Electronic Circuit Theory - Dell S1909WXf Service Manual

Electronic Circuit Theory

2.1 Inverter PWM circuit
2.1.1) Inverter Control circuit :( fig.2)
DC_5V through R1 and C1 will provide power for U1 on the pin2 (VDDA).
EN-ON/OFF voltage signal coming from scalar which is on IF BD will enable U1 to work. This
applies a level greater than approximately 2V to pin8 (ENA) enables the IC, A voltage less than 1V
will disable the IC. R3 is used to limit current.C3 and C40 is used to dump noise.
The soft-start function is provided by connecting C8 to pin12(SSTCMP), In the start-up mode,
current charges C8, its voltage controls the gradual increase in power to the transformer and
subsequently to the output load, this reduces in-rush current and provides reliable operation to the
CCFL.If no current is sensed approximately 2 seconds, U1 shuts off. Once the voltage at the pin4
(ISEN) reaches the lamp on threshold, the IC switches from the striking mode to the normal
operation mode and the PWM dimming control is activated.
DIM-ADJ duty cycle signal through R2 and C2 is on Pin 3(DIM), which adjusted will can
change the brightness of Panel.C41 is used to dump noise. Internal LPWM dimming control is
determined by R11 and C7 connected to LCT (pin11), analog dimming is implemented by providing
a DC voltage to PID (pin5).
The striking frequency is determined by R12 and C11 connected to Pin 13(CT) and R6
connected to RSTR (Pin6).The operating frequency is determined by R12 and C11 connected to
Pin 13(CT) and R7 connected to RT (pin7).
DRV1 and DRV2 of U1 are used to drive Q1 and Q2. DRV1 and DRV2 are controlled by
build-in PWM IC. Q1 and Q2 are switches which has two build-in IGBTs. The working principle of
circuit of T1 and T2.
Fig.2
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