FAN7930BM【PDF 13/22 页】IC PFC CTLR CRITICAL CONF 8-SOIC

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Sheet目录38

9页 10页 11页 12页 [13页]14页 15页 16页 17页

?2010 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FAN7930B " Rev. 1.0.3

Figure 28. Auxiliary Voltage Threshold

When no ZCD signal is available, the PFC controller

cannot turn on the MOSFET, so the controller checks

every switching off time and forces MOSFET turn on

when the off time is longer than 150 約. This restart

timer triggers MOSFET turn-on at startup and may be

used at the input voltage zero cross period.

Figure 29. Restart Timer at Startup

Because the MOSFET turn-on depends on the ZCD

input, switching frequency may increase to higher than

several megahertz due to the mis-triggering or noise on

the nearby ZCD pin. If the switching frequency is higher

than needed for critical conduction mode (CRM),

operation mode shifts to continuous conduction mode

(CCM). In CCM, unlike CRM where the boost inductor

current is reset to zero at the next switch on; inductor

current builds up at every switching cycle and can be

raised to very high current that exceeds the current

rating of the power switch or diode. This can seriously

damage the power switch. To avoid this, maximum

switching frequency limitation is embedded. If ZCD

signal is applied again within 3.3 約 after the previous

rising edge of gate signal, this signal is ignored

internally and FAN7930B waits for another ZCD signal.

This slightly degrades the power factor performance at

light load and high input voltage.

ZC D

a f t e r C O M P A R A T O R

M O S F E T G a t e

M a x f

S W

L i m i t

In h i i R i n

E r r o r O c c u r s !

I g n o r e s Z C D N o i s e

Figure 30. Maximum Switching Frequency

Limit Operation

6. Control: The scaled output is compared with the

internal reference voltage and sinking or sourcing

current is generated from the COMP pin by the

transconductance amplifier. The error amplifier output is

compared with the internal sawtooth waveform to give

proper turn-on time based on the controller.

O U T

P F C

IN V

C O M P

C la m p

C ir c u it

RE F

S ta ir

S te p

T H D o p tim iz e d

S a w to o th

G e n e r a to r

S a w t o o t h

M O S F E T O f f

R 1

C 1

C 2

6 . 2 V

Figure 31. Control Circuit

Unlike a conventional voltage-mode PWM controller,

FAN7930B turns on the MOSFET at the falling edge of

ZCD signal. The ON instant is determined by the

external signal and the turn-on time lasts until the error

amplifier output (V

COMP

) and sawtooth waveform meet.

When load is heavy, output voltage decreases, scaled

output decreases, COMP voltage increases to

compensate low output, turn-on time lengthens to give

more inductor turn-on time, and increased inductor

current raises the output voltage. This is how a PFC

negative feedback controller regulates output.

The maximum of V

COMP

is limited to 6.5 V, which

dictates the maximum turn-on time. Switching stops

when V

COMP

is lower than 1.0 V.

Z C D

a f t e r C O M P A R A T O R

V CO M P & S a w t o o t h

M O S F E T G a t e

Figure 32. Turn-On Time Determination

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