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AN3142 Arkusz danych(PDF) 28 Page - STMicroelectronics |
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AN3142 Arkusz danych(HTML) 28 Page - STMicroelectronics |
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28 / 44 page ![]() Designing a fixed-off-time PFC AN3142 28/44 Doc ID 17005 Rev 3 at minimum mains voltage, as set in Equation 1, guarantee against parameter variation, the mentioned tolerances should be taken into account, making some calculations considering the worst cases. In this case, taking into account the resistors and threshold tolerances, 1 M Ω and a 56 kΩ have been calculated, therefore the actual divider ratio is 0.946. Then the following check can be done: Equation 55 Equation 56 Equation 57 Equation 58 Pin 11 (ZCD) is the input of the zero current detector circuit. In FOT mode, it is connected to the Line-Modulated-Fixed-Off-Time circuit seen in Figure 6. Taking into account the information contained in Section 3, the starting point for the design of that circuit is the pair of the desired values for TOFF on the top of the line voltage sinusoid at minimum (TOFF @ VACmin) and maximum line (TOFF @ VACmax) obtained by setting the switching frequency on the peak of the sinusoid at low mains and considering the minimum on-time of the L6563S: Equation 59 L _ FF H _ FF L _ FF EN _ RUN ENABLE _ FF R R R V V + ⋅ = V 923 . 0 M 1 k 56 M 1 V 88 . 0 V ENABLE _ FF = Ω Ω + Ω ⋅ = multL multL multH EN _ FF START _ in R R R 2 mV 20 V V + ⋅ ⎟⎟⎠ ⎞ ⎜⎜⎝ ⎛ + = Vac 87 k 51 k 51 M 6 . 6 2 mV 20 V 923 . 0 V START _ in = Ω Ω + Ω ⋅ ⎟⎟⎠ ⎞ ⎜⎜⎝ ⎛ + = L _ FF H _ FF L _ FF DIS _ RUN DISABLE _ FF R R R V V + ⋅ = V 844 . 0 M 1 k 56 M 1 V 80 . 0 V DISABLE _ FF = Ω Ω + Ω ⋅ = multL multL multH DIS _ FF STOP _ in R R R 2 mV 20 V V + ⋅ ⎟⎟⎠ ⎞ ⎜⎜⎝ ⎛ + = Vac 9 . 79 k 51 k 51 M 6 . 6 2 mV 20 V 844 . 0 V STOP _ in = Ω Ω + Ω ⋅ ⎟⎟⎠ ⎞ ⎜⎜⎝ ⎛ + = min sw min min OFF f k ) VAC ( T = s 76 . 3 ns 220 kHz 80 32 . 0 ) VAC ( T min OFF µ = − = |
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