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Extending the operating temperature of DC-DC converter modules by understanding their components

April 11, 2011 | Ann-Marie Bayliss | 222902522
Ann-Marie Bayliss, Product Marketing Manager, Murata Power Solutions explains how to extend the operating temperature of DC-DC converter modules by understanding the mechanisms behind component derating.
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As power density increases, the internal operating temperature of key DC-DC converter components potentially becomes a major barrier to extended temperature operation. At the same time, users are increasingly keen to dispense with cooling fans that require regular maintenance. Understanding the mechanisms behind component derating allows judicious selection that ensures reliable DC-DC converter operation in high ambient temperature environments such as telecoms racks and server farms.

In recent years, with geometric growth in logic density, the general trend for integrated circuits has been towards ever-lower operating voltages that speed switching while limiting on-chip power dissipation to acceptable levels. While much peripheral logic operates at 3.3 V - itself a step down from the traditional 5 V level that many industrial users still prefer - many processor and FPGA cores operate at much lower levels, with 1.8 V being common and lower levels becoming more usual. The combination of denser logic, faster switching speeds, and lower operating voltages creates the need for DC-DC converters that handle increasing load current levels, in turn implying more heat generation from resistive and semiconductor volt-drops.

In distributed-power architectures, the converter must also accommodate increasingly large step-down ratios that can pose challenges to conversion efficiency. In large systems, designers are adopting system-level modelling techniques to establish the optimal physical points for down-converting 24 VDC and 48 VDC distribution rails to the target load voltages and to help manage thermal loads. While variable-speed fans are an extremely efficient cooling tool, avoiding their use cuts noise and simplifies equipment maintenance schedules that range from cleaning filters to replacing units before the onset of bearing failure. In many instances, specifying a DC-DC converter that can operate reliably and with a long useful life in elevated temperatures eases thermal management concerns and may dispense with the need for fans.

Appreciate component derating characteristics

The question then becomes, how can an equipment designer confidently specify a DC-DC converter for extended temperature operation, knowing that the underlying converter design issues have been addressed? Most engineers will know a rule-of-thumb that states that for each 10C decrease in operating temperature from a components maximum rating, there is an approximate halving of its failure rate during its useful life. The origins of this guidance lie within the Arrhenius equation that classically quantifies chemical reaction times at varying temperatures. This model has been empirically shown to apply equally well to the diffusion and migration processes that apply to various classes of electronic components, providing a solid basis for predicting mean-time-to-failure due to temperature rises.

Many components require derating above about 75C, but the key to design success is to appreciate the mechanisms that affect different component classes and to specify parts according to the environmental stresses that they will endure. For instance - and possibly the best example for visualising chemistry under electrical stress - the lifetime of electrolytic capacitors shows a clear correlation between operating temperature, electrical stress, and the rate of electrolyte diffusion that this generic lifetime prediction formula demonstrates:

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