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FQPF9N50C bảng dữ liệu(PDF) 11 Page - Fairchild Semiconductor |
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FQPF9N50C bảng dữ liệu(HTML) 11 Page - Fairchild Semiconductor |
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11 / 17 page  AN-8027 © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.0 • 8/26/09 11 (Design Example) Setting the crossover frequency as 22Hz: 1 2 6 6 2 2.5 5(2 ) 70 10 0.9 1.27 2.5 20 10 387 5 270 (2 22) MV BOUT MAX VC BOUT BOUT VC GI K C V Cf nF π π − − ⋅⋅ =⋅ ⋅⋅ ×⋅ ⋅ =⋅ = × ⋅⋅ ⋅ 9 1 11 362 2 222 20 10 VC VC VC R k fC π π − == = Ω ⋅⋅ ⋅⋅ × Setting the pole of the compensator at 120Hz: 2 3 11 3.7 2 2120 362 10 VC VP VC CnF fR π π == = ⋅⋅ ⋅⋅ × [STEP-10] Transformer Design for PWM Stage Figure 19 shows the typical secondary-side circuit of forward converter for multi-output of PC power application. A common technique for winding multiple outputs with the same polarity sharing a common ground is to stack the secondary windings instead of winding each output winding separately. This approach improves the load regulation of the stacked outputs. The winding NS1 in Figure 19 must be sized to accommodate its output current, plus the current of the output (+12V) stacked on top of it. To get tight regulation of 3.3V output, magnetic amplifier (MAG-AMP) is used. The saturable core of MAG-AMP prevents the diode DREC from fully conducting by introducing high impedance until it is saturated. This allows the effective duty cycle of VREC to be controlled to be regulated the output voltage. MAG AMP Control MAG AMP N p +12V +5V +3.3 V -12V Additiona l LC filter Additiona l LC filter Additiona l LC filter Additiona l LC filter N S 1 N S 2 V REC + - D REC N S 3 Figure 19. Typical Secondary-Side Circuit Once the core for the transformer is determined, the minimum number of turns for the transformer primary-side to avoid saturation is given by: MIN MIN BOUT MAX P eSW VD N Af B = Δ (44) where Ae is the cross sectional area of the core in m 2, fSW is the switching frequency, and ΔB is the maximum flux density swing in Tesla for normal operation. ΔB is typically 0.2-0.3 T for most power ferrite cores in the case of a forward converter. The turn ratio between the primary-side and secondary-side winding for the first output is determined by: 11 1 () MIN BOUT MAX P SO F VD N n NV V == + (45) where VF is the diode forward-voltage drop. Next, determine the proper integer for NS1 resulting in Np larger than Np min. Once the number of turns of the first output is determined, the number of turns of other output (n-th output) can be determined by: () () () 1 11 On F n Sn S OF VV N N VV + =⋅ + (46) The golden ratio between the secondary-side windings for the best regulation of 3.3V, 5V, and 12V is known as 2:3:7. (Design Example) The minimum PFC output voltage is 310V and the maximum duty cycle of PWM controller is 50%. By adding 5% margin to the maximum duty cycle, DMAX=0.45 is used for transformer design. Assuming ERL35 (Ae=107mm2) core is used and ΔB=0.28, the minimum turns for the transformer primary side is obtained as: 63 310 0.45 72 107 10 65 10 0.28 MIN MIN BOUT MAX P eSW VD N Af B − ⋅ == = Δ ×⋅ × ⋅ The turns ratio for 5V output is obtained as: 310 0.45 25.6 ( ) (5 0.45) MIN BOUT MAX P SO F VD N n NV V ⋅ == = = ++ The number of turns for the primary-side winding is determined as: 1 2 25.6 51.2 MIN pS P Nn N N =⋅ = × = < 11 3 25.6 76.8 3 MIN pS P S Nn N N N = ⋅= × = > ∴ = Then, the turns ratio for 12V output is obtained as: 22 21 11 12 0.7 36.99 7 50.45 OF SS OF VV NN VV + + = ⋅= ⋅ = ≅ ++ Therefore, the number of turns for each winding is obtained as: Np=78, NS1=3, NS2=7 (3+4 stack) and NS3=7. |
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