It is common for electronic circuits have a continuous energy supply, such as a steady dc voltage, during operation. However, in some cases it may be necessary to use a power source that is transitory or transient in nature, such as the energy pulse generated by a piezoelectric generator. Piezoelectric generators and other forms of energy harvesting device are finding use in a wide range of applications where it is undesirable or impossible to provide a wired power supply or where replacement of batteries is impractical. Energy harvesting is also beneficial for the environment, by reducing the need for production and disposal of batteries, which include toxic components, and by making use of renewable energy sources and “waste” energy. An example of an application using a piezoelectric generator is in powering a wireless switch. The switch may emit a radio signal when actuated, for example, to turn on or off a device configured to receive the signal. The switch may be arranged such that actuation strikes a piezoelectric crystal, generating a voltage pulse. This pulse charges a capacitor, which in turn powers a radio circuit that emits the signal, avoiding the need for a battery or other stored power source.
Where power is supplied by transient energy pulse, it is known to use the energy pulse to charge a capacitor, and the energy stored on the capacitor may then be used to power an electronic circuit or system. An exemplary arrangement is shown in FIG. 1, which shows a voltage source 100 that generates a transient signal Vin, that is passed to power management circuit 110. Power management circuit 110 supplies a current ILoad to power an electronic system 120. The power management circuit 110 is shown in more detail in FIG. 2, and includes a capacitor C which acts as a storage element and is charged by the energy pulse received at Vin. A diode 220 between the capacitor C and Vin prevents a current flowing from the capacitor to Vin when the voltage across C, VC, is greater than that at Vin, such as immediately following the energy pulse. The voltage VC is supplied to Vout, which connects to the circuit or system to be powered (ILoad).