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Large capacitance value tantalum capacitors for RF power applications in GSM wireless terminals

Posted: 09 Aug 2001     Print Version  Bookmark and Share

Keywords:power 

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Communications Design China 7 Conference Proceedings 231 Abstract The GSM wireless technology is based on a TDMA pulsed signal transmitted from the wireless terminal to the base sta- tion. The power of the signal, hence the quality of the transmis- sion, is based on several factors, one being efficient power man- agement. Efficient power management involves the use of load switching, which in turn improves talk time, standby time and battery life. Load switching uses large value capacitors for the RF power amplifier power supply. This may be achieved by using several standard surface mount tantalum capacitors in parallel.Although this solution uses standard parts, it may not be the most cost effective, and is certainly not the smallest. The alternative is the use of one conformal tantalum ca- pacitor. Conformal encapsulation technology is ideal for this application. It allows GSM phones to work on a single 10005F capacitor in a 7360, 3.5mm high package. Other versions use similar value in a 2mm high package. This paper covers the requirements for the capacitor, the standard and conformal solution and their related technologies. Future developments also shown demonstrate the ability of the conformal technology evolution to match closely the industry's requirements. Introduction The cellular phone market is now dominated by two main tech- nologies: CDMA and GSM. GSM stands for Global System for Mobile Communications. It was introduced in Europe over 10 years ago for wireless voice communications, and is now used all over the world not only for voice, but also data trans- mission.All cellular phones have a similar architecture that can be divided into digital and analog sections. The power manage- ment section is key to the operation of the phone. It supplies power to all other sections using different voltages and some- times switches the power off on a section when it is not needed. (The most obvious example is the backlit display that goes back to dark after a few seconds of pressing a key.) One section that consumes a lot of power is the RF ampli- fier, also called the PA(power amplifier). The way the PAworks is by transmitting a signal over an RF carrier frequency, usu- ally 800, 1800 or 1900MHz. The signal is transmitted over this Large capacitance value tantalum capacitors for RF power applications in GSM wireless terminals Jean L. Racine Director of Worldwide Marketing Vishay Sprague frequency only during a certain time.Alow frequency of 216Hz is used to synchronize the phone. The transmission cycle uses only one eighth of this cycle. Therefore, the transmission cycle, or pulse, only lasts 5785s every 4.6ms. during this cycle, the energy needed for the correct operation of the PA amounts to 4.3 volts at 1.5 to 2 Amperes maximum. Drawing this amount of energy from the battery while the rest of the phone is in operation can cause the battery to wear out very quickly. Phone designs use load switching to power the RF ampli- fier. Some designs use load switching on the battery so that a smaller capacitor supplies the energy to the rest of the phone while the battery powers the RF amplifier. This causes the bat- tery to be pulsed and reduce talk time and battery life. Load switching using a large capacitor to power the PA as the battery continues to power the other section of the phone is generally used in GSM phone designs. During the one-eighth of the cycle when the PAneeds power, it is supplied from a large capacitor acting as a reservoir. Dur- ing the rest of the cycle, or the seven remaining eighths of the cycle, the capacitor recharges to the appropriate level. This is usually done at slightly higher voltage (4.8 volts), but at a lower current (less than an Ampere). Requirements for the Bias Capacitor Requirements for the storage capacitor, sometimes called the bias capacitor, are as follows: 7 Operating voltage: 4.8 volts max 7 Current draw: 2 Amps max (at 4.3 volts) 7 Temperature: -400C to +700C 7 The capacitor must have a large enough capacitance to store the required amount of energy for the PA during the short non-transmit portion of the cycle. 7 The capacitor also needs to be large enough to insure flatness of the pulse for proper data transmission. The capacitor required for this function needs to be reliable at these voltage and current levels. It is likely that one of the following technologies will be used in that specific application: 232 Communications Design China 7 Conference Proceedings Miniature aluminum capacitors These types offer several thousands of microfarads of capacitance. This solution was used in early designs, but the size of today's phones prohibits the use of them in new de- signs. Molded tantalum capacitors These offer large capacitance in the E case package (EIA 7343H), but usually do not achieve more than 3305F of capaci- tance at 10 volt rated voltage. Besides, the E size package is becoming too high for some of the slimmer phone designs. In GSM applications, several of these capacitors must be used in parallel, adding to the size and the cost of this solution. Conformal coated tantalum capacitors This construction offers a 10005F at 6.3 volt rated voltage in a single R case package (EIA 7260). Although the capacitor is only rated at 6.3 volts, its reliability is as good or better than several 3005F 10 volt rated molded tantalum capacitors. This has been proven both by Vishay and the many customers that have used the device in this particular application. Tantalum technology The differences between the coated and molded technology lie in the encapsulation process and the termination system. All tantalum capacitors use an anode, or "pellet", made of pressed tantalum powder. The tantalum pentoxide (Ta2O5) is formed on the whole surface area of the pellet, using an anod- ization process. The thickness of the oxide, which is the dielec- tric, is proportional to the formation voltage. The formation voltage is dependent on the rated voltage of the final capacitor. The counter electrode, or cathode, is deposited using manga- nese nitrate solutions that decompose into manganese dioxide (MnO2) inside the capacitor, during the deposition process. Carbon and silver paste layers are added for better electrical and mechanical characteristics. At this point, the silvered pellet can be assembled either as a molded or a coated type device. As mentioned earlier, the most commonly used capacitors in the GSM power amplifier application are the molded 3305F 10v in an E case or the coated 10005F 6.3v R case capacitors. Below are the key characteris- tics of these two items for this application: Reliability The reliability for the 10005F 6.3v capacitor is calculated us- ing life test results on actual lots of capacitors. During the de- sign phase more than 600 parts from 13 different lots were tested at 85C and rated voltage (6.3v) for 2000 hours. No failures were found. Besides, the DC leakage current, the parameter that is most affected in poor reliability designs, remained very stable. The base failure rate ? is better than 10-6, or 0.1% per 1000 hours, when tested at 85C and rated voltage. Using the conditions of the GSM applications, the failure rate is then 2 FITS. These parts are also surge screened to avoid turn on fail- ures. The in rush current in tantalum capacitors may cause them to fail catastrophically. This has been largely discussed, and sometimes exaggerated at the expense of this technology. The test conditions ensure that the parts "sees" more current in the manufacturing and testing sequence than it will ever see in the application. This particular device will receive surge pulses up to 40~50 amperes, three times at rated voltage. This is enough to guarantee safe operations at lower voltages and currents. New market requirements Wireless communications developments are driven by data transmission. The GSM standard is now being upgraded to GPRS (General Packet Radio System), which uses several slots of the cycle, not just one like GSM. The capacitance needed to supply these energy levels is typically 20005F and may go up sometimes to 30005F or 40005F. The applications are, of course, cellular phones with data functions, Personal Digital Assistants (PDAs) with wireless functions, or add on devices for PDAs, PCMCIA wireless modems, LANs, etc. There is no end to the rapid growth of wireless technolo- gies. There is also no end to the search for smaller, lighter and thinner devices. These requirements translate into development of capaci- tors that are most likely to promote the coated technology. The molded technology does not adapt easily to the requirements of the market, because their sizes are limited to a few EIA con- figurations. In addition, the lead frames used for the termina- tions and the thickness required for the component walls limit the possibilities for low profile applications. Finally, some manu- facturing processes induce stress to the tantalum pellet that may be detrimental to the reliability of the device. On the other hand, the conformal technology offers the most efficient packaging solution for both high capacitance values and low profile. New coated capacitor ranges A new range of 2mm high packages has been introduced in 2000 to complement the 1.5mm high existing packages. The height increase makes it possible to obtain a lot more capaci- tance in a package that is still low pro- file by industry standards (PCMCIAs, cell phones). The form factor of these packages results in the lowest ESR of any non-polymer, low profile tanta- lum surface mount capacitor. More recently, two additional case sizes have been added to the low pro- file conformal offering. These are 14.5mm long by 7.4mm wide. The X case provides as much capacitance as the R case in a 2mm high package (10005F 6.3v and 15005F 4v). The Y case goes a step further with capaci- tance values of 15005F 6.3v and 22005F 4v), by increasing the height to 2.5mm in the same X case footprint. Communications Design China 7 Conference Proceedings 233 Conclusions The future wireless data communications devices will require low profile capacitors in higher capacitance values. For instance, the new GPRS protocol requires several times as much capaci- tance to power the RF amplifier than conventional GSM. This will become a real challenge for the molded technol- ogy, which is less flexible for new packages, especially thin ones. Conformal coated technology allows packages that can be 2mm high and still offer large capacitance values. The coated tantalum technology provides the highest CV per cc, the flattest sizes and lowest ESR for Manganese dioxide based cathode type devices. It offers the most suited compo- nents for the new portable wireless devices. Author's contact details Jean L. Racine Vishay Sprague 1754 Main Street Sanford, ME 04073 USA Phone: (1-207) 490 7295 Fax: (1-207) 490 7213 E-mail: jean.racine@vishay.com




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