Dell S1909WNf Service Manual page 8

drive Q4, this push and pull the energy in the transformer.
The voltage signal on negative pole of ZD4 sensed through R35 comes to Pin4 of U1, ISEN
(Lamp Current Detection & Control). The CCFL current is detected through R14and reaches a
regulated value. The CCFL current detected at resistor R14 is converted to a voltage level and
input to the ISEN pin.C12 is used to dump noise. Once the CCFL are ignited and current is sensed
through resistor R14 .capacitor C12 performs the loop compensation function. The voltage at IC
pin8 controls the drive duty cycle of the power MOSFET to regulate the CCFL current.
2.1.2) Output Circuit and Protection Circuit :( fig.3)
SPW-054
T1
1
3
4
6
NC/220pF/50V X 7R
ZD6
BAV99
OVP
+5V
C9 and C2 is the filter capacitance.R17 and C21 as the snubber circuit will decrease the peak
voltage on Q4 drain pole witch have bad effort for MOSFET safety area.R18 and C22 have the
same effect with R17 and C21.C8 and C27 will deeply decrease the voltage on Q4 drain pole.
The transformer (T1) secondary winding leakage inductance and output capacitance(C18)
forms a lower pass filter, which converts the square-wave driving signal into a sinusoidal output
voltage signal for CCFL.
The over-voltage protection feature is implemented by using an external capacitor
devider(C13/C14,C17/C19) to sense the output voltage.The divide-down voltage signal is
transferred though ZD5 and ZD6,witch will rise the OVP voltage ,then this OVP signal though Q3
determine the voltage of SST_CMP ,thus regulating the output voltage.
If a CCFL is removed, fails or damaged during normal operation, CCFL current is no longer
sensed and the voltage on ISEN pin drops. 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. To restart the IC, either
toggle the SST_CMP pin or recycle the power on the VDDA pin.
10 C18
15pF /3KV
7
C13
3pF/3KV
C15 C14
2700pF/50V X 7R
C17
3pF/3KV
C19
2700pF/50V X 7R
ZD5
BAV99
OVP
+5V
fig.3
7
Service Manual
CN3
1
2
CN1
1
2
R6
1K
ZD4
BAV99
390R 1%
CN2
C24
NC/220pF /50V X 7R
CN4
1
2
R2
1K
NC
4100-D02
R14
1
4100-D02
2
NC