Low-ESR and fused tantalum capacitors

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23 January 2008 Passive Components

The market demand for solutions with higher bulk capacitance and lower ESR in both commercial and established reliability applications is continuing. These solutions require increased volumetric efficiency as well as reduced size. In established reliability applications, the voltage requirements within avionics and aerospace have remained fairly stable as common bus voltages from 5 V to 28 V. However, as system integration includes more high-speed ASICs and CPUs or data processing, there is an increasing trend towards low voltage applications that require regulated power in the 1,5 V to 3,3 V range.

For low-ESR applications, a new series recently incorporated into MIL-PRF-55365 was the CWR29 family from AVX. These include a wide range of CV options up to 50 V rated, with Weibull reliability grading and optional surge current test. The typical ESR performance of these parts is usually well within specification; for example a capacitor with a 330 μF /6 V rating will have a maximum limit of 180 mΩ, but typical performance well under 100 mΩ.

Having optimised the characteristics of discrete capacitors in medium power applications, it is sometimes necessary to use a bank of capacitors in parallel, or a parallel/series combination to reach the capacitance, ESR target and required voltage derating of some higher voltage bus lines (eg 28 V), or where the derating rules are conservative. A new series of parts that allows this is the stacked tantalum module. These can be configured in standard two, four, or six unit stacks with other custom configurations available. The advantage of using such stacks is that the manufacturer can use parts within set ESR limits and, matching the ESR in a stack, ensures that current sharing is better equalised.

Given the emphasis on testing and reliability, tantalum chip capacitors have an excellent track record in avionic and space applications, but the typical failure mechanism remains short circuit. For additional safety, AVX offers a series that includes a Weibull-grade reliability internal element matched to an internal fast-acting fuse. This gives the best of both options - established reliability performance and failsafe operation. It is offered as both a discrete capacitor and in module format (and all with standard dimensions). In the module version, each element is independently fused so that in the event of an element failure, there is still redundancy in the system.

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Low-ESR and fused tantalum capacitors

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