Comparison of the electro-thermal constraints on SiC MOSFET and Si IGBT power modules in photovoltaic DC/AC inverters

doi:10.1016/j.microrel.2017.07.087

Microelectronics Reliability

Volume 78, November 2017, Pages 65-71

https://doi.org/10.1016/j.microrel.2017.07.087 Get rights and content

Highlights

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Si IGBT and SiC MOSFET with same current and voltage ratings are compared.
•
Junction temperatures in a photovoltaic inverter are estimated over one year.
•
Temperature swings and average temperatures are higher in the case of IGBT.

Abstract

This article presents a comparative study between SiC MOSFETs and Si IGBTs regarding changes in their junction temperature in a PV inverter application. The estimation of these variations is made by introducing the current mission profiles extracted from a photovoltaic plant over one year into a calculation tool. The latter is based on a losses model and a thermal model including a coupling between them. The calculation of the losses in SiC MOSFETs in the 3rd quadrant is detailed. The results are the mission profiles of the junction temperature of semiconductors, which allow for determining and comparing the thermal constraints in SiC MOSFET and Si IGBT power modules.

Introduction

Silicon carbide (SiC) semiconductor components are being used increasingly in power electronic applications, mainly because of their high switching speeds which improve the overall efficiency and/or the compactness of the inverters [1] [2] [3].

In the case of photovoltaic installations, the inverter has the highest failure rate and the anticipation of its breakdowns is an important issue. Moreover, few studies have been conducted on the reliability of this type of converter using SiC MOSFETs [4].

In this context, the junction temperature T_J of the semiconductors and its variations over time ΔT_J contribute to accelerating the ageing of the DC/AC inverter [5] [6] [7]. Many studies have proposed methods to estimate this temperature especially for IGBT power modules using thermal models [8] [9].

The aim of our study is to compare the junction temperature swings in a SiC MOSFET and in a Si IGBT power module used in a 2 level photovoltaic inverter, having the same current and voltage ratings. A numeric tool is used to estimate the junction temperature from current mission profiles.

Knowing the temperature and its variations over time, coupled with the study of degradation modes and mechanisms as well as the mission profiles, will allow to estimate the lifetime of these semiconductors in photovoltaic applications.

In this paper, the method used to estimate the junction temperature of the devices is described. This model is composed of several sub-models, the main ones being the losses estimation model and the thermal model. These two models can be coupled in order to take into account the variation of the losses depending on the temperature of the components. Temperature, estimated as a function of time, is then injected into a cycle counting algorithm named “Rainflow” that allows to obtain, for a given temperature profile, the number of occurrences for each value of ∆ T_J [10]. This approach is shown in Fig. 1, where I is the current profile, P the losses profile for a given semiconductor component, T_J the junction temperature profile, ∆ T_J the variation of this temperature, T_{J_M} the mean temperature and T_A the ambient one. All the calculations are made with analytical models and are implemented in Matlab software.

Section snippets

Selection of the components

The direct comparison of SiC and Si technologies is not obvious. In fact, the electro-thermal stresses on the packaging depend a lot on the chosen configuration. For example, using the same technology, these stresses are different when changing the current rating of the devices: the maximum temperature and the thermal management system will change. Furthermore these stresses depend a lot on the application (current level, switching frequency…). The cost would also be an important point. Thus,

Considerations for the calculation

The evolution of the losses in the semiconductor components as function of time is estimated using the mission profile presented above. Due to the large number of samples (one sample every 20 ms as explained in Section 2.2) throughout the year, the calculations are simplified using only the value of the average losses on the fundamental period T_bf. Therefore, the temperature variations within each fundamental period in our calculations [11] are not taken into account.

In the following paragraphs

Electro-thermal coupling

The electro-thermal coupling is used to take into account the dependence of some electrical parameters on the junction temperature. This coupling exists in the case of an IGBT, but its effect is much more important in the case of MOSFET, in particular for R_DSon. Fig. 12 shows the temperature dependence of R_DSon issued from the manufacturers' datasheets, plus its dependence on the current I_T. The same work was done to estimate the variation of the parameters E_T, R_T, E₀ and R_D of the IGBT power

Modeling methodology

The thermal model has, as input, the losses profile. In this model, the thermal impedances Z_thCH (Case-Heatsink), Z_thHA (Heatsink-Ambient) and Z_thJC (Junction-Case) are used. The latter is given by the following equation according to the Foster model [13]: $Z_{thJC} = \sum_{i = 1}^{i = N} R_{t h_{i}} (1 - e^{- \frac{t}{τ_{i}}})$ where R_thi is one elementary thermal resistance of the model, τ_i one elementary time constant, at the point i, and N the number of RC cells in the model.

Usually, the use of Foster models is only possible if the

Integration of the models and Rainflow algorithm

“Rainflow” is a cycle counting algorithm (cycles contained in a mission profile) [15] [16].The T_JM profiles of the various modules are the entries of this algorithm, which gives as output the number of occurrences N of each value of ∆ T_J at a given level of average T_J (T_JM), represented in form of tables and histograms N = f (T_JM, ΔT_J) [17] [18] [19].

By applying this algorithm on the temperature profiles obtained in Fig. 14, comparative histograms of T_JM(Fig. 16) and of ∆ T_J (Fig. 17) in the case of

Conclusions

In this article, a model was used in order to estimate the junction temperature (losses estimation model + thermal model), to compare its evolution between two power module technologies using a current profile measured over a year in a photovoltaic plant. These modules use Si IGBTs and SiC MOSFETs with the same current and voltage ratings.

The obtained temperature profile is then injected into an algorithm named “Rainflow”. The results allows to observe that the average junction temperature and

Acknowledgments

This project has received support from the State Program Investment for the Future bearing the reference (ANR-10-ITE-0003).

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View full text

Comparison of the electro-thermal constraints on SiC MOSFET and Si IGBT power modules in photovoltaic DC/AC inverters

Highlights

Abstract

Introduction

Section snippets

Selection of the components

Considerations for the calculation

Electro-thermal coupling

Modeling methodology

Integration of the models and Rainflow algorithm

Conclusions

Acknowledgments

Microelectron. Reliab.

Microelectron. Reliab.

Mech. Syst. Signal Process.

Int. J. Fatigue

MOSFET technology advances DC-DC converter efficiency for processor power

SiC MOSFETs Enable High Frequency in High Power Conversion Systems

SiC power devices and modules

Failure modes and robustness of SiC JFET transistors under current limiting operations

Impact of absolute junction temperature on power cycling lifetime

Quantifying the thermal fatigue of CPV modules

Presented at the 6th Int. Conf. Concentrating Photovoltaic Syst