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failure rate. For simplicity the mathematical model used to describe the reliability as a function of the failure rate is of an exponential type. In a series system, the total reliability of the system is
equal to the product of all the single reliabilities of the parts making up the series. The total failure rate though is equal to the summation of the single failure rates. This means that, as the reliability is a lower number than the unit, the total product will certainly be lower than the reliability of every single part that goes into forming it, and the failure rate will certainly be higher.
Improving reliability Product and/or system reliability should be a key focal point during the design and development process. If not, the ability to identify issues and assess concerns cannot be addressed as the design concept takes place. It is too late to consider reliability implications towards the end of the development. For a power supply to be reliable, it has to be simple.
Design work aimed at simplicity will produce a power supply that is more reliable than a complex one. For example, a basic single output low power converter will have greater calculated reliability than a multiple output high power converter. The addition of protection circuits though, will increase the actual life of the power supply. In the early stages of development, the power supply
circuit should be subdivided into two macro-blocks: critical applications and non-critical applications. This will assist the designer to look at component selection and derating coefficients. Critical applications will consist of areas where a failure will
cause the power supply to stop functioning. Non-critical areas are the auxiliary applications. For the critical applications, the
parts must be of the highest quality, while trying to minimise the use of components that will deteriorate over time; such as electrolytic capacitors, fans and relays. Figure 2 shows a possible derating plan for a given
component in a non-critical application, and Figure 3 represents the same component in a critical application. The y-axis is (S) a coefficient indicating the stress on the
component, and the x-axis shows the working temperature of the component. Zone A is the permissible zone; zone Q is the zone where it may be problematic if the component not working; and zone R is the prohibited zone. Two of the accepted standards for performing reliability
predictions are MIL-HDBK-217 and Bellcore/Telcordia Technical Reference TR-332. Both of these empirical prediction methods have several assumptions in common – constant failure rate, the use of thermal and stress acceleration factors, quality factors, and use conditions. They are both based on models developed from statistical curve fitting of historical failure data, which may have been collected in the field, in-house or from manufacturers. Probably the most widely known and used reliability prediction handbook is MIL-HDBK-217. In the military standards (MIL-HDBK-217F, MIL- HDBK-251M MIL-HDBK 781A and MIL-HDBK 338B) one can get some of the best indications on corrective coefficients applicable to failure rates based on the various components. These are dependent on the conditions of use, on the temperature, information on reliability tests and information on “design reliability”. For example, the mathematical model of the overall
failure rate for a power MOSFET according to MIL-HDBK- 217F, is its basic failure rate multiplied by the particular Temperature, Application, Quality and Environment factors.
Figure 2: Non-critical derating plan. 22
Figure 3: Critical derating plan.
www.mouser.com January 2017
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