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FAN9612 Folha de dados(PDF) 8 Page - Fairchild Semiconductor |
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FAN9612 Folha de dados(HTML) 8 Page - Fairchild Semiconductor |
8 / 37 page © 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN9611 / FAN9612 • Rev. 1.1.3 8 Theory of Operation 1. Boundary Conduction Mode The boost converter is the most popular topology for power factor correction in AC-to-DC power supplies. This popularity can be attributed to the continuous input current waveform provided by the boost inductor and to the fact that the boost converter’s input voltage range includes 0V. These fundamental properties make close to unity power factor easier to achieve. Figure 6. Basic PFC Boost Converter The boost converter can operate in continuous conduction mode (CCM) or in boundary conduction mode (BCM). These two descriptive names refer to the current flowing in the energy storage inductor of the boost power stage. Figure 7. CCM vs. BCM Control As the names indicate, the current in Continuous Conduction Mode (CCM) is continuous in the inductor; while in Boundary Conduction Mode (BCM), the new switching period is initiated when the inductor current returns to zero. There are many fundamental differences in CCM and BCM operations and the respective designs of the boost converter. The FAN9611/12 utilizes the boundary conduction mode control algorithm. The fundamental concept of this operating mode is that the inductor current starts from zero in each switching period, as shown in the lower waveform in Figure 7. When the power transistor of the boost converter is turned on for a fixed amount of time, the peak inductor current is proportional to the input voltage. Since the current waveform is triangular, the average value in each switching period is also proportional to the input voltage. In the case of a sinusoidal input voltage waveform, the input current of the converter follows the input voltage waveform with very high accuracy and draws a sinusoidal input current from the source. This behavior makes the boost converter in BCM operation an ideal candidate for power factor correction. This mode of control of the boost converter results in a variable switching frequency. The frequency depends primarily on the selected output voltage, the instantaneous value of the input voltage, the boost inductor value, and the output power delivered to the load. The operating frequency changes as the input voltage follows the sinusoidal input voltage waveform. The lowest frequency operation corresponds to the peak of the sine waveform at the input of the boost converter. Even larger frequency variation can be observed as the output power of the converter changes, with maximum output power resulting in the lowest operating frequency. Theoretically, under zero-load condition, the operating frequency of the boost converter would approach infinity. In practice, there are natural limits to the highest switching frequency. One such limiting factor is the resonance between the boost inductor and the parasitic capacitances of the MOSFET, the diode, and the winding of the choke, in every switching cycle. Another important characteristic of the BCM boost converter is the high ripple current of the boost inductor, which goes from zero to a controlled peak value in every switching period. Accordingly, the power switch is stressed with high peak current. In addition, the high ripple current must be filtered by an EMI filter to meet high-frequency noise regulations enforced for equipment connecting to the mains. The effects usually limit the practical output power level of the converter. |
Nº de peça semelhante - FAN9612 |
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Descrição semelhante - FAN9612 |
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