Publication date: 30 March 2010
Low or high tyre pressure adversely affects the contact between tyre and road surface, therefore to ensure automobile safety it is vital that the correct air pressure is maintained at all times in all tyres. However, tyre pressure is affected by many factors including ambient temperature, the tightness of air valves, how snug the tyre fits to the wheel rim or hidden defects. The problem of fluctuating ambient temperature has a simple solution: tyres can be filled with nitrogen, which has less affected by temperature changes than air, thus the pressure remains more stable. However, the other factors listed always exist and result in low tyre pressure, which often goes unnoticed by drivers because the effect is slow and gradual. It is not only adhesion between tyre and road surface which is adversely affected. Rolling resistance also increases leading to greater increasing of fuel consumption and more CO2 emissions.
The way to increase passenger safety and prevent higher fuel consumption is to use a system which automatically measures tyre pressure, warning the driver whenever pressure is above or below permitted limits. Tyre Pressure Monitoring Systems (TPMS) were used for the first time in standard passenger vehicles by Porsche in 1986. Since that time, USA legislation has regulated that all new cars must features TMPS as standard and it is also commonly found also in most new cars selling in Europe and elsewhere.
Indirect TPM systems do not feature tyre pressure sensors. Instead they use the vehicle's ABS (Anti Lock Brake) wheel speed sensors to calculate the speed of each tyre, as equally inflated tyres will travel at the same speed. A computer detects any difference in speed of an under-inflated tyre and activates the warning.
Direct TPM systems use in-wheel sensors typically powered by a 3V lithium coin battery. Pressure and temperature data is wirelessly transmitted to a central control unit which provides information and warning alerts to the driver displayed on the dashboard (Fig. 1) or the rearview mirror.
Although new types of Direct TPM systems that do not need a battery are being developed - usually requiring a separate coupling coil/antenna for each wheel - electromagnetic interference can be difficult to resolve. Therefore, battery-powered TPM systems are still preferred, with Lithium batteries often specified 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. A capacitor is usually connected parallel with a battery to avoid system failure due to insufficient voltage.
The purpose of the bulk (parallel) capacitor used in conjunction with the remote wheel tyre pressure sensor is to deliver an energy pulse when the measurement or transmission sequence is initiated, especially at very low ambient temperatures. The suitable nominal capacitance depends is typically in the range from 22 to 220µF, and a small footprint and low profile is a common requirement to match the small size of the transmitter. Excellent performance at low and very low temperatures is an obvious necessity to ensure reliable functionality. Therefore, tantalum and niobium oxide capacitors are best suited for this application.
Standby power consumption must be minimized to assure maximum battery life in the remote unit. Both active parts and passive functions must be considered, and the leakage current (DCL) of the bulk capacitor is a mechanism which directly drains a battery so reducing DCL is important.There are three possible methods which may be combined to minimize the leakage current of the bulk capacitor. Selection of the correct tantalum or niobium oxide capacitor is one possibility. Different formulas exist for different AVX product families to determine the basic DCL (specified at full rated voltage, 20degC):
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 ambient temperature and voltage derating are very important factors to consider when calculating DCL – see Figure 2 and 3. Special tantalum TAJ series capacitors have been developed to further reduce the DCL values shown in Equation 1. Voltage derating is a second way to control leakage current.
Figure 2: Temperature dependence of DCL Figure 3: Effect of voltage derating on DCL
The typical range of DCL versus rated voltage can be seen in Figure 3. This relationship can be approximated in linear measure by reverse decimal logarithmical function with offset – see Figure 4.
To achieve the optimal leakage current ratings for the application (DCLa) at room temperature we need to consider two factors. One is the calculation of basic DCL defined at rated voltage Vr as in Equation 1; the second calculation is DCL ratio vs Voltage derating – Figure 4.
where Ri = ratio of DCLa/DCL (at Vr) – Figure 4.
The maximum actual DCLa vs Va/Vr at the input conditions of the chosen capacitor series at nominal capacitance C and application voltage Va is displayed in Figure 5.
The value of maximum actual DCL varies with different input conditions (chosen capacitor series, nominal capacitance, rated voltage), however shape of the graph (Fig. 5) 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 TPMS transmitter is a lithium coin cell with a nominal voltage Va = 3V. To support operation in low temperatures, designers can choose from several different capacitor series which are available with different rated voltages. Figure 6 compares the maximum DCL of different capacitor series all with a nominal capacitance of 47µF.
In this example where the DCLa is 3V, the optimal rated voltage (Vr) = 10V (Fig. 6), which means optimal operating conditions are at 30% of rated voltage (see Fig. 5). For different capacitances the optimal rated voltage will also be 10V.
The third way to minimize the overall energy consumption of the sensor electronics is to disconnect the bulk capacitor at higher temperatures where the lithium battery has a low enough internal resistance to supply the required energy pulses.
Depending on actual measured temperature, usually when ambient temperature is below 0degC, an electronic switch in the form of a FET transistor can be used to connect the bulk capacitor in parallel to lithium battery (Fig. 7). Above the temperature where the lithium battery requires a boost, the capacitor is disconnected so it does not drain the battery. The actual DCL of the capacitor is not critical in this case, because for most of the transmitter lifetime the capacitor is disconnected, and when it is connected at low temperatures, the DCL is limited (Fig. 2).
Among tantalum and niobium oxide series - which are AEC-Q200 qualified for automotive use – AVX’s TRJ and NOJ capacitor series are suitable to support the lithium battery in TPMS transmitter applications. These devices exhibit different basic leakage currents (DCL@Vr – Equations 1), with TRJ devices have the lowest catalogue DCL. However, voltage derating can be applied to optimize the application DCL, to less than the catalogue value defined at rated voltage Vr,. The optimal condition for minimal application DCL is to use the capacitor at between 25 and 40% of rated voltage – Figure 5. For 3V TPMS applications, the optimal rated voltage is 10V – Figure 6. Therefore, comparing devices (Fig. 6) shows that 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. Special products can be also be designed with a very low basic DCL. Niobium oxide NOJ capacitors - because of their naturally higher DCL - are recommended for circuits where the bulk capacitor can be switched off at higher operating temperatures – Figure 7.
