The FAN7530 is an active power factor correction (PFC)
controller for the boost PFC application that operates in the
critical conduction mode (CRM). The critical conduction
mode boost power factor converter operates at the boundary
of continuous conduction mode and discontinuous conduction
mode. The CRM PFC controllers are of two kinds: the
current-mode CRM PFC controller and the voltage-mode
CRM PFC controller. For the current mode, a boost switch is
turned on when the inductor current reaches zero and turned
off when the inductor current meets the desired current reference.
In this case, the rectified AC line voltage should be
sensed to generate the current reference, as in the
FAN7527B; however, the sensing network can cause addi
tional
power loss. In the voltage mode, the switch turn-on is
the same as that of the current mode, but the switch turn-off
is determined by an internal ramp signal. The ramp signal is
compared with an error amplifier output and the switch turnon
time is controlled to be constant, as shown in Figure 1. If
the turn-on time is constant, the peak inductor current is proportional
to the rectified AC line voltage, as shown in Figure
2. In this way, the input current waveform follows the waveform
of the input voltage, thereby obtaining a good power
factor. The FAN7530 is a voltage-mode CRM PFC controller.
Because the voltage-mode CRM PFC controller does not
need the rectified AC line voltage information, it can save
the power loss of the sensing network.
