For PFC circuits that sense the AC input voltage, most controllers require an external two-pole filter to derive the RMS line voltage. While this is acceptable for line UVLO (also known as brown-out protection), the sluggishness of the two-pole filter results in additional line current distortion which hinders using the RMS voltage information for any part of the PWM control such as voltage feed-forward. Instead, the FAN9612 senses the peak of the AC input voltage to derive the RMS value. Since the RMS value is proportional to the peak of the line voltage, the required external circuitry is reduced from a two-pole filter to a simple resistive divider. As shown in the figure below, the FAN9612 uses the divided down, peak voltage signal for brown-out protection (V_IN(UVLO)), input over voltage protection (V_IN(OVP) ) and voltage feed-forward (V_IN(VFF) ) as part of the PWM control. The ratio between R_IN1 and R_IN2 is used to set the V_IN(OVP) trip point and brown-out protection level. Brown-out hysteresis is a programmable feature unique to FAN9612 and is selectable by the internal 2μA current source and R_IN(HYS) .

Input Voltage Sensing Circuitry Click on image to enlarge.

Voltage feed-forward offers several advantages to PFC converters. First, the control loop gain becomes independent of input voltage which greatly eases the task of compensation and helps maintain tighter output voltage regulation during line transients. Secondly, the input current remains sinusoidal, reducing current distortion, even during power limit. Third, since the user-programmable maximum on-time (MOT) is proportional to V_IN, an effective power limit function for each channel is realized. And finally, the FAN9612 is also capable of operating with DC input voltage, making it suitable for high power inverters such as those designed for solar power applications.

In addition to brown-out protection and input voltage OVP, the FAN9612 offers two levels of output voltage OVP. The feedback resistors, R_FB1 and R_FB2, shown in the figure below, divide down the output voltage and feed this signal to the input of the FAN9612 transconductance error amplifier. A non-latching output OVP circuit internally monitors this signal and is programmed to prevent switching if the feedback voltage exceeds 3.25V. So in effect, R_FB1 and R_FB2 perform the dual function of regulating the output voltage and fulfilling output OVP. Some applications may have design requirements restricting the output OVP and voltage regulation functions from sharing the same resistor string. The FAN9612 addresses this concern by supplying a second level, latching OVP function. The threshold of this latching circuit is 3.5V and can be actively set by R_OV1 and R_OV2 above the non-latching OVP level. In the unlikely event that R_FB2 were to short to ground, this second level OVP function latches DRV1 and DRV2 off.

Simplified Application Circuit Click on image to enlarge.