Publication date: 13 December 2011
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 significantly 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 influence 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 specified 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.
The suitable nominal capacitance for battery circuits is typically in the range 22 to 220µF, and a small footprint and low profile 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 (specified at full rated voltage and room temperature, 20degC):
Equations 1TAJ 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 figures 1 & 2 ambient temperature and voltage derating are very important factors to consider when calculating DCL. Special *LE suffixed 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.
To achieve the optimal leakage current ratings for the application (DCLa) at room temperature we need to consider two factors: the basic DCL defined at rated voltage Vr as in Equations 1; and the DCL ratio vs Voltage derating, see Figure 3.
Equations 2:TAJ series: DCLa = 0.01*C*Vr*Ri,TRJ series: DCLa = 0.0075*C*Vr*Ri,NOJ series: DCLa = 0.02*C*Vr*Ri,
where Ri = ratio of DCLa/DCL (at Vr) – Figure 3. The maximum DCL multiplier vs Va/Vr for a fixed application voltage Va is displayed in Figure 4.
The maximum actual DCL value varies with different input conditions (chosen capacitor series, nominal capacitance, rated voltage), however shape of the graph (Fig. 4) will be the same. So we can identify the range of Va/Vr (derating) values with minimum actual DCL as the ‘optimal’ range. Therefore the minimum DCL is obtained when capacitor is used at 25 to 40 % of rated voltage - when the rated voltage of the capacitor is 2.5 to 4 times higher than actual application voltage.
As we have said, the typical energy source of a handheld device is a lithium-ion rechargeable battery with a nominal voltage Va = 3.7V. To support device functionality, designers can choose from several different capacitor series. Figure 5 compares the maximum DCL of different capacitor series all with a nominal capacitance of 47µF.
In this example where the Va is 3.7V, the optimal rated voltage (Vr) = 10V (Fig. 5), which means optimal operating conditions are at 37% of rated voltage (see Fig. 4). For different capacitances the optimal rated voltage will also be 10V.
AVX’s TRJ, TAJ tantalum and NOJ niobium oxide capacitor series are suitable to support the battery in handheld and TPMS transmitter applications. These parts exhibit different basic leakage currents with TRJ series capacitors having the lowest DCL. However, voltage derating can be applied to reduce DCL to less than the catalogue value defined at rated voltage Vr. The optimum condition for minimum DCL is to use the capacitor at between 25 and 40% of its rated voltage. For 3.7V applications, the optimal rated voltage is 10V, therefore the best choice available from AVX is to use a TRJ capacitor with Vr = 10V, closely followed by a TAJ device with the same rating. Also special TAJ *LE suffixed tantalum capacitors exist with an even lower guaranteed maximum DCL.
