Designing a fixed-off-time PFC

AN3112

22/36

Doc ID 16820 Rev 3

The two bode plot charts are relevant to the PFC operating at the main voltage set point of

265 Vac and full load. In this condition the crossover frequency is fc = 4 Hz, the phase

margin is 50° and the third harmonic distortion is below 3%.

Pin 4 (CS): The #4 pin is the inverting input of the current sense comparator. Through this

pin, the L6564 senses the instantaneous inductor current, converted in a proportional

voltage by an external sensing resistor (Rs). As this signal crosses the threshold set by the

multiplier output, the PWM latch is reset and the power MOSFET is turned off. The

MOSFET stays in off-state until the PWM latch is set again by the ZCD signal. The pin is

equipped with 150 ns (typ.) leading-edge blanking for improved noise immunity.

The sense resistor value (Rs) can be calculated as follows. For the 400 W PFC it is:

Equation 44

Where:

●

ILpk: it is the maximum peak current in the inductor, calculated as described in 4.2

●

Vcsmin = 1.0 V, it is the minimum voltage admitted on the L6564 current sense (on the

datasheet).

Because the internal current sense clamping sets the maximum current that can flow in the

inductor, the maximum peak of the inductor current may be calculated considering the

maximum voltage Vcsmax allowed on the L6564 (on datasheet):

Equation 45

The calculated ILpkx is the value at which the boost inductor must not be in saturation and it

is used for calculating the inductor number of turns and air-gap length.

Figure 12.

Open loop transfer function-bode

plot

Figure 13.

Phase

-200

-100

0

100

0. 1

1

10

100

1000

f [Hz]

-200

-150

-100

0.1

1

10

100

1000

f [Hz]

max

pk

min

s

IL

Vcs

R

<

Ω

=

<

124

.

0

A

01

.

8

V

0

.

1

R

s

s

max

pksat

R

Vcs

IL

=

A

67

.

9

12

.

0

V

16

.

1

IL

pksat

=

Ω

=