Tantalum Polymer Capacitors Achieve Higher Voltage Ratings

Tantalum surface-mount capacitors have gained widespread favor for bulk decoupling use in both conventional and switch-mode power supplies (SMPSs) since their introduction more than 20 years ago. Today, tantalum surface-mount capacitors are primarily used in SMPSs across multiple industry segments, most often in applications that have space restrictions, long stable life expectancy and high-reliability requirements. The characteristics of high-volumetric efficiency, stable performance and the absence of a wear-out mechanism continue to drive their popularity in SMPSs, despite aggressive competition from other alternative dielectric materials such as aluminum and ceramic.

In addition to their benefits, tantalum capacitors have traditionally had two weaknesses — a susceptibility to ignition when they fail and higher equivalent series resistance (ESR) than capacitors based on other dielectrics. These drawbacks were overcome with the introduction of a new cathode material — an intrinsically conductive polymer — as a replacement for the conventional manganese dioxide (MnO₂) cathode. But while surface-mount tantalum capacitors built using the MnO₂ cathode could be used at voltages up to 28 V, the tantalums built using polymer cathode material were previously only usable up to 19 V.

However, recent advancements in polymer technology have permitted development of tantalum polymer capacitors for continuous duty at 20 V to 28 V, which enables them to address a wider range of power-supply input requirements. To assess the usability of the new tantalum polymer capacitors in power-supply applications, their electrical performance and reliability are compared against existing MnO₂ tantalum capacitors.

Tantalum Pros and Cons

All capacitor technologies have their advantages and disadvantages. Issues such as voltage coefficient and the potential cracking of high-capacitance ceramics or the dry-out concerns and incompatibility of aluminum electrolytics to reflow temperatures are weighted and compared against their advantages to arrive at a technology solution that best meets the needs of the power-supply design. Primary among the disadvantages of tantalum capacitors in SMPS use are the potential for an ignition failure mode and higher ESR when compared to some alternative dielectrics.

Both these disadvantages are linked to a single construction material within the tantalum capacitor, namely the use of MnO₂ as the cathode. High amounts of oxygen present in the MnO₂ cathode can provide a localized fuel source that, under the right failure conditions, can result in an ignition failure.

Also, since MnO₂ is a semi-conductive material, it is the largest contributor to the component's total ESR value. It is these two characteristics that most often lead power-supply designers to consider other dielectric solutions.

In the late 1990s, however, the undesirable features of tantalum surface-mount capacitors were overcome by the introduction of intrinsically conductive polymer as a replacement for the MnO₂ cathode. The use of conductive polymer offered a new material set 1000 times more conductive than MnO₂ with an absence of readily available oxygen that could potentially lead to an ignition failure.

With the risk of ignition failure addressed and ESR values significantly lower than any offerings coming from a traditional MnO₂-style tantalum capacitor, tantalum polymer capacitors rapidly gained popularity throughout the design community as engineers quickly took advantage of this new technology to replace MnO₂ tantalum capacitors and multilayer ceramic chip (MLCC) capacitors on the power supply's output rails.

Yet another advantage gained with the removal of MnO₂ was the improvement in the voltage derating requirements of tantalum surface-mount capacitors. To fully optimize the reliability of the tantalum dielectric (Ta_2O5), a voltage derating is recommended. Over many years of reliability analysis, the tantalum capacitor industry, in conjunction with reliability experts from the military sector, concluded that a 50% voltage derating of MnO₂ tantalum capacitors yielded acceptable reliability even in the most demanding applications such as aerospace.

Extensive studies concluded that the primary contributor to failures was damage induced on the dielectric during the board mounting process.^[1-2] This damage was the result of coefficient of thermal expansion mismatches in the material sets, which placed mechanical stresses on and produced fault sites in the dielectric. The physical properties of the MnO₂ play a role in this since the material is rigid and in direct contact with the dielectric, offering no protection from the expansion and contraction of the other materials around the anode.

When the MnO₂ was replaced with the softer and more elastic conductive polymer, researchers found that the dielectric condition after board mounting was much improved and the applied voltages could be greatly increased with no negative impact on predicted reliability.^[2] Over time, the recommended voltage derating for tantalum polymers was established at 20% voltage derating or less (depending on manufacturer and voltage rating). The predicted reliability of the tantalum polymer capacitor with a 20% derating was equal to that of a MnO₂ tantalum capacitor with a 50% derating.

While multiple improvements were realized with the replacement of MnO₂ with conductive polymer, tantalum polymer capacitors did have at least one weak point with regard to SMPS applications, which was the inability of manufacturers to produce a reliable design for working application voltages much above 19 V. This limited the use of tantalum polymer capacitors to the lower-voltage output applications of SMPS.

At the time tantalum polymer capacitors were introduced, MnO₂ tantalum capacitors were being safely used in voltage input applications up to 28 V. With an industry demand for higher-voltage tantalum polymer capacitors, manufacturers quickly began development activities to provide solutions for the 20-V to 28-V input voltages to replace MnO₂ and high-capacitance MLCCs with these higher-performance solutions. Despite industry pressures for a higher-voltage rating in tantalum polymer capacitors, technical challenges prevented capacitor manufacturers from delivering the higher-voltage ratings.

High Voltage Arrives

Today, that limitation has been overcome through advancements in polymer technology resulting in the release of a higher-voltage tantalum polymer device suitable for continuous duty at 20 V to 28 V. With this advancement achieved, many designers are now considering the advantages a tantalum polymer capacitor may offer over alternative solutions to address power-supply input needs.

To assess the relative advantages of the tantalum polymer capacitor, the designer must compare its performance characteristics to the current solution. Since all capacitor technologies and performance characteristics cannot be accounted for in this discussion, only the use of a tantalum polymer capacitor as a replacement for the commonly used MnO₂ tantalum capacitor will be explored. The topics will also be limited to reliability and electrical characteristics alone with a brief discussion on cost.

Reliability

In recent years, multiple studies have been conducted to quantify the reliability of tantalum polymer devices and compare them to their MnO₂ predecessors. These studies have reached similar conclusions.^[3-4] The polymer cathode systems are at least as reliable as the well-established MnO₂ cathode design when used within their recommended operating ranges.^[3-4]

To qualify the new high-voltage tantalum polymer capacitor, new reliability studies were conducted. One hundred components were placed on a conventional 85°C life test and charged at rated voltage (35 V) for 2000 hours. Initial measurements were taken immediately following board mounting of the devices. The devices were measured again after 250, 500, 1000 and 2000 hours of testing.

The reliability of the tantalum polymer capacitor can best be determined by assessing the dc leakage and ESR performance of the capacitor over the duration of the life test. Following 2000 hours of testing, the dielectric showed no signs of degradation, as evidenced by the reduction in dc leakage (Fig. 1). In addition, the ESR performance was found to be stable throughout the 2000 hours of testing (Fig. 2).

This study demonstrates that the high-voltage tantalum polymer capacitor has reliability characteristics at least as good as those of currently existing MnO₂ and tantalum polymer capacitors, thus delivering stable and reliable performance well beyond the life expectancy of any electronic hardware when used at the recommended derated condition.

Electrical Performance

The primary goal for developing an intrinsically conductive polymer formulation for higher-voltage tantalum polymer capacitors was to offer power-supply designers a device with improved performance characteristics over currently used solutions. While the benefits of a “non-ignition” failure mode resulted from the use of conductive polymers as a replacement for MnO₂ , the main objective was to reduce ESR and improve capacitance over frequency. To quantify the benefits of this technology, a comparison study was conducted using several MnO₂ tantalum capacitor technologies commonly used for higher-voltage applications such as power decoupling on 20-V to 24-V power-input rails.

Component selection for this comparison was based on the highest capacitance value and lowest ESR commonly available in a 50-V MnO₂ tantalum surface-mount design. Today, the highest CV ratings commonly available (for the targeted voltage range) in a standard commercial series design offers up to 15 µF of capacitance in a 7343-43 (7.3-mm × 4.3-mm × 4.3-mm) package size with advertised maximum ESR limits of around 700 mΩ. Low-ESR MnO₂ designs are also commonly available with ESR offerings as low as 200 mΩ. In addition, a significantly more expensive multi-anode tantalum (MAT) design of the same capacitance value and case size (but whose internal construction consists of three thinner anodes to reduce ESR) was offered by several manufacturers with an ESR limit of 75 mΩ.

Due to the polymer device's ability to perform more reliably closer to its rated voltage, a 35-V-rated polymer component built with the high-voltage polymer process with a maximum ESR limit of 100 mΩ was selected. The advantage of using a lower-rated-voltage polymer device yielded the additional benefit of a much smaller package size (low-profile 7343-19). The table summarizes the list of components that were selected for this evaluation.

ESR vs. Frequency

To establish a baseline for ESR comparison of the four component types, initial ESR measurements were taken from 10 kHz to 1 MHz. The resulting ESR measures (Fig. 3) demonstrated the advantages of the polymer cathode when compared to the commercial and low-ESR MnO₂ devices. In addition, the polymer design was found to have only slightly higher ESR than the much larger and more costly MAT MnO₂ design when operating at frequencies below 30 kHz and showed no significant difference in ESR at frequencies above 30 kHz. These results show the superior performance characteristics of the tantalum polymer device at common SMPS frequencies of 200 kHz to 800 kHz.

ESR vs. Temperature

Both MnO₂ and conductive polymer have a negative coefficient of resistivity to temperature (ESR increases with decreasing temperature). While this may be contradictory to the way in which metals conduct themselves, the reason for this negative coefficient is due to the semiconductive nature of these materials, which is a more dominant factor than the tantalum metal itself. While the use of polymer in place of the MnO₂ has slightly reduced this impact at cold temperatures (Fig. 4), the polymer's negative coefficient to temperature still outweighs the metallic structure and thus delivers a similar ESR response to temperature.

Capacitance vs. Frequency

The capacitance behavior was analyzed to determine the effect of frequency on capacitance. As shown in Fig. 5, the commercial and low-ESR MnO₂ technologies lose 67% and 40% of capacitance, respectively, at around 300 kHz, while the MAT device loses only 14% of initial capacitance. The polymer device demonstrates a capacitance response similar to that of the MAT device with only a 13% drop in capacitance at 300 kHz.

Capacitance vs. Temperature

The behavior of capacitance over temperature remains unaltered by the use of polymer in place of the MnO₂ structure since the relationship of capacitance to temperature relates to the dielectric, which is unchanged. The behavior of this dielectric correlated to a negative coefficient (capacitance is reduced as temperature is decreased).

As shown in Fig. 6, the slope of each component selection remains consistent. Minor variations can be detected between each of the four products; however, these variations relate more to the differences in tantalum particle size used within each design, which can vary between manufacturers.

Capacitance vs. Voltage

Unlike some dielectrics such as ceramic, tantalum capacitors do not experience capacitance loss with voltage and maintain the same capacitance within their voltage range. So when bias is applied to a tantalum capacitor, the user can expect to have the nameplate capacitance at all voltage levels within its voltage rating. This characteristic remains unchanged regardless of the cathode material selected.

Response of dv/dt

With the assessment of component performance completed, one can determine how this technology would be of benefit on higher-voltage power-supply input rails when compared to MnO₂ technologies. The improvements in ESR and capacitance roll-off can be viewed in a time domain as shown in the dv/dt plot in Fig. 7.

Using the same four tantalum capacitor technologies as before, a dv/dt plot was constructed to demonstrate this element. The dv is expressed as volts per ampere, as the current is another independent variable with this response. The dt is expressed in microseconds. As can be seen, there is no discernable difference in dv between the polymer and multi-anode MnO₂ device up to and beyond 90 µs. However, the commercial and low-ESR MnO₂ technologies demonstrate a more rapid decline in voltage almost immediately.

In looking at a time interval of 30 µs, it can be seen that the MAT MnO₂ and polymer capacitors experience a voltage drop of around 2.5 V/A. However, the commercial MnO₂ and low-ESR MnO₂ capacitors experience a dv of 4.3 V/A and 3.2 V/A, respectively, within the same time domain.

By targeting a specific application need, a piece count assessment can be conducted. For this exercise, a dv/dt of less than 1 V/A per 30 µs was selected. To maintain this requirement, the piece count of each capacitor technology was increased until the dv/dt was met. As shown in Fig. 8, the minimum piece count necessary to maintain this dv/dt was 5x commercial MnO₂, 4x low-ESR MnO₂, 3x MAT MnO₂ and 3x polymer capacitors.

Cost Factors

Considering that much of the cost associated with tantalum surface-mount capacitors comes from the tantalum itself, it can quickly be concluded that the use of fewer capacitors and a smaller case size will translate into a lower total-cost solution. As shown in the table, the use of the 15-μF part with a MnO₂ design required the use of a 7343-43 case size versus the use of a 7343-19 case size in a polymer design (55% less package volume).

In Fig. 8, the assessment of a specific dv/dt application need of 1 V per 30 µs has also shown the piece count reduction that may occur when replacing MnO₂ with polymer. While the added cost of polymer processing versus MnO₂ processing does shift the manufacturing costs and therefore the selling price of the polymer capacitor, many designers have concluded that in their own unique power-supply applications, the use of the high-voltage polymer part has translated into a lower-cost solution over currently used MnO₂ devices. In addition to cost, the use of the tantalum polymer capacitor has also translated into more board space, the potential for a lower-profile power-supply design and superior performance over currently utilized capacitor technologies.

What's Next

The initial release of a high-voltage tantalum polymer capacitor was limited to a 15-μF, 35-V design in the lower-profile package design with an ESR limit of 100 mΩ to 125 mΩ. Future products currently in development include higher-capacitance values (22 μF and 33 μF) in larger case sizes (7342-28 case sizes) and lower ESR values (45 mΩ to 60 mΩ). These efforts will focus on 35-V-rated devices intended for use in 20-V to 28-V power-supply applications.

Looking even further ahead, there also are development activities to deliver higher-voltage ratings, which could potentially lead to the use of tantalum polymer capacitors in power-supply applications with operating voltages as high as 46 V.

References

1. Prymak, J. “Derating Differences in Tantalum-MnO2 vs. Tantalum-Polymer vs. Aluminum-Polymer,” 2003 CARTS Conference.

2. Prymak, J. “Improvements with Polymer Cathodes in Aluminum and Tantalum Capacitors,” IEEE 2001-APEC Conference 2001.

3. Reed, E. “Characterization of Tantalum Polymer Capacitors,” NASA Electronic Parts and Packaging Program, NEPP Task 1.21.5, Phase 1, FY05.

4. Reed, E. “Characterization of Tantalum Polymer Capacitors,” NASA Electronic Parts and Packaging Program, NEPP Task 1.21.5, Phase 2, 2006.