With the increase in the simulation of power electronics applications, accounting accurately for temperature is becoming increasingly important. Thermal analysis of switch-mode power supply allows students to study the effect of power dissipation and operating temperature on the response of the device. This characterization is common in senior and masters level power electronics courses particularly with the need to consider factors that could affect the expected lifetime of power switching devices.
When a semiconductor conducts current, there is a non-zero voltage drop across it. This results in losses that are converted almost entirely into heat. Consider the following simplified structure of a typical use case of an IGBT silicon chip and a diode silicon chip, mounted to a case that is mounted to a heat sink.
For both the IGBT and diode, the heat power originates in the junction, where its value is the highest. The instantaneous value of power is equal to the resistance (I x V) of the IGBT or diode. The heat flows through the thermal impedance of the structure and dissipates in the ambient environment. The lower the thermal impedance, the lower the rise of the silicon temperature above ambient is.
One reasonable and commonly-applied simplification of this process is the modeling of heat flow only along one dimension. Another simplification is the modeling of the thermal path using lumped thermal impedances at the interface of any two surfaces, such as the junction-to-case interface.
With these simplifications, the thermal process can be represented and solved using circuit elements and circuit simulators. The power, as it flows through the structure in the form of heat, is represented as current. The I x V products can therefore be represented using current sources. The temperature, as it exists in various physical points along the thermal network, is represented as a voltage. The ambient environment is represented as a voltage source. And the thermal impedances are represented using R-C elements. These R-C elements can be found within the component database:
Database: Master Database
The following circuit is a representation of the above thermal structure.
In the physical switch structure shown at the start of this topic, the IGBT and diode share the case surface. It is assumed that the IGBT and diode chips are so close together that the temperature of the case is the same regardless of whether it is probed closer to the IGBT or diode. Therefore, in the model, the case node Tc, which represents the temperature inside the case layer, is shared between the IGBT and diode. The same reasoning applies to the Th node, which represents the temperature inside the heat sink.
Just as electrical current raises the voltage across a electrical impedance, the power flow raises the temperature across a thermal impedance. As power flows, it raises the temperature of each of the nodes by an amount equal to the power multiplied by the thermal impedance. By inspection, the nodes closer to the power generators are at a higher temperature than the nodes closer to the ambient source.