In a conventional double power supply system, a first power supply, which can be any type of solar cell, fuel cell, wind power generator, or power grid, can receive external energy for conversion to energy that is stored within the first power supply. A second power supply can be power supply that can store energy, such as battery. When energy stored in the first power supply is sufficient, the first power supply may transfer energy to the second power supply and to the load. However, when the energy of the first power supply is insufficient, the second power supply may transfer energy to the load for driving separately.
Thus, conventional approaches transfer energy to the load from either the first power supply or the second power supply. However, disadvantages of this approach include increased circuit complexity and/or lower reliability, possibly resulting in failure to meet requirement of current applications.