Research On The Three-level NPC Inverter Power Device Junction Temperature Calculation And Modeling

Multilevel inverter can effectively reduce the output voltage harmonic component,improve the waveform quality.Among them,the three-level NPC inverter,from both practical and control complexity,has great competitive advantages.With the improvement of the inverter capacity,loss of power devices have a sharp increase,the cooling problem has become one of the key issues in the process of inverter design.Actual application shows that the cooling performance is good or bad will directly affect the life of the inverter.The heat of inverter is mainly caused by the loss of power devices,at the same time the characteristics of power device itself is sensitive to temperature,temperature changes will affect the characteristics of switching devices,which would affect the performance of inverter.So the accurate analysis of the power device can optimize the design of inverter heat dissipation,and improve the reliability of the inverter to run for a long time.Firstly,the modulation strategies of the three-level NPC inverter are introduced,the output voltage waveforms under different modulation strategies are obtained from simulation,and their harmonic characteristic are analyzed.Clarified the connection of SPWM and SVPWM in essence,it is concluded that SVPWM can be equivalent to SPWM modulation added in three harmonics.Then through the analysis of three-level NPC inverter power devices working characteristic curve,the reasons of the loss of power devices are summarized.The losses are classified and single power device loss calculation method is presented.The current flow pathes of three-level NPC inverter are analyzed,and the work laws of power device are summarized.Deducing power device loss calculation formula under the SPWM modulation and SVPWM modulation.Formula shows that the loss of power devices are related to modulation,switch frequency,and load current,load impedance Angle and moderate static parameters.Each power device losses are analyzed in detail with the load impedance Angle,modulation,and the changing rule of the switching frequency.Finally introduces two thermal models of power device,each power device's junction temperature is calculated in detail on the basis of the previous chapter loss calculation formula by simulation and puts forward two important factors affecting the service life of power device:average junction temperature and junction temperature swings.The average junction temperature distribution devices in four kinds of extreme conditions are analyzed.As we can see from the simulation results,there is a serious imbalance of the distribution of junction temperature embodied in the three-level NPC inverter:some power devices'junction temperature are very high,some power devices'junction temperature are very low,this increases the difficulty of the cooling system design.The junction temperature fluctuation curves of VT₁?VT₂are given as example,and decreasing the switch frequency method was proposed to reduce the average junction temperature.