Loads

7.3 LOADS

For optimum performance and efficiency, a power source and its load should be matched in both voltage and current requirements. For example, a 1.5-V primary power source should be used to supply a circuit requiring 1.5 V, and if the circuit requires 20 μA of continuous current, the primary power source should be able to produce that current. When either the voltage or current requirements are not met, a conversion stage must be introduced, the inefficiency of which causes an increase in average power consumption over what otherwise might be obtained. In many low-power applications, this efficiency loss may equal the power consumption of the load because the conversion circuits often include fixed overhead (i.e., power-consuming items such as voltage and current references, storage capacitor leakage currents, etc. that do not change with power supplied to the load). Much discussion often takes place on the need for power sources with specific capabilities; in this section, the tables are turned, and an examination is made of what opportunities exist to adapt the circuits and other loads in a wireless sensor network node to particula power sources.

When voltage is considered, "adapting the circuit to the power source" usually means identifying ways to lower the required supply voltage for the load, for, with the exception of lithium cells and piezoelectric generators, nearly all sources of energy available to the wireless sensor network node designer have voltage quanta substantially less than standard CMOS logic levels. This is especially true for scavenged energy sources; for example, silicon photovoltaic cells have an open-circuit terminal voltage on the order of 500 mV or so. Thermoelectric generators may have a Seebeck coefficient (the ratio of voltage generated per unit temperature differential across the thermoelectric element) as low as 200 μV/K, requiring hundreds of elements to be placed in series to develop a useable output voltage.^[68] If circuits could be operated directly on a 500-mV supply generated from a photovoltaic cell, or on a 300-mV supply generated from a thermoelectric generator, without the need for voltage multiplication, total system efficiencies could be greatly increased. Existing energy scavenging methods could be implemented more easily, and the wireless sensor network market would grow.