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FAN7530 데이터시트(PDF) 19 Page - Fairchild Semiconductor |
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FAN7530 데이터시트(HTML) 19 Page - Fairchild Semiconductor |
19 / 37 page © 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN9611 / FAN9612 • Rev. 1.1.3 19 3. Adjusting the Output Voltage with Load In some applications, the output voltage of the PFC boost converter is decreased at low power levels to boost the light load efficiency of the power supply. Implementing this function with a circuit external to the FAN9611/12 is straightforward because the error amplifier reference (the positive input) is available on the soft-start (SS) pin, as shown in Figure 28. In the FAN9611/12 architecture, the power of the converter is proportional to the voltage on the COMP pin, minus a small offset. The voltage on the COMP pin is monitored to determine the operating power of the supply. Therefore the voltage on the SS pin can be adjusted lower to achieve the desired lower output voltage. Several possible implementations to adjust the output voltage of the boost stage at light load are described in the application note AN-8021. It includes the universal output voltage adjust implementation which is modulated by input voltage to avoid the boost converter becoming a peak rectifier at high line and light load. Figure 28. FAN9611/12 Error Amplifier Configuration 4. Adjusting the Output Voltage with Input Voltage In some applications, the output voltage of the PFC boost converter is adjusted based on the input voltage only. This boost follower implementations increases the efficiency of the downstream DC-DC converter and therefore of the overall power supply. Implementations for both the two-level boost and the linear boost follower (or tracking boost) are described in application note AN-8021. 5. Adjusting the Phase-Management Thresholds In any power converter, the switching losses become dominant at light load. For an interleaved converter where there are two or more phases, light-load efficiency can be improved by shutting down one of the phases at light load (also known as phase-shedding or phase-dropping operating). The initial phase-management thresholds are fixed at approximately 13% and 18% of the maximum load power level. This means when the output power reaches 13%, the FAN9611/12 automatically goes from a two-phase to a single-phase operation (phase shed or phase drop). When the output power comes back up to 18%, the FAN9611/12 automatically goes from the single-phase to the two-phase operation (phase-add). The default thresholds can be adjusted upward based on the application requirement; for example, to meet the Energy STAR 5.0 or the Climate Savers Computing efficiency requirements at 20% of the load. The phase drop threshold can be adjusted upward (for example to 25%) by adjusting the maximum on time. Figure 29. Adjusting Phase Management Thresholds Since the phase management threshold is fixed at 13% and 18% of the maximum power limit level, the actual power management threshold as a percentage of nominal output power can be adjusted by the ratio between nominal power and maximum power limit level as shown in Figure 29. The second plot shows an example where the maximum power limit level is 1.4 times of nominal output power. By adjusting the maximum on-time (using RMOT), the phase management thresholds can be adjusted upward. Phase management is implemented such that the output of the error amplifier (VCOMP) does not have to change when the system toggles between single-phase and two-phase operations, as shown in Figure 30. The output of the error amplifier is proportional to the output power of the converter independently, whether one or both phases are operating in the power supply. Furthermore, because the maximum on-time limit is applied independently to each pulse-width modulator, the power handling capability of the converter with only one phase running is approximately half of the total output power that can be delivered when both phases are utilized. Additional details on adjusting phase management are provided in the application note AN-6086. Figure 30. VCOMP vs. tON MAX |
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