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FAN6921 数据表(PDF) 3 Page - Fairchild Semiconductor |
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FAN6921 数据表(HTML) 3 Page - Fairchild Semiconductor |
3 / 16 page AN-6921 APPLICATION NOTE © 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.1 • 8/24/10 3 Figure 4. Frequency Variation of BCM PFC Since the design of line filter and inductor for a BCM PFC converter with variable switching frequency should be done at minimum frequency condition, it is worthwhile to examine how the minimum frequency of BCM PFC converter changes with operating conditions. Figure 5 shows the minimum switching frequency, which occurs at the peak of line voltage, as a function of the RMS line voltage for different output voltage settings. For universal line application, the minimum switching frequency occurs at high line (265VAC) as long as the output voltage is lower than about 405V. Figure 5. Minimum Switching Frequency vs. RMS Line Voltage (L = 780µH, POUT = 100W) 3. Operation Principle of Quasi- Resonant Flyback Converter QR flyback converter topology can be derived from a conventional square wave, pulse-width modulated (PWM), flyback converter without adding additional components. Figure 6 and Figure 7 show the simplified circuit diagram of a quasi-resonant flyback converter and its typical waveforms. The basic operation principles are: During the MOSFET on time (tON), input voltage (VIN) is applied across the primary-side inductor (Lm). MOSFET current (IDS) increases linearly from zero to the peak value (Ipk). During this time, the energy is drawn from the input and stored in the inductor. When the MOSFET is turned off, the energy stored in the inductor forces the rectifier diode (D) to turn on. During the diode ON time (tD), the output voltage (Vo) is applied across the secondary-side inductor and the diode current (ID) decreases linearly from the peak value to zero. At the end of tD, all the energy stored in the inductor has been delivered to the output. During this period, the output voltage is reflected to the primary side as Vo× Np/Ns. Then, the sum of input voltage (VIN) and the reflected output voltage (Vo× Np/Ns) is imposed across the MOSFET. When the diode current reaches zero, the drain-to- source voltage (VDS) begins to oscillate by the resonance between the primary-side inductor (Lm) and the MOSFET output capacitor (Coss) with an amplitude of Vo× Np/Ns on the offset of VIN, as depicted in Figure 7. Quasi-resonant switching is achieved by turning on the MOSFET when VDS reaches its minimum value. This reduces the MOSFET turn-on switching loss caused by the capacitance loading between the drain and source of the MOSFET. Figure 6. Schematic of QR Flyback Converter |
类似零件编号 - FAN6921 |
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类似说明 - FAN6921 |
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