Thermal Modeling of Power-electronic Systems

The article describes new SPICE and SABER simulation models which contain a dynamic link between electrical and thermal component descriptions. On the one hand, operating states in which a relevant inherent heating occurs can be simulated under a good approximation of actual conditions. On the other hand, the models offer defined nodes which provide a link to the thermal environment of the component and enable, for example, the study and optimization of heat sink options. Following a list of the basic properties of the two common thermal equivalent circuit diagrams is a description of the implementation of a dynamic temperature-dependent model in SPICE and SABER using a power MOSFET model as an example. Various possibilities for determining the thermal parameters are demonstrated. The possibilities and limitations of the new models are presented with application-based examples. Introduction Today, circuit simulators are standard tools in the development and optimization of electronic systems. However, simulation has until now been limited to electronic functions because, in the simulation models available today, temperature dependence can at best be taken into account by changing the static global temperature. In power-electronic systems in particular, the temperature is one of the critical parameters due to the fact that many properties of power semiconductors are very strongly temperature-dependent – following are some examples: 7KHUPDO 6\VWHP 0RGHOLQJ 2 04/00.doc Mz q A maximum junction temperature is specified for all semiconductor components which, when exceeded, can lead to destruction or permanent damage of the component. Even when temporary events such as avalanche or short-circuit conditions occur, it must be ensured that the maximum permissible junction temperature is not exceeded – a problem which is almost impossible to solve by conventional means, i.e. using a Zth-diagram. q Within the safe operating area, the lifetime of semiconductor components is strongly affected by temperature fluctuations due to loading. Each change in temperature causes mechanical stress in the component which, in particular, affects solder and bond connections. Here it is not the absolute temperature which is decisive but the temperature cycling. As a rule of thumb it can be assumed that the aging of a component is proportional to the fourth (! ) power of the temperature deviation [1]. q The ON-resistance of a MOSFET and thus the conduction losses are roughly doubled with a temperature increase from 25°C to 150°C. q The threshold voltage of a MOSFET drops with increasing temperature which reduces the signal-to-noise margin at the control node. Ignoring this effect can lead to an undesired – even catastrophic – turn-on of the transistor when it should be inhibited, especially in bridge circuits with high slew rates for the drain-source voltage. In addition to these aspects, the question of the fluctuation in the junction temperature is also becoming more and more important in the course of circuit design. Some of the overdimensioning for example in the switching of lamps or motors to cover temporary overloads is no longer necessary when the brief, excessive power dissipation can be compensated by suitably coupled thermal capacitances. Similarly, this applies when one wants to do away with the usual protection circuits when relying on the high robustness of todays power semiconductors. Avalanche energy of modern power MOSFETs for example is specified in such a way that any load current pulse pattern below the rated current is allowed as long as it is ensured that the peak junction temperature does not exceed the maximum value. In order to be able to simulate the time-dependent temperature curves which occur in all operating states, it is necessary to couple the electrical model of a component dynamically with the description of its thermal properties. For use in an electric circuit simulator, the thermal description by an electric analog model is to be preferred. 7KHUPDO 6\VWHP 0RGHOLQJ 3 04/00.doc Mz Electrical description of thermal systems Basically, the propagation of heat in a system can take place in three different ways, convection, heat radiation or heat conduction. Electronic components usually have only heat conduction which is described in a homogeneous isotropic material by the equation t T c T th ∂ ∂ λ ρ ∂ ∂ ⋅ ⋅ = 2 2