AN2782 datasheet(21/39 Pages) STMICROELECTRONICS

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AN2782

Designing a fixed off-time PFC

Doc ID 14763 Rev 2

21/39

voltage by an external sense 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 reset by the ZCD signal. The pin is

equipped with 200 ns leading-edge blanking for improved noise immunity.

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

where:

●

ILpkmax is the maximum peak current in the inductor, calculated as described in (28)

●

Vcsmin = 1.0 V is the minimum voltage allowed on the L6562A current sense (in 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 is calculated considering the maximum

voltage Vcsmax allowed on the L6562A (in the datasheet):

The calculated ILpksat is the limit at which the boost inductor saturates and it is used for

calculating the inductor number of turns and air gap length.

The power dissipated in Rs is given by:

It does not exceed 1% of the rated output power (10), that is, 4 W.

According to the result, four parallel resistors of 0.47

Ω with 1 W of power rating have been

selected.

●

Pin 3 (MULT): The MULT pin is the second multiplier input. It is connected, through a

resistive divider, to the rectified mains to get a sinusoidal voltage reference. The

multiplier can be described by the relationship:

where:

●

VCS (multiplier output) is the reference for the current sense

●

k = 0.38 (typ) is the multiplier gain

●

VCOMP is the voltage on pin 2 (E/A output)

●

VMULT is the voltage on pin 3

(56)

(57)

(58)

(59)

max

min

Vcs

124

max

pksat

Vcs

pksat

rms

ISW

⋅

(

)

⋅

MULT

COMP

)

(

⋅

−

⋅

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