FL7930C(2013) 查看數據表（PDF） - Fairchild Semiconductor

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FL7930C
(Rev.:2013)

Single-Stage Flyback and Boundary-Mode PFC Controller for Lighting

Fairchild Semiconductor 

VZCD

tON

tON is typically constant over 1 AC line frequency,

but tON is changed by ZCD voltage.

tON not shorter

t

tON get shorter

VZCD at FET on

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 FL7930C 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 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, FL7930C

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

Though VIN is

eliminated, operation of

controller is normal due

to the large bypass

capacitor.

VOUT

VIN

Though VIN is

eliminated, operation of

controller is normal due

to the large bypass

capacitor.

VAUX

MOSFET gate

DMAX

fMIN

DMIN

fMIN

NewVCOMP

VIN Absence Detected

IDS

Smooth

Soft-Start

t

Figure 43. With VIN-Absent Circuit

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 0.8 V, the

over-current protection comparator generates a

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

is included to filter switching noise.

11. Gate Driver Output: FL7930C 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 / -800 mA peak current with a typical rise and

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

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

VCC voltage is higher than 13 V.

VAUX

MOSFET gate

fMIN

DMAX

VCOMP

IDS

High drain

current!

t

Figure 42. Without VIN-Absent Circuit

© 2011 Fairchild Semiconductor Corporation

FL7930C • Rev. 1.0.2

16

www.fairchildsemi.com

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