# A Peak Absorption Filtering Method for Radiated EMI from a High-Speed PWM Fan
## 3 Mechanism of Fan Radiation and Analysis of Radiated Source
### Analysis of Radiated Interference from Winding and Control PCB
For the electrical connections, a star-type arrangement was implemented for the three-phase winding system, with the completed 3D winding assembly illustrated in Figure 12. The magnetic circuit incorporated high-performance NdFeB permanent magnets, while the core structure was assembled from stacked silicon steel laminations to optimize magnetic effects. The components in the circuit  
Figure 12: Hybrid modeling approach for windings. were linked to the field model through an external circuit, where the FIT method in the time domain with field-circuit coupling was employed for a numerical solution. The full-wave field-circuit simulation model of the winding and inverter circuit was established in CST, as shown in Figure 13.  
The control PCB was then imported into CST Microwave Studio in ODB++ format. The winding port Touchstone model obtained through parameter extraction was imported into CST Design Studio as a circuit block to establish the field-circuit radiation simulation model of the PCB and the inverter circuit, as shown in Figure 14.  
The influence of the fan frame and fan impeller on the radiation of the internal winding and PCB was also not negligible. Therefore, the 3D model of the fan was imported into CST for combined calculations. The HPWM-LON control strategy for the fan drive system was implemented using MATLAB/Simulink2021b to generate the gate signals for the MOSFETs. The simulation conditions were configured according to the experimental measurement setup. The measurements were conducted in an anechoic chamber, with the boundary conditions set as a \"Conducting Wall\" for the bottom surface and \"Open\" for the remaining boundaries. An adaptive meshing method was employed, with the convergence of electromagnetic energy serving as the stopping criterion for the simulation iterations. A time-domain solver was used for the simulation, and the accuracy threshold was set to \(-35\) dB, meaning the iterations stopped once the post-calculation energy decay fell below \(-35\) dB. The electric field probe was set at the same position as the receiving antenna in  
Figure 14: Field-circuit radiation simulation model for control PCB-inverter circuits. Figure 13: Field-circuit radiation simulation model for winding-inverter circuits. the measurement. Considering the influence of the impeller and the fan frame on the internal radiation, the full-wave simulation models of the winding and the control PCB as the radiation sources are shown in Figure 15, respectively. A comparison of the winding and PCB radiation simulation results was obtained as shown in Figure 16.  
According to the simulation results, the peak frequency point of the radiated electric field was consistent with the experimental measurements. The radiated electric field intensity of the windings was much higher than that of the control PCB, which was close to the radiated electric field measurement results of the overall fan. The calculated surface current of the PCB at \(f=15.5\) kHz is shown in Figure 16. The position of maximum surface current is where the PCB was connected to the three-phase winding, i.e., the output side of the inverter circuit, marked by the red box in Figure 12.0 Therefore, it can be concluded that the windings were the main component that caused excessive radiated interference from the fan. The input voltage to the ground of the three-phase winding was measured using an oscilloscope, and the time-domain measurement results are shown in Figure 12.1 Each time the MOSFET turned on and off, there was a significant voltage peak, as marked by the red circle in the figure. Therefore, a filter circuit was able to be designed at the winding input port to absorb voltage peaks and prevent them from entering the winding and causing radiated interference. Figure 12.2 Comparison of the simulation results of the winding-radiated EMI and the PCB-radiated EMI with the measurement results. Figure 12.3 Full-wave simulation models of the winding and the control PCB with the impeller and the fan frame.