10.1.5 Estimating Junction Temperature

Using the thermal metrics Ψ_JT and Ψ_JB, as shown in the table, the junction temperature can be estimated with the corresponding formulas (given in Equation 5).

Equation 5.

where

• P_D is the power dissipation calculated with Equation 2,
• T_T is the temperature at the center-top of the device package, and
• T_B is the PCB temperature measured 1 mm away from the device package on the PCB surface (as shown in Figure 23).

Figure 23. Measuring Points for T_T and T_B

For more information about measuring T_T and T_B, see the application note, Using New Thermal Metrics, SBVA025.

According to Figure 24, the thermal metrics (Ψ_JT and Ψ_JB) have very little dependency on copper area. Using Ψ_JT or Ψ_JB with Equation 5 is a good way to estimate T_J by simply measuring T_T or T_B on an application board.

Figure 24. Ψ_JT and Ψ_JB vs Board Size