Many DC electrical systems require multiple DC voltages in order to operate effectively. In order to supply the different DC voltages, some systems incorporate a variety of batteries and/or other energy storage mediums that operate at the different DC voltage levels. Frequently, the power source that supplies the system operates at only one of the desired voltage levels. In order to supply power at the other operating voltages it is necessary to transfer energy from one DC voltage level to another.
Typically, the provision of various DC voltages from a single DC source is accomplished by implementing a DC/DC switch mode converter that functions to transfer energy from one voltage level to the other. The energy transfer can be either one directional or bi-directional depending on the requirements of the particular system and the design of the converter. These converters generally require multiple semiconductor devices along with magnetic components and relatively large capacitors. As the required power level increases the cost, size, and complexity of the converter also increases. Additionally, in order to keep the size of the components to a minimum the switching frequency of these converters range from 10's of Khz up to several Mhz depending on the design of the converter. The higher frequencies often cause EMC issues with surrounding components and can make it difficult for the converters to meet RF radiation regulations.
The above described challenges can be difficult to overcome for many systems. The challenges are compounded in applications such as automotive applications where compact, inexpensive, and lightweight components are desired. In the automotive context cost is a significant driver due to the large volumes and already high prices of automobiles. Weight is very critical to automotive applications as higher weights directly translate into lower miles per gallon of fuel usage. Additionally, the automotive industry has very strict requirements on EMC performance that are often difficult to meet when using switch mode power supplies. The difficulty in EMC performance is due in part to the close proximity of components within the vehicle as well as the safety critical functions of the systems.
What is needed is a battery system that can provide multiple voltage levels while providing a means to transfer energy between the different portions of the system. It would be beneficial if such a battery system provided a simplified energy transfer process using low switching frequencies. It would be further beneficial if such a system could be realized while reducing weight, size, cost, and complexity compared to conventional systems.