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Service Manual
When current flow from the Diode D858 and D859 through R871, R872,
R873, R874 and R831, R832,
R833, R834 gets to VRMS and IAC pin to start up U850, Meanwhile, the
VDD supply current is as low as
100 uA thus most of the HV current is utilized to change the VDD capacitor
R858, C822.For normal operation, this input is use to provide current
reference for the multiplier
RI pin is to program the switching frequency, By connecting R814 to
ground to set the switching frequency, f =(1560/R814kohm)
(KHz)=1560/24KHz=65KHz.
PFC output voltage control. For an universal input (90VAC~264VAC)
power supply applying active boost PFC and Fly-back as a second stage, the
output voltage of PFC is usually designed around 250V at low line while it is
380V at high line. This is to improve efficiency for low-line input. In U850
(SG6901A), the RANGE pin is used for the two-level output voltage setting.
The RANGE output is shorted to ground when the Vrms voltage exceeds
1.95V while it is of high impedance whenever the Vrms voltage drops below
1.6V. The output voltage can be designed using below equation .
Range=Open Voutput =〔(R981+R982+R983+R984+R822)/R821〕*3V
Range=Ground Voutput =〔(R981+R982+R983+R984)+(R822// R821)/
(R822// R821)〕*3V
PFC Over-voltage Protection. When the OVP feedback voltage exceeds the
over-voltage threshold, the U850 will inhibit the PFC power converter form
operating abnormally while the FBPFC pin is open.
U850 provides an OTP pin for over-temperature protection. A constant
current is output from this pin. When the OTP voltage drops below 1.2V,
U850 (SG6901A) will be disable. Until OTP voltage exceeds 1.4V.During
startup of PWM stage, the SS pin will charge an external capacitor C816 with
a constant current source.U850 (SG6901A) output stage is a fast totem-pole
gate driver. The output driver is clamped by an internal 18V Zener diode in
order to protect the power MOSFET.
15
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