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AN4138 Datasheet(PDF) 4 Page - Fairchild Semiconductor |
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AN4138 Datasheet(HTML) 4 Page - Fairchild Semiconductor |
4 / 20 page AN4138 APPLICATION NOTE 4 ©2002 Fairchild Semiconductor Corporation condition. Therefore, Lm is obtained in this condition as where VDC min is specified in equation (2), D max is specified in equation (5), Pin is specified in step-1, fs is the switching frequency of the FPS device and KRF is the ripple factor in full load and minimum input voltage condition, defined as shown in Figure 6. For DCM operation, KRF = 1 and for CCM operation KRF < 1. The ripple factor is closely related to the transformer size and the RMS value of the MOSFET current. In the case of low power applications such as battery chargers, a relatively large ripple factor is used in order to minimize the transformer size. It is typical to set KRF = 0.5- 0.7 for the universal input range and KRF = 1.0 for the European input range. Once Lm is determined, the maximum peak current and RMS current of the MOSFET in normal operation are obtained as where Pin, VDC min, D max and Lm are specified in equations (1), (2), (5) and (6) respectively and fs is the FPS switching frequency. Figure 6. MOSFET Drain Current and Ripple Factor (KRF) (5) STEP-5 : Choose the proper FPS considering input power and peak drain current. With the resulting maximum peak drain current of the MOSFET (Ids peak) from equation (7), choose the proper FPS of which the pulse-by-pulse current limit level (Iover) is higher than Ids peak. Since FPS has ± 12% tolerance of I over, there should be some margin in choosing the proper FPS device. (6) STEP-6 : Determine the proper core and the minimum primary turns. Table 2 shows the commonly used cores for battery chargers with output power under 10W. The cores recommended in table 2 are typical for the universal input range and 100kHz switching frequency. With the chosen core, the minimum number of turns for the transformer primary side to avoid the core saturation is given by where Lm is specified in equation (6), Iover is the FPS pulse- by-pulse current limit level, Ae is the cross-sectional area of the core as shown in Figure 7 and Bsat is the saturation flux density in tesla. Figure 8 shows the typical characteristics of ferrite core from TDK (PC40). Since the saturation flux density (Bsat) decreases as the temperature goes high, the high temperature characteristics should be considered. If there is no reference data, use Bsat =0.3~0.35 T. Since the MOSFET drain current exceeds Ids peak and reaches I over in a transition or fault condition, Iover is used in equation (11) instead of Ids peak to prevent core saturation during transition. Figure 7. Window Area and Cross Sectional Area L m V DC min D max ⋅ () 2 2P infsKRF ---------------------------------------------- = (6) I ds peak I EDC ∆I 2 ----- + = (7) I ds rms 3I EDC () 2 ∆I 2 ----- 2 + D max 3 -------------- =8 () I EDC P in V DC min D max ⋅ -------------------------------------- = (9) ∆I V DC min D max L mfs ----------------------------------- = (10) I ∆ EDC I EDC RF I I K 2 ∆ = CCM operation : K RF < 1 I ∆ EDC I EDC RF I I K 2 ∆ = DCM operation : K RF =1 peak ds I peak ds I N P min L mIover B satAe ------------------- 10 6 (turns) × = (11) Aw Aw Aw Aw Ae Ae Ae Ae |
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