# A Peak Absorption Filtering Method for Radiated EMI from a High-Speed PWM Fan
## 3 Mechanism of Fan Radiation and Analysis of Radiated Source  
### Generation Mechanism and Path Analysis of Conducted Interference  
In EMC design, the three elements of interference sources, propagation paths, and sensitive equipment are included. Radiated interference is generated externally by conducted interference in circuits through components with an antenna effect. The generation of conducted interference in the fan is closely related to the high-speed on/off characteristics of power MOSFETs. Assuming that the amplitude of the PWM wave is \(U\), the period is \(T\), the rising and falling edge times are t, and the turn-on time of the switching device in one cycle is ton, the duty cycle can be expressed as  
\[d=t_{\text{on}}/T \tag{1}\]  
According to the Fourier transform, the amplitude-frequency characteristics of the interference source can be obtained:  
\[|V(f)|=2 dU\left|\frac{\sin(n\pi d)}{n\pi d}\right|\left|\frac{\sin(\pi tf)}{ \pi tf}\right| \tag{2}\]  
where \(n\) is the harmonic number. According to Equation (2), the PWM signal contains many high harmonics in addition to the fundamental waveform for normal operation, and these harmonics propagate through the conduction path to other components.  
The conducted paths of EMI can be divided into common mode conducted paths and differential mode conducted paths. Each time a switching action occurs, the voltage at the  
Figure 7: Measurement results of radiated interference.  
midpoint of the bridge arm relative to the negative line of the power supply changes rapidly, thus generating common mode interference; at the same time, the switching process of the switching device also causes a sharp change in the current of the bridge arm, resulting in differential mode interference.  
The common mode interference conduction path is shown as the red dashed line in Figure 8. Common mode interference is generated by transient voltage trips during switching dynamics, and the resulting current to ground forms a closed loop through the windings, ground, and DC line.  
The differential mode interference conduction path is shown by the dotted line in Figure 9.  
Path 1: Assuming that the switching state is that Q1 and Q5 are open, the differential mode current flows into the DC side from the U phase; flows through the DC cable, LISN, and flows out from the V phase; and finally passes through the winding, forming a closed loop.  
Path 2: Differential mode current flows from the U phase into C\({}_{\text{dc}}\) and out from the V phase, and it finally passes through the winding, forming a closed loop.  
In the conducted path of electromagnetic interference, the DC power lines, windings, and PCB have antenna effects and can be highly efficient sources of electromagnetic radiation, and the interference current will radiate energy outward through these pathways, as shown in Figure 10. Therefore, before investigating the suppression methods of radiated interference, the intensity of radiated interference from the components in the loop needs to be further analyzed to target the proposed interference suppression methods.  
Figure 8: Paths of CM interference.  
Figure 9: Paths of DM interference.