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FAN9612 数据表(PDF) 5 Page - Fairchild Semiconductor |
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FAN9612 数据表(HTML) 5 Page - Fairchild Semiconductor |
5 / 18 page AN-6086 © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.4 • 4/22/10 5 Therefore, once the output voltage and minimum switching frequency are set, the inductor value is given as: 2 ,, ,, 2 2 LINE MINF OUT LINE MINF OUT CH SW MIN OUT VV V L Pf V η ⋅− =⋅ ⋅⋅ (6) where VLINE,MINF is the RMS line voltage that results in minimum switching frequency. For universal input range, VLINE,MINF is the maximum line voltage (265VAC) when VOUT is set at lower than 405V; while VLINE,MINF is minimum line voltage (85VAC) when VOUT is set at higher than 405V. As the minimum frequency decreases, the switching loss is reduced, while the inductor size and line filter size increase. Thus, the minimum switching frequency should be determined by the trade-off between efficiency and the size of magnetic components. The minimum switching frequency must be above the minimum frequency of FAN9611/12, which is set at 16.5kHz to prevent audible noise. Once the inductance value is decided, the maximum peak inductor current at the nominal output power is obtained as: , . , 22 OUT CH LPK LINE MIN P I V η ⋅ = ⋅ (7) where VLINE,MIN is the minimum line voltage. The number of turns of boost inductor should be determined considering the core saturation. The minimum number is given as: , LPK BOOST e I L N AB ⋅ ≥ ⋅Δ (8) where is Ae is the cross-sectional area of core and ΔB is the maximum flux swing of the core in Tesla. ΔB should be set below the saturation flux density. Figure 8 shows the typical B-H characteristics of ferrite core from TDK (PC45). Since the saturation flux density ( ΔB) decreases as the temperature increases, the high temperature characteristics should be considered. Figure 8. Typical B-H Curves of Ferrite Core (Design Example) Since the output voltage is 400V, the minimum frequency occurs at high-line (265VAC) and full-load condition. Assuming the efficiency is 95% and selecting the minimum frequency as 52kHz, the inductor value is obtained as: 2 ,, ,, 2 3 2 2 0.95 265 400 2 265 202 400 2 200 52 10 LINE MINF OUT LINE MINF OUT CH SW MIN OUT VV V L Pf V H η μ ⋅− =⋅ ⋅⋅ ⋅− ⋅ =⋅ = ⋅⋅ × The maximum peak inductor current at nominal output power is calculated as: , , , 22 2 2 200 7 0.95 85 OUT CH LPK LINE MIN P I A V η ⋅ ⋅ == = ⋅⋅ Assuming PQ3230 core (PC45, Ae=161mm 2) is used and setting ΔB as 0.3T, the primary winding should be: 6 , 6 7 202 10 29 161 10 0.3 LPK BOOST e IL N turns AB − − ⋅ ⋅× ≥= = ⋅Δ ×⋅ Thus, the number of turns (NBOOST) of boost inductor is determined as 30 turns. [STEP-2] Inductor Auxiliary Winding Design Figure 9 shows the inductor current and voltage waveforms of a BCM boost converter. FAN9611/12 indirectly detects the inductor zero current point using an auxiliary winding of the boost inductor. Since the zero current detection (ZCD) circuit in FAN9611/12 is designed to turn on the MOSFET when the slope of auxiliary winding voltage becomes zero, no special consideration for timing delay is required for the auxiliary winding design. AUX IN BOOST N V N () AUX OUT IN BOOST N VV N − Figure 9. ZCD Detection Waveforms The voltage of the ZCD pin is clamped near zero and the resistor RZCD limits the current of the ZCD pin below 1mA as: |
类似零件编号 - FAN9612 |
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类似说明 - FAN9612 |
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