Circuit Description - ViewSonic VS11444 Service Manual

4. Circuit Description

4.1 Switching Mode Power Supply
4.1.1 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.
4.1.2 High Voltage to Low Voltage Control Circuit
C804 is used to smooth the wave from rectifier. IC802 is a highly integrated PWM controller. When rectified
DC high voltage is applied to the HV pin during start-up, the MOSFET Q804 is initially off, and the Vcc pin
capacitor is charged. When the Vcc pin voltage reaches approximately 10V, the control circuitry is activated
and the soft-start begins. The soft-start circuit gradually increases the duty cycle of the MOSFET from zero to
the maximum value over approximately 4ms. If no external feedback/supply current is fed into the FB pin by
the end of the soft-start, the current Setpoint will be above the fault level, FAULT flag is raised, if the FAULT
duration exceeds 80ms, the output controller disable
Resistor R808, R809, R810, R811 are for line over voltage shutdown(OVP)
When PWM is turned off, the main current flow will be consumed through R804 and D802, This will
prevent MOSFET Q804 from being damaged under large current impulse and voltage spike.
D803 and C807 to provide internal Auxiliary voltage to Vcc pin during normal operation. Otherwise, error
amplifier and feedback current input the FB pin for duty cycle control.
4.1.3 DC_5V and DC_14V Output Circuit
For DC 5V, D805 is used to rectify the inducted current. R828 and C814 are used to store energy when
current is reversed. The parts including C818, C822, C820,L803 are used to smooth the current waves.
For DC 14V, D803 is used to rectify the inducted current. R827 and C813 are used to store energy when
current is reversed. The parts including C815, C817 and L802 are used to smooth the current waves.
4.1.4 Feedback and OVP Protect Circuit
Pin R of IC803 is supplied 2.5-v stable voltage. It connects to 5V and 14V output through R822, R823
and R824. R822, R823 and R824 are output sampling resistor. When the sampling voltage more than 2.5V or
less than 2.5V, current of FB IC802 will change, this can change the voltage from T801.
OVP Protect Circuit: When output is overvoltage, the auxiliary winding voltage will be increased, when it
reaches about 14V. Q803 is triggered . It makes the IC802 Pin 1 exceed 5V, then the IC802 output will be
disabled.
Q801, R816, R817 and ZD803 make up of dummy loading circuit. For start-up sequence, during 5V
output take place high loading first, this dummy loading circuit operated to insure 14V not be increased.
4. 2 Inverter Circuit
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