DELL MONITOR 2208WFP _ SMPS [Power Supply] _ WORKING
CN850 is a connector for connecting AC Power. F850 is a fuse to protect all the circuit AC. Input voltage is from 90Vto 264V. R850/3851are joined between two inputting main circuit to prevent man from shock. L850 is used to filter low frequency noise. C850and C851are used to discharge the noise that L850 produced. High frequency waves are damped by C852.
High Voltage to Low Voltage Control Circuit
D850 is a rectifier in which there are 4build-in diodes, inverting AC to DC. C854 is used to smooth the wave from rectifier. U850 is a highly integrated PWM controller. Typical start-up current for U850 is only 20uA, When current flow through R8 1/R842/R843 gets to Pin 3of U850,with VDD hold-up capacitor C855, U850 is enough for starting up.
WhenU850 begins to operate Pin8of U850 will output square wave to drive Q850, 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. In the same time, the current inducted by wires which
connected T850 Pin 1 and Pin 3, with components of D852, R856 and C855 ,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 of two 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 V- (PWMcopm) = (Vcomp-2Vf) / 3, A switch cycle will be terminated immediately, Vcomp is internally clamped to a variable voltage around 0. 85V for output power limit.
When Q850 are turned off, the main current flow will be consumed through D851, C875, R853/R854/R855 and D853, This will prevent Q850 from being damaged under large current impulse and voltage spike.
DC_24V ,12Vand 5VOutput Circuit and Feedback circuit.
D854&D855 are used to rectify the inducted current. C862 are used to store energy when current is reversed. The parts including L852 are used to smooth the current waves that are from D854&D855, and then 24Vvoltage is supplied. D880&D881 are used to rectify the inducted current. C881 is used to store energy when current is reversed. The parts including U880 are used to change the voltage from about 13Vto steady 12V, and then 12Vvoltage is supplied. D856 is used to rectify the inducted current. C874, C864&C875are used to store energy when current is reversed. The parts including L851, and C876are used to smooth the current waves that are from D856, and then 5Vvoltage is supplied. 5Vsupply voltage feed back to PWM controller U850 via R861, R860,and I850,I851.R859,C859 and C863 are used to control response time. LD7575 implements an OVP function on Vcc .Whenever the Vcc voltage is higher than the OVP threshold voltage, the output gate drive circuit will be shutdown simultaneously thus to stop the switching of the power MOSFET until the next on.