FAN7930CM Ver la hoja de datos (PDF) - Fairchild Semiconductor

Número de pieza

componentes Descripción

Fabricante

FAN7930CM

Critical Conduction Mode PFC Controller

Fairchild Semiconductor 

(see Equation 1). Positive voltage is induced (see

Equation 2) when the power switch turns off.

VAUX

=

−

TAUX

TIND

⋅

VAC

(1)

( ) VAUX

=

TAUX

TIND

⋅

VPFCOUT

− VAC

(2)

where:

VAUX is the auxiliary winding voltage;

TIND is boost inductor turns;

TIND auxiliary winding turns;

VAC is input voltage for PFC converter; and

VOUT_PFC is output voltage from the PFC converter.

Figure 25. Two Cases of RDY Triggered HIGH

Figure 27. Circuit Near ZCD

Because auxiliary winding voltage can swing from

negative to positive voltage, the internal block in ZCD

pin has both positive and negative voltage clamping

circuits. When the auxiliary voltage is negative, internal

circuit clamps the negative voltage at the ZCD pin

around 0.65V by sourcing current to the serial resistor

between the ZCD pin and the auxiliary winding. When

the auxiliary voltage is higher than 6.5V, current is

sinked through a resistor from the auxiliary winding to

the ZCD pin.

Figure 26. Two Cases of RDY Triggered LOW

4. Zero-Current Detection: Zero-current detection

(ZCD) generates the turn-on signal of the MOSFET

when the boost inductor current reaches zero using an

auxiliary winding coupled with the inductor. When the

power switch turns on, negative voltage is induced at the

auxiliary winding due to the opposite winding direction

Figure 28. Auxiliary Voltage Depends on

MOSFET Switching

The auxiliary winding voltage is used to check the boost

inductor current zero instance. When boost inductor

current becomes zero, there is a resonance between

boost inductor and all capacitors at the MOSFET drain

pin: including COSS of the MOSFET; an external

capacitor at the D-S pin to reduce the voltage rising and

falling slope of the MOSFET; a parasitic capacitor at

inductor; and so on to improve performance. Resonated

voltage is reflected to the auxiliary winding and can be

used for detecting zero current of boost inductor and

© 2010 Fairchild Semiconductor Corporation

FAN7930C • Rev. 1.0.0

12

www.fairchildsemi.com

Share Link: