“Power supply designers face two main problems today: eliminating harmful input harmonic currents and ensuring that the power factor is as close to unity as possible. Harmful harmonic currents can cause the transmission equipment to overheat and cause interference problems that must be resolved later; both of these can also adversely affect the size and/or efficiency of the circuit. If the load applied to the line is not purely resistive, there will be a phase shift between the input Voltage and current waveforms, which will increase the apparent power and reduce the transmission efficiency. If the nonlinear load distorts the input current waveform, it will cause current harmonics, which will further reduce the transmission efficiency and introduce interference into the mains grid.
Author: Edward Ong, Senior Product Marketing Manager, Power Integrations
Power supply designers face two main problems today: eliminating harmful input harmonic currents and ensuring that the power factor is as close to unity as possible. Harmful harmonic currents can cause the transmission equipment to overheat and cause interference problems that must be resolved later; both of these can also adversely affect the size and/or efficiency of the circuit. If the load applied to the line is not purely resistive, there will be a phase shift between the input voltage and current waveforms, which will increase the apparent power and reduce the transmission efficiency. If the nonlinear load distorts the input current waveform, it will cause current harmonics, which will further reduce the transmission efficiency and introduce interference into the mains 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 a rectifier circuit. The rectifier tube converts the AC voltage into an AC signal with a fixed polarity and its peak voltage is equivalent to a fixed VDC voltage. The signal is fed into a large capacitor to form a filter that can smooth the ripple 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 poles. However, the input current appears as a series of spikes, which increase as the input voltage increases. This is because the diode conduction (and therefore the flow of current) only occurs when the large capacitor is charging, when the VAC input voltage exceeds the DC voltage stored on the capacitor. When VAC is lower than the stored capacitor voltage, the charge stored on the large capacitor will support the output of the power supply. During this period, energy is transferred from the capacitor to the load, which will cause the capacitor voltage to drop. Once the AC voltage again exceeds the (now lower) voltage on the energy storage capacitor, the capacitor will recharge. This short charging window means that the input current is provided in the form of triangular pulses rather than sine waves.
Figure 1: The input current of the rectifier stage appears as a series of spikes containing a large number of harmonic components,
This will pollute the AC line
This spike current waveform is composed of a series of power frequency harmonics. The harmonic content is restricted by various national and international regulations to protect the distribution network. The power factor of the circuit shown in Figure 1 is often very low, about 0.5, which is far from the ideal 1.
This problem can be solved in several different ways. The easiest way is to add an Inductor to offset the capacitive component of the circuit-this technique is called passive power factor correction. However, the role of passive power correction is limited. 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 make the power factor of the circuit closer to 1, without negatively affecting the size of the circuit. Active power factor correction is composed of PFC diode, inductor and MOSFET. The MOSFET is used as a high-frequency switch and is driven by a controller that executes a power factor correction algorithm.
The switching circuit forces the input current to follow the rectified VAC input and again becomes an appropriate sine wave. Ideally, the sine wave has low distortion to eliminate harmonic currents that can pollute the AC line. Since the voltage and current waveforms are in phase, the power factor also rises to close to the ideal value of 1.
Figure 2: The rectifier stage with active power factor correction function changes the input current into a sine wave
An easy way to implement an active PFC circuit is to use Power Integrations’ HiperPFS-4 solution (see Figure 3). The HiperPFS-4 device includes an ultra-low reverse recovery charge diode, which achieves high efficiency by minimizing diode conversion losses. It also has a low R that reduces conduction lossDS(ON)MOSFET and advanced continuous conduction mode controller integrated with many safety functions.
Figure 3: HiperPFS-4 of Power Integrations
HiperPFS-4 devices integrate power factor correction diodes, MOSFETs, and controllers at the same time. This high level of integration helps shorten development time and speed time to market. Another advantage of integrating key components in one package is to minimize the parasitic inductance in the wiring. The reduction of circuit inductance helps to reduce the voltage stress at both ends of the PFC diode and the peak drain-source voltage of the MOSFET, thereby improving the reliability of the circuit. In addition, the diodes used have soft recovery characteristics, which can reduce ringing, thereby reducing EMI. Integrating diodes and MOSFETs in one package can significantly reduce the loop size and further reduce EMI.
Figure 4: HiperPFS-4 device integrates key components for active power factor correction in the same package,
To minimize the parasitic inductance in the connection, thereby reducing the di/dt induced voltage stress on the power switch
Active power factor correction is the best way to reduce harmful input harmonic currents and improve power factor. Power Integrations has developed the HiperPFS-4 solution, which integrates the key components required for active power factor correction into the same package. This solution can greatly reduce the input current harmonics and improve the power factor, while solving many common layout problems in traditional circuit design, such as reducing voltage stress, EMI and parasitic losses.