FAN7930C 데이터 시트보기 (PDF) - Fairchild Semiconductor

부품명

상세내역

제조사

FAN7930C

Critical Conduction Mode PFC Controller

Fairchild Semiconductor 

valley position of MOSFET voltage stress. For valley

detection, a minor delay by the resistor and capacitor is

needed. A capacitor increases the noise immunity at the

ZCD pin. If ZCD voltage is higher than 1.5V, an internal

ZCD comparator output becomes HIGH and LOW when

the ZCD goes below 1.4V. At the falling edge of

comparator output, internal logic turns on the MOSFET.

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 and result in burn down. To

avoid this, maximum switching frequency limitation is

embedded. If ZCD signal is applied again within 3.3μs

after the previous rising edge of gate signal, this signal

is ignored internally and FAN7930C waits for another

ZCD signal. This slightly degrades the power factor

performance at light load and high input voltage.

Figure 31. Maximum Switching Frequency

Limit Operation

5. 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.

Figure 29. 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 150μs. This is called the

restart timer, which triggers MOSFET turn-on at startup

and may be used at the input voltage zero-cross period.

150 μs

Figure 30. 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 miss-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

Figure 32. Control Circuit

Unlike a conventional voltage-mode PWM controller,

FAN7930C turns on the MOSFET at the falling edge of

ZCD signal. On-instance is determined by the external

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

output (VCOMP) 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 PFC negative feedback

controller regulates output.

The maximum of VCOMP is limited to 6.5V, which dictates

the maximum turn-on time, and switching stops when

VCOMP is lower than 1.0V.

© 2010 Fairchild Semiconductor Corporation

FAN7930C • Rev. 1.0.0

13

www.fairchildsemi.com

Share Link: