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Designing battery-powered equipment with low leakage Tantalum and NbO capacitors
by Radovan Faltus, AVX tantalum technical marketing
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Leakage current (DCL) is an effect common to all capacitors and its value and behavior under varying electrical and environmental conditions are related to the capacitor technology used. The leakage current
in tantalum and niobium oxide capacitors consists of the dielectric absorption current and the fault current that results due to impurities and irregularities within the dielectric. Since operating currents are signifi cantly higher than a capacitor’s leakage current the functionality of a circuit remains unaffected by DCL. However, if the application is battery operated, such as consumer applications like mobile phones, mp3/mp4 players, DVD players etc or in automotive applications where capacitors are used in a battery operated transmitter, the leakage of the capacitor will directly infl uence standby time as it directly discharges energy from the battery. In battery-powered handheld equipment, capacitors
are commonly used with 3.7V lithium-ion rechargeable batteries for several functions: to backup data and settings when the battery is being replaced or the charger is unplugged; to smooth the voltage and current peaks at the instant the battery is inserted and when the charger is plugged/unplugged; and to support the battery with stored energy when an increased current is demanded. In automotive applications, tyre pressure management
systems wirelessly transmit pressure and temperature data from in-wheel sensors to a central control unit which provides information and warning alerts to the driver. A bulk (parallel) capacitor is used in conjunction with the sensor is to deliver an energy pulse when the measurement or transmission sequence is initiated, especially at very low ambient temperatures. 3V lithium coin cells are often specifi ed for tyre pressure warning systems because of their exceptional shelf life of over ten years. Lithium batteries also function well at low temperatures, however in such conditions they exhibit an increased internal resistance resulting in a larger voltage drop.
28 EIU
Capacitor requirements The suitable nominal capacitance for battery circuits is typically in the range 22 to 220µF, and a small footprint and low profi le is a common requirement to match the small size of the end device. Excellent performance at low and very low temperatures is an obvious necessity to ensure reliable functionality. Therefore, tantalum and niobium oxide capacitors are the best choice. Standby power consumption must be minimized
to maximize maximum battery life. Both active parts and passive functions must be considered, and - as we have previously stated - the leakage current of the bulk capacitor is a mechanism which directly drains a battery so reducing DCL is important. The correct selection of the correct tantalum or niobium oxide capacitor is imperative if leakage current is to be minimized. Different formulas exist for the various AVX capacitor families to determine the basic DCL (specifi ed at full rated voltage and room temperature, 20degC):
Equations 1 TAJ series (Standard Tantalum): DCL = 0.01*C*Vr, TRJ series (Professional Tantalum): DCL = 0.0075*C*Vr, NOJ series (Niobium Oxide): DCL = 0.02*C*Vr, where C = nominal capacitance; Vr = rated voltage. Tantalum TRJ professional series
capacitors have a lower DCL in similar conditions than standard TAJ products. Niobium oxide NOJ OxiCap® devices exhibit a higher DCL. However, as shown in fi gures 1 & 2 ambient temperature and voltage derating are very important factors to consider when calculating DCL. Special *LE suffi xed tantalum TAJ series capacitors have been developed to further reduce the DCL values shown in Equations 1; voltage derating is a further way to reduce leakage current. The typical range of DCL versus rated voltage can be
seen in Figure 2. This relationship can be approximated in linear measure by reverse decimal logarithmical function with offset – see Figure 3.
www.electronicscomponentsworld.com /
www.electronicsproductionworld.com January 14th 2012
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