Physics of Failure Models for Capacitor Degradation in DC-DC Converters

TitlePhysics of Failure Models for Capacitor Degradation in DC-DC Converters
Publication TypeConference Paper
Year of Publication2010
AuthorsKulkarni, C., G. Biswas, X. Koutsoukos, G. Kai, and C. Jose
Refereed DesignationDoes Not Apply
Conference NameThe Maintenance & Reliability Conference, MARCON 2010
Date Published02/2010
Conference LocationKnoxville, TN

This paper proposes a combined energy-based and physics of failure model for degradation analysis and prognosis of electrolytic capacitors in DC-DC power converters. Electrolytic capacitors and MOSFET’s have higher failure rates than other components in DC-DC converter systems. Currently our work focuses on analyzing and modeling electrolytic capacitors degradation and its effects on the output of DC-DC converter systems. The output degradation is typically measured by the increase in ripple current and the drop in output voltage at the load. Typically the ripple current effects dominate, and they can have adverse effects on downstream components. For example, in avionics systems where the power supply drives a GPS unit, ripple currents can cause glitches in the GPS position and velocity output, and this may cause errors in the Inertial Navigation (INAV) system causing the aircraft to fly off course.

A model based approach to studying degradation phenomena enables us to combine the energy based modeling of the DC-DC converter with physics of failures models of capacitor degradation, and predict using stochastic simulation methods how system performance deteriorates with time. This more systematic analysis may provide a more general and accurate method for computing the remaining useful life (RUL) of the component and the converter system.

We have employed a topological energy based modeling scheme based on the bond graph (BG) modeling language for building parametric models of multi-domain physical systems. The BG approach captures relationship between component parameters, system behavior and performance. Component degradation models are constructed using empirical physics of failure models that have been presented in the literature, and validating these models using data collected from accelerated degradation studies. The physics of failure models provide mathematical formulations that are directly linked to component parameters.

Literature reports a number of operating conditions that may cause capacitor degradation. These include High Voltage conditions, Transients, Reverse Bias, Strong Vibrations and high ripple current. In our work, we have studied the effects of capacitor degradation on DC-DC converter performance by developing a combination of converter system model and a physics of failure model of electrolytic capacitor degradation when subjected to thermal and electrical stresses. Thermal stress occurs when the capacitors operate in high temperature environments, while electrical stress conditions occur due to high operating voltages and even ripple currents above the rated values. In our work we are developing models to capture the failure phenomenon in these components.

Our current work adopts a physics of failure model (Arrhenius Law) for equivalent series resistance (ESR) increase in electrolytic capacitors subjected to electrical and thermal stresses. Under stress conditions the ESR gradually increases and capacitance of the capacitor gradually decreases with time thus resulting in the capacitors ability to filter out AC components in the output voltage. As a result, the output ripples current and ripple voltages of the converter increases over time. The output DC voltage also decreases over time, but the ripple current effects on the load are more significant. High ripple currents may lead to frequent resets or even damage in the systems that are downstream from the power supply. We present a combined model- and data-driven approach for estimating and validating the parameters of our physics of failure models for capacitor degradation. We use Monte Carlo simulation methods to develop prognostic methods that predict remaining useful life based on degradation in the power supply output.

For model simulation study the derived degradation model of the capacitors are reintroduced into the DC-DC converter system model to study changes in the system performance using Monte Carlo methods. The simulation results observed under different stress conditions are recorded and compared with the hardware experiments. We have designed different hardware setups for capturing the data of actual degradation phenomenon under thermal and electrical stress. In the first setup capacitors are subjected to only thermal stress. Under this condition output ripple voltage and increase in ESR is monitored over the period of time. In the second setup experiment the capacitors are subjected to electrical stress by continuous charging/discharging cycle. The ESR parameter is monitored regularly over this period of time. The data from these experiments is used to verify results from the models developed and also for refining the model parameters for more accuracy. The paper concludes with comments and future work to be done.

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