I. Types and Applications of Switching Power Supplies
There are two types of modern switching power supplies: DC switching power supplies and AC switching power supplies. This article mainly introduces DC switching power supplies, whose function is to convert raw power sources (crude electricity) with poor power quality, such as AC mains power or battery power, into high-quality DC voltage (refined electricity) that meets the requirements of the equipment.
The core of a DC switching power supply is the DC/DC converter. Therefore, the classification of DC switching power supplies depends on the classification of DC/DC converters. In other words, the classification of DC switching power supplies and the classification of DC/DC converters are essentially the same; the classification of DC/DC converters is basically the classification of DC switching power supplies.
A switching power supply is a type of power supply that uses modern power electronics technology to control the on and off time ratio of switching transistors to maintain a stable output voltage. Switching power supplies are generally composed of pulse width modulation (PWM) control ICs and switching devices (MOSFETs, BJTs, etc.). The development direction of switching power supplies is towards higher frequencies. Higher frequencies enable the miniaturization of switching power supplies and allow them to enter a wider range of application fields, especially in the field of high technology, which promotes the miniaturization and portability of high-tech products.
Furthermore, the development and application of switching power supplies are of great significance in saving energy, conserving resources, and protecting the environment. Especially in the application of high-tech fields, the development of switching power supplies has been propelled forward, with an annual growth rate exceeding double digits, moving towards lighter, smaller, thinner, lower-noise, higher-reliability, and more interference-resistant designs.
Compared to linear power supplies, the cost of both increases with output power, but at different rates. At a certain output power point, the cost of a linear power supply is actually higher than that of a switching power supply. However, with the development and innovation of power electronics technology, switching power supply technology is constantly innovating, and this cost inversion point is increasingly shifting towards lower output power levels, providing ample room for the development of switching power supplies. Higher frequency operation is the future direction of switching power supply development.
II. Filtering Function of Switching Power Supplies
Passive filters have relatively simple circuits, are inexpensive, and have reliable performance, making them an effective way to suppress electromagnetic interference in switching power supplies. A passive filter consists of inductors, capacitors, and resistors, and its direct function is to address conducted emissions.
Because the filter capacitor in the original switching power supply circuit has a relatively large capacitance, pulse spike currents are generated in the rectifier circuit. These currents are composed of many high-order harmonic currents, which can interfere with the power grid. In addition, the switching transistors in the switching power supply circuit and the primary coil of the switching power supply transformer will generate pulsating currents. Because the current change rate is very high, different frequencies of induced currents will be generated in the surrounding circuits, including differential-mode and common-mode interference signals. These interference signals can be conducted through the two power lines to other power grid lines and interfere with other electronic devices.
The differential mode filtering section can not only reduce the differential mode interference signal inside the switching power supply, but also greatly attenuate the electromagnetic interference signal generated by the switching power supply itself during operation and transmit it to the power grid.
The pulse current loop is an interference signal generated by electromagnetic induction from other switching power supply circuits and the ground or chassis, which is a common-mode signal. In the switching power supply circuit, a strong electric field can be generated between the collector of the switching transistor and other circuits, and the circuit will also generate displacement current, which is also a common-mode interference signal.
Active filtering technology
Active filtering technology is an effective method for suppressing common-mode interference. This method addresses the noise source of the switching power supply. Its basic idea is to extract a compensation signal from the main circuit that is equal in magnitude but opposite in phase to the electromagnetic interference signal from the switching power supply to balance the original interference signal, thereby reducing the interference level. When designing and selecting switching power supply filters, we should pay attention to frequency characteristics, withstand voltage performance, rated current, impedance characteristics, shielding, and reliability.
The filter must be installed in the right location and the installation method must be correct in order to achieve the expected filtering effect against interference.
