BenQ G700 Service Manual page 29

BenQ G700
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.
2.1.3 DC_5V and DC_12V 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 12V, 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.
2.1.4 Feedback and OVP Protect Circuit
Pin R of IC803 is supplied 2.5V stable voltage. It is connected to 5V and 12V 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 12V output voltage becomes up to 15V, 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 12V not be increased.
Low voltage to high voltage circuit
14VDC provides the power for IC501; the control signals Brightness and ON/OFF come from I/F board.
ON/OFF signal connect to pin10 of IC501 and makes IC501 enable. Brightness signal connect to pin4 of IC501
and regulates the panel brightness, R501, R518, C518 make up a network of delaying time circuit and R512,
R514 make up a divided voltage network, C510 is used to dump noise. The operation frequency is determined
by the external Resistor R508 and capacitor C512 connected to pin13 of IC501. BURST MODE regulated
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Service Manual
output
DC