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 R_θJA, Ψ_JB, and Ψ_JT. 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 R_θJA 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 2 uses Ψ_JB 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 Ψ_JB value accounts for this (as opposed to the R_θJB value) and results in the simple equation.

Equation 3 uses Ψ_JT 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 Ψ_JT value accounts for this (as opposed to the R_{θJC (top)} value) and results in the simple equation.

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 2-1 shows a Safe Operating Area (SOA) curve for the same TPSM82866A power module. SOA curves show the maximum recommended temperature versus load current, as a quick aid to check if a device is thermally suitable for a given application. This particular curve uses the ambient temperature and the EVM’s R_θJA value to determine the Safe Operating Area. Using these two values, combined with the power loss at each operating point, Equation 1 creates the boundary lines in the SOA curve. The top of the curve at 6 A reflects the recommended maximum output current due to the device’s rated current, while the sloped portion of the line reflects the recommended maximum output current due to the power losses, and resulting temperature rise, at that operating point. Operate below the lines to keep the device within its rated junction temperature.

Table 2-1 shows two operating conditions for powering an SoC with an input voltage of 5 V and output voltage of 1.2 V. Figure 2-2 demonstrates that the first operating condition (red dot) is outside of the SOA curve, at the elevated ambient temperature. The second operating condition (blue dot) shows one solution to operate within the SOA curve: lower the output current.

One way to reduce the output current is to reduce the processing speed of the SoC. Another solution to operate within the SOA curve would be to reduce the maximum ambient temperature or to reduce the R_θJA by adding airflow to the system.

The SOA curves are usually created from measured efficiency data on the EVM. From Equation 1, the power loss at various ambient temperatures creates the temperature rise required to reach the power module’s maximum operating temperature of 125°C.

Equation 4 calculates the power loss from the data sheet’s efficiency curves:

Because efficiency at high loads decreases with increasing temperature, the efficiency values at an elevated temperature (such as 85°C) are used. As an example, Figure 3-1 shows the efficiency curve at 85°C for the same 5 V_in and 1.2 V_out condition. At 5.5 A load with nearly 84% efficiency, Equation 4 calculates the power loss as 1.25 W. Multiplying by the 25.4 °C/W R_θJA value gives a temperature rise of 32°C. Subtracting this from the 125°C maximum temperature results in a maximum ambient temperature of 93°C. Thus, the SOA curve in Figure 2-1 crosses 5.5 A near 93°C.