The disclosures of Japanese Patent Application No.  2000-004944 filed on Jan. 13, 2000, and No.  2000-156547 filed on May 26, 2000, including the specifications, drawings and abstracts are incorporated herein by reference in their entirety. 
1.  Field of the Invention
The invention relates generally to a power supply circuit, and more specifically to a power supply circuit for supplying predetermined electric power during of a load. 
2.  Description of Related Art
In general, vehicles and so forth are known to have a circuit that supplies electric power from a battery in order to operate a load such as bulbs and motors. 
For example, Japanese Patent Application Laid-open Publication No.  HEI 09-289707 describes a power supply system for an electric vehicle, disclosing a technology where a high voltage battery is used as a common power supply and its voltage is distributed to a plurality of power supply groups, each of which is provided with a DC-DC converter that is in accordance with the type and magnitude of a load belonging thereto.  According to this technology, when the driver turns ON an ignition switch of a vehicle to operate each of the loads, the DC-DC converter that is in accordance with the type and magnitude of the load belonging to a power supply pertaining to the load is activated and electric power that meets the load is supplied. 
In recent years, computerization has also progressed in vehicles, which now have many loads installed that have memory functions.  In this case, it is necessary to supply constant electric power (a dark current) thereto when the load is not being operated to maintain the content of the memory.  Moreover, there may be loads which are on stand-by that require a constant dark current even when the load is not being operated to ensure quick operation thereof.  When a power supply system is used that does not consider electric power supply when the load is being operated, when the user wishes to supply a specified amount of electric power (the dark current) to a certain load when the load is not being operated, the user has no other choice but to activate the DC-DC converter of a power supply group to which the load belongs.  Therefore, because of power consumption of the DC-DC converter having a large capacity (i.e. service capacity) exceeding a capacity necessary for supplying the dark current, there is a fear that the capacity (i.e. remaining capacity) of the battery might drop. 
Moreover, in the above-mentioned related technology, since the DC-DC converter having a capacity required to operate the load is used, once an abnormality occurs and the DC-DC converter is activated erroneously, the load which is not being operated is supplied with an amount of electric power corresponding to the capacity of the DC-DC converter; therefore there is also a fear that the capacity of the battery might drastically drop. 
In light of the above-mentioned problems, it is the object of the invention to provide a circuit that can supply a specified amount of electric power when the load is not being operated or when the load is being operated but requires a smaller amount of electric power than for normal operation while suppressing capacity reduction of the battery, as well as suppress the capacity reduction of the battery even when supplying the load with electric power due to erroneous activation. 
In order to achieve the above-mentioned object, an embodiment according to the invention is a power supply for a vehicle for supplying electric power to a load, comprising a battery, a first converter, and a second converter. 
The first converter changes a voltage of the battery and supplies main electric power to the load.  The second converter changes the voltage of the battery and supplies auxiliary electric power to the load. 
By providing the second converter in addition to the first converter, required electric power can be supplied to the load even when the load is not being operated without greatly reducing the capacity of the battery.