大功率LLC变换器IGBT等效结电容特性分析

Characteristic Analysis of IGBT Equivalent Junction Capacitance for High Power LLC Converter

轨道交通车辆中高频辅助变流器通常采用大功率LLC变换器作为其高频隔离环节。利用其良好的软开关性能,可以有效提升开关频率并减小开关损耗,从而实现辅助变流器的减重增效。在实际应用过程中,受IGBT等效结电容大小的影响,LLC变换器的软开关区间会发生变化,为了确保其在全负载范围内实现软开关,需要在设计阶段准确获取其等效结电容信息。但大多数IGBT器件手册并未明确提供等效结电容参数,采用仪器测量也仅能获得低压小电流激励下的数据,无法反映真实工况时IGBT的结电容特性。此处详细分析了LLC变换器谐振参数与等效结电容放电阶段的耦合关系,提出了一种IGBT等效结电容参数提取方法。最后,搭建了100 kW高频辅助变流器样机,对实测数据进行曲线拟合分析,并获得了额定工况下IGBT结电容的特性曲线,验证了该方法的可行性。

High power LLC converter is usually used in rail transit vehicle as the isolation unit of high frequency auxiliary converter.By utilizing good soft-switching performance,the switching frequency will increase and switching loss will reduce which can help improve efficiency and power density of the auxiliary converter.In the practical application,the soft-switching area of LLC converter will change with the variation of IGBT equivalent junction capacitance.To guaranee the soft-switching in full load range,the accurate junction capacitance should be acquired.However,the information of junction capacitance cannot be found in many ICBT datasheets.Measured by the instrument,the capacitance data under low voltage and low current excitation can be obtained which cannot reflect the junction capacitance in the working condition.The coupling relationship between the resonant parameters and the discharge stage of the equivalent junction capacitance is analyzed in detail and the extraction method of the IGBT equivalent junction capacitance is proposed.Finally,in order to verify the feasibility of this method,a 100 kW high-frequency auxiliary converter prototype is built and the characteristic curve of IGBT junction capacitance under rated working condition is obtained by curve fitting.