Research On Power Losses And Junction Temperature Of Power Devices For EV Converters

With the increase of power density of power converter,the power devices are facing increasing reliability problems,especially in industrial production and mass consumption related areas,such as electric vehicles(EV).Electric vehicles with electric power transmission systems are superior to diesel locomotives with mechanical transmission in terms of energy cleanliness and energy transfer efficiency,but electronic devices are worse in terms of reliability than metal machinery.In the electric vehicle power transmission system,the power converter(inverter)is undoubtedly one of the most important part,due to the processing of high-power energy conversion,the inverter also faces more severe reliability problems.The results show that the influence of device temperature is most prominent in many factors that affect the reliability of power converter devices.In order to study the power loss and junction temperature of power device under various operating conditions of EV,and to take effective measures to improve its thermal reliability.In this paper,the electric-thermal simulation model of the inverter power device based on the motor control model is established.Then,the specific influence effects of the relevant factors affecting the power loss of power device are analyzed.Combined with the operating state(speed and electromagnetic torque)of the driving motor,The power losses of the device and the changing law of the junction temperature are studied,and finally the relevant improvement measures are put forward.The motor and its control model is the basis of simulating the operation of electric vehicles.Therefore,the motor modeling process is described in detail.The rotor flux oriented control and the field weakening control which will be applied later are also introduced.Then,The associated modulation method,especially the space vector modulation(SVM)that will be used in this paper,are analyzed.Then,the power loss calculation of power device is deduced in detail.The analysis of power loss calculation is divided into two parts.The first part introduces the power loss of the device in the range of linear modulation,discusses the effect of the influence factors of the power loss,and analyzes changing law of power device’s power losses and junction temperature under different operating state of the drive motor,which specify the direction of improving the reliability of power device for EV inverter;the second part calculates the device loss in the over-modulation range,study the influences of over-modulation to device power losses,especially the switching power losses.And also,the impact of over-modulation to output voltage and current harmonics of the inverter are considered.In the previous section,the power losses and the junction temperature of the power device under different operating state of the motor are studied.Combined with the relevant factors that affect the power loss of the device,the control strategy of the variable DC link voltage during the low-speed operation of the motor is put forward.And during the high-speed operation of the motor,a constant modulation factor(m = 0.952)control strategy is applied.Because of the proposed control strategy,the power device losses and junction temperature can be effectively reduced.In addition to the simulation verification,the article finally validates the above theoretical research with the active load experiment.The active load test method is simple and can effectively simulate the various working states of the power device.The results of the experiment verify the correctness of the theories.