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FAN9612MX データシート(PDF) 9 Page - Fairchild Semiconductor |
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FAN9612MX データシート(HTML) 9 Page - Fairchild Semiconductor |
9 / 37 page © 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN9611 / FAN9612 • Rev. 1.1.3 9 2. Interleaving The FAN9611/12 control IC is configured to control two boost converters connected in parallel, both operated in boundary conduction mode. In this arrangement, the input and output voltages of the two parallel converters are the same and each converter is designed to process approximately half the total output power. Figure 8. Interleaved PFC Boost Operation Parallel power processing is penalized by the increased number of power components, but offers significant benefits to keep current and thermal stresses under control and to increase the power handling capability of the otherwise limited BCM PFC control solution. Furthermore, the switches of the two boost converters can be operated 180 degrees out of phase from each other. The control of parallel converters operating 180 degrees out of phase is called interleaving. Interleaving provides considerable ripple current reduction at the input and output terminals of the power supply, which favorably affects the input EMI filter requirements and reduces the high-frequency RMS current of the power supply output capacitor. There is an obvious difficulty in interleaving two BCM boost converters. Since the converter’s operating frequency is influenced by component tolerances in the power stage and in the controller, the two converters operate at different frequencies. Therefore special attention must be paid to ensure that the two converters are locked to 180-degree out-of-phase operation. Consequently, synchronization is a critical function of an interleaved boundary conduction mode PFC controller. It is implemented in the FAN9611/12 using proprietary and dedicated circuitry called Sync-Lock™ interleaving technology. 3. Voltage Regulation, Voltage Mode Control The power supply’s output voltage is regulated by a negative feedback loop and a pulse width modulator. The negative feedback is provided by an error amplifier that compares the feedback signal at the inverting input to a reference voltage connected to the non-inverting input of the amplifier. Similar to other PFC applications, the error amplifier is compensated with high DC gain for accurate voltage regulation, but very low bandwidth to suppress line frequency ripple present across the output capacitor of the converter. The line frequency ripple is the result of the constant output power of the converter and the fact that the input power is the product of a sinusoidal current and a sinusoidal voltage thus follows a sine square function. Eliminating the line frequency component from the feedback system is imperative to maintain low total harmonic distortion (THD) in the input current waveform. The pulse width modulator implements voltage mode control. This control method compares an artificial ramp to the output of the error amplifier to determine the desired on-time of the converter’s power transistor to achieve output voltage regulation. Figure 9. PWM Operation In FAN9611/12, there are two PWM sections corresponding to the two parallel power stages. For proper interleaved operation, two independent 180- degree out-of-phase ramps are needed; which necessitates the two pulse width modulators. To ensure that the two converters process the same amount of power, the artificial ramps have the same slope and use the same control signal generated by the error amplifier. 4. Input-Voltage Feedforward Basic voltage-mode control, as described in the previous section, provides satisfactory regulation performance in most cases. One important characteristic of the technique is that input voltage variation to the converter requires a corrective action from the error amplifier to maintain the output at the desired voltage. When the error amplifier has adequate bandwidth, as in most DC-DC applications, it is able to maintain regulation within a tolerable output voltage range during input voltage changes. On the other hand, when voltage-mode control is used in power factor corrector applications; the error amplifier bandwidth, and its capability to quickly react to input voltage changes, is severely limited. In these cases, the input voltage variation can cause excessive overshoot or droop at the converter output as the input voltage goes up or down. To overcome this shortcoming of the voltage-mode PWM circuit in PFC applications, input-voltage feedforward is often employed. It can be shown mathematically that a PWM ramp proportional to the square of the input voltage rejects the effect of input voltage variations on the output voltage and eliminates the need of any correction by the error amplifier. |
同様の部品番号 - FAN9612MX |
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同様の説明 - FAN9612MX |
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