1.  Technical Field
The present invention relates to a power generation unit for taking out the charge, which is generated when a piezoelectric material such as a piezoelectric device is deformed in response to an external force, as electrical energy, a method of controlling the power generation unit, a battery, an electronic apparatus, and a transportation device including the power generation unit. 
2.  Related Art
When a piezoelectric material such as lead zirconium titanate (PZT), quartz crystal (SiO2), or zinc oxide (ZnO) is deformed, electrical polarization is induced inside the material, and positive and negative charges appear on the surface.  Such a phenomenon is called a so-called piezoelectric effect.  There has been proposed an electrical power generation method of vibrating a cantilever to thereby make a weight repeatedly act on the piezoelectric material, and thus taking out the charge generated on the surface of the piezoelectric material as electricity using such a characteristic provided to the piezoelectric material. 
For example, by vibrating a metal cantilever having a mass disposed at the tip and a thin plate made of the piezoelectric material bonded thereto, and taking out the positive and negative charges alternately generated on the piezoelectric material due to the vibration, an alternating current is generated.  JP-A-7-107752 proposes a technology of rectifying the alternating current using diodes, and then storing the result in a capacitor, and then taking it out as electricity.  JP-A-2005-312269 proposes a technology of arranging that a junction is closed only during the period in which the positive charges are generated in a piezoelectric device to thereby make it possible to obtain a direct current without causing a voltage loss in the diodes.  Since it is possible to miniaturize the power generation unit by using these technologies, there is expected an application of, for example, incorporating the power generation unit in, for example, a small-sized electronic component instead of a battery. 
However, in the proposed technology according to the related art, there arises a problem that the obtainable voltage is limited up to the voltage generated by the electrical polarization of the piezoelectric material.  Therefore, in most cases, an additional step-up circuit is required, and there arises a problem that it is difficult to sufficiently miniaturize the power generation unit.