AN3142 datasheet(29/44 Pages) STMICROELECTRONICS

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Giải thích chi tiết về linh kiện	Solution for designing a 400 W fixed-off-time controlled

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AN3142

Designing a fixed-off-time PFC

Doc ID 17005 Rev 3

29/44

Equation 60

Where fswmin is the switching frequency on the top of the sinusoid of the input voltage at

VACmin = 90 VAC (Figure 18) and 220 ns is a corrector factor in order to consider the delay

between the ZCD and GD signal.

Considering the ratio between

Equation 60, Equation 59:

Equation 61

In the formula

Equation 59, Equation 60, the delay between the ZCD signal and the gate

drive signal is taken into account in order to increase the accuracy of the mathematical

model.

From the theory of the line modulation fixed off-time, TOFF is increasing with the line voltage

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

important in order to avoid line distortion [

5].

Figure 18.

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:

Equation 62

max

min

max

OFF

)

VAC

(

−

⋅

220

450

)

VAC

(

max

OFF

−

⋅

)

VAC

(

)

VAC

(

min

OFF

max

OFF

x =

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