AN3112 datasheet(27/36 Pages) STMICROELECTRONICS

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AN3112

Designing a fixed-off-time PFC

Doc ID 16820 Rev 3

27/36

From the theory of the line modulation fixed off-time, TOFF increases with the line voltage so

that at maximum line voltage the condition TON>TONmin [4] is always true. This is important

in order to avoid line distortion [

5].

Figure 16.

Switching frequency function on the peak of the sinusoid input voltage

waveform and the corresponding off- time value

Now considering the two discharging resistors R and R0 of the circuit in

Figure 6, the ratio is

defined as:

Equation 57

where 0 < K1 < 1. Through the definition of the k2 parameter the expected time constant

τ =(R//R0)C is underlined, necessary for achieving the desired TOFF@90 Vac.

Equation 58

Finding a way to obtain K1 and K2 means to gain the values of R and R0 and the

discharging time constant of the capacitor C.

The following section describes the mathematical way of obtaining the two parameters K1

and K2. Combining

Equation 56, Equation 57, Equation 58 with the expression of the off-

time (

Equation 5), the following expressions are obtained:

)

VAC

(

min

OFF

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