SOLUTION FOR INCREASING CAPACITANCE DEMAND HIGH CV TANTALUM CAPACITORS

Transcription

1 SOLUTION FOR INCREASING CAPACITANCE DEMAND HIGH CV TANTALUM CAPACITORS T. Zednicek, AVX Czech Republic s.r.o., Dvorakova 328, Lanskroun, Czech Republic Phone: Fax: W.A.Millman, J.Gill AVX Limited, Long Road, Paignton, Devon TQ4 7ER, United Kingdom Phone: Fax: ABSTRACT The latest development of high CV Capacitors opens a way towards new applications and firmly positions tantalum capacitors against competing technologies, such as multilayer ceramic and aluminium electrolytic. Despite many studies into replacement of small case size tantalum by ceramic capacitors, a massive increase in demand for high CV tantalum capacitors has been seen. High CV ceramics are not the total answer to designers problems in applications such as mobile phones, disk drive, notebooks and many others. Designers are looking for reliable, high temperature and voltage stable noise-free capacitors for clear audio and video signal filtering. capacitors such as the 1µF at 6.3v rating in R case (85 [212M] equivalent), are an ideal space effective solution. At the other end of the scale, a huge increase in demand for large case size (E & V case) parts is taking place due to successful competition against electrolytic capacitors. Meanwhile, space/size critical applications, such as required for portable equipment, are driving the demand for low profile capacitors, downsizing to small 63 [168M] and even 42 [15M] microchip. This paper gives practical examples of how flexible tantalum technology can be in answering such customer driven demand. designers. The tantalum capacitor industry has shown a high level of flexibility in adopting to these recent trends leading to the development of new capacitor series during the last few years. The TPS Low ESR series answered the first power supply manufacturers demand a couple of years ago and has become today s favourite series. Continuous downsizing and IC packaging height reduction are the main application drivers for low profile tantalum capacitors. Hearing aids, pacemakers and other medical implantable applications are suited to new microchip tantalum capacitors. This series includes the highest capacitance ratings ever available in 63 and 42 dimensions for any technology. So far, high CV ceramics tantalum replacement series - have not seen the extended use predicted by many papers and studies. The experience of many audio/video equipment manufactures has lead them to return to the use of tantalum capacitors on all applications where clear signal filtering is required. The main reason is that the well-known piezo effect of ceramic materials can degrade the signal quality down to a customer noticeable level. In other applications, including automotive and portable, the temperature characteristic of high CV ceramic dielectrics means that they may not be suitable for tantalum replacement. High reliability, low inductance, high capacitance and low ESR are the key parameters for the latest generation of fast response processors. capacitors are recognised as a best option combining these requirements and occupy a superior position compared to aluminium capacitors used in the applications. INTRODUCTION The history of solid tantalum capacitors reaches back more than fifty years and its Surface Mount Technology package option has been available for more than fifteen years. Short design time, high performance in small sizes and light weight are more and more important factors for electronic

2 HIGH CV CERAMICS VERSUS TANTALUM CAPACITORS High CV formulations of barium titinate (the base ceramic material for most dielectric systems) exhibits a microphonic effect. For example, if one takes a Y5V capacitor and subjects it to a DC bias with a superimposed signal (e.g. 1kHz sine wave), the capacitor will start to sing. This mechanism is also reversible, which means a 1kHz external signal will cause generation of a 1kHz noise to the electrical signal. capacitors exhibit no such microphonic effect. Experiment AVX Application Laboratories in 3 applications have tested different behaviour of Ceramic and capacitors. The first application was designed to record signal / noise from the tested capacitor placed close to a piezo speaker. The piezo speaker was used as a microphone to record a signal from the tested capacitor under AC voltage (and with DC bias in the second application). The third application was just a comparison of AC signal filtering capabilities by the capacitors. Tested Capacitors: MLC ceramic 126 (A) 1 µf / 16 V, Y5V dielectric A case (126) 1 µf / 1 V Microphonic effect The capacitors abilities to resist an external hum / noise signal were tested by 1kHz Sine shape signal generated by a piezo speaker placed to within 1mm of Glue Generator Piezo transducer Glass board 1mm MLC cap. 1uF/16V (Y5V) or tantalum cap. 1uF/1V AC amplifier 3x Source without distortion (battery) 9V Memory scope Fig.1 Block diagram of microphonic effect measurement superimposed signal was measured by oscilloscope with an AC amplifier. Output signal in real voltage values were measured with ceramic and tantalum capacitors connected see figures 1. and 2. Fig.2. Output with ceramic capacitor 9VBIAS and 1kHz speaker signal Voltage (uv) Output level - 1Hz input sinus signal MLC capacitor 1uF/16V (Y5V) time (ms) Fig.3. Output with tantalum capacitor 9VBIAS and 1kHz speaker signal Voltage (uv) Output level - 1Hz input sinus signal capacitor 1uF/1V time (ms) Piezo effect The reciprocal physical mechanism to the microphonic effect is the piezo effect. The ceramic capacitor behaves like a piezo speaker if an AC voltage is applied. The dependence of the piezo effect on the DC bias was investigated in this application. Glue Generator with DC bias voltage +5V - 1Hz 1V/2V MLC cap. 1uF/16V (Y5V) Piezo transducer Low frequency milivoltmeter Glass board 1mm Fig.4: Block diagram of piezo effect measurement the capacitor. The capacitors were subjected to a 9v bias from a standard battery power supply and the

3 Fig5. Dependence of audio signal from ceramic capacitor to DC BIAS at a frequency of 1kHz Filtering The ceramic and tantalum capacitors were compared when filtering a simple sinusoidal signal of 4mV AC, frequency 1kHz superimposed on a 1v DC bias. Fig.6: Block diagram of filtering measurement Voltage (V) Output level from piezo senzor (mv) MLC cap. 1uF/16V - Y5V Bias voltage (V) MLC 1uF/16V Tant. 1/1V Input 1V DC/4mV AC 1kHz Time Output from MLC capacitor Output from Ta capacitor Fig.7: Filtering of 1kHz sine wave 1V BIAS by 1µ/16V ceramic and 1µ/1V tantalum capacitors Ceramic capacitors summary Generator with DC bias voltage 1V AC 4mV ef. 1kHz Memory scope Waveforms of AC voltage of 1V source with using different capacitors. 1Hz - 1V 1Hz - 2V The latest developments in computer applications, e.g. super fast processors and new fast responding memories, are enabling new features and applications for Notebooks, Mobile Phones, Disc Drives and many other industries. One of the main trends for future development is to improve Audio and Video quality. This demand calls for even smaller active or passive components to allow for more space on board for huge data storage/ transport/ processing units. However, small, close placed components may significantly degrade Audio/Video signal quality by being either a source or receiver of noise or hum. Board designers must carefully select components which do not degrade high quality Audio/Video signals. capacitors are ideal noise-free component for Audio/Video applications due to their high volumetric efficiency and resistance to noise and hum. Ceramic capacitors may cause noticeable degradation of signal quality if used in critical positions for processed signals. Harmful Microphonic and Piezo effects of ceramic capacitors 126 1µF / 1V Y5V to sine signals were experimentally confirmed by this paper. ALUMINIUM VERSUS TANTALUM CAPACITORS The latest development in aluminium capacitors has lead to the replacement of wet electrolyte by conductive polymer material. This change has significantly improved many parameters of wet aluminium capacitors including reliability, ESR, temperature and frequency behaviours. Despite this change, tantalum capacitors are still preferable in many capacitance hungry applications. Reliability capacitors have excellent reliability far superior to aluminium electrolytics because the tantalum dielectric has no known wear-out mechanism. Failure Rate vs OS-CON Infant Failure Period Life Period Wear out Period os-con Time Fig.8: versus life time comparison

4 Life was carried out at an independent ISO 9 approved test house to practically compare the reliability of tantalum and aluminium capacitors. Forty aluminium conductive polymer capacitors and forty tantalum capacitors were placed on each test. Table 1. below summarises the results. Qty Qty of failures i Initial Final Life: 1kHrs, 85 C,Ur 4 Life: 1kHrs, 125 C,Uc 4 DHSS : 1kHrs, 6 C, 95%r.h.,Ur 4 Qty Qty of failures conductive i polymer Initial Final Life: 1kHrs, 15C,Ur DHSS : 1kHrs, 6 C, 95%r.h.,Ur 4 Table1. Life test results of aluminium conductive polymer and tantalum capacitors Inductance Current tantalum capacitor design offers lower inductance as compared to conventional conductive polymer aluminium capacitors. Decoupling digital circuits with faster and faster transition times requires very low reaction time. Higher inductance can cause voltage overshoot. and aluminium capacitors rated at 47uF / 16v were tested as decoupling capacitors at 3.3v with load increasing from.3amps to 1.8Amps. 47/16 1MHz 3 nh 1 nh Table2. Typical 1MHz inductance of 47/16 capacitors TPSD476M16R15 Fig.1 Transient Load Response of tantalum capacitor Time base of both Fig.9 and Fig 1. records are 2ms/division and the scale is 5mV/division. Capacitor Summary Manufacturers are seeing increasing demands for product reliability, either to meet new legislation requirements or to maintain market position. capacitors are better suited to high MTBF or long life applications when compared to aluminium capacitors because tantalum capacitors have no known wear out mechanism, as mentioned above. Inductance is a critical parameter in applications such as decoupling digital circuits with fast transition times. Inductance affects the response time of the energy supply necessary for the application, as seen in Pentium TM processors where the response time needs to be very low. Higher inductance of aluminium capacitors can cause voltage overshoot. TANTALUM TECHNOLOGY FLEXIBILITY UNDER CUSTOMER DRIVEN DEMAND 'SPIKE 47µF 2V OS-CON The main driving factors of the electronics industry are high performance, portability and higher reliability. capacitors are a part of the supply chain for most electronic industries. The ability of tantalum technology to address the space/size limitations of end equipment illustrates how flexible this technology can be. Fig.9 Transient Load Response of aluminium capacitor

5 Space and Volumetric efficiency Mobile phones and notebook applications are currently the biggest drivers for size limited high performance applications. The tantalum industry has responded to this and, in the last few years, developed new product families to address these specific demands. Following IC packaging height reduction trends, D case footprint low profile products (including 2. mm high Y case and 1.5 mm high X series) have now been released. High CV tantalum powder development combined with new assembly techniques have enabled the industry to reach nearly the same capacitance in these new case sizes as was available only in D or E case size 2 years ago. A typical example is the 22µF, 1v rating, originally released in E case, but now available in the Y case (D case footprint with 2. mm height). Microchips, using new semiconductor-like assembly techniques, are another example of the ability of tantalum technology to address the demand for the smallest possible capacitor dimensions. Such tantalum capacitors with package sizes of 63, and even 42, previously dedicated to ceramic capacitors only, can offer the highest capacitance ever seen in these dimensions. Combined with the unique characteristics of tantalum dielectric to provide stable, reliable and clear noise-free filtering, Microchips create a space for new application ideas. For example, the next generation of many hearing aid, pacemaker and other human implantable devices was designed based on this new tantalum technology. high temperature, humidity and mechanical stress conditions. Extended reliability, usually for more than fifteen years, is an additional requirement under these conditions. Specially designed tantalum capacitors are available for such automotive industry requirements. Standard tantalum capacitor operation is defined in the temperature range from -55 to +125 C with basic reliability 1% per 1 hours. Design modification of tantalum capacitors has allowed the expansion of continuous operation temperature to +15 C and improved the basic reliability to.5% per 1 hours. CONCLUSIONS This paper investigated the main reasons for increasing interest for tantalum capacitors in alternative applications and addressed the question of why tantalum capacitors have not replaced by high CV ceramics and aluminium capacitors as predicted by many papers and analyses. The key advantages found are summarised below: 1] Versus ceramic technology: High volumetric efficiency Clear audio/video signal filtering 2] Versus aluminium technology: Higher reliability Lower inductance 3] For new designs: Flexibility in answering customer driven demand Low ESR GENERAL REFERENCES Clean power supply output for DC/DC converters has been the driver for low ESR versions of tantalum capacitors for the past several years. The filtering efficiency factor for a capacitor can be defined as the ESR per unit capacitance. All major tantalum capacitor manufacturers compete in special designs and new materials offering a best ratio between price and low ESR value to find an answer to the ever increasing demand for low ESR capacitors. Reliability J.Gill: Capacitor Technology Comparison, AVX Technical Publications, AVX Ltd, UK W.A.Millman: Application Specific SMD Capacitors, AVX Technical Publications, AVX Ltd, UK J.Cain: Comparison of Multilayer Ceramic and Capacitors, AVX Technical Publications, AVX Myrtle Beach, USA CARTS EUROPE 97 Seminar notes Capacitor Application in Switch Mode Power Supplies Automotive customers have special requirements for the design of electronics for under hood applications. The electronic circuits placed near to engines are faced with