Enhanced thermal management in high frequency ferrite-based inductor cores via grain boundary nanoengineering strategies

Abstract

Ferrite materials possess a unique combination of properties including permeability, permittivity, and low RF losses. There exist no other materials with such wide-ranging value to electronic applications in terms of power generation, conditioning, and conversion. These power management functions are required by not only enormous systems, such as our national power grid, but also our smaller systems, such as mobile communication platforms and components, where microinductors are integrated with semiconductor circuitry. These seemingly desperate needs provide bookends for the U.S. interests in size, frequency, and technology maturity to address societal needs in energy conservation and performance. Today, ferrites play an essential role in today's society. Nearly every consumer product has one or more ferrites embedded into its systems. And nearly all future products considered today are anticipated to contain ferrites. A principal challenge in the design and production of ferrite components is the management of escalating heat dissipation and its impact upon overall system efficiency. The heat dissipation derives from excessive eddy current and residual losses that become dominant at high frequencies. The power dissipation mechanisms of these ferrites include hysteresis, eddy current and residual losses. At high frequencies, in applications such as switching power supplies, these losses result in heat dissipation that can be only mitigated by increasing the resistivity of the current flow path and tuning of the domain wall resonance away from the operational frequency, respectively. The thesis presented here pursues the mitigation of heat generation in RF systems from magnetic components at its core. Specifically, we have endeavored to minimize heat generation by power losses by minimization of eddy current and residual losses. However, our aim is higher in that we hope to realize such losses without concomitant degradation to functional properties, most namely, the permeability. This is a lofty goal that has stymied researchers for decades. In our approach to minimize eddy current losses, we have aimed to disrupt long-range eddy currents by introduction of insulating magnetic nanoparticles (i.e., YIG) to the grain boundary regions. We demonstrate that the total core loss, Pv, decreases with increasing weight percent additive in comparable measure for both BTO and YIG by about 71 and 77% for the highest concentration of inclusions. This is largely attributed to a reduction in eddy current losses where a total reduction in Pe relative to the parent compound is found to be for x= 0.08 wt% of YIG 79.2% and 76.9%, respectively. This accounts for as much as 87.5% of the total loss dissipation in response to the introduction and collocation of these inclusions to the grain boundary region. Most importantly, permeability values for the YIG-modified samples show a striking contrast when compared with that of the BTO-modified samples. The former retains a high permeability, with a 24.5% reduction for the highest concentration of additives, as oppose to the latter that experienced a 64.3% reduction for the same additive weight percent. When ferrite-containing power management components are driven to higher powers and frequencies, residual losses, Pr, become particularly detrimental to performance. For these operational conditions, we have opted to design the ferrite compound with key additives of Ni2+ and Ga3+. We adopted Ni substitutions based on the work of others that showed that this resulted in the minimization of hysteretic losses, Ph, in the vicinity of the device operating temperature. We have reproduced these results. The introduction of Ga substitution is our innovation and is intended to shift the domain wall resonance frequency (i.e., the spectrum region of most residual losses) to higher frequencies far from the operating frequency thus minimizing the impact of Pr to the total core loss. Our principle finding was that with optimal doping of Ga, the relative role of Pr at the typical operating temperature of power inductor cores 80 C, was suppressed by 36%. In summary, we have successfully demonstrated for the first time the ability to mitigate high frequency eddy current losses while maintaining high permeability. The approach requires only a small alteration to chemistry and processing and is therefore adaptable to industrial scale processing at low cost. Further, the substitution of Ga3+ for Zn2+ was shown to shift the resonance frequency far from the operating conditions and resulted in substantial reduction in residual losses. Taken together, these innovative strategies represent a substantial improvement in the performance of ferrites as inductor cores for power electronic applications. The improvements are not merely incremental, but some might say groundbreaking.