# Influence of Electrical Transients and A/D Converter Dynamics on Thermal Resistance Measurements of Power MOSFETs

## TL;DR

This paper shows that electrical transients and A/D converter dynamics can affect thermal resistance measurements in power MOSFETs, especially those made of silicon carbide.

## Contribution

The study reveals that measurement errors vary with semiconductor material and can be reduced by waiting for electrical transients to decay.

## Key findings

- Electrical transients and oscillations last up to several milliseconds, affecting thermal resistance measurement accuracy.
- Silicon carbide and gallium nitride MOSFETs show higher thermal resistance measurement errors than silicon devices.
- Waiting 2.5 ms after transients decay reduces measurement error to a few percent.

## Abstract

What are the main findings?
An analysis of datasheets of power MOSFETs indicates that the accuracy of thermal resistance measurements depends on the shortest thermal time constants of the tested transistors.Measurements performed using the system built by the authors indicate that electrical transients and oscillations are observed for up to several milliseconds, which may significantly affect measurement accuracy.The measurement error of thermal resistance is generally higher for power MOSFETs made of silicon carbide and gallium nitride than for silicon transistors.

An analysis of datasheets of power MOSFETs indicates that the accuracy of thermal resistance measurements depends on the shortest thermal time constants of the tested transistors.

Measurements performed using the system built by the authors indicate that electrical transients and oscillations are observed for up to several milliseconds, which may significantly affect measurement accuracy.

The measurement error of thermal resistance is generally higher for power MOSFETs made of silicon carbide and gallium nitride than for silicon transistors.

What is the implication of the main finding?
The measurement of the thermal sensitive electrical parameter (TSEP) during thermal resistance measurements of power MOSFETs should start only after electrical transients and oscillations have faded out. In the investigated system, this occurs up to 2.5 ms after the switchover from transistor heating to TSEP recording.The measurement error of thermal resistance for power MOSFETs made of silicon carbide is higher than for devices made of silicon or gallium nitride.

The measurement of the thermal sensitive electrical parameter (TSEP) during thermal resistance measurements of power MOSFETs should start only after electrical transients and oscillations have faded out. In the investigated system, this occurs up to 2.5 ms after the switchover from transistor heating to TSEP recording.

The measurement error of thermal resistance for power MOSFETs made of silicon carbide is higher than for devices made of silicon or gallium nitride.

When designing power electronic systems, it is crucial to correctly estimate the junction temperature of semiconductor devices, particularly power MOSFETs, under actual operating conditions. Thermal resistance is a parameter that characterizes the ability of these devices to dissipate internally generated heat under steady-state conditions. Determining the value of this parameter under specific cooling conditions requires dedicated measurements. This paper considers the widely used indirect electrical method of measuring thermal resistance. The influence of the dynamic properties of the measurement system, including the A/D converter, on the measurement error of the thermal resistance of power MOSFETs was analyzed. Using the constructed measurement system, it was demonstrated that, depending on the semiconductor material of the tested transistors, different error values were obtained, even with the same system configuration. The largest errors were observed for transistors made of silicon carbide. It was further shown that, with the applied A/D converter module, the measurement error can be limited to a few percent if recording of the thermal sensitive electrical parameter (TSEP) begins soon enough after the transients caused by the switchover from heating to TSEP measurement have fully decayed.