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2019

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Temperature Measurement of Semiconductors

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A new methode patented by ZF allows more accurate calculation of key variables in the development of power electronics.
Dr. Marco Denk, January 17, 2019
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Dr. Marco Denk is responsible for advanced engineering of power electronics at ZF. The mechatronics technician previously worked on power semiconductors for his doctorate project.
In every hybrid drive as well as every purely electric drive system, the power electronics are the central switching point. They regulate the flow of power and control the conversion of DC battery voltage into three-phase AC voltage as required by the electric motor. Power electronics are therefore not only responsible for ensuring the function of an electric driveline but also dictate its efficiency, its reliability and, last but not least, its costs.

In power electronics, semiconductors made of silicon or silicon carbide are among the most important and cost-intensive components. The semiconductors are used for transistors known as Insulated Gate Bipolar Transistors (IGBTs), which are indispensable for an electric drive. Depending on operational conditions, these modules are subjected to high thermal loads. This makes it particularly challenging to correctly dimension the semiconductor surface: on the one hand, reliable operation of the system must be ensured for 15 years or more, and on the other hand, the use of materials must be optimal in terms of installation space and costs. It is therefore difficult for developers to find a balance between safety reserves and cost-effectiveness. The reason for this is the lack of precise data on operating loads and aging of semiconductors under real conditions.

A demanding project description

A demanding project description

This was precisely the starting point of a ZF research project that I was able to implement at the University of Bayreuth: The characteristic for the actual load is the barrier-layer temperature of the semiconductor. The aim was to measure this parameter in the power semiconductor during operation and to document the temperature thresholds as well as to analyze the temperature fluctuations. In this way the operational load of the semiconductor and aging indicators can be evaluated. The situation was made more difficult by the fact that all of this needed to be done without installation of additional sensors or implementation of other modifications to the power module.

An unusual solution

An unusual solution

To measure the temperature during operation without making structural changes, we use the internal gate resistor of the semiconductor as a sensor. To do this, we modify a control circuit so that a sinusoidal identification signal can be modulated onto the gate voltage in the switched-off condition. The system responds by regulating the drop in voltage prompted by the external gate resistor, converting it into temperature values with a calibration curve and ultimately sending it to the controller board.
We verified the new temperature measurement method by setting up a ZF hybrid transmission equipped with a thermographic camera for test purposes. A comparison of the measurement via infrared camera with the temperature determined on the basis of the internal gate resistance showed a high degree of alignment between the two measurement methods.

Where do you send the data to?

Where do you send the data to?

Another challenge was recording and storing the measured data. Complex additional modules, such as solid-state drives (SSDs) with high storage capacity, were out of the question for reasons of space and cost. Without compression, the data already has a storage requirement of one megabyte for a relatively short journey. However, a method developed within this project makes it possible to compress the obtained data in such a way that the temperature profile of even extensive journeys can be transferred to EEPROM with 8 kilobytes of storage.
The thermal load profiles thus obtained with different test drivers or car models can subsequently be evaluated. In this way the new process lays the foundation for simulations with which we can optimize the dimensioning of semiconductors in power electronics.