New developments of THERMOS3, a tool for 3D electro-thermal simulation of smart power MOSFETs
Introduction
The simulation of power electron devices is a complex numerical problem and it has been faced in many different ways in the recent past as the issue of power dissipation together with the knowledge of electro-thermal interactions in MOSFETs has become relevant. Being coupled, but on a different timescale with respect to the thermal problem, the electrical problem is usually reduced to few DC equation that are sufficient to model the temperature dependent static behaviour of the single cell. The time-dependent heat equation is solved, numerically or analytically, in a way such as to take into account layout geometries, boundary conditions and heatsink influence.
The THERMOS3 simulator we propose, entirely written in MATLAB, is based on a forward iterative finite difference time domain scheme [1] where the electrical equivalent of the thermal problem and the electrical quantities (drain current, source voltages) are solved self-consistently at each discrete time step. Non linearity in the thermal conductivity together with voltage depolarization due to finite resistance of the source metallization is explicitly taken into account. The ID = f(VGS, VDS,T) behaviour of the unit cell is modelled considering temperature dependence of the threshold voltage, mobility and MOSFET internal resistances. Convection on the surface and sides and isothermal boundary condition at the heatsink have been used.
As an exhaustive example of the capability of this new tool, we present the simulation of a smart power device produced by STMicroelectronics (Fig. 1). This device is protected during permanent short circuit by a current feedback loop which keeps the dissipated power inside the SOA together with a thermal shutdown circuit which shuts the device of if temperature on chip exceeds a reference value. Dynamic temperature behaviour of this device has been fully characterized by fast transient infrared imaging [2], therefore a quantitative comparison with the simulation is available for validation. One main issue is the optimization of time stepping since the overall behaviour of the device has to be observed in the 100 ms (or longer) timescale while sensitive information such as fast temperature transients can be as short as 100 μs. In our approach, an adaptive time stepping strategy together with switching event prediction and detection has been implemented.
Section snippets
The simulator
A multicellular power VDMOS can be treated as a number of equivalent single cells all of them sharing the same voltage on the drain and gate terminal and with the sources connected through a metal layer with a finite resistivity. Therefore, both VDS and VGS are dependent not only on temperature (the resistivity of the metal layer is itself a function of temperature) but on the layout of the device and position of the source bond wires. The single cell is modelled by the following set of
Experimental results
To compare and validate the outcome of THERMOS3, we have experimental temperature map at our disposal. The detection of the temperature distribution across the area of the device in transient conditions has been made possible by the use of a custom developed radiometric 2D measurement system that allows acquiring the transient temperature maps of the device surface [2]. By means of this equipment, we have obtained the thermal transient analyses used to calibrate the output of the simulator.
Results, comparisons and comments
The device has been discretized in Nx × Ny × Nz unit cells each with its own thermal parameters and operated in short circuit to the usual battery voltage Vbat = 12 V. Current feedback fixes the total current at Ilim = 10 A. In Fig. 4, we report a comparison between experimentally detected temperature distribution and the result of the simulation. As we can note, a good quantitative agreement is obtained both in shape and in peak temperature value.
To further investigate the features of the simulator, we
Conclusions and future developments
In our study on these kind of devices and in all the experience acquired in the electro-thermal simulation of power devices, we have understood that not only the localization of the bond wires is important in simulations but also a proper electro-thermal description of these components must to be taken into account. Into the next new version of the THERMOS3 simulation tool will be inserted also the possibility to well simulate and describe how the power bond wires, described in terms of
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