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AN-6920MR Datasheet(PDF) 2 Page - Fairchild Semiconductor |
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AN-6920MR Datasheet(HTML) 2 Page - Fairchild Semiconductor |
2 / 17 page AN-6920 APPLICATION NOTE © 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.0 • March 10, 2011 2 2. Operation Principles of BCM Boost PFC Converters The most widely used operation modes for the boost converter are continuous conduction mode (CCM) and boundary conduction mode (BCM). These refer to the current flowing through the energy storage inductor of the boost converter, as depicted in Figure 2. As the names indicate, the inductor current in CCM is continuous; while in BCM, the new switching period is initiated when the inductor current returns to zero, which is at the boundary of continuous conduction and discontinuous conduction operations. Even though the BCM operation has higher RMS current in the inductor and switching devices, it allows better switching condition for the MOSFET and the diode. As shown in Figure 2, the diode reverse recovery is eliminated and a fast silicon carbide (SiC) diode is not needed. MOSFET is also turned on with zero current, which reduces switching loss. Figure 2. CCM vs. BCM Control The fundamental idea of BCM PFC is that the inductor current starts from zero in each switching period, as shown in Figure 3. When the power transistor of the boost converter is turned on for a fixed 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, the input current of the converter follows the input voltage waveform with a 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. A by-product of the BCM is that the boost converter runs with variable switching frequency that 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 current follows the sinusoidal input voltage waveform, as shown in Figure 3. The lowest frequency occurs at the peak of sinusoidal line voltage. Figure 3. Operation Waveforms of BCM PFC The voltage-second balance equation for the inductor is: . ( ) ( ( )) IN ON O PFC IN OFF V t t V V t t ⋅ = − ⋅ (1) where VIN(t) is the rectified line voltage. The switching frequency of BCM boost PFC converter is obtained as: . . , . ( ) 1 1 | sin(2 ) | 1 O PFC IN SW ON OFF ON OUT O PFC IN PK LINE ON O PFC V V t f t t t V V V f t t V π − = = ⋅ + − ⋅ = ⋅ (2) where VIN,PK is the amplitude of the line voltage and fLINE is the line frequency. Figure 4 shows how the MOSFET on time and switching frequency change as output power decreases. When the load decreases, as shown in the right side of Figure 4, the peak inductor current diminishes with reduced MOSFET on time and the switching frequency increases. |
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