This article describes a novel multi-anode configuration that has been developed to lower the height of the components and reduce the ESR and manufacturing costs. This approach will be compared to standard single anode designs. The 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 Equivalent Series Resistance (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 where more anode elements are used within one capacitor body (Figure 1). MnO2 technology provides excellent field performance, environmental stability and high electrical and thermal stress resistance over a wide voltage range from 2.5 to 50V. 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. The single anode is the standard general-purpose design due to its excellent cost versus performance ratio. The multi-anode design offers the lowest possible ESR; however the downside to this approach is a higher manufacturing cost compared to a single anode solution. The fluted anode design using standard chip assembly processes is a compromise between low ESR and low cost. So the flute design is used in price-sensitive, low ESR designs, and the multi-anode concept has been used in applications where low ESR and high reliability is required without compromise, such as telecom infrastructure, networking, servers or military/aerospace projects. Aside from the aforementioned differences between single anode, multi-anode and flute anode capacitors, the multi-anode concept has two additional advantages:
1) Because the multiple anodes design has a 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.
2) Compared to the single anode the volumetric efficiency (the active zone) of multi-anode capacitors is lower which can lead to a presumption that multi-anodes can not reach the same Capacitance Voltage (CV) factor. In practise, 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.
New multi-anode construction
Conventional tantalum multi-anodes available on the market today mostly use three to five anodes inside one body in a vertical configuration. 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. The cost of the multi-anode design grows exponentially with the number of anodes. The three anodes design currently used in most designs is close to the optimum cost versus ESR ratio. The 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.
Horizontal layout
The difference in ESR performance between horizontal and vertical configurations is shown in Figure 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.
Compared to the horizontal construction the vertical design has the disadvantage that it only has a limited potential for height reduction -- currently capacitors measure between 3.5 to 4.5mm high. Today, this factor is increasing in importance when miniaturisation 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 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.
Symmetrical layout
The ESR performance of the two anode mirror design is slightly inferior to the three vertical anode equivalent, however it is cheaper to make. However, the main benefit achieved by this new Mirror design is that this configuration enables multi-anode capacitors to be reduced in height down to 3.1mm for the 7343-31 D case size and even 2.0 maximum height for 7343-20 Y case sizes in the very near future. The other advantage of the mirror design is its symmetrical layout which helps to reduce self inductance (ESL). Figure 3 compares the ESR performance of a typical 22microFarads, 35V capacitor using different internal design configurations. As described above, the greater the surface area, the better the ESR performance. Also the ESR distribution range is much tighter in low ESR designs. Hence, low ESR parts are recommended for circuits with a bank of parallel capacitors due to a more equal load share amongst the individual capacitors. It should be also noted that there is no direct comparison in the case of vertical multi-anode as it is available in the taller E case size only, compared to the other designs whach are available in the lower D case size. Three vertical D case size anodes in the vertical multi-anode style would show a higher ESR value compared to the E case size (data available for this comparison only). The other benefit of mirror design is its symmetrical internal design, which was mentioned briefly earlier. The symmetrical construction helps to compensate part of the inductance loop, therefore ESL is lower than aa design which uses a classical lead frame with a pocket. The catalogue ESL value for a D case single anode design is 2.4nH -- typical values are around 2.1nH. The mirror design ESL is about 1nH -- half that of the conventional design. This moves the resonant frequency of mirror multi-anodes to higher values (see Figure 4) which measures the resonant frequency of the mirror design at 500kHz, while the single anode is 340kHz. The capacitance drop with frequency is lower in case of the mirror structure due to the thinner anodes that can be used.
Power dissipation
The change in resonant frequency of the mirror design due to the lower ESL significantly improves its working range for today's favourite DC/DC converter switching frequency range 250kHz to 500kHz.
Another benefit of the mirror design lies in its improved power dissipation capability. Heat generated in the anode by ripple current is cooled through leads and tantalum wire to PCB pads. Thus while the single anode D case capacitor has the capability to continuously dissipate only 150mW, a mirror construction capacitor of the same case size can handle 255mW. This represents a ripple current handling capability of 2.7A -- the single anode design handles only 1.0A (D 330microFarads 10V 150mOhms). Mirror type horizontal multi-anode capacitors currently reach capacitance values in TPM D case of from 220microFarads to 1000microFarads with voltages ranging from 2.5 to 10V and ESR from 25 to 35mOhms. Further developments will extend the voltage range up to 35 and 50V, making the new capacitors attractive for telecommunications applications where design height is becoming a crucial parameter. Capacitance values of 10 to 22microFarads and ESR performance of 65 to 140mOhms on a single 35 to 50V capacitor are difficult to attain within the 3.1mm maximum height by any other technology.
Conclusion
A 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 (3.1mm max height) with potential down to Y case 7343-20 (2.0mm)
reduced manufacturing costs
lower ESL (symmetrical design) expands significantly the working frequency up to 500kHz (D case)
lower ESL is achieved on the standard footprint without need for PCB layout change
significantly higher ripple current capability
A patent covering the mirror assembly of solid electrolytic capacitors has been filed as U.S. Serial No.11/602,451 in the U.S. Patent and Trademark Office.
Hall B6.624
Figure1: Multi-anode construction
Figure 2: ESR of horizontal and vertical layout
Figure 3: Typical ESR of different internal anode design configurations
Figure4: Capacitance and ESR versus frequency for mirror multi-anode and single anode D case 330F 4V capacitors
