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AN2761 bảng dữ liệu(PDF) 13 Page - STMicroelectronics |
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AN2761 bảng dữ liệu(HTML) 13 Page - STMicroelectronics |
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13 / 36 page ![]() AN2761 Designing a TM PFC Doc ID 14690 Rev 2 13/36 Ton and Toff are respectively the ON-time and the OFF-time of the power MOSFET, ILpk is the maximum peak inductor current in a line cycle, and θ is the instantaneous line phase in the interval [0, Π]. Note that the ON-time is constant over a line cycle. As previously stated, ILpk is twice the line-frequency peak current (16), 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: The switching frequency is the minimum at the top of the sinusoid ( θ = Π /2 rad => sin θ =1), maximum at the zero-crossings of the line voltage (θ = 0 rad or Π rad=> sin θ =0), where Toff =0 µs. 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: After calculating the values of the inductor at low mains and at high mains L(VACmax), L(VACmin) (35), the minimum value has to be taken into account. It becomes the maximum inductance value for the PFC dimensioning. For this application a 0.7 mH boost inductance has been selected. (34) (35) (36) (37) ( ) 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 73 . 0 V 400 W 02 . 86 kHz 35 2 ) Vac 85 2 V 400 ( Vac 85 ) VAC ( L 2 min = ⋅ ⋅ ⋅ ⋅ − ⋅ = () mH 83 . 0 V 400 W 02 . 86 kHz 35 2 ) Vac 265 2 V 400 ( Vac 265 ) VAC ( L 2 max = ⋅ ⋅ ⋅ ⋅ − ⋅ = |
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