Electronic Circuit Theory - Dell E2209Wf Service Manual

Electronic Circuit Theory

2.1 Inverter PWM circuit
2.1.1)Inverter Control circuit:(fig.2)
27VDC provides the power for U1; the control signals Brightness and ON/OFF come from I/F
board. ON/OFF signal connect to pin
pin of U1 and regulates the panel brightness, make up a network of delaying time circuit and R23
make up a divided voltage network, C is used to
frequency is determined by the external Resistor R16 and capacitor C8 connected to pin3 of U1.
C7 is used for soft start and compensation.
The output drives, include DRV1, DRV2 (pins8,9 respectively) output square pulses to drive
MOSFET Q6, Q7 and each of Q6, Q7 is consist of a N+P channel MOSFET. Q6 and Q7 work as All
Bridge structure, it is high efficient, PWM switching.
During start up, VSEN (pin1) senses the voltage at the transformer secondary. When VSEN
reaches 3.0V, the output voltage is regulated. If no current is sensed approximately 2seconds U1 shut
off.
The current flowing through CCFL is sensed and regulated through sense resistor R6, R19. The
feedback voltage connected to Pin1 (ISEN), then compared with a reference voltage (1.5V) via a
current amplifier, resulting in PWM drive outputs to All Bridge switches.
2.1.2 Protection Circuit
Fig.2
of U1 and makes U1 enable. Brightness signal connect to
7
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. The operation