Samsung ML-920 Service Manual page 171

3-8-2 AC INPUT STAGE
AC Input power path is consist of the Fase (F501) for AC current limit, the Varistor (TNR501) for by-passing high Voltage
Surge, the discharge resistor(R508), the AC Impalse Noise Filtering Circuit (C501, LF501, C503), the Common Mode
Grounding Circuit (C504, C505), the 2'nd noise filter (LF502), and the thermistor (TH501).
Wher power is turned on, TH 501 limits Inlush-Current by it's high resistanle, and When it's temperature rise, it's resistance
become about Zero ohm.
3-8-3 SMC(SWITCHED MODE CONTROL)
The AC input voltage is rectified and filtered by BD552 and C507 to create the DC high voltage applied to the primary wind-
ing of T501. TR01 pin #1 is driven by the SMPS device U502. U502. auto-starts and chops the DC voltage. The U502 is PWM
SMPS IC and has internally a SMC(switched mode control) IC and a MOSFET output stage. The SMC IC has a Auto-restart
without a Power Supply for the IC and a Thermal Shutdown function and so on. C509, R512, C510, D505 clamp leading-
edge voltage spikes caused by transformer leakage inductance.
The power secondary winding(pim #11-12)is rectified and filtered by D507, C552, L551, and C554 to create the 5V output
voltage. The bias winding(pin #4-5)is rectified and filltered by D506 and C511 to create U502 bias voltage. The secondary
output 5V is regulated through the path of the voltage divide by R553, R556-U503 switching PC252-the bias voltage of U502-
U503 PWM duty cycle-T501 secondary voltage. C508 filters internal pin, determines the auto-restart frequency, and togeth-
er with R506, compensates the control loop. U552 of the secondary stage -12Vis the Low Power-loss Regulator with built-in
overcurrent protection function
3-8-4. FIXED TEMPERATURE CONTROL
3-8-4-1. Fixed Lamp Control Circui
AC Neutral
CON502
Samsung Electronics
R503
C502
THY501
R502
<Fixed Lamp Control Circuit>
- This Document can not be used without Samsung's authorization -
CIRCUIT DESCRIPTION
AC Power Live
L501
Zero crossing circuit
Logic Unit
Fuser On
4
2
R505
1
6
SMPS Unit
U501
DC Power
Repair Manual
3-41