T.Zedníč ek, L.Marek, S.Zedníč ek AVX Czech Republic s.r.o., Dvorakova 328, 563 01 Lanskroun, Czech Republic Phone: +420 465 358 126 Fax: +420 465 358 128 e-mail: [email protected]ABSTRACT Traditional tantalum capacitors have been known for their excellent reliability, robustness and stable parameters. This is why conventional tantalum capacitors with counter MnO2 electrode are still a popular type of capacitor used for long life and high reliability applications. One of the downsides of the MnO2 electrode system is its higher ESR compared to the polymer type of tantalum capacitors. The multi-anode concept (i.e. use of more 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 reduce the height of the components, reduce manufacturing costs and self inductance ESL. The low ESL extend the working frequency range up to the common DC/DC switching frequency 250 - 500kHz (D case) New Low Profile Low ESL Multi-Anode „Mirror“ Tantalum Capacitor
A B S T R A C TTraditional tantalum capacitors have b en known for their excellent reliability,robustness and stable parameters. This is why conventional tantalumcapacitors with counter MnO2 electrode re still a popular type of capacitor usedfor long life and high reliability applica ions. One of the downsides of the MnO2
electrode system is its higher ESR co pared to the polymer type of tantalumcapacitors. The multi-anode concept (i. . use of more node elements within onecapacitor body) significantly reduces SR and is an ideal choice for mostdemanding applications such as serv rs and high power telecommunicationboards. This paper describes a novel ulti-anode configuration that has beendeveloped to reduce the height of the mponents, reduce manufacturing costsand self inductance ESL. The low ESL extend the working frequency range upto the common DC/DC switching freque cy 250 - 500kHz (D case)
New Low Pr file Low ESLMulti-Ano e „Mirror“Tantalum apacitor
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Introduction
One common trend in switch-mode powersupplies, micro-processors, and digital circuitapplications is the reduction of noise whileoperating at higher frequencies. In order tomake these possible components with lowEquivalent Series Resistance (ESR), highcapacitance and high reliability are required.One way to significantly reduce the ESR oftantalum capacitors has been a multi-anodeapproach where more anode elements areused within one capacitor body [ref.1- 6] – seeFig.1.
MnO2 technology provides excellent fieldperformance, environmental stability and highelectrical and thermal stress resistance over awide voltage range from 2.5 to 50 volts. Thesecapacitors are designed for operation intemperatures up to 125°C.
Fig.1. Multi-anode Construction
The overall surface area of a tantalumcapacitor anode, particularly its surface-to-volume ratio, is one of the key parameters thatdefine its ESR value - the higher the overallsurface area, the lower the ESR.
2a] single 2b] fluted anode 2c] multi-anode
Figure.2. Anode design in cross section
The single anode (Figure 2a) is the standardused for general capacitor designs because ofcost versus performance efficiency. A multi-anode design (Figure 2c) offers the lowestpossible ESR. Downside of this approach is ahigher manufacturing cost compared to asingle anode solution. The fluted anode design(Figure 2b) using standard chip assembly
processes is a compromise between low ESRand cost requirements. While the flute designis used in price sensitive low ESR designs,multi-anode concept has been used inapplications where low ESR and high reliabilityis required without compromises such astelecom infrastructure, networking, servers ormilitary/aerospace designs.
Apart from the mentioned differences betweensingle anode, multi-anode and flute anodeabove there are two additional advantages ofthe multi-anode concept: 1] Due to a betterthermal dissipation of multiple anodescompared to single anodes the multi-anodecapacitor can be loaded to higher continuouscurrent and it is also more robust againstcurrent surges where heat is dissipated moreefficiently. When compared to the singleanodes of the same case size the powerdissipation of conventional multi-anodes ishigher. 2] Compared to the single anode thevolumetric efficiency (the active zone) of multi-anodes is lower and that can lead to apresumption that multi-anodes can not reachthe same Capacitance times Voltage (CV)factor. In practise, thinner anodes are easier toprocess and better penetrated by the secondMnO2 electrode system, hence enabling theuse of higher CV tantalum powders andtherefore in the case of multi-anode capacitorsthe same or even better CV of the capacitor isachieved.
New Multi-anode Construction
Conventional tantalum multi-anodes on themarket mostly use three to five anodes insideone body in a vertical configuration as shownin Fig 3a]. This is practical from themanufacturing point of view, however from theESR standpoint this solution is inferior to ahorizontal layout – see Fig.3b] where thinnerflat anodes can reduce the ESR even further.The cost of the multi-anode design growsexponentially with number of anodes. Thethree anodes currently used in most designsare close to the optimum cost versus ESRratio.
a] vertical b] horizontal construction
Figure.3. Multi-Anode concept cross section
Multianode D case ESR
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25
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2 anodes 3 anodes 4 anodes
No of anodes
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R[m
Oh
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] horizontalvertical
The individual anodes in the vertical designconfiguration are connected to the secondelectrode by a silver glue epoxy to a secondelectrode lead frame. The same system isused in standard single anode capacitorshence the manufacturing technology is similarto the established process and no majorinvestment into new technology flow isrequired for the multi-anode design. Thehorizontal design on the other hand wouldrequire a “solution” to connection betweenthe anodes and costly modifications ofestablished technology. Hence to date thisdesign has not been used in volumeproduction of the single body multi-anodecapacitor. Horizontal designs are used moreoften in special applications by “stacking” oftwo or more finished capacitors together bysoldering or jigging systems into arrays ormodules.
ESR differences between horizontal andvertical configurations are shown in Fig. 4. Thisexample is based on theoretical calculation forD case and it is showing the estimation of ESRfor 2, 3 and 4 anode systems in horizontal andvertical configurations. It is can be seen thatthe two anode horizontal layout has a similarESR to the three anode system in verticalconfiguration. The ESR versus cost value ishowever better for the horizontal multi-anodestructure.
Compared to the horizontal the verticalconstruction has the disadvantage of limitedheight reduction potential which is currently at3.5 to 4.5mm. Today, this factor is increasingin importance when miniaturisation ofelectronics even in applications like telecominfrastructure or military is becoming an issue,where this has not been so in the past.
Figure.4. ESR of horizontal and vertical layout
A novel multi-anode construction has beendeveloped using two anodes in horizontal“mirror” configuration – see Fig.5]. The mirrorconstruction uses a modified lead frame shapewhere the lead frame is in the middle betweenthe two anodes. This configuration solves theconnection issues of the horizontal anodes andbrings the manufacturing modification cost toacceptable level.
cross section of 2.0mm height 7343case
3D internal construction
Fig.5. Multi-Anode “Mirror” Horizontal Design
The ESR level of the two anode mirror designis slightly inferior to the three vertical anodeequivalent however the cost versusperformance value is better (2-anode system ischeaper to make compare to 3-anodes).However, the main benefit achieved is that thisconfiguration enables the reduction ofcomponent height of multi-anode designsdown to 3.1mm 7343-31 D case size and even2.0 maximum height 7343-20 Y case size innear future. The other advantage of the mirrordesign is its symmetrical layout which helps toreduce self inductance ESL.
Figure 6. shows a comparison of 22F 35Vtypical ESR between the different internaldesign configurations. As described in theprevious chapter, the higher the surface area,the better ESR is achieved. Also the ESRdistribution range is much tighter in low ESRdesigns. Hence the low ESR parts arerecommended for circuits with a bank ofparallel capacitors due to a more equal loadshare among the individual capacitors. Itshould be also noted that there is no directcomparison in the case of vertical multi-anodeas it is available in the taller E case size onlycompared to the other designs made in the
lower D case size. Three vertical D case sizeanodes in the vertical multi-anode style wouldshow a higher ESR value compared to the Ecase size data available for this comparisononly.
Fig 6. Typical ESR of different internal anodedesign configurations
The other benefit of mirror design is itssymmetrical internal design. The symmetricalconstruction helps to compensate part of theinductance loop (Fig.7) and thus selfinductance ESL is lower compare to a classicallead frame with a pocket. Table 1. showstypical ESL values of a D case size mirror andsingle anode design capacitor. Catalogue ESLvalues of D case single anode design is 2.4nH,typical values are around 2.1nH. The mirrordesign ESL is about 1nH - half of theconventional self inductance. This movesresonance frequency of mirror multi-anodes tohigher values – see Fig.8. where resonancefrequency of mirror design was measured at500kHz, the single anode at 340kHz.Capacitance drop with frequency is lower incase of the mirror due to thinner anodes used.
Table 1, Fig.7 ESL of D330/4 capacitor inmirror design and single anode construction
The move of the resonance frequency in mirrordesign due to the lower ESL significantlyimprove its working range to today’s favoriteDC/DC converter switching frequency range250kHz – 500kHz.
Fig.8. Capacitance and ESbehaviour of mirror multanode D case 330F 4V
The other benefit of the mimproved power dissipatheat generated in anodecooled through leads anPCB pads. The coolingshown in Figure 9.
Fig.9a. cooling effect
Fig.9b. cooling effect
Mirror
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Thus the single anode D case capacitor iscapable to continuously dissipate 150mWpower compare to 255mW in case of mirror Dcase. This represent ripple current capability ofsingle design 1.0A (D 330F 10V 150m) withsignificantly higher 2.7A ripple in the case ofmirror design (D 330F 10V 35m).
The mirror type of horizontal multi-anodes canreach capacitance values in TPM D case from10F to 1000F in voltage 2.5 to 50V with ESRrange 25 – 140m. Especially high voltage 35and 50V capacitors will be attractive fortelecommunication application where designheight is becoming an important parameter.The capacitance values 10-22F ESR 65-140m on a single 35-50V capacitor aredifficult to attain within the 3.1mm maximumheight by any other technology.
Summary & Conclusion
A novel “mirror” design approach of horizontalmulti-anode tantalum capacitors has beendeveloped. The new construction excels in thefollowing fields:
- better low ESR configuration- lower profile – D case 7343-31
(3.1mm max height) with potentialdown to Y case 7343-20 (2.0mm)
- manufacturing cost lower compareto the conventional 3-verticalanode multi-anode design
- lower ESL (symmetrical design)expands significantly the workingfrequency up to 500kHz (D case)
- significantly higher ripple currentcapability
Solid Electrolytic Capacitor Assembly usingmirror approach has been filed as U.S. SerialNo.11/602,451 in the U.S. Patent andTrademark Office on November 20, 2006.
References
1] E.Reed, J.Marshall “18mOhms and Falling –New Ultra Low ESR Tantalum ChipCapacitors” CARTS USA 1999 New Orleanspp 133-141
2] J.Ladd, “Lowest Available ESR Conformally-Coated Multiple-Anode Tantalum Capacitor”CARTS USA 2000 California pp 228-233
