中频变压器是大功率隔离DC/DC变换器高效可靠运行的关键。本文研究了双主动桥（DAB, Dual Active Bridge）变换器中频变压器优化控制和设计方法，以防止中频变压器直流偏磁饱和，优化中频变压器效率和功率密度。变压器直流偏磁会增加损耗并带来可靠性问题。本文对中频变压器直流偏磁进行了分析。对使用新型碳化硅(Silicon Carbide, SiC)金属-氧化物半导体场效应晶体管(Metal-Oxide-Semiconductor Field-Effect Transistor, MOSFET)器件的DAB变换器的稳态直流偏磁以电压激励和等效电阻的方式进行了数学建模，分析了暂态过程中导致直流偏磁的伏秒积不平衡，及分布参数对直流偏磁的影响。使用纳米晶材料制作的中频变压器饱和电流低，易偏磁饱和。为了解决其直流偏磁问题，提出了含前馈控制的闭环偏磁控制策略。通过使用差分式励磁电流测量和高精度脉宽调制（Pulse Width Modulation，PWM），实现了稳态励磁电流高精度控制。通过使用前馈控制考虑移相比变化导致的主要直流偏磁激励，预先补偿占空比，从而实现了暂态过程直流偏磁的快速消除。变压器的优化设计既应保证直流偏磁裕量，又要实现高效率和高功率密度。本文对直流偏磁裕量进行了建模分析，提出了变压器磁芯材料的优化选择和结构参数优化设计方法。以一台900V 150kW DAB变换器的中频变压器为例，对比分析了移相电感集成等的变压器设计方案。使用低损耗闭口纳米晶磁芯材料进行变压器优化设计，并使用偏磁控制，既可以避免直流偏磁饱和，又可实现高效率和高功率密度。基于所提出的直流偏磁控制和优化设计的变压器，开发了一套用于高铁辅助供电300kW电力电子变压器，介绍了其优化设计与控制方法。通过使用SiC MOSFET，优化设计参数，并进行功率元件集成，实现了高效率和高功率密度。本文介绍了该样机的控制策略，并提出了一个简化的软启动策略。对该电力电子变压器进行了系统测试，分析了其效率、温升和控制性能等。对中频变压器进行优化控制和设计，既可防止变压器偏磁饱和导致的可靠性问题，又可以提高系统功率密度，降低成本，对大功率隔离DC/DC变换器的发展有着重要意义。
Medium frequency transformer (MFT) is the key of high efficiency and reliable operation of high power isolated DC/DC converters. This paper focuses on optimization of design and control of MFT in high power DAB (Dual Active Bridge) converters, to prevent DC saturation of MFT and optimize efficiency and power density of MFT.MFT DC bias will cause increased loss and reliability issue. This paper analyzes the flux DC bias problem of MFTs. A DC bias model is proposed for Silicon Carbide (SiC) Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) based DAB converters, using DC voltage sources and equivalent resistances to describe DC bias mechanism. The transient DC bias due to volt-second unbalance is analyzed, and impact of parasitics is considered.The nanocrystalline core based MFTs feature low saturation current and are easy to saturate. To solve the DC bias problem, a unified flux balancing control (UFBC) method including a Predictive Bias Suppression (PBS) control is proposed. Differential current measurement and high resolution Pulse Width Modulation (PWM) are used to achieve high accuracy control of steady state DC bias current. The DC bias in transients due to change of phase shift is considered with PBS, and duty cycle change is applied predictively, achieving fast suppression of DC bias in transients. Design optimization of MFT should not only provide adequate DC bias capacity, but also achieve high efficiency and high power density. This paper models and analyzes DC bias capacity of MFTs. Optimized selection of core material and optimization of structural parameters are proposed. Taking the MFT in a 900V 150kW DAB converter as an example, a comparison of different MFT design approaches including embedding phase shift inductance as leakage inductance is done. Optimizing MFTs with low loss ungapped nanocrystalline core material and using flux balancing control, MFT DC saturation can be avoided while achieving high efficiency and high power density.Based on the proposed flux balancing control and optimized MFT, a 300kW solid state transformer for the auxiliary power supply of high-speed railway is developed, and optimized design and control method are introduced. With SiC MOSFET, optimized parameters and integration of power components, high efficiency and high power density are achieved. The control strategy of the prototype is introduced, and a simplified soft starting strategy is proposed. The solid state transformer is tested, and the efficiency, temperature rise, control performance, etc. are analyzed.With optimal control and design of MFT, reliability issue due to DC bias can be avoided, efficiency and power density can increase, which is meaningful for power density improvement, cost reduction and reliability improvement of high power isolated DC/DC converters.
