磁性元件的功率損失仍然為切換式電源供應器功率損失的主要來源，因此為追求更高性能與轉換效率的切換式電源供應器，磁性材料與磁性元件的研究與改進是一項重要的課題。軟磁錳鋅鐵氧鐵芯，具備高的電阻係數、初導磁率、飽和磁通密度、居里溫度及低的鐵損等優點，因此廣泛運用在切換式電源供應器上。目前切換式電源供應器的切換頻率主要仍以500 kHz以下為主，在此頻率範圍的錳鋅鐵氧鐵芯損失主要為磁滯損與渦流損，當切換頻率低於100 kHz時，磁滯損為磁性元件損失的主要來源，而當切換頻率提高到100 kHz至500 kHz後，渦流損失變成磁性元件損失的重要來源。
本論文主旨為研究軟磁錳鋅鐵氧磁體鐵芯。論文首先以二階段式的田口直交表實驗法製備軟磁錳鋅鐵氧鐵芯，藉由製程控制與改變多種摻雜物的濃度，提高錳鋅鐵氧磁體電阻係數，研製鐵芯及試片，並應用此鐵芯繞製成電感，做導磁率與阻抗量測。再者，經由磁性元件的研製、試片電阻率及元件電與磁特性測量，提出以等效電路觀念與方法，利用阻抗(導納)軌跡分析，計算此等效電路上各元件参數，如此可簡單且有效地分析錳鋅鐵氧多晶磁性材料與元件其相關電與磁性能。
最後文中提出一個估算磁性元件渦流損失的計算方法。影響渦流損失的變因大致包含鐵芯的大小、形狀、工作頻率、感應磁通密度與鐵芯電阻係數等。但，對多晶結構的錳鋅鐵氧磁體鐵芯而言，晶粒的大小決定渦流路徑的長短，亦即影響渦流損的大小，因此本文將錳鋅鐵氧磁體鐵芯微結構中晶粒的平均半徑導入做為一項參數。除此之外，為了簡化量測方式，利用外加電流為參數取代感應磁通密度，推導適合多晶結構的錳鋅鐵氧磁體鐵芯的渦流損失估算公式。所以在實務操作上，電力電子工程師即可簡單的以具電流探棒之示波器測得工作電流值，估算磁性元件在此操作模式下的渦流損，設定適合電路最佳的切換頻率。

This dissertation presents a study of MnZn soft ferrite cores for power converters. The core losses of magnetic devices are the major source of power dissipation in switching-mode power supplies (SMPSs). Therefore, the study and improvement of magnetic materials as well as inductive devices are important issues in the pursuit of higher conversion efficiency. Due to the characteristics of high resistivity, high permeability, sufficient magnetic flux density, high Curie temperature, and low loss, MnZn soft ferrite cores are extensively used for the magnetic devices in power converters. At present, the switching frequency of the SMPS is generally below 500 kHz. Within this frequency range, the dissipation of the ferrite core mostly comes from hysteresis and eddy-current losses. The eddy-current loss plays a critical role at the 100-500 kHz frequency, whereas the amount of hysteresis loss dominates the core loss of magnetic devices below 100 kHz.
This study firstly prepares high-resistivity MnZn soft ferrite cores using a two-stage Taguchi orthogonal array method. The first stage involves the use of processing control and the concentrations of various additives to prepare high-bulk-resistively sintered cores and discs; these cores are then made into the toroidal inductors. Afterwards, based on the production of magnetic devices and the measurements of bulk resistivity, and electrical and magnetic properties of the samples, an equivalent lumped-circuit method is presented for examining the relationship between material and electrical properties. The equivalent lumped circuit, combined with equivalent lumped resistances and capacitance, is applied to investigate the effect of microstructure on electrical and magnetic properties of the ferrite cores. The values of lumped resistances and capacitance are determined by analyzing the admittance loci of the sintered samples.
Furthermore, a new estimation model for the eddy-current loss of polycrystalline ferrite cores is proposed. The factors that cause such loss include the shape and size of core, driving frequency, induced magnetic flux density, and the resistivity of the material. Apart from these factors, for the polycrystalline MnZn soft ferrite cores, the grain size determines the length of the eddy path, which affects the amount of eddy-current loss. Therefore, to enhance calculation accuracy, the mean radius of the grain is taken as a parameter to deduce a new estimation model of the MnZn ferrite core. In addition, to simplify measurements, the driving current, as another parameter, replaces the induced magnetic flux density. As a result, a power electronics engineer can practically use an oscilloscope with current probe to measure the operating current and simultaneously obtain the eddy-current loss of magnetic devices, and therefore determine the appropriate switching frequency for the SMPS.

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