Derating

Derating

DOI: 10.4018/978-1-4666-9429-3.ch010
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Abstract

In previous chapters, we discussed the converter with or without fault. The common similarity between them is that they continue to operate without reduction of their nominal specification. In this chapter, uninterrupted operation of a faulty power conversion system with catastrophic damages in some of its parts is investigated. It is shown that a faulty electric power converter can continue to work with degraded specifications. This algorithm is named derating for accessibility. This technique can be used for both a faulty system because of its uninterrupted operation and a normal system because of extensive life time. Algorithms for derating of a faulty electric machine and a power supply are described. Derating for increasing the useful life of a motor drive system is presented. A novel method for switching frequency selection in a switching power supply is proposed based on derating concept. Derating is introduced as a technique to compensate additional losses in an electric power converter operating in harsh environment (for example: a motor drive which is supplied with a non sinusoidal voltage waveform). Industrial examples are presented in details for better understanding of the derating concept. Some of the presented examples contain novel idea for derating and others are well known in industry.
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Introduction: Derating To Continue The Operation

Protection techniques prevent catastrophic failures in electric power converters. However, as it was described before, protection systems can not prevent any fault. There are two main approaches to a faulty converter. Isolating the faulty converter is one of the approaches and it is used when the converter mission can be interrupted for repair and maintenance. If the mission of the converter is important at the time of failure, users should use another method. Derating is a method for allowing the faulty converter to continue its mission. Figure 1 shows the state of this chapter in the flowchart of the book.

Figure 1.

State of chapter 10 in the flowchart of the book

Although, derating method is mainly a method for faulty systems, it can also be used for life extending in normal cases. Component failure rates generally decrease as applied stress levels decrease. Thus, derating or operating components at levels below their ratings (for current, voltage, power dissipation, temperature, etc.) will increase reliability. You can achieve this derating by circuit design (minimizing applied stress), component selection (using components with ratings well above the applied stress), and thermal design (reducing the component's operating temperature). Derating is a technique usually employed in electrical power and electronic devices, wherein the devices are operated at less than their rated maximum power dissipation, taking into account the case/body temperature, the ambient temperature and the type of cooling mechanism used. In this article, we will briefly explain the theoretical background of derating and how it is applied.

Derating increases the margin of safety between part design limits and applied stresses, thereby providing extra protection for the part. By applying derating in an electrical or electronic component, its degradation rate is reduced. The reliability and life expectancy are improved.

Intuitively, if a component or system is operated under its design limit, it will be more reliable than if it is operated at or above the design limit. Theoretically, the benefit of derating can be explained using load-strength interference theory.

Most estimates of failure rate are based on an operating temperature, input voltage, and output power. Using the converter at an output power or current different from that used for the reliability calculation can yield a reliability different from that predicted. We can use these data for operating the converter at an output power less than the maximum permissible value. The power dissipated in many internal components — including most of the high-power devices in the power conversion chain — is approximately proportional to the output power or current. This means that derating the output current by x% will reduce the component temperature rise by at least x% and by more for components such as MOSFETs and magnetics, where the power dissipation is proportional to the current squared. The lower power dissipation leads to a corresponding lower failure rate. Consequently, the power system designer can decrease the converter's failure rate by reducing its output power.

A necessary procedure for manufacturing that enhances reliability is burn-in. Burn-in keeps infant mortality in the factory, rather than allowing it in the field. This can be done at the part, board, or system level. All dc-dc converters should go through a burn-in process. Failure rates and times need to be recorded and analyzed to ensure the burn-in period is long enough to bring out all cases of infant mortality.

If you can't achieve the required level of predicted reliability solely by derating and thermal management of one dc-dc converter, then you can add more converters in parallel. Although more components are present to fail, the consequence of one failure may be completely overcome, requiring the system to have two failures before compromising functionality

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Load-Strength Interference

Usually, failure happens when the applied load exceeds the strength. Load and strength should be considered in a general way. For electronic parts, “load” might refer to voltage, power or an internal stress such as junction temperature. “Strength” might refer to any resisting physical properties.

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