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Simulating Thermal Effects in Semiconductors
• Junction thermal mass can only be set to zero if the junction-case resistance is also set
to zero.
• If both case and junction thermal masses are defined, but junction-case resistance is
zero, then the initial temperatures assigned to junction and case must be identical.
Thermal Mass Parameterization
Datasheets usually quote both of the thermal resistances, but rarely give values for
thermal masses. There are two parameterization options for the thermal masses:
• By thermal time constants — Parameterize the thermal masses in terms of
thermal time constants. This is the default.
• By thermal mass — Specify the thermal mass values directly.
The thermal time constants t_J and t_C are defined as follows:
t_J = M_J · R_JC
t_C = M_C · R_CA
where M_J and M_C are the junction and case thermal masses, respectively, R_JC is
the thermal resistance between junction and case, and R_CA is the thermal resistance
between port H and the device case.
You can determine the case time constant by experimental measurement. If data is not
available for the junction time constant, you can either omit it and set the junction-case
resistance to zero, or you can set the junction time constant to a typical value of one tenth
of the case time constant. The alternative is to estimate thermal masses based on device
dimensions and averaged material specific heats.
Electrical Behavior Depending on Temperature
For blocks with optional thermal ports, there are two simulation options:
• Simulate the generated heat, device temperature, and the effect of temperature on the
electrical equations.
• Simulate the generated heat and device temperature, but do not include effect
of temperature on the electrical equations. Use this option when the impact of
temperature on the electrical equations is small over the temperature range to
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