Reliability-Oriented Assessment of a DC/DC Converter for Photovoltaic Applications
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18-02-2011, 02:41 PM

Abstract –
There is a trend toward increasing the performanceof power electronics converters used in photovoltaic systems.However, parameters such as reliability, efficiency, andcomplexity might be in conflict. In this paper, the design of asimple DC/DC interleaved boost converter is assessed. It takesadvantage of state-of-the-art power devices that have fastswitching times. It is shown that requirements of low complexity,high reliability, and high efficiency can be satisfied, withoutresorting to switching aids, provided that the devices are selectedwith reliability in mind. A 1 kW prototype attained a peakefficiency as high as 98%, and a predicted mean time betweenfailures of 100,000 hours at 80C.I. INTRODUCTION
There is currently a widespread interest in photovoltaic(PV) systems, with powers that typically range from 1 kW toabout 5 kW. A major cause of concern is the reliability of thepower-electronics stage, because it has been found that mostof the failures, up to 66%, occur in this stage [1, 2]. Also, thepower stage average time to failure is much shorter than theoperational life of a PV cell, which typically exceeds a 20years span. Thus, there is an effort to design and manufacturehigh reliability power stages, with 10 years mean timebetween failures (MTBF) [3].In PV systems it is always desirable to operate thephotovoltaic array at the point of maximum power delivery, ormaximum power point (MPP), and it is a well known fact thatthis point varies as a function of temperature and irradiance. Inorder to achieve this goal, it is necessary to draw a constant,ripple-free current from the array. The simplest DC/DCconverter topology that can be used for this purpose is theboost converter. When the converter is connected to a welldesignedcontroller circuit, the MPP can be properly tracked[4]. Any switching converter draws a source current withsome amount of superimposed ripple. The ripple amplitudecan be reduced by operating the converter at high frequencybut, as the frequency is increased, so does the switching losses.In order to maintain the efficiency at acceptable levels, severalmodifications, such as the interleaved boost configuration,have been proposed. The main advantage is that the inputripple can be greatly reduced without resorting to highfrequency switching, as long as the gating signal to theindividual switches are properly timed [5]. A drawback of thisapproach is that the number of switching devices is increased.Losses can still be kept at acceptable levels if some switchingaids,such as non-dissipative snubbers, are included [6, 7].Another approach to have a ripple-free current involves usingcoupled inductors in the paralleled stages [8, 9]. On the other hand, there has been a steady improvement in thecharacteristics of solid-state power switches, and newergeneration devices have faster switching times [10]. It seemspossible, then, to achieve performances in hard switchingconfigurations, comparable to those previously obtained withcircuits that included switching aids.This paper is aimed at verifying this assumption. A hardswitchingtwo-stage interleaved boost converter is designedand tested to measure its efficiency. The results are comparedwith those reported in the technical literature. In order to havea realistic comparison, the converter efficiency was testedboth in a controlled laboratory environment, and connected toa PV array. When comparing two converters designed for thesame application, as a rule-of-thumb, the one with fewercomponents will have a higher reliability, but the reliability isalso a function of the stresses on the devices. Therefore, inorder to assess how well the converter complies with theoperating-time goal, the mean time between failures (MTBF)of the hard-switching converter is calculated
.II. DESIGN OBJECTIVESA boost converter draws an almost constant current fromthe source, without requiring further stages. The currentincludes a ripple at the switching frequency fS, whosemagnitude can be greatly reduced by using the interleavedconfiguration shown in Fig. 1. For proper operation, the gatingsignals provided to Q1 and Q2 must have a 180º phase shiftbetween each other. The current drawn from the supply isdivided between the paralleled stages. Therefore, for the sameoutput power, an interleaved converter requires devices with alower current rating than the conventional boost converter. Nominal output voltage VO = 230V
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03-02-2012, 12:07 PM

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