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FAN6204A

• Rev. 1.1

9

Functional Description

V

OUT

V

DET

I

M

V

Primary

MOSFET

V

GS

Synchr onous Rectifier

MOSFET

V

CT

V

LPC

t

PM.ON

I

DS

I

V

t

L.DIS

t

CT.DIS

Body diode of

SR MOSFET

Body diode of

SR MOSFET

I

DS

Primary

MOSFET

V

OUT

V

DET

I

M

V

Primary

MOSFET

V

GS

Synchr onous Rectifier

MOSFET

V

CT

V

LPC

t

PM.ON

I

DS

I

V

t

L.DIS

t

CT.DIS

Body diode of

SR MOSFET

Body diode of

SR MOSFET

Blanking time

Figure 18. Typical Waveforms of Linear-Predict Timing Control in CCM and DCM/QR Flyback

Linear Predict Timing Control

The SR MOSFET turn-off timing is determined by

linear-predict timing control and the operation principle

is based on the volt-second balance theorem. The volt-

second balance theorem states that the inductor

average voltage is zero during a switching period in

steady state, so the charge voltage and charge time

product is equal to the discharge voltage and discharge

time product. In flyback converters, the charge voltage

the

discharge

voltage

is

as

the

typical

waveforms show in Figure 18. The following equation

can be drawn:

..

IN

PM ON

OUT

L DIS

V

t

n V

t



 



(1)

inductor discharge time.

FAN6204A uses the LPC and RES pins with two sets of

of SR MOSFET, can be predicted by Equation (1). As

shown in Figure 18, the SR MOSFET is turned on when

the SR MOSFET body diode starts conducting and DET

voltage drops to zero. The SR MOSFET is turned off by

linear-predict timing control.

Circuit Realization

The linear-predict timing-control circuit generates a

replica

of

magnetizing

current

of

flyback

shown in Figure 19. Using the internal capacitor voltage,

to zero, the SR controller turns off the SR MOSFET.