Start-up circuit with low minimum operating power for microwatt energy harvesters

Remote sensors powered by energy harvesting need to restart successfully after long periods of no available energy, during which all stored energy may have been depleted. This start-up is affected by known phenomena such as `lock-up` and `voltage collapse`. In this study, the authors address these phenomena in the context of energy harvesting where the energy required for a sense and transmit cycle is accumulated over long time periods at low power. A voltage-detecting switch with very low power consumption is proposed, which avoids system lock-up. This start-up circuit uses an array of discrete MOSFETs operated in their sub-threshold regions. A performance metric for start-up circuits is proposed. `Minimum operating power` is the harvester power level below which the load does not start up. Reducing this minimum operating power thus reduces the required size of harvester or increases the application range. For the proposed circuit, a minimum operating power of 3.5 μW is derived experimentally, and recommendations are provided to further reduce this. The minimum operating power of the proposed circuit is shown to be a strong function of capacitive surge current, and surge current associated with undefined logic states. The transient contribution of components to the quiescent power is analysed through experiment. Simulation shows that increased temperature not only reduces the minimum operating power, but also reduces the energy available to the load.