Bridge Rectifier

The AC power stream, after it passes the EMI/Transient filter is rectified by one or more bridge rectifiers. So AC is converted to DC with increased voltage (e.g. if we have 230V input then the DC output of the bridge rectifier will be v2 × 230= 325.27V DC). Afterwards the DC signal is fed to the APFC stage.

Active Power Factor Correction (APFC)

Before we talk about the Power Factor Correction stage let's take a quick look at the concept of power factor. Power factor is defined as the ratio of real power to apparent power (kW/kVA) and power is the product of voltage and current (P = V × I).

There are two basic types of loads, resistive (the load consists only of resistors) and reactive (the load consists of inductors, capacitors or both).
In a system with only linear load both current and voltage curves are sinusoidal (the sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation). If the load is purely resistive then both current and voltage reverse their polarity at the same time (the phase angle between voltage and current is 0 degrees), so at every instant the product of voltage and current is positive, meaning that the "direction" of energy flow does not reverse, so only real power is transferred to the load.



In case that the load is purely reactive, there is a time shift (maximum theoretically 90 degrees, typical 45 degrees) between voltage and current so the product of these two for half of each cycle is positive and for the other half is negative (when voltage is at its peak, either positive or negative, current is zero and vice-versa). Thus, on average as much energy flows to the load as flows back to the source (power grid). If we analyze a whole cycle then we will see that there is no net energy flow and only reactive energy flows since there is no net transfer of energy towards the load.



However both scenarios above are only theoretical, as in real life all loads/circuits present resistance, inductance and capacitance at the same time, so both real and reactive power will flow to them. Apparent power is the vector sum of real and reactive power or the product of the root-mean-square of voltage and current. As we already mentioned above, power factor is the ratio between real and apparent power. Ideally we want the power factor to be close to 1. Here we must note that residential consumers pay only for the real power (Watts) they use and not for apparent power. On the contrary business consumers (e.g. factories) pay for apparent power usage, too.

Although we residential consumers do not pay for apparent power, in order to minimize apparent power usage the EU standard EN61000-3-2 states that all switched-mode power supplies with output power of more than 75 W must include passive PFC, at least. In addition 80 PLUS power supply certification requires a power factor of 0.9 or more. Some years ago many PSU manufacturers used Passive PFC (PPFC) in their products. PPFC uses a filter that passes current only at line frequency, 50 or 60 Hz, so the harmonic current is reduced and the non-linear load is transformed to a linear one. Then with the usage of capacitors or inductors the power factor can be brought close to unity. The downfalls of PPFC are the smaller attained power factors compared to APFC and the need for a voltage doubler, for the PSU to be compatible with 115/230V (the proper voltage is manually selected via a switch near the AC input). On the other hand passive PFC has higher efficiency compared to active PFC!


Active PFC is basically an AC/DC converter which controls the current supplied to the PSU via PWM (Pulse Width Modulation). At first the AC voltage is rectified by the bridge rectifier. Then the PWM triggers the APFC MOSFETs (usually two) which separates the intermediate DC voltage into constant pulse sequences. These pulses are smoothened by the smoothing capacitor and are fed to the main switches. Right before the smoothing capacitor we always find an inductor (coil) that has the ability to limit the sudden rise of current without dissipating energy, because it's a reactive component. This coil is necessary because all capacitors that are connected directly to a DC signal show uncontrolled inrush current, so the inductor limits this inrush. In addition sometimes there is also a thermistor used with active PFC to further limit the inrush current especially in the switch-on phase of the PSU, where the smoothing capacitor is fully discharged.

In active PFC two different types of control are mostly used, Discontinuous Conduction Mode (DCM), where the PFC MOSFETs are turned on only when the inductor current has reached zero and Continuous Conduction Mode (CCM) where the MOSFETs are turned on when inductor current is still above zero and therefore all reverse recovery energy is dissipated in the MOSFETs. In the APFC stage of PSUs the second mode (CCM) is mostly used since it is ideal for over 200W output power, because it offers the lowest peak to average current ratio for the converter throughput power. The main drawbacks of CCM are the losses and EMI generation associated with the turn-off of the boost diode (the reverse recovery currents of the diode causes significant power dissipation to the MOSFETs and increased EMI). Because of that we usually see an X capacitor after the bridge rectifier.