A more accurate thermal metric given by Ψ_JT can be used to closely predict junction temperature based on measuring T_C. Unlike R_ΘJC, Ψ_JT is measured using the UCC14240EVM-052 evaluation module (EVM) that more closely represents how the IC is expected to be used in a real-world PCB design. The EVM can therefore be used to estimate IC junction temperature with reasonable accuracy for packages mounted in a non-JEDEC environment. This thermal metric has been adopted by the industry under the JEDEC standard (JESD51-2) and since Ψ_JT is not a true thermal resistance, it is measured by the Greek letter psi (Ψ) to differentiate it from theta (Θ). The calculation for determining T_J from Ψ_JT gives a more accurate result and is similar in form to R_ΘJA given in Equation 37.

R_ΘJA and Ψ_JT are thermal parameters based on test standards defined and developed for a single die IC package. An extension of single die package standards was introduced in JESD51-31 to include thermal test methods covering multi-die packages. However, UCC14240-Q1 is an isolated, DC-DC multi-source package (MSP) containing a primary and secondary die and an integrated planar transformer consisting of a primary and secondary transformer winding. Due to the nature of the MSP, a single set of JEDEC standards cannot be used to characterize the UCC14240-Q1. Since T_J for an MSP doesn’t carry the same meaning as a single die or even a multi-die IC, the two die and two transformer windings are treated as four separate potential sources of heat generation and a thermal matrix is derived which accurately describes the temperature relationship between each of the four internal elements mentioned.

The thermal matrix is a system of linear equations written into a 4x4 matrix and for the purpose of deriving the UCC14240-Q1 data sheet thermal parameters is given by Equation 39.

Where the nomenclature of Equation 39 is defined as:

• 1, 2, 3, 4 denote primary die, secondary die, primary winding and secondary winding, respectively
• T is the die or transformer temperature rise
• P is power dissipation
• R is thermal resistance with subscript as follows:
  • R₁₁ is temperature rise per unit power (°C/W) of primary die due to dissipation from primary die
  • R₁₂ is temperature rise per unit power (°C/W) of primary die due to dissipation from secondary die
  • R₁₃ is temperature rise per unit power (°C/W) of primary die due to dissipation from primary transformer winding
  • R₁₄ is temperature rise per unit power (°C/W) of primary die due to dissipation from secondary transformer winding

The model represented by the thermal matrix is simulated using the EVM in 125°C still ambient air. Knowing the predicted temperature and power dissipation values, the thermal resistance numbers are determined by solving the four equations in the thermal matrix. It is not expected that the user would attempt to validate the distributed thermal matrix solution but rather, it is presented here simply to outline the procedure used to establish confidence in the lumped thermal parameters published in the data sheet.

The maximum temperature contribution inside the UCC14240-Q1 is coming from the internal transformer windings. Since T_J is the primary concern, the transformer is allowed to rise to a temperature that can exceed 150°C. The heat generated from the transformer flows through the thermal impedance related to the primary and secondary die as determined by the thermal matrix. The cumulative resulting temperatures at the primary and secondary dies are monitored to shut down the UCC14240-Q1 around 160°C and maintain T_J < 150°C.

Measuring the case temperature with a thermal camera and calculating the IC power dissipation, we can have high confidence for estimating the maximum T_J according to Equation 38.