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AN-8024 Datasheet(PDF) 6 Page - Fairchild Semiconductor |
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AN-8024 Datasheet(HTML) 6 Page - Fairchild Semiconductor |
6 / 10 page AN-8024 APPLICATION NOTE © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.1 • 9/18/09 6 The typical voltage and current margins for the rectifier diode are: 1.3 RRM DO VV >⋅ (18) 1.5 RMS FDO II >⋅ (19) where VRRM is the maximum reverse voltage and IF is the current rating of the diode. (Design Example) The diode voltage and current are calculated as: 373 5 25.5 18.18 MAX IN DO O V VV V n =+ = + = 1 10.47 18.18 0.36 6.9 0.47 RMS RMS MAX DO DS MAX D In I D A − =⋅ − =⋅ = Two 5A and 40V diodes in parallel are selected for the rectifier diode. [STEP-10] Feedback Circuit Configuration Since FSBH-series employs current-mode control, the feedback loop can be implemented with a one-pole and one- zero compensation circuit. The current control factor of FPS, K is defined as: 3.2 LIM LIM SAT FB I I K V == (20) where ILIM is the pulse-by-pulse current limit and VFB SAT is the feedback saturation voltage. which is typically 3.2V. As described in step 4, it is typical to design the flyback converter to operate in CCM for heavy load condition. For CCM operation, the control-to-output transfer function of a flyback converter using current mode control is given by: ˆ ˆ (/ ) (1 / )(1 / ) 2(1 / ) ωω ω = ⋅⋅ +− =⋅ ++ o vc FB LIN P S Z RZ RO IN P v G v KR V N N ss VV s (21) where RL is the load resistance and the pole and zeros of Equation (21) are obtained as: 2 2 (1 ) 1(1 ) , (/ ) L ZRZ P CO M S P L O RD D and R CDL N N R C ωω ω − + == = where D is the duty cycle of the FPS and RC is the ESR of CO. Notice that there is a right half plane (RHP) zero (ωRZ) in the control-to-output transfer function of Equation (21). Because the RHP zero reduces the phase by 90 degrees, the crossover frequency should be placed below the RHP zero. Figure 7 shows the variation of a CCM flyback converter control-to-output transfer function for different input voltages. This figure shows the system poles and zeros together with the DC gain change for different input voltages. The gain is highest at the high input voltage condition and the RHP zero is lowest at the low input voltage condition. Figure 7. CC M Flyback Converter Control-to Output Trans- fer Function Variation for Different Input Voltages Figure 8 shows the variation of a CCM flyback converter control-to-output transfer function for different loads. This figure shows that the low frequency gain does not change for different loads and the RHP zero is lowest at the full load condition. Figure 8. CCM Flyback Converter Control-to Output Transfer Function Variation for Different Loads When the input voltage and the load current vary over a wide range, it is not easy to determine the worst case for the feedback loop design. The gain, together with zeros and poles, vary according to the operating conditions. Moreover, even though the converter is designed to operate in CCM or at the boundary of DCM and CCM in the minimum input voltage and full load condition, the converter enters into DCM, |
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