FAN7930C Ver la hoja de datos (PDF) - Fairchild Semiconductor

Número de pieza

componentes Descripción

Fabricante

FAN7930C

Critical Conduction Mode PFC Controller

Fairchild Semiconductor 

Figure 41. Effect of THD Optimizer

By THD optimizer, turn-on time over one AC line period

is proportionally changed, depending on input voltage.

Near zero cross, lengthened turn-on time improves THD

performance.

9. VIN Absent Detection: To save power loss caused by

input voltage sensing resistors and to optimize THD, the

FAN7930C 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 a 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 live 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, FAN7930C

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.

10. Current Sense: The MOSFET current is sensed

using an external sensing resistor for 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.

11. Gate Driver Output: FAN7930C 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.

VOUT

VIN

Though VIN is

eliminated, operation of

controller is normal due

to the large bypass

capacitor.

VAUX

MOSFET gate

DMAX

fMIN

NewVCOMP

DMIN

fMIN

VIN Absence Detected

IDS

Smooth

Soft-Start

FAN7930 Rev.00

t

Figure 42. Operation without VIN Absent Circuit

Figure 43. Operation with VIN Absent Circuit

© 2010 Fairchild Semiconductor Corporation

FAN7930C • Rev. 1.0.0

16

www.fairchildsemi.com

Share Link: