FAN7930BM Ver la hoja de datos (PDF) - Fairchild Semiconductor

Número de pieza

componentes Descripción

Fabricante

FAN7930BM

Critical Conduction Mode PFC Controller

Fairchild Semiconductor 

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.

Figure 28. Auxiliary Voltage Threshold

When no ZCD signal is available, the PFC controller

cannot turn on MOSFET, so the controller checks every

switching off time and forces MOSFET turn on when the

off time is longer than 150μs. It is called restart timer.

Restart timer triggers MOSFET turn on at startup and

may be used at the input voltage zero cross period.

Figure 31. Control Circuit

150μs

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 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

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 FAN7930B waits for another

ZCD signal. This slightly degrades the power factor

performance at light load and high input voltage.

Unlike a conventional voltage-mode PWM controller,

FAN7930B turns on the MOSFET at the falling edge of

ZCD signal. On instance is decided 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.

0.155 V / μs

Figure 32. Turn-On Time Determination

© 2010 Fairchild Semiconductor Corporation

FAN7930B • Rev. 1.0.2

13

www.fairchildsemi.com

Share Link: