New developments in device construction are improving ESR for a host of applications
BY T. ZEDNICEK, L. MAREK, and S. ZEDNICEK
AVX Czech Republic
Lanskroun, Czech Republic
http://www.avx.com
One common trend in switch-mode power supplies, microprocessors, and digital circuit applications is reduced noise at higher operating frequencies. In order to make this possible, components with low ESR, high capacitance, and high reliability are required.
The overall surface area of a tantalum capacitor anode, particularly its surface-to-volume ratio, is one of the key parameters that defines its ESR value — the higher the overall surface area, the lower the ESR. One way to significantly reduce ESR in tantalum capacitors is to use a multi-anode approach, where more anode elements are used within one capacitor body.
Traditional approaches
Conventional tantalum capacitors with counter MnO2 electrodes are still a popular choice for long-life and high-reliability applications. MnO2 technology provides excellent field performance and environmental stability, as well as high electrical and thermal stress resistance over a wide voltage range from 2.5 to 50 V. Devices are designed for operation in temperatures of up to 125C.
However, one of the downsides of the MnO2 electrode system is its higher ESR when compared with polymer tantalum capacitors.
Anode options
Single-anode technology is the standard general-purpose choice due to its excellent cost vs. performance ratio. Multi-anode designs offer the lowest possible ESR, but the downside to this approach is that its manufacturing cost is higher than that of a single-anode solution.
Fluted anode designs — using standard chip assembly processes — is a compromise between low ESR and low cost. So, the fluted design is typically used in price-sensitive, low-ESR designs while multi-anode technology is suitable for applications where low ESR and high reliability is required, such as telecom infrastructure, networking, servers, or military/aerospace projects.
Multi-anode tantalum capacitors can significantly reduce ESR compared with single-anode devices.
Aside from the aforementioned differences, the multi-anode concept has two additional advantages:
1. Multiple-anode designs offer better thermal dissipation than single-anode devices, which means a multi-anode capacitor can be loaded to a higher continuous current; for the same reason, multi-anode capacitors are also more robust against current surges.
2. Compared to the single anode, the volumetric efficiency of multi-anode capacitors is lower, which can lead to a presumption that multi-anodes can not reach the same CV factor. In practice, thinner anodes are easier to process and better penetrated by the second MnO2 electrode system, enabling the use of higher CV tantalum powders, and therefore multi-anode capacitors achieve the same or even better CV levels.
Available multi-anode types
Conventional tantalum multi-anodes available on the market today typically use three to five anodes inside one body in a vertical configuration (see Fig. 1 ). This is practical from a manufacturing point of view, however from an ESR standpoint this solution is inferior to a horizontal layout, where thinner flat anodes will reduce the ESR even further.
Fig. 1. Multi-anode devices use two or more anode elements within one capacitor body.
New multi-anode devices
The cost of the multi-anode design grows exponentially with number of anodes. The three-anode design currently used in most designs is close to the optimal cost versus ESR ratio.
Individual anodes in the vertical design configuration are connected to the second electrode by silver glue epoxy to a second electrode lead frame. The same system is used in standard single-anode capacitors, hence the manufacturing technology is similar to the established process and no major investment into new technology flow is required for the multi-anode design.
The horizontal design, on the other hand, requires a new solution to the problem of connection between the anodes—resulting in costly modifications of established technology. Therefore, to date this design has not been used for a single body multi-anode capacitor in volume production. Horizontal designs are used more often in special applications by stacking two or more finished capacitors together by soldering or jigging systems into arrays or modules.
The difference in ESR performance between horizontal and vertical configurations is shown in Fig. 2 . This example is based on a theoretical calculation for D case capacitors. It shows that the two anode horizontal layout has a similar ESR to the three anode system in vertical configuration. However the ESR versus cost value is better for the horizontal structure.
Fig. 2. Horizontal and vertical configurations have similar performance but cost can be a deciding factor.
Compared to the horizontal construction the vertical design has a limited potential for height reduction—currently capacitors measure between 3.5 and 4.5 mm high. Today, this factor is increasing in importance when miniaturization of electronics even in applications like telecom infrastructure or military is becoming an issue, where this has not been so in the past.
A novel multi-anode construction has been developed using two anodes in a horizontal “mirror” configuration. The mirror construction uses a modified lead-frame shape, where the lead frame is positioned in the middle between the two anodes. This configuration solves the connection issues of the horizontal anodes and brings the manufacturing modification cost down to acceptable level.
The ESR performance of the twoanode mirror design is slightly inferior to the three-vertical-anode equivalent, however it is cheaper to make. The main benefit achieved by the mirror design is it enables multi-anode capacitors to be reduced in height, down to as low as 3.1 mm.
The other advantage of the mirror design is its symmetrical layout which helps to reduce self inductance (ESL). The symmetrical construction helps to compensate part of the inductance loop enabling ESL that is lower than a design using a classical lead-frame with a pocket.
The catalog ESL value for a D-case single-anode design is 2.4 nH, with typical values are around 2.1 nH. The mirror design ESL is about 1 nH – half that of the conventional design. This moves the resonance frequency of mirror multi-anodes to higher values (see Fig. 3 .)
Fig. 3. Graphs showing mirror design performance present (aA) capacitance drop vs frequency is lower than single-anode solutions, and (bB) ESR is slightly inferior.
The capacitance drop with frequency is lower in case of the mirror structure due to the use of thinner anodes. The change in resonant frequency of the mirror design due to lower ESL significantly improves its working range for today’s favorite dc/dc converter switching frequency range (250 to 500 kHz).
Another benefit of the mirror design is in its improved power dissipation capability (see Fig. 4 ) Heat generated in the anode by ripple current is cooled through leads and tantalum wire to PCB pads.
Fig. 4. Power dissipation improves for mirror designs (a) over single-anode devices (b).
Thus, while the single-anode D case capacitor can continuously dissipate only 150 mW, a mirror-construction capacitor of the same case size can handle 255 mW.
Mirror-type horizontal multi-anode capacitors currently reach capacitance values from 220 to 1,000 µF with voltages from 2.5 to 10 V and ESR from 25 to 35 mΩ. Further developments will extend the voltage range up to 35 and 50 V, making the new capacitors attractive for telecommunications applications where design height is becoming a crucial. Capacitance values of 10 to 22 µF and ESRs from 65 to 140 mΩ on a single 35 to 50-V capacitor are difficult to attain within the 3.1-mm maximum height by any other technology. ■
For more on tantalum capacitors, visit http://www2.electronicproducts.com/Passive.aspx.
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