By Edward Ong, Senior Product Marketing Manager, Power Integrations
Power supply designers face two major problems today: eliminating unwanted input harmonic currents and ensuring that the power factor is as close to unity as possible. Harmful harmonic currents can cause overheating of transmission equipment and subsequent interference challenges that must be addressed; both of which can also adversely affect circuit size and/or efficiency. If the load applied to the line is not purely resistive, a phase shift will occur between the input voltage and current waveforms, increasing apparent power and reducing transfer efficiency. If the nonlinear load distorts the input current waveform, it will cause current harmonics, further reducing the transmission efficiency and introducing disturbances into the utility grid.
If you want to solve these problems, you need to understand the basic principles of power conversion. In the power supply, the AC voltage from the wall socket is usually connected to the rectifier circuit, and the rectifier tube converts the AC voltage into an AC signal of fixed polarity and its peak voltage is equivalent to a fixed VDC voltage. This signal is fed into a large capacitor that forms a filter that smooths out the ripples in the voltage waveform. The newly generated DC signal is fed to the DC/DC converter stage of the power supply to achieve the low voltage DC required for the final output.
If we go back to the original rectifier stage and look at the waveform, the input AC voltage is a traditional symmetrical sine wave with alternating positive and negative electrodes. However, the input current appears as a series of spikes that increase as the input voltage increases. This is because diode conduction (and therefore current flow) only occurs when the bulk capacitor is charging, when the VAC input voltage exceeds the DC voltage stored on the capacitor. When VAC is below the stored capacitor voltage, the charge stored on the bulk capacitor will support the output of the power supply. During this time, energy is transferred from the capacitor to the load, which causes the capacitor voltage to drop. Once the AC voltage exceeds the (now lower) voltage on the storage capacitor again, the capacitor will recharge. This short charging window means that the input current is delivered as a triangular pulse rather than a sine wave.
Figure 1: The input current of the rectifier stage appears as a series of spikes containing a large number of harmonic components, which can pollute the AC line
This peak current waveform consists of a series of power frequency harmonics. Harmonic content is limited by various national and international regulations enacted to protect electrical distribution networks. The power factor of the circuit shown in Figure 1 tends to be very low, around 0.5, a far cry from the ideal 1.
This problem can be solved in a few different ways. The easiest way to do this is to add an Inductor to cancel out the capacitive component of the circuit – a technique called passive power factor correction. However, passive power correction has a limited role. In applications with high power output, the physical size of the required inductor makes it impractical. In this case, an active PFC circuit is usually used to bring the power factor of the circuit closer to unity without negatively affecting the size of the circuit. Active power factor correction consists of PFC diodes, inductors and MOSFETs. The MOSFETs act as high frequency switches and are driven by a controller that implements a power factor correction algorithm.
The switching circuit forces the input current to follow the rectified VAC input and become a proper sine wave again. Ideally, sine waves have low distortion to eliminate harmonic currents that can pollute AC lines. Since the voltage and current waveforms are in phase, the power factor also rises to near the ideal value of 1.
Figure 2: A rectifier stage with active power factor correction turns the input current into a sine wave
An easy way to implement an active PFC circuit is to use the HiperPFS-4 solution from Power Integrations (see Figure 3). The HiperPFS-4 device includes an ultra-low reverse recovery charge diode that achieves high efficiency by minimizing diode switching losses. It also features a low RDS(ON) MOSFET that reduces conduction losses and an advanced continuous conduction mode controller that integrates many safety features.
Figure 3: HiperPFS-4 by Power Integrations
The HiperPFS-4 device integrates power factor correction diodes, MOSFETs and a controller at the same time. This high level of integration helps reduce development time and speed time to market. Another advantage of integrating key components in one package is minimizing parasitic inductance in the traces. The reduction in circuit inductance helps to reduce the voltage stress across the PFC diode and the peak drain-source voltage of the MOSFET, thereby increasing the reliability of the circuit. In addition, the diodes used have soft recovery characteristics that reduce ringing and thus reduce EMI. Integrating the diode and MOSFET in one package can significantly reduce the loop size, further reducing EMI.
Figure 4: The HiperPFS-4 device integrates key components for active power factor correction in the same package to minimize parasitic inductance in the wiring, thereby reducing di/dt induced voltage stress on the power switch
Active power factor correction is the best way to reduce unwanted input harmonic currents and improve power factor. Power Integrations has developed the HiperPFS-4 solution, which integrates key components required for active power factor correction into the same package. This approach dramatically reduces input current harmonics and improves power factor, while addressing many layout issues common to traditional circuit designs, such as reduced voltage stress, EMI, and parasitic losses.