Power Converter Thermal Impedance – A Guide to the Essentials

At a certain ambient temperature, the converter reaches its maximum internal temperature limit, and any further increase in ambient temperature must be compensated for by reducing the power dissipation inside the converter through a lower load. This is called thermal derating¹.

Calculating Thermal Impedance and Temperature Rise

The thermal impedance of a converter from case to ambient can be determined using the following relationship:


If the power dissipation is known (the difference between input power and output power), measuring the case temperature rise above ambient allows the thermal impedance to be calculated. In practice, this simple relationship is difficult to determine. Measuring the converter’s temperature rise above ambient requires high precision to obtain reliable results. One way to increase measurement reliability is to take multiple readings at different ambient temperatures, as the ΔT (temperature rise above ambient) should remain consistent for each temperature step.

For true convection cooling operation, thermal impedance must also be determined in a draught-free environment, as air movement caused by the thermal chamber’s fan can influence the results. If the device-under-test (DUT) is placed inside a cardboard box within the temperature-controlled chamber, and a four-wire (Kelvin contact) system is used for terminal voltage measurements, reasonably accurate results can be obtained:



Temperature measurements must be performed in a draught-free environment because thermal impedance varies significantly under different conditions. It is highly dependent on the efficiency of heat transfer to the surrounding fluid (typically air). Poor heat transfer coefficient leads to higher case temperature for the same power dissipation, resulting in higher thermal impedance. Conversely, efficient heat transfer reduces temperature rise for the same dissipated power, resulting in low thermal impedance. Thus, equation 1 is valid, but only under a specific set of conditions (convection cooling only). Maintaining consistent testing conditions is essential for reproducible results—the documentation of the set-up, the equipment list, and the calibrations performed are just as important as the test results.

For most real-life applications, the convection cooling thermal impedance figure is most relevant, as this reflects the typical use of PCB-mounted converters. These converters are soldered onto a circuit board, and the heat dissipated inside the converter spreads into the surroundings through free air convection cooling. Some heat is also conducted through the electrical connections to the PCB tracks, so temperature derating graphs in datasheets are accurate only when the PCB layout recommendations are followed. A small portion of heat is additionally dissipated by radiation, but this effect is minimal.

Heat Dissipation and Cooling Methods: Convection vs. Forced Air

The most effective method to improve heat transfer efficiency to the surroundings is to move the fluid (air) around, which explains why the thermal chamber fan affects readings. This principle has been known since Isaac Newton published a paper in 1701 establishing the relationship:


Where q is the heat transfer rate, h is the heat transfer coefficient, A is the DUT surface area, and ΔT is the temperature difference between the DUT and the ambient.