Methods for Suppressing Noise in Switching Power Supplies
The three elements that form electromagnetic interference are the interference source, propagation path, and disturbed equipment. Therefore, suppressing electromagnetic interference should also start from these three aspects. Firstly, interference sources should be suppressed and the cause of interference should be directly eliminated; Secondly, it is to eliminate the coupling and radiation between the interference source and the disturbed equipment, and cut off the propagation path of electromagnetic interference; The third is to improve the anti-interference ability of the disturbed equipment and reduce its sensitivity to noise. The third point is not within the scope of this article's discussion.
The use of Power Factor Correction (PFC) technology and soft switching power conversion technology can greatly reduce noise amplitude.
(1) Measures on the circuit
The main reason for electromagnetic interference generated by switching power supplies is the rapid changes in voltage and current, so it is necessary to minimize the rate of change (du/dt, di/dt) of voltage and current in the circuit as much as possible. Adopting an absorption circuit is also a good way to suppress electromagnetic interference. The basic principle of an absorption circuit is to provide a bypass for the switch when it is disconnected, absorbing the energy accumulated in parasitic distribution parameters, and thereby suppressing interference. Commonly used absorption circuits include RC, RCD, LC passive absorption networks, and active absorption networks.
Filtering is a good method for suppressing conducted interference. For example, installing a filter at the input end of the power supply can suppress the interference generated by the switching power supply and feedback to the power grid, as well as suppress the impact of noise from the power grid on the power supply itself. In the filtering circuit, many specialized filtering components are also used, such as through core capacitors, three terminal capacitors, and ferrite magnetic rings, which can improve the filtering characteristics of the circuit. Proper design or selection of filters and correct installation of filters are important components of anti-interference technology.
