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Thermal Modeling of Infineon MOSFETs in Multisim
Hello Circuit Designers,
In power electronics design, the thermal behavior of switching elements is critical to evaluate the performance of a Switched Mode Power Supply (SMPS).
Thermal behavior means the performance of the semiconductor switching element as the conducting p-n junction heats up.
In the most recent update release of Multisim, over 500 new MOSFET models from Infineon Technologies have been added to Multisim's library of components. Many of these models include the SPICE thermal models.
SPICE thermal modeling is an interesting topic that will be discussing in a future post. For now, let's go over the different levels (types) of thermal modeling of the Infineon MOSFETs in Multisim.
Level 1 Models: Fixed Junction Temperature during the Simulation
The circuit below is a simple buck converter stepping down a 10V input to a 5V output. In Level 1 models, during Transient Analysis, the junction temperature is fixed using the TEMP setting
within the simulator's SPICE options. Alternatively you can run Temperature Sweep Analysis which overrides this setting as it sweeps its value.
Drag and drop .png image into Multisim to load the same circuit
Note that in Multisim, if you go to Simulate>Analyses>Transient Analysis under the Analysis Options tab, select to use custom settings and click on customize to change the parameter TEMP indicating the junction temperature used in any transient simulation (by default it is set to 27 degrees Celsius)
Select Simulate>Analyses>Temperature Sweep analysis to perform two transient analysis runs - at 27°C and 60°C. This will run until reaching a steady state at both the load voltage and current display.
Note that Level 1 models are not electro-thermal models. If the goal is to predict the junction temperature value, you need to use the Level 3 models.
Level 3 Models: Simulation of Electro-Thermal Models
This example is the same buck converter except that the switching MOSFET is a level 3 model that includes electro-thermal behavior of the junction (i.e heating up of the junction during transient simulaiton). The ambient temperature is modeled using the DC voltage TAmb and the MOSFET case thermal resistance is modeled using RthCA.
Drag and drop .png image into Multisim to load the same circuit
If you run a transient analysis on the circuit, you can see the junction heating up from the ambient temperature (defined in SPICE) of 27 degrees to about 32.4 degrees after 20msec of running the supply.
