Implementing the line-modulated fixed-off-time

AN3112

6/36

Doc ID 16820 Rev 3

3

Implementing the line-modulated fixed-off-time

The circuit that implements LM-FOT control with the L6564 PFC controller is shown in

Figure 6. During the on-time of the MOSFET the gate voltage V

0.7 V typ.) which causes the switch to turn on. The time needed for the ZCD voltage to go

Figure 6.

Circuit implementing FOT control with the L6564

The circuit in

Figure 6. makes T

holding effect of T1 (which acts as a buffer) along with R and C whose time constant is

significantly longer than a line half-cycle. With the addition of R0 and T1, as long as the

following the law:

Equation 1

evolution from that point on is described as:

Equation 2

()

()

()

BE

mult

0

C

RR

)

R

R

(

t

BE

mult

0

ZCDclamp

ZCD

V

V

R

R

R

e

V

V

R

R

R

V

)

t

(

V

0

0

+

⋅

+

+

⋅

⎥

⎦

⎤

⎢

⎣

⎡

+

⋅

+

−

=

′

⋅

+

⋅

−

()

C

R

t

BE

mult

0

ZCD

e

V

V

R

R

R

)

t

(

V

⋅

−

⋅

+

⋅

+

=

′′