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FAN7930C Datasheet(PDF) 14 Page - ON Semiconductor |
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FAN7930C Datasheet(HTML) 14 Page - ON Semiconductor |
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14 / 22 page  www.onsemi.com 14 VOUT PFC + - INV 1 COMP 3 Clamp Circuit + - VREF Stair Step THD-Optimized Sawtooth Generator Sawtooth MOSFET Off R1 C1 C2 1V 6.2V Figure 32. Control Circuit Unlike a conventional voltage- mode PWM controller, FA N7930C turns on the MOSFET at the falling edge of ZCD signal. The “ ON” instant is deter mined by the external signal and the turn-on time lasts until the error amplifier output (VCOMP) and saw tooth w aveform meet. When load is heavy, output voltage decreases, scaled output decreases, COMP voltage increases to compensate low output, turn-on time lengthens to give more inductor turn-on time, and increased inductor current raises the output voltage. This is how a PFC negative feedback controller regulates output. The maximum of VCOMP is limited to 6. 5 V, w hich dictates the max imum turn-on time. Sw itching stops w hen VCOMP is low er than 1.0 V. ZCD after COMPARATOR VCOMP & Sawtooth MOSFET gate t s / V 155 . 0 Figure 33. Turn-On Tim e Determ ination 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 34. 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 35. Gain Characteris tic 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 36. 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 is triggered dur ing startup phase. Operation on and off by OV P at startup may cause audible noise and can increase voltage stress at startup, |
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Similar Description - FAN7930C |
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