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FAN100 Datasheet(PDF) 10 Page - Fairchild Semiconductor |
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FAN100 Datasheet(HTML) 10 Page - Fairchild Semiconductor |
10 / 21 page AN-6067 APPLICATION NOTE © 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.1 • 1/26/10 10 Application Information Transformer Design The transformer inductor current must operate in DCM under any conditions. A typical output V-I curve is shown in Figure 24. For discontinuous current mode operation, the transformer inductor should be small enough to meet this condition. Point “B” is the lowest output voltage within the CC regulation and the widest discharge time of the transformer inductor due to the reflected voltage on the primary inductor. It is the easiest into CCM condition for transformer inductor. Point “A” is the maximum output power of the power system. Ensure that the magnetic flux density falls within 0.25~0.3 Tesla, considered a safe range. The number of turns for primary transformer inductor can be determined on point “A.” Figure 25 shows the characteristic curve of turn ratio and transformer inductance. o V o I ( ) maximum output power determine turn number A B determine primary inductor Figure 24. Critical Operating Points to Determine the Transformer 0 0.5 1 1.5 2 2.5 3 3.5 5 6 7 8 9 10 11 12 13 14 15 n(turn ratio) B=0.5V B=1V B=1.5V B=2V 1 , 0.45 IoA Vf V == Figure 25. Characteristic Curve of Turn Ratio and Inductance Determine Maximum and Minimum Input Voltage Figure 26 shows the corrected input voltage waveform. The red line shows ripple voltage on the bulk capacitor and the minimum and maximum voltage on the bulk capacitor is expressed in equations 7 and 8, respectively. The CBULK is the input capacitor and a typical value is 2-3µF per watt of output power for wide range input voltage (90-264V). 2.5 Assume mS .min in V Figure 26. Bridge Rectifier and Bulk Capacitor Voltage Waveform 2 .min ,min 2(1- 0.3) 2- 120 oo in ac bulk VI VV C η ⋅⋅ ⋅ =⋅ ⋅⋅ (7) .max .max 2 in ac VV =⋅ (8) Determine the Turn Ratio The transformer turn ratio (n p=Npri/Nsec) is an important parameter of the flyback converter; it affects the maximum duty ratio when the input voltage is at a minimum value. It also influences the voltage stresses on the MOSFET and the secondary rectifier. The permissible voltage stresses and the maximum voltage stresses on the MOSFET, as well as the secondary rectifier, can be expressed as: .max .max () DS in p o f VV n V V =+ ⋅ + (9) .max .max in Fo p V VV n = + (10) The leakage spike due to leakage inductance on the MOSFET and rectifier must also be taken into account. Determine Transformer Inductance Determine the VDD voltage level and if the output voltage is defined. The turn ratio between auxiliary winding and secondary winding can be calculated as: DD fa a Of VV n VV + = + (11) where VDD is voltage on VDD cap, usually ranging from about 15V~20V. In the CC regulation region, on point “B,” the power system shuts down if the output voltage is too low and the VDD |
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