Acer AL1702 AL1702 Service Guide - Page 5

AL1702 1) Circuit Description 1. PowerBoard Operation 1.1 Line filter consists of C801, T801, C802, C803, C804. It eliminates high frequency interference to meet EMI's requirement. 1.2 Rec & Filter Bridge diode D801 converts AC source into pulsed DC. This pulsed DC is smoothed and filtered by C805. R802 is an NTC ( negative thermal coefficient ) resistor, used to reduce inrush current to be within safe range. 1.3 Power transformer : T802 converts energy for square wave from power source C805 to secondary side to generate +12V and +22V. 1.4 Output : The square wave from T802 is rectified by D809, D811, then filtered by C822, C817 to generate +12V and +22V respectively. I804 is the regulator for +5V output. 1.5 Driver : Q803 drive T802 from PWM control of I801 for power converted. 1.6 FB : Negative feedback CKT consists of photo coupler I802 and adjustable regulator I803. It can maintain output voltages +22V and +12V at a stable level. 1.7 PWM : 1.7.1 Start : When power is turned on, Q801 conducts due to bias from C805 and R805,R803. C807 is charged a 16 volt and a starting current about 0.3mA to pin 7 of I801. I801 starts to oscillate and outputs a pulse train through pin 6 to drive Q803. 1.7.2 OPP : When Q803 turns on, C805 supplies a linearly increasing triangle current through the primary inductance of T802 to the driver Q803, once the peak value of this current multiplied by R811 exceeds1 volt, pulse train will be turn off immediately to protect Q803, T802 from being burned out. 1.7.3 Regulation : If output voltage +22V or 12V goes up, the R terminal of I803 gets more bias, accordingly photo transistor and photo diode flows more current. The voltage of pin 2 of I801 goes up too, making the pulse width of pin 6 of I801 to become narrower. So the output voltage +5V will be pulled down to a stable value. 1.7.4 OVP : If +22V or 12V goes up too much, the induced voltage on pin 4 of T802 becomes large also. Suppose that it is over 18 volts, ZD801 conducts, pin 3 of I801 is pulled up over 1 volt. The pulse train at pin 6 of I801 goes down to zero, shutting Q803 off immediately. 1.7.5 SCP : If output terminal is short to ground, photo transistor does not conduct, hence Q806 does not conduct either. Then oscillation of I801 is stop, shutting Q803 off immediately. Power Board Block Diagram Line Filter Rec. & Filter Power Transformer Outpur Rec. & Filter PWM Driver FB Page 3