Selecting NbO capacitors

Relatively new, niobium oxide is giving traditional aluminum electrolytics a run for their money in audio applications

BY CHRIS REYNOLDS
AVX
yrtle Beach, SC
http://www.avx.com

How does taking your through-hole, aluminum-capacitor-based PCB design and using niobium oxide capacitors to convert it to a smaller SMT assembly sound? Pretty good, if the latest feedback from some audio system manufacturers is considered.

You have to go back some time into the archives to find a Pease Porridge article analyzing the sound characteristics of aluminum versus other capacitor technologies in audio circuits. The conclusion was that, while there were no differences in scope traces or measured characteristics, aluminum capacitors gave a smoother auditory experience.

Now low-ESR niobium oxide electrolytic capacitors are gaining ground on aluminum. Recent trials by leading Japanese audio manufacturers concluded that very good sound quality is achievable even with low-capacitance ratings in the 10-µF range, while 100-µF devices readily deliver high-end sound quality.

Subjective audiophile analysis aside, a lot of hard measurables also support converting from aluminum-leaded to niobium oxide SMT for a wide range of power applications including:

Combined high capacitance, low ESR, and low residual inductance (ESL)No issue with low ac rippleImproved stability with time and temperatureWider temperature and humidity environmental rangeNiobium oxide capacitors in audio applicationsEstablished SMT EIA standard sizesNo issue with vibration or shockRoHS compliant, high temperature reflow capableEnhanced reliabilityImproved ripple performance.

When comparing the performance of aluminum verses niobium oxide in power supplies, ESR vs. frequency is a key factor. In many applications, niobium oxide capacitors can replace aluminum at a lower capacitance value with equivalent filtering.

For power distribution and ASIC decoupling in motherboards, low inductance is also important. In situ analysis shows that upgrading from a through-hole to SMT design gives an immediate ESL improvement, which can be translated into both transient load changes and ripple handling.

Another characteristic of niobium oxide capacitors is that they can be operated with minimum voltage derating (see reliability section below), so a 6.3-V-rated device is suitable for use on 5.5-V output rails. Coupled with the availability of low voltage ratings designed specifically for core applications, the capacitance can be maximized further, with ratings to 680 µF available for 2.5-V applications.

Improved stability

In terms of operation, standard aluminum has an operating range of 40 to 85C with long-life, standard-reliability applications restricted to 105C max. Niobium oxide capacitors, on the other hand, are operational at rated voltage from 55 to 85C and to 125 max with derating.

Parametrically, over their operational temperature range, aluminum electrolytic capacitors will have a maximum capacitance change (∆C) of 20% at 40C and 20% at 105C, typically performing 10% to 15%. Niobium oxide has tighter temperature characteristics over a wider range, with ∆C of 10% at 55C and 12% at 125C, but typically well within these limits at 5% over the range.

Another temperature related parameter is ESR. In niobium oxide capacitors, ESR actually decreases at elevated temperatures, providing improved filtering in power applications. Importantly, the materials used in niobium oxide capacitors have essentially no wear-out mechanism, so these parameters will remain within limits over time.

Temperature and humidity

A critical part of the end user manufacturing process is PCB attachment, typically by reflow solder or wave solder. Niobium oxide technology is based on a solid electrolyte system packaged in molded epoxy with a compliant lead frame.

This construction can withstand multiple Pb-free reflow profiles with a moisture-sensitivity-level rating of 1, meaning that no desiccant packaging or special handling is required on the SMT assembly line. Niobium oxide capacitors use niobium pentoxide dielectric, a sister material to tantalum, with identical package assembly into standard EIA case sizes: A (3216), B (3528), C (6032), D (7343) and E (7343H).

Enhanced reliability

With a nominal reliability of 0.2% /1000 hours, the niobium oxide failure-rate specification is a factor of five times lower than commercial aluminum electrolytic. Another advantage is the low specific heat index of the anode material, which allows it to dissipate more electrical power without overheating. However, a concern with all components is what happens when you take them over the edge.

This is where niobium oxide provides another advantage. Using niobium oxide as the base material for the capacitor anode results in a benign failure mode compared to other electrolytic technologies. If overstressed up to rated voltage, induced breakdown is typically resistive, so parametric leakage may increase, but the part can still operate as a capacitor in the circuit. In fact, niobium oxide is the only electrolytic technology that has a non-short-circuit failure mode due to electrical stress.

Back to audio

There are many technical requirements for capacitors used in audio (that is, low-frequency/low-noise) circuits, many of which have been discussed above. In these applications, high fidelity and noiseless audio filtering is needed, but typically at lower capacitance values than required for power filtering. This opens up additional capacitor technologies, including multilayer ceramic chip.

Typically, the midrange capacitance values needed require Class II ceramic materials, for example, X7R, which can have a piezo effect resulting in electrical noise in the circuit. Niobium oxide technology, on the other hand, does not suffer from this piezo effect, and also has the advantage of retaining full capacitance with applied dc bias (no voltage coefficient). ■

For more information on selecting capacitors or multimedia and entertainment, visit http://electronicproducts-com-develop.go-vip.net/passives.asp