FAN7930B 데이터 시트보기 (PDF) - Fairchild Semiconductor

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FAN7930B

Critical Conduction Mode PFC Controller

Fairchild Semiconductor 

10. Input Voltage Absent Detection: To save power

loss caused by input voltage sensing resistors and to

optimize THD easily, the FAN7930B omits AC input

voltage detection. Therefore, no information about AC

input is available from the internal controller. In many

cases, the VCC of PFC controller is supplied by an

independent power source like standby power. In this

scheme, some mismatch may exist. For example, when

the electric power is suddenly interrupted during two or

three AC line periods; VCC is still alive during that time,

but output voltage drops because there is no input

power source. Consequently, the control loop tries to

compensate for the output voltage drop and VCOMP

reaches its maximum. This lasts until AC input voltage is

live again. When AC input voltage is live again, high

VCOMP allows high switching current and more stress is

put on the MOSFET and diode. To protect against this,

FAN7930B internally checks if the input AC voltage

exists. If input does not exist, soft-start is reset and

waits until AC input is live again. Soft-start manages the

turn-on time for smooth operation when it detects AC

input is applied again and applies less voltage and

current stress on startup.

VOUT

VIN

VAUX

MOSFET Gate

NewVCOMP

IDS

Though VIN is eliminated,

operation of controller is

normal due to the large

bypass capacitor.

DMAX

fMIN

DMIN

fMIN

Input Voltage Absent Detected

Smooth Soft-

Start

FAN7930 Rev.00

t

Figure 42. Operation with Input Voltage

Absent Circuit

11. Current Sense: The MOSFET current is sensed

using an external sensing resistor for the over-current

protection. If the CS pin voltage is higher than 0.8V, the

over-current protection comparator generates a

protection signal. An internal RC filter of 40kΩ and 8pF

is included to filter switching noise.

12. Gate Driver Output: FAN7930B contains a single

totem-pole output stage designed for a direct drive of

the power MOSFET. The drive output is capable of up

to +500/-800mA peak current with a typical rise and fall

time of 50ns with 1nF load. The output voltage is

clamped to 13V to protect the MOSFET gate even if the

VCC voltage is higher than 13V.

Figure 41. Operation without Input Voltage

Absent Circuit

© 2010 Fairchild Semiconductor Corporation

FAN7930B • Rev. 1.0.2

16

www.fairchildsemi.com

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