Electronic Circuit Theory - Dell 1907FPf Service Manual

2. Electronic Circuit Theory

2.1Switching Mode Power Supply
2.1.1 AC Current Input Circuit
CN850 is a connector for connecting Ac Power. F850 is a fuse to protect all the circuit Ac. Input
voltage is from 90v to 264V. R850 and R851 in serial to prevent man from shock. L851 is used to clear up
low frequency wave. C850 and C851 are used to discharge the waves that L851 produced. High frequency
waves are damped by C852 and C853. D850 is a rectifier in which there are 4 build- in diodes, inverting
AC to DC.
2.1.2 High Voltage to Low Voltage Control Circuit
C854 is used to smooth the wave from rectifier. R852 is a fuse to protect the following circuit when
current flow is too large. U850 is a highly integrated PWM controller. Typical start-up current for U850 is
only 30 uA, R853 and R854 are serial circuit to limit current flow. When current flow through R853 and
R854 gets to Pin 3 of U850, with VDD hold-up capacitor C857, U850 is enough for starting up.
When U850 begins to work Pin8 of U850 will output about 18v to drive Q850,and then the main
current flow get to GND bypassing through T850, Q850. Because of the change of current flow, wires in
the other side of T850 will induct current.At the same time, the current inducted by wires which connected
T850 Pin 1 and Pin 2, with components of D852, R855, C857 and C856, will be supplied to U850 for
normal operating.
When the sense voltage across the sense resistor R859, reaches the threshold voltage around 0.85v,
the output GATE drive will be turned off. Every time when the output of power supply is shorted or over
loaded, the FB voltage will increase, the build- in PWM output will then be turned off. Both will prevent
the power supply from being overheated under over loading condition. The PWM duty cycle is determined
by this current sense signal and VFB, the feedback voltage. When the voltage on sense pin reaches Vcomp
=(VFB-1.0)/3, A switch cycle will be terminated immediately. Vcomp is internally clamped to a variable
voltage around 0. 85 V for output power limit
When Q850 are turned off, the main current flow will be consumed through D851, C880 and
R880. This will prevent Q850 from being damaged under large current impulse and voltage spike.
2.1.3 DC_12V Output Ci r cuit
D854 is used to rectify the inducted current. R860 and C861 are used to store energy when current is
reversed. The parts including C862, C864, C863 and L853 are used to smooth the current waves that are
from D854, and then 12- v voltage is supplied
12-v supply voltage feed back to PWM controller U850 via R863, R868, and I850,I851.
2.1.4 Protect Circuit of U850
When Vdd or 12V output come out of SPEC , the voltage on D856 will be rised over 22 volts, the current
via D856 is large enough to star up protect circuit internal U850, U850 will be in Auto Restart mode until
the matter is got rid of .
2.2 Inverter Circuit
2.2.1 Low voltage to High voltage circuit
DC_5V through R2 will provide power for U1 on the pin6. EN-ON/OFF voltage signal coming from
gm2621- LF which is on IF BD will enable Q1 and Q2 to work. R5 and R30 are used to limit current. C5 is
used to dump noise, C6 is used to soft start, and then the voltage on pin 1 will enable U1. The operating
frequency is determined by the external resistor (R4) connected to Pin 2.
During start up, VSEN-DIM (pin7) senses the voltage on the transformer secondary. When VSEN
reaches 3V, the output voltage is regulated. If no current is sensed approximately 2 seconds, U1 shuts off.
DIM-ADJ duty cycle signal through R3 and C5 is on Pin 7(ISEN), which adjusted will can change the
brightness of Panel.
DRV1 and DRV2 of U1 are used to drive Q6 and Q7 . DRV1 and DRV2 are controlled by build- in
PWM IC. Q6 and Q7 are switches which has two build-in IGBTs. The working principle of circuit of T1
and T2, Q6 AND Q7 are the same to circuit of T850 and Q850 described ahead.
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