Electronic Circuit Theory - Dell 1909Wf Service Manual

Service Manual

Electronic Circuit Theory

2.1 Inverter PWM circuit
2.1.1)Inverter Control circuit:(fig.2)
DC_5V through R3 will provide power for U1 on the pin6.
ON/OFF voltage signal coming from IF BD will enable Q1 to work.C4 is used to
dump noise, C5 is used to soft start, and then the voltage on pin 1 will enable U1.
During start up, SST_CMP (pin1) senses the voltage on the transformer secondary.
If no current is sensed approximately 2 seconds, U1 shuts off. DIM duty cycle signal
through R3 and C3 is on Pin 7(DIM), which adjusted will can change the brightness
of Panel.
The striking and operating frequency is determined by the external resistor (R4)
connected to Pin 2(RT).
DRV1 and DRV2 of U1 are used to drive Q3 . DRV1 and DRV2 are controlled by
build-in PWM IC. Q3 is switches which has two build-in IGBTs. The working
principle of circuit of T1, Q3 is the same to circuit of T850 and Q850 described after.
ISEN pin is used for lamp current sence feedback, Once the voltage at the ISEN
pin is less than the lamp" on" threshold, the shutdown timer is activated. The IC
maintains the output voltage for approximately 2-3seconds and once the timer expires,
the IC will shutdown.
Fig 2
7