Datenblatt-Suchmaschine für elektronische Bauteile |
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SB340 Datenblatt(PDF) 7 Page - Fairchild Semiconductor |
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SB340 Datenblatt(HTML) 7 Page - Fairchild Semiconductor |
7 / 12 page AN-9719 APPLICATION NOTE © 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.0 • 11/2/10 7 (Design Example) EE-16 core is selected, whose effective cross-sectional area is 19.2mm2. Choosing the saturation flux density as 0.3T, the minimum number of turns for the primary-side is obtained as: · · 10 540 10 ·0.8 0.3 · 19.2 10 75 [STEP-7] Determine the Number of Turns for each Winding Figure 12 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 (VRO) determined in Step-3 as: (13) where NP and NS are the number of turns for primary-side and secondary-side, respectively; VO is the output voltage; and VF is the output diode (DO) forward-voltage drop. Next, determine the proper integer for NS such that the resulting NP is larger than NP MIN obtained from Equation 12. The number of turns for the auxiliary winding for VDD supply is determined as: · (14) where VDD is the supply voltage nominal value and VFA is the forward-voltage drop of DDD as defined in Figure 12. Since VDD increases as the output load increases, set VDD at 5~8V higher than VDD UVLO level (8V) to avoid the over- voltage protection condition during the peak-load operation. Figure 12. Simplified Transformer Diagram (Design Example) Assuming the diode forward- voltage drop is 0.85V, the turn ratio is obtained as: 74 12 0.85 5.8 Then determine the proper integer for NS such that the resulting NP is larger than NP MIN as: 13, · 75 Setting as 12V, the number of turns for the auxiliary winding is obtained as: · 12 0.85 12 0.5 ·13 13 [STEP-8] Determine the Wire Diameter for Each Winding Based on the RMS Current of Winding The maximum RMS current of the secondary winding is obtained as: · 1 (15) The current density is typically 3~5A/mm2 when the wire is long (>1m). When the wire is short with a small number of turns, a current density of 5~10A/mm2 is acceptable. Avoid using wire with a diameter larger than 1mm to avoid severe eddy current or ac losses. For high-current output, it is better to use parallel windings with multiple strands of thinner wire to minimize skin effect. (Design Example) The RMS current of primary-side winding is obtained from Step-4 as 0.31A. The RMS current of secondary-side winding is calculated as: · 1 5.8 · 0.31 1 0.48 0.48 1.87 0.3mm (5A/mm2) and 0.4mm (8A/mm2) diameter wires are selected for primary and secondary windings, respectively. [STEP-9] Choose the Rectifier Diode in the Secondary- Side Based on the Voltage and Current Ratings The maximum reverse voltage and the RMS current of the rectifier diode are obtained as: (16) · 1 (17) |
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