The Generation Mechanism and Suppression Technologies of Electromagnetic Interference in Switching Power Supplies
Suppression of Electromagnetic Interference in Switching Power Supply
The three elements that form electromagnetic interference are the interference source, propagation path, and disturbed equipment. Therefore, suppressing electromagnetic interference should be addressed from these three aspects. The purpose is to suppress interference sources, eliminate coupling and radiation between interference sources and disturbed devices, and improve the anti-interference ability of disturbed devices, thereby improving the electromagnetic compatibility performance of switching power supplies.
Using filters to suppress electromagnetic interference
Filtering is an important method for suppressing electromagnetic interference, which can effectively suppress the entry of electromagnetic interference into equipment in the power grid, and also suppress the entry of electromagnetic interference from equipment into the power grid. Installing switch power filters in the input and output circuits of switch power supplies can not only solve the problem of conducted interference, but also be an important weapon for solving radiation interference. The filtering suppression technology is divided into two methods: passive filtering and active filtering.
Source filtering technology
Passive filtering circuits are simple, cost-effective, and reliable, making them an effective way to suppress electromagnetic interference. Passive filters are composed of inductive, capacitive, and resistive elements, and their direct function is to solve the problem of conduction emission.
Active filtering technology
Active filtering technology is an effective method for suppressing common mode interference. The basic idea of this method, which takes measures from the noise source (as shown in Figure 2), is to extract a compensation signal from the main circuit that is equal in size and opposite in phase to the electromagnetic interference signal to balance the original interference signal, in order to reduce the interference level. As shown in Figure 2, by utilizing the current amplification effect of the transistor, the current at the emitter is converted to the base and filtered in the base circuit. The filter composed of R1 and C2 reduces the ripple at the base, which in turn reduces the ripple at the emitter. Due to the smaller capacity of C2 compared to C3, the volume of the capacitor has been reduced. This method is only suitable for low-voltage low-power power supplies. In addition, when designing and selecting filters, attention should be paid to frequency characteristics, withstand voltage performance, rated current, impedance characteristics, shielding, and reliability. The installation position of the filter should be appropriate and the installation method should be correct in order to achieve the expected filtering effect on interference.
