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FAN6300H Datasheet(PDF) 4 Page - Fairchild Semiconductor |
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FAN6300H Datasheet(HTML) 4 Page - Fairchild Semiconductor |
4 / 13 page AN-6300 APPLICATION NOTE © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.2 • 5/21/10 4 To determine the primary-side inductance (LP), the following variables should be determined beforehand: The minimum switching frequency ( fs,min): The maximum average input current occurs at the minimum input voltage and full-load condition. Meanwhile, the switching frequency is at minimum value during QR operation. The falling time of the MOSFET drain voltage (tf): As shown in Figure 4, the falling time of MOSFET drain voltage is half of the resonant period of the MOSFET effective output capacitance and primary- side inductance. If a resonant capacitor is added to be paralleled with Coss, tf can be increased and EMI can be reduced. However, this forces a switching loss increase. The typical value of tf for NB adaptor application is about 0.5~1μs. After determining fs,min and tf, the maximum duty cycle is calculated as: ×× od max s,min f od in nV +V D(1 - f t ) n V +V +V () = () (3) where Vin,min is specified at low-line and full-load. According to Equation 1, the maximum average input current Iin,max is determined as oo in,max in,min VI I V η = (4) According to Figure 3, Iin,max can be obtained as: = pk in,max max ds,max 1 ID I 2 (5) Ids,max pk can be determined as: = pk in,min max ds,max ms,min VD I Lf (6) In Equation 5, replace Ids,max pk by Equation 6, then combine Equations 4 and 5 to obtain LP: 2 in,min max P in s,min (V D L 2P f ) = (7) where Pin, and Dmax are specified in Equations 1 and 3, respectively, and fs,min is the minimum switching frequency. Once LP is determined, the RMS current of the MOSFET in normal operation are obtained as: rms peak max ds,max ds,max D II 3 = (8) [d] Determine the Proper Core and the Minimum Primary Turns When designing the transformer, consider the maximum flux density swing in normal operation (Bmax). The maximum flux density swing in normal operation is related to the hysteresis loss in the core, while the maximum flux density in transient is related to the core saturation. From Faraday’s law, the minimum number of turns for the transformer primary side is given by: × pk 6 Pds,max P,min max e LI N10 BA = (9) where: LP is specified in Equation 7; Ids,max pk is the peak drain current specified in Equation 6; Ae is the cross-sectional area of the core in mm 2; and Bmax is the maximum flux density swing in tesla. Generally, it is possible to use Bmax =0.25~0.30 T. Determine the Number of Turns for Auxiliary Winding The number of turns for auxiliary winding (Na) can be obtained by: DD D1 a od V+V N= V+V (10) where: VDD is the operating voltage for VDD pin; VD1 is the forward voltage drop of D1 in Figure 5; and Vo and Vd as determined in Equation 2. |
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