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Increased voltage capability of tantalum polymer capacitors opens up new applications

Mon, 04/02/2012 - 4:55am
Martin Bárta, Field Application Engineer, AVX
Tantalum polymer capacitors -- widely used for more than 15 years -- have benefited from significant increases in CV and reductions in ESR. However, due to the maximum achievable breakdown voltage (VBD) of such capacitors, the working voltage of tantalum-polymer capacitors has been limited to around 20V so far. Even increasing the forming voltage and thus increasing dielectric thickness has not resulted in a significant improvement and the maximum breakdown voltage has remained low. Now, with the introduction of new polymer materials and process techniques, the maximum achievable breakdown voltage has been increased, thus extended the voltage range, creating new opportunities for tantalum polymer capacitors capable of offering high capacitance, low ESR and medium range voltage solutions.

Figure 1 presents a roadmap for the development of the next generation of high voltage tantalum polymer capacitors. Recently released 63V and 75V rated tantalum polymer devices have extended the range and further developments suggest that devices with voltages as high as 125V will be introduced to mass production during 2012. 

Figure 1. Roadmap showing next generation of tantalum polymer capacitors

The increase in uniform distribution of breakdown voltage is a key parameter that reflects an enhanced ability to withstand higher working voltage. An example can be seen in Figure 2 where 3.3?F, 125V rated tantalum polymer capacitors are shown to achieve VBD values higher than 220V. 

Figure 2. VBD distribution of 3.3?F 125V capacitor samples

Polymer technology brings benefits such as a significant reduction in ESR thanks to the higher conductivity of organic polymer (the second electrode is a major contributor to ESR) - as well as reduced ignition failure mode.

63 and 75V rated tantalum polymer capacitors
The release of 63V and 75V tantalum polymer capacitors significantly increases the working range of such parts and opens the technology up to a number of new applications. The combination of high capacitance, high voltage, small case size and low ESR may enable the development of new generation power supplies. An example of ESR versus frequency characteristics of the newly-released, E case 10?F 63V tantalum polymer capacitors is shown in Figure 3. The ESR median value is measured at 100kHz, 25degC. 

Figure 3. ESR median vs frequency, tantalum polymer 10?F 63V capacitor

Due to the ability of polymer capacitors to withstand surge currents, derating can be maintained at 20%; therefore the new 63V rated tantalum polymer capacitors are suitable for 48V rails, commonly used in telecomms systems, LED drivers and medium voltage power supplies.

LED lighting drivers
A wide range of lighting applications use LED technology. For high and medium power applications, constant-current mode driver topology with LEDs connected in series is preferred, as this approach delivers high overall system efficiency and enables PWM dimming. Light levels can be easily matched to the needs of the environment by turning the LED chips on and off at low frequency, starting from 120Hz. The voltage rail in this application is typically in the range of 28-60 VDC. High voltage tantalum polymer capacitors are well suited for such application voltage as 63V rated devices can be used up to 50V; 75V products are suitable for use up to 60V. Additionally, tantalum polymer devices do not exhibit any piezo effect and thus avoid the typical short-comings of ceramics capacitors at such low frequencies. Another advantage that polymer tantalum capacitors have over their ceramic counterparts is an improved mechanical robustness.

As rated voltages increase, tantalum polymer devices may also be suitable as input capacitors, as they are more reliable than aluminum electrolytic devices (E-caps) and have no wear out mechanism. This would make them more suitable to match the increasing life expectancy of power supplies in LED lightning systems.

Lower and medium voltage tantalum polymer capacitors are well established in Notebooks and LCD TVs, with D (7343-31) and Y (7343-20) being the favorite case sizes used in DC/DC converters and backlight LED drivers. However, some modern LCD TV architectures with LED backlighting now require a higher output voltage. Typically, a 5V output is dedicated for the microprocessor and the logic circuitry, 12V supplies the TV panel, 24V is used for the audio power amplifiers, and 24V and higher voltages for the backlight.

Telecomms
Currently-available 63V rated tantalum polymer capacitors with a capacitance range of 0.47 to 15uF and ESR ratings as low as 100m? ideally suit applications in telecommunications systems which often use 48V rails and where long lifetime, low ESR and high ripple current capability are the key requirements.

Summary
The increasing capability of tantalum polymer capacitors to operate at higher voltages delivers a combination of highest capacitance and smallest case size at these voltage ranges; this has not previously been possible using other capacitor technologies. These and other benefits, including high stability and excellent reliability, are opening up opportunities for designers to shrink the size of their products and add new functionality in wide range of applications. Additionally, tantalum polymer devices can eliminate the shortcomings associated with conventional capacitor technologies that operate in similar voltage ranges by eliminating the piezo effect and offering better mechanical robustness than ceramic capacitors, achieving higher capacitance and voltage per volume than film capacitors and better reliability and lifetime than aluminum polymer technology devices.
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