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FAN7930B Datasheet(PDF) 13 Page - Fairchild Semiconductor |
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FAN7930B Datasheet(HTML) 13 Page - Fairchild Semiconductor |
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13 / 22 page  © 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN7930B • Rev. 1.0.2 13 Figure 28. Auxiliary Voltage Threshold When no ZCD signal is available, the PFC controller cannot turn on MOSFET, so the controller checks every switching off time and forces MOSFET turn on when the off time is longer than 150μs. It is called restart timer. Restart timer triggers MOSFET turn on at startup and may be used at the input voltage zero cross period. s 150 μ Figure 29. Restart Timer at Startup Because the MOSFET turn on depends on the ZCD input, switching frequency may increase to higher than several megahertz due to the miss-triggering or noise on the nearby ZCD pin. If the switching frequency is higher than needed for critical conduction mode (CRM), operation mode shifts to continuous conduction mode (CCM). In CCM, unlike CRM where the boost inductor current is reset to zero at the next switch on; inductor current builds up at every switching cycle and can be raised to very high current, that exceeds the current rating of the power switch or diode. This can seriously damage the power switch and result in burn down. To avoid this, maximum switching frequency limitation is embedded. If ZCD signal is applied again within 3.3μs after the previous rising edge of gate signal, this signal is ignored internally and FAN7930B waits for another ZCD signal. This slightly degrades the power factor performance at light load and high input voltage. Figure 30. Maximum Switching Frequency Limit Operation 6. Control: The scaled output is compared with the internal reference voltage and sinking or sourcing current is generated from the COMP pin by the transconductance amplifier. The error amplifier output is compared with the internal sawtooth waveform to give proper turn-on time based on the controller. Figure 31. Control Circuit Unlike a conventional voltage-mode PWM controller, FAN7930B turns on the MOSFET at the falling edge of ZCD signal. On instance is decided by the external signal and the turn-on time lasts until the error amplifier output (VCOMP) and sawtooth waveform 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 PFC negative feedback controller regulates output. The maximum of VCOMP is limited to 6.5V, which dictates the maximum turn-on time, and switching stops when VCOMP is lower than 1.0V. s / V 155 . 0 μ Figure 32. Turn-On Time Determination |
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