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ELECTRONICS SIMULATION 3D Electro-Thermal Modelling


Simulation goes beyond the semiconductor components to provide accurate cooling considerations in power dense electronics.


O


ften a typical 3D cooling simulation of an electronic system assumes that all power is dissipated in the semiconductor components. In the case of products with high-current


power packages or modules, the power dissipation in the electrical distribution network has become a significant factor. Power dissipation in the copper traces and connections can be in the range of 30% of total input power, so it is essential to consider this heating effect in the simulation for highest accuracy prediction in complex, power dense electronics. According to a white paper on the subject released by Siemens PLM Software, the power dissipation value, and the assumption it is dissipated solely in the semiconductor, can result in substantial errors in temperature-rise predictions. These errors can be overcome by using an electrothermal simulation approach in which the full electrical circuit is simulated, predicting the resistive heating in the power delivery network as well as the dissipation in the semiconductor. Solving for both the electric and thermal behaviour of a system allows for power levels and distribution to be predicted, thus improving temperature-rise prediction accuracy. Making such predictions is becoming increasingly


important as a result of trends towards decreasing drain-source on resistance. As a result, the relative electrical resistivity and hence power dissipation of the power delivery system will become increasingly significant.


ELECTROTHERMAL SIMULATION Legacy thermal simulation approaches assume that all the consumed power is dissipated in the semiconductor and so an approach that takes into account the whole circuit power consumption needs to be used for achieving accurate predictions. For its study, Siemens PLM used the example of a simple IGBT power inverter module and used the Simcenter Flotherm simulation software calibrated using T3STER, in which the circuit was modelled. According to Siemens PLM, an accurate, steady-


state, 3D electrothermal model requires well-defined electrical and thermal resistance properties, which means the geometry, electrical resistivity and thermal conductivity values need to be determined and precisely described.


10 /// Electronics Testing 2019


❱ ❱ 3D Electro- Thermal


Modelling helps to predict the thermal behaviour of the whole electrical circuit beyond the semiconductor


To verify this for the subject being used for the


test, the company tuned the 3D electrothermal model against a combination of the measured chip temperature, the chip’s transient temperature response to a unit power step and point voltage drops. The model provided a sufficiently detailed representation, including the chip metallisation, active layer, chip, solder, direct copper bond (DCB) substrate, etc. All the inactive IGBT and diode active layers, as well as the ceramic layer were defined as dielectric, isolating the electrical circuit to the power delivery network, metallisation layers, bond wires and the chip and active layers of the IGBT. Similarly, the heatsink on the test subject was modelled using its contact thermal resistance rather than its individual pin and water jacket characteristics explicitly. The electrical resistivity material properties of all the metallic objects were well-characterised, including temperature-dependent coefficients.


SIMULATION RESULTS In testing, it was found that under low current conditions (such as sensing), the electrical resistivity of the IGBT was far greater than the rest of the circuit. The vast majority of the dissipated power occurred at the chip. However, at high currents, the relative resistivity of the chip decreased with respect to the rest of the circuit, and the assumption that all the consumed power was dissipated at the chip was incorrect. Out of the total consumed power of 912W, 64 percent was dissipated on the active layers of the two IGBTs, 4.7 percent in the bond wires, 1.4 percent in the metallisation layers and the remaining 29.6 percent in the rest of the power delivery circuit. By using Flotherm total system simulation and


verifying it with T3STER, Siemens PLM was able to show the advantages of predicting accurate thermal behaviour over the results of using linear electrical models .


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