) and compared to an internal 2.5V reference. 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 , C912. The equation is approximately : Fosc =1.8/ (R912*C912). The external signal "FBP" that comes from horizontal circuit can synchronize the working frequency of power circuit by through cross R919,R918 and D911. F-5. Power Saving Ckt The monitor has ON, Suspend/Standby and OFF states. The monitor enters the 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 +13Vs. In this state, input power should be less than 15W and D960 LED illuminate amber. The monitor enters OFF mode if both H-sync and V-sync are removed. When in the OFF State, PS1 and PS2 should be low simultaneously to turn off +13Vs and +7.5Vs outputs. In this state, input power should be less than 5W and D960 LED illuminate amber. When the monitor wakes up from the OFF mode to the ON mode , the +13Vs will provide power to heat the heater of CRT for a time via Q970 and R971, that can make sure the recovery time less than 5 sec. 14