Reliability as a Figure of Merit

Reliability as a Figure of Merit

DOI: 10.4018/978-1-4666-9429-3.ch006
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An electric power converter is specified by its system performance indices. Many system performance indices like efficiency are important in design process of system. However, that doesn't mean the converter performance is necessarily adequate for a practical application. At the end of design process of a converter, some desired specifications may not be achieved. In this chapter, reliability as a figure of merit in design of a system is presented and compared with other indices. We want to highlight the effect of reliability considerations on the design methodology of a power converter. The most important specification of a power supply or power converter is its robustness. Because any failure in power supply leads to failure of the whole of the system. A power converter may have poor performance but operate reliably and vice versa. In fact, this is a reliability based design approach to achieve a long useful life. It is shown that in many systems, high efficiency is not a good choice for selection of system operating point. A system can be inefficient but very reliable. Two complex examples are presented to show undesired results of neglecting reliability in design process. Methods for more reliable operation of electric power converters than high performance operation are proposed. A discussion about correct and intelligent optimization of a system parameters and operating set point is presented.
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Quality of any system is expressed by some indices which are named as figure of merits. These indices help users to have a smart selection among some systems with a common application (Rajaei, Kaboli, & Emadi, 2008). For example, both electric and engine motors can be used for driving a vehicle. Selection between these two choices can be done by difference of their efficiency as a figure of merit. An electric power converter as a system also has various figure of merits (Kaboli, Haddadi, & Khaligh, 2008). Some of the most important figure of merits in a power electronic converter are listed as follows:

  • Efficiency

  • Power factor

  • Total harmonic distortion of voltage and current (input or output)

  • Duty cycle

There are similar indices for electric machines too (Nasirian, Kaboli, & Davoudi, 2012). However, some of these indices are more important in electric machines or in power electronics. For example, regarding to switching nature of power electronic converters, harmonic distortion indices are more important than electric machines.

It is a common scenes to use the above mentioned indices for characterizing the converters in many literatures. In this chapter, we want to open another window for reader and it is reliability as a figure of merit. Figure 1 shows the state of this chapter in flowchart of the book. Importance of this concept is due to its conflict with some of well stablished indices like efficiency.

Figure 1.

State of chapter 6 in the flowchart of the book

Reliability as a Forgotten Quality Index

The performance of power electronic converters, especially in terms of efficiency and power density, has been continuously improved by the intensive research and advancement in circuit topologies, control schemes (Hasanzadeh, Zolghadri, Kaboli, & Homaifar, 2003), semiconductors, passive components, and system integration technologies. In recent years, the automotive and aerospace industries have brought stringent reliability constraints on power electronic systems because of safety requirements. The industrial and energy sectors are also following the same trend to more reliable power electronic systems with cost-effective and sustainable solutions.

Of course, high efficiency is an important goal; however, that doesn't mean the converter's performance is necessarily adequate for a practical application. The published efficiency is usually a typical value measured at 25°C ambient, at the nominal input voltage. As the temperature rises, losses for semiconductor (usually MOSFETs) and copper traces can rise dramatically. At low line, there's increased input current loss. At high-line switching, losses increase; thus, efficiency goes down. With this in mind, worst-case efficiency is more important than the quoted typical efficiency. Whereas efficiency values can assist in comparing similar-sized converters with equal voltage and current ratings, they aren't worth much if the size, output voltage, or the output currents are different.

Design for Reliability (DFR) is not a new concept, but it has begun to receive a great deal of attention in recent years. In this chapter, we will try to answer these questions and, at the same time, we will propose a general DFR process that can be adopted and deployed with a few modifications across different industries in a way that will fit well into the overall Product Development Process


Reliability Oriented Approach

In the first part of this chapter, it will be discussed the challenges and exciting new opportunities in the research on reliability of power electronics.

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