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FAN9611MX Datasheet(PDF) 11 Page - Fairchild Semiconductor |
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FAN9611MX Datasheet(HTML) 11 Page - Fairchild Semiconductor |
11 / 35 page © 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN9611 / FAN9612 • Rev. 1.0.1 11 8. Power Limit The architecture and operating principle of FAN9611/12 also provides inherent input power limiting capability. 50 100 150 200 250 300 0 0.5 1.0 VINRMS [V] Figure 14. On-Time vs. VIN_RMS When the slope of the PWM ramp is made proportional to the square of the input RMS voltage, the maximum on-time of the boost power switch becomes inversely proportional to the square of VIN,RMS, as represented in Figure 14. In boundary-conduction mode, the peak current of the boost transistor is proportional to its on time. Therefore, controlling the maximum pulse width of the gate drive signal according to the curve shown is an effective method to implement an input-voltage independent power limit for the boost PFC. 9. Light-Load Operation (Phase Management) One of the parameters determining the operating frequency of a boundary conduction mode converter is the output power. As the load decreases, lower peak currents are commanded by the pulse width modulator to maintain the output voltage at the desired set point. Lower peak current means shorter on-time for the power transistor and shorter time interval to ramp the inductor current back to zero at any given input voltage. As a result, the operating frequency of the converter increases under light load condition. As the operating frequency and corresponding switching losses increase, conduction losses diminish at the same time. Therefore, the power losses of the converter are dominated by switching losses at light load. This phenomenon is especially evident in a BCM converter. To improve light-load efficiency, FAN9611/12 disables one of the two interleaved boost converters automatically when the output power falls below approximately 12% of the maximum power limit level. By managing the number of phases used at light load, the FAN9611/12 can maintain high efficiency for a wider load range of the power supply. Normal interleaved operation of the two boost converters resumes automatically once the output power exceeds approximately 19% of the maximum power limit level of the converter. By adjusting the maximum on-time (using RMOT), the phase management thresholds can be adjusted upward, as described in the “Adjusting the Phase- Management Thresholds” section of Application Information in this datasheet. Output Power Normalized to PO MAX 1 2 12% 19% 100% 50% 0% Figure 15. Automatic Phase-Control Operation 10. Brownout Protection with Soft Recovery An additional protection function usually offered by PFC ICs is input brownout protection to prevent the converter from operating below a user-defined minimum input voltage level. For this function to work, the input voltage of the converter is monitored. When the voltage falls below the brownout protection threshold, the converter stops working. The output voltage of the boost converter falls until the load stops drawing current from the output capacitor or until the input voltage gets back to its nominal range and operation resumes. As the output falls, the voltage at the feedback pin falls proportionally, according to the feedback divider ratio. To facilitate soft recovery after a brownout condition, the soft-start capacitor – which is also the reference voltage of the error amplifier – is pulled lower by the feedback network. This effectively pre-conditions the error amplifier to provide closed-loop, soft-start-like behavior during the converter’s recovery from a brownout situation. Once the input voltage goes above the brownout protection threshold, the converter resumes normal operation. The output voltage rises back to the nominal regulation level following the slowly rising voltage across the soft-start capacitor. 11. Soft Starting the Converter During startup, the boost converter peak charges its output capacitor to the peak value of the input voltage waveform. The final voltage level, where the output is regulated during normal operation, is reached after the converter starts switching. There are two fundamentally different approaches used in PWM controllers to control the startup characteristics of a switched-mode power supply. Both methods use some kind of soft-start mechanism to reduce the potential overshoot of the converter’s output after the desired output voltage level is reached. The first method is called open-loop soft-start and relies on gradually increasing the current or power limit of the converter during startup. In this case, the voltage error amplifier is typically saturated, commanding maximum current until the output voltage reaches its final value. At that time, the voltage between the error amplifier inputs changes polarity and the amplifier slowly comes out of saturation. While the error amplifier recovers and before it starts controlling the output voltage, the converter operates with full power. Thus, output voltage |
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