Draft: Diagnostic/prognostic Experiments for Capacitor Degradation and Health Monitoring in Dc-dc Converters

Studying and analyzing the ageing mechanisms of electronic components avionics in systems such as the GPS and INAV are of critical importance. In DC-DC power converter systems electrolytic capacitors and MOSFET’s have higher failure rates among the components. Degradation in the capacitors under varying operating conditions leads to high ripples output voltages and currents affecting downstream components leading to cascading faults. 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 The work in this paper proposes a detail experimental and systematic study on analyzing the degradation phenomenon is electrolytic capacitors under high stress operating conditions. The output degradation is typically measured by an increase in ESR (Equivalent Series Resistance) and decrease in the capacitance value. . We present the details of our accelerated ageing methodology along with analysis and comparison of the results. INTRODUCTION In DC-DC converter power supply hardware electrolytic capacitors and MOSFET’s have higher failure and degradation rates than other components in the systems. Variety of factors, ∗Address all correspondence to this author. such as High Voltage conditions, Operating Temperature, Transients, Reverse Bias, Strong Vibrations and high ripple current attribute to the failure in these components. These degraded units affect the performance and efficiency of the DC-DC converters in a significant way. Degradation and failures in the components occurs due to prolong operation periods under normal conditions or operations under extreme stress conditions like high temperature and high voltage. The paper develops a method for studying the degradation effects of electrolytic capacitors subjected to loading under extreme operating conditions i.e. high voltage stress and observe their impact on overall system performance. The degradation in the DC-DC converters is typically measured by the increase in ESR (Equivalent Series Resistance) and decrease in capacitance value which leads to high 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. The work in this paper is specifically directed towards DC-DC converters in Avionics systems. In these systems the power supply drives a GPS unit, and ripple currents at the converter output can cause glitches in the GPS position and velocity output, and this, in turn, may cause errors in the Inertial Navigation (INAV) system causing the aircraft to fly off course [1]. Switched-mode power supplies are widely used in DCDC converters because of their high efficiency and compact size. DC-DC converters are important in portable electronic de1 Copyright c © 2010 by ASME FIGURE 1. BUCK BOOST CONVERTER SCHEMATIC CIRCUIT vices, which derive their power primarily from batteries. Such electronic devices often contain several sub-circuits with different voltage requirements (sometimes higher and sometimes lower than the supply voltage, and possibly even negative voltage). DC-DC converters can provide additional functionality for boosting the battery voltage as the battery charge declines. A typical buck-boost DC-DC converter schematic circuit is illustrated in Fig. 1. In the literature it has been reported that electrolytic capacitors are the leading cause for breakdowns in power supply systems [2, 3]. The performance of the electrolytic capacitor is strongly affected by its operating conditions, which includes voltage, current, frequency, and working temperature. For degraded electrolytic capacitor the impedance path for the ac current in the output filter keeps increasing, thus introducing a ripple voltage on top of the desired DC voltage [4]. Continued degradation of the capacitor leads the converter output voltage to also drop below specifications and in some cases the combined effects of the voltage drop and the ripples may damage the converter itself in addition to affecting downstream components. In general, capacitor degradation has been studied under nominal conditions as well as under extreme stress conditions, such as high voltage, high ripple, and adverse thermal conditions [5, 6]. Our overall goal in this work is to perform a systematic study of capacitor degradation stress conditions by replicating and extending some of the experimental studies that have been carried out in the past. Our approach is to perform empirical studies and then link them to theoretically-derived physics of failure models. This paper presents a first step by systematically collecting accelerated capacitor degradation data at high voltage operation. The results are observed, analyzed and are further compared with data observed from degradation under normal operating conditions. We also discuss and present our preliminary work for capacitance degradation in this paper as a first step to studying capacitance degradation using physics of failure models. The experimental studies, conducted at the NASA Ames Prognostics Centre of Excellence Lab, are discussed in greater detail along with observed result analysis later in the paper. The rest of this paper is organized as follows. The the following section discusses the mechanisms for capacitor degradation in DC-DC converters. The next section discusses accelerated degradation experiments conducted on electrolytic capacitors under high voltage stress . The following sections discusses the analysis methods and comparison of accelerated degradation with normal degradation. The paper concludes with discussion of the results and future work. ELECTROLYTIC CAPACITOR DEGRADATION This section discusses in detail the conditions under which the capacitor degrades leading to faults in the system. We study the adverse effects of the load conditions, operating conditions, ripple currents, which cause degradation by raising the temperatures in the capacitor core. Physical Model of the Capacitor An aluminum electrolytic capacitor, illustrated in Fig. 2 consists of a cathode aluminum foil, electrolytic paper, electrolyte, and an aluminum oxide layer on the anode foil surface, which acts as the dielectric. When in contact with the electrolyte, the oxide layer possesses an excellent forward direction insulation property [7]. Together with magnified effective surface area attained by etching the foil, a high capacitance is obtained in a small volume [8]. Since the oxide layer has rectifying properties, a capacitor has polarity. If both the anode and cathode foils have an oxide layer, the capacitors would be bipolar [9]. In this paper, we analyze the ”non-solid” aluminum electrolytic capacitors in which the electrolytic paper is impregnated with liquid electrolyte. There is another type of aluminum electrolytic capacitor, that uses solid electrolyte but we will not include these types of capacitors in this discussion [10]. Degradation Mechanisms There are several factors that cause degradation in electrolytic capacitors. As the degradation increases time the component fails , and this impacts the overall system functionality. The definition of failure and some of the failure modes are discussed below. Failures in a capacitor can be one of two types: (1) catastrophic failures, where there is complete loss of functionality due to a short or open circuit, and (2) degradation failures, where there is gradual deterioration of capacitor function. Degradations are linked to an increase in the equivalent series resistance (ESR) and decrease in capacitance over time [11, 12]. Capacitor degradation is typically attributed to: 2 Copyright c © 2010 by ASME FIGURE 2. PHYSICAL MODEL OF ELECTROLYTIC CAPACI-