Samsung LW32A23W Service Manual page 8

PFC Circuit
Operating Principles
As the stand-by power circuit starts oscillating,the control IC of the PFC circuit (L4981) is applied with Vcc
and starts oscillating. The oscillation frequency is determined by the resistance (RP824) connected to 17Pin
and the condenser (CP816) connected to 18Pin. For regulation,the output voltage is divided by resistors and
monitored by 14Pin. The 4Pin voltage, which is entered after dividing this voltage and the AC voltage by
resistors, is calculated in the IC and the command value of the main FET is prepared. Therefore, the output
voltage is controlled by the voltage determined by the dividing resistors, and the input current has the
same waveform (sine wave) as the AC voltage, producing a power factor of almost 100%.
Protections
1. Overvoltage
The circuit has an overvoltage protection function of the voltage limit type. In the event of a sudden
interruption of the input, of overshooting [overload?] during a sudden change of load,or non-control
status, this function protects the circuit by narrowing the ON width of the FET and preventing the
voltage from rising above the overvoltage setting. The overvoltage can be set with RP834, RP835, RP836
and RP840. (The power source is set to approximately 385V)
2. Overcurrent
The current is converted into the voltage value through RP831 and RP832,and monitored by 2 Pin and 8
Pin of the control IC. If the voltage of the detecting resistor rises above the set value due to an increase in
current, the pulse width is narrowed to limit the output.
3. Thermal Shutdown
Although the IC has a thermal shutdown function, a thermistor (TH) is installed on the heat-radiating fin
of the main FET and the circuit is stopped if overheated by overload, etc.
FET VDS Waveform (Input of 100V)
LW32A23W/LW40A23W
FET VDS Waveform (Input of 230V)
Block Diagrams