Circuit Description - ViewSonic VA2016W - 20" LCD Monitor Service Manual

4. Circuit Description

1. Switching Mode Power Supply
1.1 AC Current Input Circuit
P801 is a connector for connecting AC Power. F801 is a fuse to protect all the circuit. AC
input voltage is from 90V to 264V. R801 and R802 joined between two inputting main circuit
to prevent man from shock. L801 is used to clear up low frequency wave. C801 and C802
are used to discharge the waves that L801 produced. High frequency waves are damped by
C801 and C802. D801 is a rectifier which composed of 4 build-in diodes, it inverts AC to
DC.
1.2 High Voltage to Low Voltage Control Circuit
C804 is used to smooth the waveform from rectifier. IC802 is a highly integrated PWM
controller, which control the power MOSFET Q804. When rectified DC high voltage is
applied to the DRAIN pin during start-up, the MOSFET is off initially, and the capacitor C807
be charged through D802,R803 and the HV pin of IC802,when the voltage VCC reaches
the threshold level 12.8V,IC 802 start up and create a PWM signal to control the power
MOSFET, then energy is transferred to secondary terminal through the transformer
T801,the auxiliary voltage 12V and the output voltage 5V/14V be created ,the auxiliary
voltage supply a continue current to IC802,the level of output voltage is feedback to FB pin
of IC802 through R823,R824,R822,IC803 and IC803 witch control the duty of the PWM
signal, then all the convert circuit go to a stable operating station.
R805 R806 R835 ZD801 ZD806 and Q806 formed a under input voltage protection
circuit , only the input AC voltage over the threshold level approximately 63V AC, the switch
Q801 can be on and then the auxiliary voltage can supply a continue current to
IC802;R808,R807,R836,R812,R815,R836,D806 and Q805 formed a over line current
protection circuit witch limited the input power under approximately 63W. ZD805 will be on
when the output voltage is too high or the feedback circuit open, the current will drive
transistor Q805 open through R830,D814 and made IC802 off the PWM waveform; the high
voltage spike created by transformer's primary winding during the transistor turn off will be
consumed through D804 R813 R814 and C806, This will prevent MOSFET which built-in
IC802 from being damaged under large current impulse and voltage spike.
1.3 DC_5V and DC_14V Output Circuit
For DC 5V, D808 is used to rectify the inducted current. R817 and C815 are used to store
energy when current is reversed. The parts including C824,C818,C822 and L803 are used
to smooth the current waves.
For DC 14V, D807 is used to rectify the inducted current. R816 and C814 are used to
store energy when current is reversed. The parts including C816,C817,C819 and L802 are
used to smooth the current waves.
1.4 Feedback and OVP Protect Circuit
Pin R of IC803 is supplied 2.5V stable voltage. It is connected to 5V and 14V output through
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