7-6
Rectifier and filter circuit: The AC input is changed to a high DC voltage. The magnitude is a product of 1.414
to AC after it is converted by a bridge rectifier (BD901) and filtered by the capacitor (C905).
Start-up circuit: After being converted and filtered, the high DC voltage travels through R906, R907 as they
dissipate a portion of the voltage. Start-up begins when the lowered voltage is supplied at a current of 30uA to
IC901 (SG6841D). The output pulse makes the switch-power circuit work. The voltage produced at the two
terminals of T901 is sent to PIN 7 of IC901 after being commuted by D902 and filtered by C907. The start-up
process is ended.
SG6841: PWM. It requires low start-up current (30uA) and low working current (3mA). It contains functions
such as low-voltage protection, over-current protection, over-voltage protection, Temperature protection, etc. The
function of each pin and the inside circuit diagram are as follows:
Pin NAME FUNCTION Pin NAME FUNCTION
1 GND Ground 5 RT Temperature protection
2 FB Feedback voltage input 6 SENSE Current test pin
3 VIN Start-up current input 7 VDD Power supply
4 RI Reference setting 8 GATE PWM drive output
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