Advances in Power Electronics

Volume 2015, Article ID 652389, 14 pages

http://dx.doi.org/10.1155/2015/652389

## Online Junction Temperature Cycle Recording of an IGBT Power Module in a Hybrid Car

Department of Mechatronics, University of Bayreuth, Universitätsstraße 30, 95447 Bayreuth, Germany

Received 14 July 2014; Accepted 16 January 2015

Academic Editor: Pavol Bauer

Copyright © 2015 Marco Denk and Mark-M. Bakran. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The accuracy of the lifetime calculation approach of IGBT power modules used in hybrid-electric powertrains suffers greatly from the inaccurate knowledge of application typical load-profiles. To verify the theoretical load-profiles with data from the field this paper presents a concept to record all junction temperature cycles of an IGBT power module during its operation in a test vehicle. For this purpose the IGBT junction temperature is measured with a modified gate driver that determines the temperature sensitive IGBT internal gate resistor by superimposing the negative gate voltage with a high-frequency identification signal. An integrated control unit manages the measurement during the regular switching operation, the exchange of data with the system controller, and the automatic calibration of the sensor system. To calculate and store temperature cycles on a microcontroller an online Rainflow counting algorithm was developed. The special feature of this algorithm is a very accurate extraction of lifetime relevant information with a significantly reduced calculation and storage effort. Until now the recording concept could be realized and tested within a laboratory voltage source inverter. Currently the IGBT driver with integrated junction temperature measurement and the online cycle recording algorithm is integrated in the voltage source inverter of first test vehicles. Such research will provide representative load-profiles to verify and optimize the theoretical load-profiles used in today’s lifetime calculation.

#### 1. Introduction

The combination of an internal combustion engine and an electric machine enables the improvement of the efficiency and the performance of the drivetrain of personal cars, busses, and utility vehicles [1]. In view of the reliability and the lifetime of voltage source inverters used in hybrid-electric powertrains the IGBT power module can be considered as the most lifetime critical component. This is especially true if power modules with conventional linking and packaging technology are used. Those modules are characterized by a bond-wire connection, a direct copper bonded Al_{2}O_{3}-substrate where the chip is soldered on, and a copper base-plate. This results in a complex structure whose materials have different coefficients of thermal expansion CTE. In case of temperature cycles this CTE mismatch causes thermomechanical stresses in the modules interconnections and leads to the lift-off or the heel-cracking of bond wires or the degradation of the die-attach or the substrate solder joint [2].

To estimate the lifetime of an IGBT power module in a hybrid car a simple lifetime calculation approach has become dominant in recent years [3]. This calculation approach is derived from the lifetime estimation of mechanical parts and demands the linkage of an application typical load-profile with a lifetime model of the IGBT power module using a cycle counting algorithm and a linear damage accumulation rule. For mechanical parts like shafts or gearwheels in transmissions this lifetime calculation approach could be verified over the years and today it is possible to design their lifetime with a high accuracy. On the contrary the lifetime calculation of IGBT power modules in hybrid cars is in a very early stage and currently it is not possible to quantify the accuracy of the lifetime calculation approach. What is known, however, is that the calculation approach suffers from different factors of uncertainty like the interaction of different failure mechanisms [4] and the information loss due to the cycle counting. However, the most critical point in lifetime calculation is the limited representation accuracy of today’s load-profiles. These theoretical profiles originate from simulation, but it is hard to consider different types of driver, different areas of operation, different hybrid strategies, and varying ambient conditions in a load-profile that is rather short in relation to the vehicle lifetime. Because of these uncertainties, there is a need to verify the theoretical load-profiles and the lifetime calculation approach with data from field studies. For this reason this paper presents a temperature cycle recorder that can be implemented in hybrid cars to record the exposure of the IGBT power module during its real operation. The recorded load history of test vehicles or first field returns can be used to create an experience base and to optimize the load-profiles and the lifetime calculation approach of IGBT power modules. This paper shows new results of the IGBT driver and the recording algorithm and combines it with the results of the following publications [5–7]. In the following the state-of-the-art approach to calculate the lifetime of an IGBT power module is briefly summarized.

#### 2. State of the Art

The lifetime calculation of an IGBT power module in a hybrid car requires basically the linkage of an application typical load-profile with an empirical lifetime model of the power module using a cycle counting algorithm. Today this load-profile originates from an application typical velocity profile like the New European Driving Cycle (NEDC) and the simulation of the entire hybrid-electric powertrain. A subsequent electrothermal model of the power module delivers the transient IGBT junction temperature over a certain timespan. This transient temperature curve is called the load-profile of the power module. It is analyzed with a cycle counting algorithm and valued with an empirical lifetime model. In recent years the following lifetime models and counting algorithms have been presented.

##### 2.1. Empirical Lifetime Models

Empirical lifetime models originate from the accelerated ageing of power modules. They specify the number of temperature cycles a power module can bear until a failure criterion is reached. In recent years various lifetime models were publicized that differ primarily in the number of parameters used to describe a temperature cycle. The elementary lifetime model is a simple Coffin-Manson law [8] that states that the number of temperature cycles to failure depends solely on the size of the amplitude of a temperature cycle:
Today there are extended lifetime models [9–11] which consider additional parameters to describe a temperature cycle. In 1997 the LESIT [9] project investigated the temperature cycle stability of power modules with conventional packaging technology from European and Japanese suppliers. It was found that the medium cycle temperature has a notable influence on the sustainable number of cycles. For this reason the Coffin-Manson law was extended by an Arrhenius term. Equation (2) shows the LESIT model, where the number of cycles to failure is a function of the cycle amplitude and the medium cycle temperature . The parameters and were derived from accelerated ageing and J/mol·K is the gas constant and J·mol^{−1} is the activation energy:
Since the technologies of conventional IGBT power modules have been improved, in 2008 the number of sustainable temperature cycles to failure was reinvestigated by power cycling of several Infineon IGBT modules. It became apparent that many additional parameters have an impact on the module lifetime. The developed CIPS08 [10] lifetime model describes the number of cycles to failure as a function of the amplitude , the minimum temperature , and the heating time of a temperature cycle:
Moreover the current per bond wire , the nominal voltage , and the bond wire diameter were taken into account. The parameters and to and their validity ranges are given in [12]. For instance the heating time of a temperature cycle must be set to s for s. Equation (3) shows the CIPS08 lifetime model. For lifetime calculation the current per bond wire can be set to A. The diameter of the bond wire and the voltage class of the power module are constants, so that the lifetime of the power module depends solely on the temperature cycles the power module is exposed to during its operation. This comparison of different lifetime models shows that the accuracy of the empirical models used for lifetime calculation could be improved due to the more accurate specification of a temperature cycle. The present state of the art is the parameterization of a temperature cycle with its amplitude , its minimum temperature , and its heating time .

##### 2.2. Cycle Counting Algorithm

Counting algorithms enable the evaluation of an application typical load-profile that consists of several different temperature cycles with an empirical lifetime model. For this purpose they extract and parameterize all temperature cycles within the load-profile and store them in a data vector. Widely accepted counting methods are the half-cycle counting, the maximum-edge counting, and the Rainflow counting [13, 14]. Figure 1 shows the application of these counting algorithms on an exemplary temperature profile.