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FAN7930C Datasheet(PDF) 14 Page - Fairchild Semiconductor |
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FAN7930C Datasheet(HTML) 14 Page - Fairchild Semiconductor |
14 / 22 page ![]() © 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN7930C • Rev. 1.0.0 14 s / V 155 . 0 μ Figure 33. Turn-On Time Determination The roles of PFC controller are regulating output voltage and input current shaping to increase power factor. Duty control based on the output voltage should be fast enough to compensate output voltage dip or overshoot. For the power factor, however, the control loop must not react to the fluctuating AC input voltage. These two requirements conflict; therefore, when designing a feedback loop, the feedback loop should be least 10 times slower than AC line frequency. That slow response is made by C1 at compensator. R1 makes gain boost around operation region and C2 attenuates gain at higher frequency. Boost gain by R1 helps raise the response time and improves phase margin. Figure 34. Compensators Gain Curve For the transconductance error amplifier side, gain changes based on differential input. When the error is large, gain is large to make the output dip or peak to suppress quickly. When the error is small, low gain is used to improve power factor performance. mho 250 μ mho 115 μ Figure 35. Gain Characteristic 6. Soft-Start: When VCC reaches VSTART, the internal reference voltage is increased like a stair step for 5ms. As a result, VCOMP is also raised gradually and MOSFET turn-on time increases smoothly. This reduces voltage and current stress on the power switch during startup. Figure 36. Soft-Start Sequence 7. Startup without Overshoot: Feedback control speed of PFC is quite slow. Due to the slow response, there is a gap between output voltage and feedback control. That is why over-voltage protection (OVP) is critical at the PFC controller and voltage dip caused by fast load changes from light to heavy is diminished by a bulk capacitor. OVP is easily triggered at startup phase. Operation on and off by OVP at startup may cause audible noise and can increase voltage stress at startup, which is normally higher than in normal operation. This operation is better when soft-start time is very long. However, too much startup time enlarges the output voltage building time at light load. FAN7930C has overshoot avoidance at startup. During startup, the feedback loop is controlled by an internal proportional gain controller and, when the output voltage reaches the rated value, it switches to an external compensator after a transition time of 30ms. This internal proportional gain controller eliminates overshoot at startup and an external conventional compensator takes over successfully afterward. |
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