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AN4129 Datasheet(PDF) 5 Page - Fairchild Semiconductor |
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AN4129 Datasheet(HTML) 5 Page - Fairchild Semiconductor |
5 / 12 page APPLICATION NOTE AN4129 5 ©2003 Fairchild Semiconductor Corporation Figure 11. DC Link Voltage and Current Waveforms The bridge diode conduction time can be calculated by (5) and the diode RMS current can be calculated by (6). The calculated value is 1.3A and the selected bridge diode is KBP206(600V/2A). 2. Transformer Design Since 2001, the European Commission has been regulating no load losses for AC adapters, battery chargers and external power supplies under 75W. Table 1 shows the regulation target specification. Vdc_link Vac Iac tc 1 2 π fac ⋅ -------------------- = arc Vdc_min 2 Vac_min ⋅ -------------------------------------- cos × (5) IBD RMS () 2 = 2 Vac_min ⋅ Vdc_min – () Cdc 2fac ⋅ 3tc ⋅ ---------------- ⋅⋅ ⋅ (6) Table 1: European Commission Regulation Specification Rated Input Power No Load Power Consumption Phase 1 1.1.2001 Phase 2 1.1.2003 Phase 3 1.1.2005 ≥ 0.3W and < 15W 1.0W 0.75W 0.30W ≥ 15W and < 50W 1.0W 0.75W 0.50W ≥ 50W and < 75W 1.0W 0.75W 0.75W At light and no load, the FAN7601 operates in burst mode to reduce the power loss. To meet the regulation specification the most important thing is to minimize the number of MOSFET switchings. The flyback converter transfers energy during the switch-off time. As the primary inductance of the flyback transformer increases, the energy transferred to the secondary side increases during one switching cycle. There- fore it is better to use a higher inductance transformer, but inductance is restricted by the size and cost of the trans- former. In this design example, a 600uH transformer is selected and the ferrite core is EER2828. The transformer turns ratio is calculated when the input line voltage is lowest and the output power is maximum. The maximum duty ratio must be lower than 0.45 to prevent subharmonic oscillation. In this design example, the turns ratio must be higher than 0.174 by (7). Once the minimum turns ratio is determined, then the numbers of primary and secondary turns is calculated when input line voltage is highest and the output power is maximum. If the converter operates in the CCM (Continuous Conduction Mode) then the turn-on time can be calculated by (8). Then the number of primary turns can be obtained as in (9). Ae is the effective cross sectional area of the core and Bmax is the maximum flux density. Ae of EER2828 is 82.1mm2 and Bmax is 0.15T. The calculated number of primary turns is 54. Then the number of secondary turns can be calculated by (10). The calculated number of secondary turns is 10. The air gap length can be calculated by (11). If the primary inductance is not high enough, the converter can operate in the DCM(Discontinuous Conduction Mode) when the input line voltage is highest and the output power is maximum. For the DCM, the turn-on time can be calculated as in (12). n 1 Dmax – Dmax ----------------------- > Vo VDiode + Vdc_min -------------------------------- ⋅ (7) ton Vo VDiode + n Vdc_max ⋅ Vo VDiode ++ ------------------------------------------------------------------------- = 1 fsw --------- ⋅ (8) Np Vdc_max ton ⋅ Ae Bmax ⋅ --------------------------------------- = (9) Ns Np = Vo VDiode + Vdc_min -------------------------------- 1Dmax – Dmax ----------------------- ⋅ (10) l g4 π =10 7 – Ae N 2 P L ---------- ⋅⋅ ⋅ (11) |
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