Circuit Description - ViewSonic VA702-1 Service Manual

4. Circuit Description

Electronic Circuit Theory
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. R820 and R821 joined between two inputting main circuit to prevent man from
shock. L801 is used to clear up low frequency wave. C801 and C806 are used to discharge the waves
that L801 produced. High frequency waves are damped by C801 and C806. 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
C805 is used to smooth the wave from rectifier. IC802 is a highly integrated PWM controller, which
build-in a power MOSFET. When rectified DC high voltage is applied to the DRAIN pin during start-up,
the MOSFET is off initially, and the CONTROL pin capacitor is charged through a switched high
voltage current source connected internally between the DRAIN and CONTROL pins. When the
CONTROL pin voltage Vc reaches approximately 5.8V, 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 10ms. If no external feedback/supply current is fed into the
CONTROL pin by the end of the soft-start, the high voltage current source is turned off and the
CONTROL pin will start discharging in response to the supply current drawn by the control circuitry.
Resistor R803, R807, R824 and R825 are for line over voltage shut-down (OVP) and line under-voltage
detection (UVP).Resistors R801, R805, R822, and R823 are for external current limit adjustment, and
used to reduce the current limit externally to a value close to the operating peak current of primary about
1.35A. The mean is power will protected when the primary current over about 1.35A.
When PWM is turned off, the main current flow will be consumed through D804 and ZD802, This will
prevent MOSFET which built-in IC802 from being damaged under large current impulse and voltage
spike.
D806 and C815 provide internal Auxiliary current to CONTROL pin during normal operation. In
addition, error amplifier and feedback current to the CONTROL pin are for duty cycle control.
1.3 DC_5V and DC_14V Output Circuit
For DC 5V, D805 is used to rectify the inducted current. R806 and C811 are used to store energy when
current is reversed. The parts including C812, C814, C822, C821, B801 and L803 are used to smooth
the current waves.
For DC 14V, D803 is used to rectify the inducted current. R802 and C802 are used to store energy when
current is reversed. The parts including C808, C810 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 R811,
R810 and R818. R811, R810 and R818 are output sampling resistor. When the sampling voltage more
than 2.5V or less than 2.5V, feedback current of IC802 will change, this can change the voltage from
transformer T801.
For 5V output OVP, ZD803 is a zener diode, when 5V output voltage becomes up to 5.6V, the zener
DC
current cause R819 voltage become up to 0.7V, Q801 is triggered and OVP starts. For 12V
OVP, ZD804 is a Zener Diode, when 14V output voltage becomes up to 16V, the zener current cause
R819 voltage become up to 0.7V, Q801 is triggered and OVP starts. The collector current of Q801 is
used to make build-in diode light. FB Current of IC802 will be changed; it can change the voltage from
T801.
Q802, R827, R828 and ZD801 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.
ViewSonic Corporation
Confidential - Do Not Copy
18
output
DC
VA702-1_VA702b-1