SLUAAJ1 May   2022 TPS62860 , TPS62861 , TPS62864 , TPS62866 , TPS62868 , TPS62869 , TPS62870 , TPS62870-Q1 , TPS62871 , TPS62871-Q1 , TPS62872 , TPS62872-Q1 , TPS62873 , TPS62873-Q1 , TPS62874-Q1 , TPS62875-Q1 , TPS62876-Q1 , TPS62877-Q1 , TPSM82810 , TPSM82813 , TPSM82816 , TPSM82864A , TPSM82866A , TPSM8287A06 , TPSM8287A12 , TPSM8287A15

 

  1.   Trademarks
  2. 1Understanding Different Thermal Metrics
  3. 2Understanding SOA Curves
  4. 3How to Create the SOA Curve
  5. 4Designing for Optimal Thermal Performance
  6. 5Summary

1 Understanding Different Thermal Metrics

Texas Instruments data sheets provide numerous thermal values to quantify the thermal performance of a particular device. The most commonly used thermal values for power modules are RθJA, ΨJB, and ΨJT. Their basic usage for assessing power modules is described below, while Semiconductor and IC Package Thermal Metrics explains the different thermal metrics in detail.

Equation 1 uses RθJA to calculate the rise in the device’s temperature (its junction temperature) from a fixed ambient temperature with a given power loss. This equation and thermal value are used when the application’s ambient temperature is controlled.

Equation 1. TJ=TA+(RθJA ×Power Loss )

Equation 2 uses ΨJB to calculate the rise in the device’s temperature from a fixed PCB temperature with a given power loss. This equation and thermal value are used when the application’s PCB temperature is controlled. Even though all of the device’s power loss does not go into the PCB, the ΨJB value accounts for this (as opposed to the RθJB value) and results in the simple equation.

Equation 2. TJ=TPCB +(ΨJB ×Power Loss)

Equation 3 uses ΨJT to calculate the rise in the device’s temperature from the temperature on the top of its case, as measured by a thermal camera for example. This equation and thermal value are used to determine the junction temperature from a measurement of the case temperature. Even though all of the device’s power loss does not go up through the top of the case, the ΨJT value accounts for this (as opposed to the RθJC (top) value) and results in the simple equation.

Equation 3. TJ=Tcase_top +(ΨJT ×Power Loss)

The thermal performance not only depends on the device itself, but also on the PCB on which it is routed. The power module’s data sheet sometimes gives two sets of thermal values: one for a standard JEDEC PCB and one for the EVM. Unlike the standard JEDEC PCB, the EVM incorporates design techniques to better allow the PCB to work together with the power module to improve the thermal performance. These techniques are discussed in Section 4.

Figure 1-1 shows these three thermal values, from both the JEDEC PCB and the EVM, for a 6-A power module TPSM82866A.

Figure 1-1 Different Thermal Metrics