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FAN6747 Fiches technique(PDF) 5 Page - Fairchild Semiconductor |
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FAN6747 Fiches technique(HTML) 5 Page - Fairchild Semiconductor |
5 / 12 page AN-6747 APPLICATION NOTE © 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.1 • 9/16/10 5 [STEP-6] Determine the Minimum Primary Turns With a given core, the minimum number of turns for the transformer primary side to avoid the core saturation is given by: 6 e SAT CS M 6 e SAT LIM M MIN P 10 A B R / 825 . 0 L 10 A B I L N × • = × = (19) where A e is the cross-sectional area of the core in mm 2, ILIM is the pulse-by-pulse current limit level determined by 0.825V threshold, RCS is current sensing resistor, and B SAT is the saturation flux density in Tesla. The pulse-by-pulse current limit level is included in Equation (19) because the inductor current reaches the pulse-by-pulse current limit level during the load transient or overload condition. Figure 6 shows the typical characteristics of ferrite core from TDK (PC40). Since the saturation flux density (B SAT) decreases as the temperature rises, the high-temperature characteristics should be considered. If there is no reference data, use B MAX =0.3T. Figure 6. Typical B-H Characteristics of Ferrite Core (TDK/PC40) (Design Example) An EF25/13/11 core is selected with effective cross-sectional area of 78mm2. Choosing the saturation flux density as 0.27T, the minimum number of turns for the primary side is obtained as: 60 10 78 27 . 0 33 . 0 / 825 . 0 10 508 10 A B R / 825 . 0 L N 6 6 6 e SAT CS M MIN P = × • • × = × • = − [STEP-7] Determine the Number of Turns for Each Winding Figure 7 shows a simplified diagram of the transformer. First, calculate the turn ratio (n) between the primary side and the secondary side from the reflected output voltage determined in step 3 as: F O RO S P V V V N N n + = = (20) where N P and NS are the number of turns for primary side and secondary side, respectively, V O is the output voltage; and V F is the diode (DO) forward-voltage drop. Determine the proper integer for N S such that the resulting NP is larger than NP min obtained from Equation (19). The number of turns for the auxiliary winding for VDD supply is determined as: S F O FA DD A N V V V * V N • + + = (21) where V DD is the nominal value of the supply voltage and VFA is the forward-voltage drop of DDD as defined in Figure 7. Since VDD increases as the output load increases, it is proper to set V DD at 3~5V higher than VDD UVLO level (9V) to avoid the over-voltage protection condition during the peak load operation. Figure 7. Simplified Transformer Diagram (Design Example) Assuming the diode forward- voltage drop is 1V, the turn ratio is obtained as: 03 . 3 1 32 100 V V V N N n F O RO S P = + = + = = Then, determine the proper integer for N S such that the resulting N P is larger than NP min as: MIN P S P S N 61 N n N , 20 N > = • = = Setting VDD* as 13V, the number of turns for the auxiliary winding is obtained as: 9 20 1 32 1 13 N V V V * V N S F O FA DD A = • + + = • + + = |
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