Traditionally rectification is simply a diode bridge and usually followed by capacitor filtering. It has peak detection characteristics, which means only small pulses of current are drawn at the peak voltages. The drawback is that not entire cycle is used, so power draw is not maximized. In other words, the power factor is low. We know that a resistive load achieves unity power factor. If we can design rectifiers so that the load appears resistive, i.e. the current is proportional to the input voltage, we maximize the power factor. We know in the switching mode converter the output voltage is a function of the duty cycle of pulse width modulation (PWM). If we adjust the duty cycle based on the AC input voltage, we can get a constant voltage output; this way power is extracted for the entire cycle.
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