Power Factor Correction (PFC) - In a conventional power supply with switching regulators, a rectifying circuit that converts AC input source into a DC source for the primary circuit is used. In this rectifying circuit, a capacitor with a large capacitance is used to soften transient response and reduce ripple so that the switching regulator is not over-stressed. However, the peak charge of the capacitor becomes greater with greater capacitance, and this leads to non-linear bursts of peak over current into the primary circuit.

Such peaks of current distort output voltage, create harmonic frequencies, and reduce power factor. There is now an international standard for controlling harmonics and PFC is mandatory for home appliances consuming over 70W or more power in Europe. PFC circuitry is classed as either Active or Passive.

Active PFC uses switching regulator technology called boost-up. Using active elements such as IC, FET and diodes to create the PFC circuit. This circuit has a theoretical power factor of over 95%, accepts a full range (90VAC-260VAC) of AC input, and reduces total harmonics noticeably. However, it needs a complicated EMI filter and an input source circuit, and is costly to build. The ZM300A-APF is based on this technology.

The other system is called Passive PFC and this uses passive elements such as an iron core inductor on the input source to create a countering reactance. While this can easily be applied to existing power circuitry without much modification, the power factor is low (60-80%), the AC input must be chosen (115VAC/230VAC), and the harmonics produced from the difference between the capacitance and the inductance are hard to control. There can be large amounts of electromagnetic noise with this system from the 115VAc input source.

Ripple Voltage - One of the other important factors in choosing a power supply is the level of DC ripple voltage. A ripple is normally defined as the peak-to-peak voltage caused by an imperfect rectification of an AC source. In the case of the typical switching regulator, ripple factors from the low-frequency AC input source, the high-frequency switching, and impulse noise contribute to the DC output ripple.

This ripple and impulse noise can be reduced to below regulation limit by inductive canceling within the rectifier circuit, but if the output ripple exceeds the limits and is carried into the system, the logic level of active elements becomes unstable and the system can malfunction.