Research on IGBT Limit Frequency Based on Heat Balance Analysis

Insulated gate bipolar transistor (IGBT) is the most widely used fully-controlled power electronic device at present. Its limit frequency is generally determined by the maximum junction temperature and maximum power consumption given in the manual, and it is difficult to reflect the nature of thermal failure. Based on the thermal balance analysis of IGBT power consumption-temperature curve and heat transfer curve, the thermal stability point, unstable point and critical point of junction temperature are obtained, thus the IGBT limit power consumption at the critical point is obtained, the design method of IGBT limit frequency is obtained from the tangent of the two curves, and finally the experimental verification is carried out. Introduction IGBT is a kind of compound power semiconductor device which combines the structure of field effect transistor and bipolar power transistor. It has the advantages of high input impedance, fast switching speed, low driving power, reduced saturation voltage, large current bearing, etc. It is widely used in various medium and high power electronic devices and is currently the most widely used full control power electronic device [1] . In the existing power electronic devices, IGBT safe operating area is generally designed based on experience and parameters and curves provided by device manufacturers, and a large margin is usually reserved for ensuring reliability in application. At present, the basic idea of studying IGBT limit frequency at home and abroad is based on the steady-state thermal resistance calculation formula, which is realized through the maximum junction temperature given in the manual. Literature [2] studied the IGBT operating limit according to the relationship between maximum junction temperature, thermal resistance and limiting power consumption. Literature [3] analyzed the on-state limit current and the on-state limit power consumption, and points out that the on-state limit power consumption is the power consumption corresponding to the maximum junction temperature. Literature [4] calculated IGBT switch power consumption and conduction power consumption, and estimated the junction temperature at switch works by using the relationship between power consumption and thermal resistance. Based on the thermal balance analysis of IGBT power consumption-temperature curve and heat transfer curve, this paper obtains the stable point, unstable point and critical point of junction temperature, obtains the limit power consumption of IGBT at the critical point, obtains the limit frequency under certain circuit conditions from the limit power consumption, and finally carries out experimental verification. Theoretical Analysis IGBT thermal failure mainly includes long-term thermal accumulation failure and short-term thermal shock failure. Among them, thermal accumulation failure is mainly caused by poor heat dissipation, excessive current and frequency, and there is a process of thermal accumulation. Heat Balance Analysis of IGBT The internal structure of IGBT is shown in fig. 1. inside the dotted line frame is a cellular structure of a through-type planar gate. Figure 1. IGBT cell structure. According to the working mechanism of IGBT, the switching and conduction process of IGBT are electron and hole currents formed by the continuous movement and recombination of carriers in the base region, and the heat generated is mainly concentrated in the active base region of IGBT. In the actual reverse PN junction curve, the reverse current will increase slightly with the increase of the reverse voltage and will increase exponentially with the increase of temperature due to the influence of the generated current in the space charge region and the surface leakage current. When the reverse bias voltage of the PN junction increases, the heat loss caused by the reverse current causes the junction temperature to rise, and the rise of the junction temperature causes the reverse current to increase. If the heat sink cannot transfer heat in time, the junction temperature rise and the increase of the reverse current will cycle alternately, and eventually the PN junction will break down. This breakdown caused by thermal effect is called thermal breakdown. In the same principle, the thermal failure mechanism of IGBT can also be analyzed from the heat balance relationship between the generated heat and the heat that can be dissipated, so IGBT power consumption curve and heat transfer curve need to be obtained. For the common sinusoidal pulse width bipolar modulated two-level H-bridge inverter circuit, IGBT conduction power consumption, switching power consumption and off-state power consumption are respectively obtained as follows.