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FAN7930B Datasheet(PDF) 14 Page - ON Semiconductor |
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FAN7930B Datasheet(HTML) 14 Page - ON Semiconductor |
14 / 22 page ![]() www.onsemi.com 14 The roles of PFC controller are regulating output voltage and input current shaping to increase pow er factor. Duty control based on the output voltage should be fast enough to compensate output voltage dip or overshoot. For the pow er factor, how ever, the control loop must not react to the fluctuating A C input voltage. These tw o requirements conflict; therefore, w hen designing a feedback loop, the feedbac k loop should be least ten times slow er than AC line frequency. That slow response is made by C1 at the 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. Freq. C1 R1 Proportional Gain C2 Integrator High-Frequency Noise Filter Gain Figure 33. Com pensators Gain Curve For the transconductance error amplifier side, gain changes based on differential input. When the error is large, gain is large to suppress the output dip or peak quickly. When the error is small, low gain is used to improve pow er factor performance. ICOMP Powering Braking mho 250 mho 115 Figure 34. Gain Characteristic 7. Soft-Start: When VCC reaches VSTART, the internal reference voltage is increased like a stair step for 5 ms. As a result, VCOMP is also raised gradually and MOSFET turn-on time increases smoothly. This reduces voltage and current stress on the pow er sw itch during startup. VREF SS gM VINV=0.4V ISOURCE COMP VCOMP ISOURCE COMP RCOMP=VCOMP t (VREF SS-V INV) gM=ISOURCE COMP VREF END=2.5V 5ms VCC VSTART=12V Figure 35. Soft-Start Sequence 8. Startup w ithout Overshoot: Feedback control speed of PFC is quite slow . Due to the slow response, ther e is a gap betw een output voltage and feedback control. That is w hy over-voltage protection ( OV P) is critical at the PFC controller and voltage dip caused by fast load changes from light to heavy is diminished by a bulk capacitor. OV P can be triggered during startup phase. Operation on and off by OV P at startup may cause audible noise and can increase voltage stress at startup, w hich is nor mally higher than in nor mal operation. This operation is improved w hen soft-start time is very long. How ever, too much startup time enlarges the output voltage building time at light load. FAN7930B has overshoot protection at startup. During startup, the feedback loop is controlled by an internal proportional gain controller and w hen the output voltage reaches the rated value, it sw itches to an external compensator after a transition time of 30 ms. This internal proportional gain controller eliminates overshoot at startup and an external conventional compensator takes over successfully afterw ard. Depend on Load VOUT VCOMP Startup Overshoot Internal Controller t Conventional Controller Overshoot-less Startup Control Control Transition Figure 36. Startup w ithout Overshoot |
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