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AN4213 数据表(PDF) 13 Page - STMicroelectronics |
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AN4213 数据表(HTML) 13 Page - STMicroelectronics |
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13 / 42 page ![]() DocID023984 Rev 2 13/42 AN4213 Designing a TM PFC The switching frequency is minimum at the top of the sinusoid and it is maximum at the zero-crossing of the line voltage ( ϑ =0, π = sin ϑ =0) where t off = 0 μs. The absolute minimum frequency f SWmin can occur at either the maximum V ACmax or the minimum mains voltage V ACmin , so the inductor value is calculated by the formula: Equation 18 where V AC can be either V ACmin or V ACmax , whichever gives the lower value for L. The values of the inductor at low mains and at high mains, respectively L(V ACmax ) and L(V ACmin ) are given by following equations: Equation 19 Equation 20 At this point the minimum value has to be taken into account. It becomes the maximum inductance value for PFC dimensioning. For this application a 0.5 mH boost inductance has been selected. 3.3.5 Power MOSFET selection The main switch selection is driven by the amount of allowable power dissipation. It is important to choose a device that minimizes gate charge and capacitance and minimizes the sum of switching and conduction losses at a given frequency. The breakdown voltage is fixed just by the output voltage, plus the allowable overvoltage and a safety margin. The conduction losses are given by: Equation 21 = = 1 sin 2 ϑ π ϑ () () 2 * 2 * 2 AC OUT AC AC V V V V L − = () () () () mH V L V P f V V V V L AC OUT IN SW AC OUT AC AC 36 . 1 400 * 129 * 10 * 35 * 2 185 * 2 400 * 185 * * * 2 * 2 * 3 2 max min min 2 min max = − = − = () () () () mH V L V P f V V V V L AC OUT IN SW AC OUT AC AC 51 . 0 400 * 129 * 10 * 35 * 2 265 * 2 400 * 265 * * * 2 * 2 * 3 2 max min max 2 max min = − = − = () ()2 * AC SWrms DSon COND V I R P = |
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