Technical Information Site of Power Supply Design

2022.10.19 Thermal Design

Estimating TJ: Calculation Example
Using ΨJT

Thermal Design of Semiconductor Components in Electronic Equipment

In the previous article, θJA was used in an example of calculation to estimate TJ. This time, an example of calculation to estimate TJ using ΨJT is presented. As the IC used in the example, similarly to the previous article, the BD450M2EFJ-C LDO linear regulator is used.

An Example of Calculation to Estimate TJ Using ΨJT

In order to use ΨJT to determine TJ, the PCB conditions, the value of ΨJT provided on the data sheet for the IC or elsewhere, the power consumption P of the IC, and the value of the temperature TT at the center of the top surface of the IC package in the state of actual use, are all needed. As explained in the previous article, P is calculated from the consumption current given on the data sheet, or else using the actual measured consumption current. In this example, the value P = 0.85 W calculated in the previous article is used. TT must be measured.

First, the PCB conditions are taken to be as follows, as in the previous article, and from the graph, ΨJT for a copper foil area of 1000 mm2 is found.

Number of layers: 1
PCB material: FR4
Copper foil area: 20mm×50mm=1000mm2

The value of ΨJT read from the graph is 6°C/W, and the measured value of TT is taken to be 115°C. A photo of measurement of TT is shown on the right; a thermocouple is used to measure the temperature at the center of the top surface of the package. Below are the calculation equation and calculation result.

TJMAX for the IC used in the example is 150°C, so that the conditions of use can be judged to be within the allowed limits.

As explained previously, estimation of TJ using ΨJT has the advantage that TJ can be estimated for operating conditions, since the measured value of TT is used.

In the next article, an example of estimation of TJ using the transient thermal resistance will be presented.

Key Points:

・For estimation of the thermal resistance ΨJT using TJ, the power consumption P and the temperature TT at the center of the top surface of the IC package in the state of actual use are required.

・Actual measurement of TT is necessary.

・TJ is found from a calculation equation, and is confirmed to be less than TJMAX.

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