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FAN6751HLMY Arkusz danych(PDF) 5 Page - Fairchild Semiconductor |
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FAN6751HLMY Arkusz danych(HTML) 5 Page - Fairchild Semiconductor |
5 / 13 page AN-6073 APPLICATION NOTE © 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.0 • 9/26/08 5 Figure 11. FAN6751HL Burst-mode Operation Switching Disabled Switching Disabled Vds Ids VFB VO time 1.1 Switching Disabled Switching Disabled Figure 12. FAN6751MR Burst-Mode Operation Built-in Slope Compensation A flyback converter can be operated in either discontinuous current mode (DCM) or continuous current mode (CCM). There are many advantages when operating the converter in CCM. With the same output power, a converter in CCM exhibits a smaller peak inductor current than in DCM. Therefore, a small-sized transformer and a low-rated MOSFET can be applied. On the secondary side of the transformer, the RMS output current of DCM can be twice that of CCM. Larger wire gauge and output capacitors with larger ripple current ratings are required. DCM operation also results in a higher output voltage spike. A large LC filter is added. Therefore, a flyback converter in CCM achieves better performance with lower component cost. Despite the above advantages of CCM operation, there is one concern—stability. In CCM operation, the output power is proportional to the average inductor current, while the peak current remains controlled. This causes sub-harmonic oscillation when the PWM duty cycle exceeds 50%. Adding slope compensation (reducing the current-loop gain) is an effective way to prevent oscillation. The FAN6751 introduces a synchronized positive-going ramp (VSLOPE) in every switching cycle to stabilize the current loop. Therefore, the FAN6751 can be used to design a cost- effective, highly efficient, compact flyback power supply operating in CCM without additional external components. The positive ramp added is: D V V SL SLOPE • = (4) where VSL = 0.33V and D = duty cycle. Figure 13. Synchronized Slope Compensation Over-Power Compensation The maximum output power of a flyback converter can generally be designed by the current-sense resistor RS. When the load increases, the peak inductor current increases accordingly. As the current-sense signal of the SENSE pin exceeds the internal limit VSENSE, 0.83V typically, as VIN=1V, FAN6751 stops the PWM pulse immediately. The output power of a flyback power supply in DCM is calculated as follows: η • • • • = S PK P OUT f I L P 2 2 1 (5) where: Lp is the transformer primary-side inductance; IPK is the peak inductor current; fS is the PWM frequency; and η is the conversion efficiency. |
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