Suppression of Radiated Emissions from Inductive-Resonant Wireless Power Transfer Systems by Using Spread-Spectrum Technique

Abstract

Inductive-resonant wireless power transfer systems are often used for wireless transfer of electric power. However, they are significant sources of radiated electromagnetic emissions. The effect of spread-spectrum approach based on classical switching frequency modulation of inductive-resonant wireless power transfer system inverter operating frequency and multi-frequency technique on the radiated emissions and efficiency is studied experimentally in detail. The influence of the classical frequency modulation and multi-frequency technique parameters on the peak radiated emission levels and the efficiency of the inductive-resonant wireless power transfer system are investigated more comprehensively. It is shown in the paper that the spread-spectrum approaches can lead to an appreciable radiated emissions reduction with small or large impact on the system’s efficiency. Some useful recommendations on how to choose parameters of the periodic switching frequency modulation or the multi-frequency technique considering a trade-off between the radiated emissions reduction and the efficiency are also proposed.

Wireless Battery Chargers Operating at Multiple Switching Frequencies with Improved Performance

“…When designing a wireless battery charger, electromagnetic compatibility (EMC) issues should also be considered, because they are significant sources of conducted and radiated electromagnetic interference (EMI) to sensitive electronic devices [5][6][7][8][9][10], as is depicted in Figure 1a. Therefore, different national or international EMC standards specify the limits and measurement methods of EMI of the wireless battery chargers.…”

“…Although less effective (in terms of EMI suppression), the spread-spectrum approach has gained substantial popularity over the last two decades because it can be used for suppressing conducted and radiated EMI simultaneously and can be implemented by using a suitable program code for a microcontroller. The spread-spectrum technique was initially used for the suppression of EMI from traditional switching power converters [11,12], and recently, it was applied to inductive-resonant WPT systems (including wireless battery chargers) [5,[7][8][9]. Several types of the spread-spectrum technique exist for EMI reduction from wireless battery chargers, but the multi-switching frequency technique is preferable to other spread-spectrum techniques for wireless battery chargers because it can be implemented by using a relatively inexpensive microcontroller that can be used simultaneously for implementing constant current (CC) or constant voltage (CV) battery charging modes [9].…”

“…One of the drawbacks of the multi-switching frequency technique (as well as the other spread-spectrum techniques) is that the peak value of coil currents may increase significantly [9]. Another drawback of the technique is that EMI reduction is only moderate (up to 8.3 dB for the conducted EMI (if the standard 9 kHz resolution bandwidth of a spectrum analyzer is used) [9] and up to 4 dB for the radiated EMI (if the close-to-standard 10 kHz resolution bandwidth of a spectrum analyzer is used) [5]). Higher EMI reduction can still be achieved with the multi-switching frequency technique, if a higher number of frequencies are used, but this would inevitably lead to a larger difference between the minimum and maximum switching frequencies ∆f and it would result in a higher drop in the efficiency in CC mode (at lower value of the equivalent input resistance of a battery) and a more significant increase in the peak and RMS values of currents of power components of the wireless battery charger [9].…”

“…Note: the radiated EMI here represents the output voltage (expressed in dBµV) of the near-field probe without considering the antenna factor of the probe. If it is necessary to show the radiated EMI expressed in dBµA/m and to compensate for the probe frequency characteristics, one should take into account the antenna factor of the probe as described, e.g., in[5].…”