# A Peak Absorption Filtering Method for Radiated EMI from a High-Speed PWM Fan
## 1 Introduction  
Axial flow fans are extensively employed in electronic devices for thermal management due to their compact structure, high efficiency, and reliable airflow generation. A typical axial flow fan comprises three key components: a brushless DC (BLDC) motor, an impeller, and a fan frame, as illustrated in Figure 1. The impeller is directly coupled to the motor shaft, and when energized, the motor drives the impeller to rotate, producing an axial airflow that enhances convective heat dissipation within electronic enclosures. Figure 1: Structure of the axis flow fan. To accommodate varying thermal loads under different operating conditions, axial flow fans often employ dynamic speed regulation. The most prevalent control method is pulse width modulation (PWM), which adjusts the motor's rotational speed by modulating the duty cycle of the input voltage. While PWM ensures efficient thermal management, it introduces rapid switching transients in the motor windings, leading to abrupt changes in current and voltage. These high-frequency switching actions generate conducted and radiated electromagnetic interference (EMI), manifesting as distinct spectral peaks at specific harmonic frequencies [1; 2; 3; 4], as evidenced by the measurement results in Figure 2.  
The EMI emissions from BLDC motors pose a critical challenge in modern electronic systems, particularly in applications where electromagnetic compatibility (EMC) is strictly regulated. In industries such as aviation, aerospace, and medical electronics, even minor EMI disturbances can disrupt sensitive communication systems, navigation equipment, or diagnostic instruments [5; 6]. Consequently, regulatory bodies have established stringent EMI limits for cooling fans and other electromechanical components. Additionally, standardized testing methodologies, including MIL-STD-461G, CISPR 25, and DO-160, have been developed to assess and mitigate EMI risks in critical environments. Given the increasing demand for high-performance and low-EMI cooling solutions, a thorough investigation into the generation mechanisms, propagation paths, and suppression techniques of motor-induced EMI is imperative. It is of great significance to study the mechanism of motor radiated interference and develop effective suppression methods. At present, research mainly focuses on the mechanism of conducted interference, focusing on the modeling and suppression of conducted interference in electrical machines. Conducted interference models for switch devices were established in [7], and a numerical model of a power transfer system was developed and used in simulation [8]. The conducted interference of a motor drive system was analyzed and modeled in [9; 10; 11]. The EMI suppression method of the DC motor was studied in [12; 13] to attenuate both common-mode (CM) and differential-mode (DM) interference. Research on radiated interference has mostly focused on modeling and predicting the radiated interference from components in power systems [14; 15]. Ref. [16] presents an equivalent modeling method for the electromagnetic radiation of PWM fan. Many publications analyzing EMI sources and propagation provide a better understanding of the EMI generation mechanism [17; 18; 19]. For suppression of radiated interference, the main methods include changing the PWM control strategy and optimizing the system structure [20; 21; 22; 23]. The design process of the methods above is complex due to the redesign of the system structure. Figure 2: Radiated interference generated in the axial flow fan. Designing EMI filters is an effective approach to mitigate radiated interference. [PERSON] [24] designed the filter to attenuate both conducted and radiated interference. And the installation location and topology of the filter are also a problem worth investigating. The prevalent filter topologies often incorporate inductive components, leading to bulky filter designs that are not suitable for small-sized axial flow fans.