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AN3027 bảng dữ liệu(PDF) 13 Page - STMicroelectronics |
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AN3027 bảng dữ liệu(HTML) 13 Page - STMicroelectronics |
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13 / 41 page ![]() AN3027 Designing a TM PFC Doc ID 16134 Rev 4 13/41 As previously stated, ILpk is twice the line-frequency peak current Equation 4, which is related to the input power and the input mains voltage. Substituting this relationship in the expressions of ton and toff, after some algebra it is possible to find the instantaneous switching frequency along a line cycle: Equation 21 The switching frequency is minimum at the top of the sinusoid ( θ = Π /2 ==> sin θ =1), maximum at the zero crossings of the line voltage ( θ = 1 or Π ==> sin θ =0), where toff =0. The absolute minimum frequency fswmin can occur at either the maximum VACmax or the minimum mains voltage VACmin, thus the inductor value is defined by the formula: Equation 22 After calculating the values of the inductor at low mains and at high mains L(VACmax), L(VACmin) Equation 23, the lowest value must be used. It becomes the maximum inductance value for the PFC dimensioning. Equation 23 For this application a 0.52 mH boost inductance has been selected. Figure 4. Switching frequency, fixing the line voltage The figure above shows the switching frequency versus the θ angle calculated with Equation 22, a 0.52 mH boost inductance and fixing the line voltage at minimum and maximum ( ) out out 2 in off on sw V ) sin( VAC 2 V VAC P L 2 1 T T 1 ) , VAC ( f θ ⋅ ⋅ − ⋅ ⋅ ⋅ ⋅ = + = θ out in min sw out 2 V P f 2 ) VAC 2 V ( VAC ) VAC ( L ⋅ ⋅ ⋅ ⋅ − ⋅ = () mH 642 . 0 V 400 W 38 . 106 kHz 40 2 ) Vac 90 2 V 400 ( Vac 90 ) VAC ( L 2 min = ⋅ ⋅ ⋅ ⋅ − ⋅ = () mH 515 . 0 V 400 W 38 . 106 kHz 40 2 ) Vac 265 2 V 400 ( Vac 265 ) VAC ( L 2 max = ⋅ ⋅ ⋅ ⋅ − ⋅ = |
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