How Curves Are Used to Calculate Losses in the Thermal Module


It can be a misunderstanding that thermal models are modeling and simulating the losses of a switching device. The correct way of understanding them is to think of them as a calculator that uses information from the datasheet to obtain losses.

Thermal switch models are based on an ideal switch model and rely on the datasheet’s curves to calculate conduction and switching losses.

In this article the loss calculation will be explained.

Thermal Model Frequency Parameter

Thermal models have a Frequency parameter at their input, this is the calculation frequency for losses and will be denoted by the letter .

Skip Voltage Update Flag

In PSIM v2024, thermal models have a parameter called “Skip Voltage Update Flag” that provides the option to skip the update of the conduction voltage of the device. If it is set to 0, the conduction voltage is updated in the simulation based on the conduction voltage curve. If it is set to 1, the conduction voltage is not updated and is kept at 0 in the simulation.

Transistors

In this section, the calculation of losses for each transistor will be explained. Regarding the antiparallel and free-wheeling diodes, these are modeled equally and will be explained in the “Diode” section of this article.

IGBT

PSIM gets the operating point considering IC and Tj and calculates the voltage drop to use in the next time step in the simulation. At every time step, conduction loss is calculated with VCE and IC. Then, the calculated loss will be the averaged value over the calculation period :

Both turn on and off losses are calculated from the Eon/Eoff curves interpolated/extrapolated for the different values of IC, Tj, and VCE. Losses for each switching transient are calculated and are averaged over the calculation period .

 

RG is considered with a scaling factor using the Eon/Eoff vs. RG curves and interpolated for Tj:

 

If the Eon/Eoff curves are defined for a single value of VCE, the losses will be scaled by:

MOSFET_(EON)

PSIM gets the operating point considering IDS and Tj and calculates the voltage drop to use in the next time step in the simulation. At every time step, conduction loss is calculated with VDS and IDS. Then, the calculated loss will be the averaged value over the calculation period :

 

Both turn on and off losses are calculated from the Eon/Eoff curves interpolated for the different values of IDS, Tj, and VDS. Switching losses are calculated for each switching transient and are averaged over the calculation period .

 

RG is considered with a scaling factor using the Eon/Eoff vs. RG curves and interpolated for Tj:

 

If the Eon/Eoff curves are defined for a single value of VDS, the losses will be scaled by:

 

 

Diodes

PSIM gets the operating point considering IF and Tj and calculates the conduction voltage drop to use in the next time step in the simulation. Every timestep, conduction loss is calculated with VF and IF. Then, the calculated loss will be the averaged value over the calculation period :

 

The turn on loss is neglected in the loss calculation. The turn off loss is calculated for each switching transient and is averaged over the calculation period .

 

 

 

 

VF is considered by using the following scaling factor:

 

RG is considered with a scaling factor using the Err vs. RG curves and interpolated for Tj: