- THE MAGAZINE
AbstractTraditional tantalum capacitors are known for their excellent reliability, robustness and stable parameters. This is why conventional tantalum capacitors with counter MnO2 electrodes are still a popular choice for long-life and high-reliability applications. However, one of the downsides of the MnO2 electrode system is its higher equivalent series resistance (ESR) when compared with polymer tantalum capacitors. The multi-anode concept (i.e., the use of several node elements within one capacitor body) significantly reduces ESR and is an ideal choice for most demanding applications, such as servers and high-power telecommunication boards. This paper describes a novel multi-anode configuration that has been developed to lower the height of the components and reduce the ESL and manufacturing costs. This approach will be compared to standard single-anode designs.
IntroductionOne common trend in switch-mode power supplies, micro-processors and digital circuit applications is the reduction of noise while operating at higher frequencies. In order to make this possible, components with low ESR, high capacitance and high reliability are required. One way to significantly reduce the ESR of tantalum capacitors is to use a multi-anode approach in which more anode elements are used within one capacitor body (see Figure 1).1-6
MnO2 technology provides excellent field performance, environmental stability, and high electrical and thermal stress resistance over a wide voltage range (from 2.5 to 50 volts). Devices are designed for operation in temperatures of up to 125°C. The overall surface area of a tantalum capacitor anode, particularly its surface-to-volume ratio, is one of the key parameters that define its ESR value; the higher the overall surface area, the lower the ESR.
Aside from the aforementioned differences between single-, multi- and fluted-anode capacitors, the multi-anode concept has two additional advantages:
- Because the multiple-anode design provides better thermal dissipation than the single-anode, 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. When compared to the single-anodes of the same case size, the power dissipation of conventional multi-anode devices is higher.
- Compared to the single-anode design, the volumetric efficiency (the active zone) of multi-anode capacitors is lower, which can lead to a presumption that multi-anodes cannot reach the same capacitance voltage (CV) factor. In practice, thinner anodes are easier to process and better penetrated by the second MnO2 electrode system, which enables the use of higher CV tantalum powders and allows multi-anode capacitors to achieve the same or even better CV levels.
New Multi-Anode ConstructionMost conventional tantalum multi-anodes available on the market today use three to five anodes inside one body in a vertical configuration, as shown in Figure 3a. This is practical from a manufacturing point of view but inferior to a horizontal layout where thinner, flat anodes will reduce the ESR even further (see Figure 3b). The cost of the multi-anode design grows exponentially with the number of anodes. The three-anode configuration currently used in most designs is close to the optimum cost vs. ESR ratio.
The individual anodes in the vertical design configuration are connected 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, which can result in costly modifications of established technology. To date, this design has therefore 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 through soldering or jigging systems into arrays or modules.
The ESR performance of the two-anode mirror design is slightly inferior to the three vertical anode equivalent, but it is cheaper to make. However, the main benefit achieved by this new mirror design is that the configuration enables multi-anode capacitors to be reduced in height down to 3.1 mm for the 7343-31 D-case size and, in the very near future, even 2.0 maximum height for 7343-20 Y-case sizes. The other advantage of the mirror design is its symmetrical layout, which helps to reduce self inductance (ESL).
Thus, while the single-anode D-case capacitor has the capability to continuously dissipate only 150 mW, a mirror construction capacitor of the same case size can handle 255 mW. This represents a ripple current handling capability of 2.7 A; the single-anode design handles only 1.0 A (D 330 µF 10 V 150 mΩ).
Mirror-type horizontal multi-anode capacitors currently reach capacitance values in TPM D-case of 220 µF to 1000 µF, with voltages ranging from 2.5 to 10 V and ESR from 25-35 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 parameter. Capacitance values of 10-22 µF and ESR performance of 65-140 mΩ on a single 35-50 V capacitor are difficult to attain within the 3.1 mm maximum height by any other technology.
Summary & ConclusionA novel mirror design approach for horizontal multi-anode tantalum capacitors has been developed. The new construction excels in the following fields:
- Better low-ESR configuration
- Lower-profile D-case 7343-31 (3.1 mm maximum height) with potential down to Y-case 7343-20 (2.0 mm)
- Reduced manufacturing costs
- Lower ESL (symmetrical design) significantly expands the working frequency up to 500 kHz (D-case)
- Lower ESL is achieved on the standard footprint without the need for PCB layout change
- Significantly higher ripple current capability
For additional information, contact AVX Czech Republic, s.r.o., Dvorakova 328, 563 01 Lanskroun, Czech Republic; (420) 467-558-111; fax (420) 467-558-128; e-mail email@example.com; or visit www.avx.com.