Circuit Operation Theory - BenQ G2220HD Service Manual

5.6 Circuit operation theory

5.6.1. Low voltage to high voltage circuit
24V DC provides the power for IC501; the control signals Brightness and ON/OFF come from I/F board.
ON/OFF signal connect to R502 to control The Q502 and Q501 To finished control pin4 of IC501 and makes
IC501 enable. Brightness signal connect to pin8 of IC501 and regulates the panel brightness. Delaying time
circuit is setting by the IC501 internal; C505 is used to dump noise. The operation frequency is determined by
the external Resistor R505 connected to pin6 of IC501. BURST MODE regulated dimming frequency is control
from the IF Baord.C503 is used for soft start and compensation, C502, C507 are used for dump noise.
The output drives, include TG, BG (pins11,14 respectively) output square pulses to drive MOSFET U501, U502,
and each of U501, U502 , is consist of single N channel MOSFET. U501,and U502 work as Half bridge-
topology, it is high efficient, PWM switching. During start up, C520, C521, C522, C523 senses the voltage at
the transformer secondary. When OV1 OR OV2 reaches 13V Level, the output voltage is regulated. If no
current is sensed approximately 2seconds IC501 shut off.
The current flowing through CCFL is sensed and regulated through sense resistor R515, R516, R517, R518.
The feedback voltage connected to Pin2, and Pin3 (LI), then compared with a reference voltage via a current
amplifier, resulting in PWM drive outputs to Half-bridge switches.
5.6.2 Power board diagram:
Operation theory
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.
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