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AN2782 Datasheet(PDF) 25 Page - STMicroelectronics |
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AN2782 Datasheet(HTML) 25 Page - STMicroelectronics |
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25 / 39 page  AN2782 Designing a fixed off-time PFC Doc ID 14763 Rev 2 25/39 From (70) and (71), solving the following equation: And then substituting the value of K1 into the expression in (70), the K2 parameter is obtained: From the values of K1 and K2 it is possible to calculate the time constant τ =(R1//R2) C necessary to achieve the desired TOFF at 90Vac: Now selecting a capacitor C in the hundred pF or few nF, for example C =120 pF, it is possible to determine the required equivalent resistance value: From (67) R and R0 are found: (70) (71) (72) (73) (74) (75) (76) [] [] [] ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + + ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ ⋅ + − − ⋅ + ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎜ ⎝ ⎛ + ⋅ + ⎥ ⎥ ⎥ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎢ ⎢ ⎢ ⎣ ⎡ ⋅ ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ + ⋅ − − ⋅ ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ + ⋅ = ρ − − F min mult ZCDtrigger k 1 1 F min mult ZCDclamp 1 F min mult F min max min mult ZCDtrigger k 1 1 F min max min mult ZCDclamp 1 F min max min mult 1 min mult V V V ln ) k ( V V V ) k 1 ( V V ln V VAC VAC V V ln ) k ( V VAC VAC V V ) k 1 ( V VAC VAC V ln ) k , V ( 1 1 [] [] ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + + ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ ⋅ + − − ⋅ + ⋅ − − = − F min mult ZCDtrigger k 1 1 F min mult ZCDclamp 1 F min mult 1 1 min mult 2 V V V ln k V V V ) k 1 ( V V ln k 1 1 ) k , V ( k 1 0 ) k , V ( x 1 min mult = ρ − ρ 91 . 0 K1 = ) k , V ( k K 1 min mult 2 2 = 46 . 12 K2 = 2 min OFF K ) VAC ( T = τ ns 7 . 336 46 . 12 s 2 . 4 = μ = τ C Req τ = Ω = = k 81 . 2 pF 120 ns 7 . 336 R eq 1 eq K 1 R R − = Ω = − Ω = k 5 . 31 94 . 0 1 k 81 . 2 R |
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