Mechanical-Thermal Coupling Mechanism And Stress Optimization Design For Press-Pack Power Semiconductor Module

Voltage-source-converter based high-voltage-direct-current(VSC-HVDC)transmission technology has been widely applied in major projects such as large-scale renewable energy grid connection,long-distance power transmission and asynchronous large power grid interconnection.It is developing rapidly towards higher voltage level,larger system capacity and more network-node interconnection.Compared to the plastic modular power module,the press-pack power semiconductor module is featured by small parasitic parameters,double-sided heat dissipation,and being easily connected in series,which has become preferred packaging solution for the core equipment of VSC-HVDC transmission system,such as converter valve and DC circuit breaker.However,within the state-of-the-art press-pack power semiconductor module,there exists strong coupling behavior among mechanical,electral,magnetic,thermal fields where mechanical force determins the distribution of thermal field.In this way,the operation reliability of press-pack power module has been seriously affected by the complex working configuration such as VSC-HVDC.Therefore,to address the key scientific problems and technical difficulties existing in the state-of-the-art press-pack packaging technology,this paper focuses on the mechanical-thermal field coupling mechanism and optimization design of press-pack power module.On the one hand,based on the theory of contact mechanics,the model of mechanical stress field inside the module is constructed to reveal the inherent characteristics of uneven temperature distribution in the existing typical commercial large-capacity press-pack power module.On the other hand,to achieve the balanced thermal distribution in the module,it is in need to break through the existing packaging technology which includes lumped-press-pack structure and divided fixture design.The collaborative design of distributed-press-pack package with integrated fixture assembly is proposed.In detail,the research content of this paper mainly includes the following three aspects.First,the representative press-pack insulated-gate-bipolar-transistor(IGBT)module applied in major VSC-HVDC projects is chosen as the study target,and the orthogonal thermal impedance network model of press-pack IGBT model is established.In order to explore the inherent mechanical-thermal imbalance characteristics of press-pack power semiconductor module,this paper first analyzes the structure of internal multichip layout and lumped press-pack power stack.The asymmetric heat distribution on the vertical dissipation boundary for press-pack chip unit is discovered,and also the unbalanced mechanical stress distribution in the horizental chip surface.Then,the thermal impedance parameter distribution model of double-sided heat dissipation boundary is proposed.With multichip layout,the orthogonal thermal impedance network of the press-pack IGBT module is formed.By analyzing the asymmetric heat dissipation distribution,it is pointed out that the thermal imbalance on the vertical boundary of press-pack IGBT module is serious while the mechanical-thermal coupling stress is concentrated on the middle of multichip layout.Finally,by establishing the type test platform of press-pack power stack,the unbalanced distribution of thermal stress on double sides of press-pack IGBT module is measured,which not only verifies the accuracy of the orthogonal thermal impedance model,but also solves the problem of on-line thermal stress test of press-pack IGBT module.Second,the noval press-pack structure of even mechanical stress distrbution and integrated fixture structure is proposed.In order to solve the strong coupling and uneven distribution of physical field stresses inside the press-pack power module,this paper first studies the mechanical field which plays a dominant role in physical field coupling environment.With the elastic-half-space theory of contact mechanics,the structural characteristics of power modules under the lumped load is analyzed and the mechanical force distribution of power module is obtained.On this basis,the lumped load is converted to the distributed load equally and the load array within the pressed space limit is formed.Hence,the balanced stress distribution under the distributed pressed load is obtained.Due to the distributed load array,the demand for fixture is reduced and integrated fixture design method is put forward.The bus-bar,heatsink and fixture can be integrated.Therefore,not only the inherent imbalance of heat dissipation of existing commercial press-pack power semiconductor modules is solved,but also the power density of press-pack power stack is improved.According to the above packaging design,a distributed-press-pack rectifier prototype is implemented.The feasibility of distributed load and integrated fixture packaging technology is verified by static characteristics test and thermal stress experiment.Finally,based on the characteristics of multichip parallel connection in the power module,a pressure-dependent composite thermal network considering the thermal coupling effect of the parallel chip is proposed.The thermal coupling effect of the chip unit in the press-pack power semiconductor module using the integrated fixture design method is obvious,and the alternate mechanical pressure has great influence on the thermal field distribution.In order to accurately describe the mechanical-thermal coupling stress distribution of the parallel chip under the influence of mechanical field,the effect of mechanical deformation on the heat transfer path between parallel chips is equivalent to the function describing the relation between the thermal coupling impedance and the mechanical pressure.Then a composite thermal network of the parallel chip with pressure-dependent parameters is formed with the contact thermal resistance and bulk thermal resistance of the single chip.In order to analyze the temperature response of the composite thermal network,an equivalent simplification transformation is carried out and a per-unit model is constructed,and the output of typical deformed states under the calibrated power input is worked out.Finally,according to the distributed-press-pack packaging technology,the prototype of the pressure-dependent power module was assembled,and the experimental results of thermal stress distribution under different pressure characteristics were obtained.By analyzing the thermal stress distribution and the thermal coupling impedance characteristics,the feasibility of the press-dependent composite thermal network model considering the thermal coupling of parallel chips is verified.