Datenblatt-Suchmaschine für elektronische Bauteile |
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FAN9611 Datenblatt(PDF) 11 Page - Fairchild Semiconductor |
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FAN9611 Datenblatt(HTML) 11 Page - Fairchild Semiconductor |
11 / 18 page AN-6086 © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.4 • 4/22/10 11 Figure 18. Control-to-Output Transfer Function Proportional and integration (PI) control with high frequency pole is typically used for compensation, as shown in Figure 19. The compensation zero (fCZ) introduces phase boost, while the high frequency compensation pole (fCP) attenuates the switching ripple. The transfer function of the compensation network is obtained as: 1 ˆ 2 2 ˆ 1 2 COMP CZ I OUT CP s vf f s vs f π π π + =⋅ + (30) where , , , 380 / , 2 1 , 2 1 2 I OUT COMP LF CZ COMP COMP LF CP COMP COMP HF AV f VC f RC f RC μ π π π =⋅ ⋅ = ⋅⋅ = ⋅⋅ , 1 2 CZ COMP COMP LF f RC π = ⋅ ^ ^ COMP OUT v v , 1 2 CP COMP COMP HF f RC π = ⋅ ,, 10 COMP HF COMP LF CC ≤ , 2 12 SS REF FB FB FB OUT V R RR V = + Figure 19. Compensation Network The procedure to design the feedback loop is as follows: (a) Determine the crossover frequency (fC) around 1/10~1/5 of line frequency. Since the control-to-output transfer function of power stage has -20dB/dec slope and -90 o phase at the crossover frequency, as shown in Figure 20; it is required to place the zero of the compensation network (fCZ) around the crossover frequency so that 45 ° phase margin is obtained. Then, the capacitor CCOMP,LF is determined as: , 2 80 / 3 4.1 (2 ) OUT MAX COMP LF OUT OUT C AV I K C V Cf μ π ⋅⋅ =⋅ ⋅⋅ (31) To place the compensation zero at the crossover frequency, the compensation resistor is obtained as: , 1 2 COMP CCOMP LF R fC π = ⋅⋅ (32) (b) Place compensator high-frequency pole (fCP) at least a decade higher than fC to ensure that it does not interfere with the phase margin of the voltage regulation loop at its crossover frequency. It should also be sufficiently lower than the switching frequency of the converter so noise can be effectively attenuated. Then, the capacitor CCOMP,HF is determined as: , 1 2 COMP HF CP COMP C fR π = ⋅⋅ (33) Figure 20. Compensation Network Design |
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