The term powertrain, or powerplant, describes the main components that generate power and deliver power to the road surface, water, or air. In a wider sense, the powertrain includes components used to transform a stored (chemical, solar, nuclear, kinetic, potential, etc.) energy into kinetic energy for propulsion purposes.
The powertrain of the current and upcoming electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs), cumulatively called plug-in electric vehicles (PEVs), is composed of a high-power battery pack, as the Energy Storage Sub-System (ESS), and an Inverter followed by a Propulsion Machine for electric propulsion.
Electric vehicles are vehicles propelled by electricity as opposed to the conventional vehicles which operate by liquid or gaseous fuels. Electric vehicles may use a combination of different energy sources such as, for example, fuel cells (FCs), batteries, and super-capacitors (or ultra-capacitors) to power an electric drive system as shown in FIG. 1. Electric vehicles (EVs) operate with high energy conversion efficiency, produce a lower level of exhaust emissions, and lower level of acoustic noise and vibration than conventional vehicles. The electricity needed for electric vehicles' operation may be produced either external the vehicle and stored in the battery, or produced onboard with the help of fuel cells.
A plug-in electric vehicle (PEV) is any motor vehicle that can be recharged from an external source of electricity, such as wall sockets. The electricity stored in the rechargeable battery packs drives or contributes to drive the wheels. PEV is a superset of electric vehicles that includes all-electric or battery electric vehicles (BEVs), plug-in hybrid vehicles (PHEVs), and electric vehicle conversions of hybrid electric vehicles as well as conventional internal combustion engine vehicles.
In electric vehicles, the main energy source is assisted by one or more energy storage devices. A combination of batteries and super-capacitors are often used as Energy Storage Sources which can be connected to the fuel cell stack in a number of ways. The voltage characteristics of the two devices must match perfectly, and only a fraction of the range of operation of the Energy Storage devices can be utilized. For example, in a fuel cell/battery configuration, the fuel cell must provide substantially constant power all the time due to the fixed voltage of the battery, and in a battery/super-capacitor configuration, only a fraction of the energy exchange capability of the super-capacitor (also referred to herein as ultra-capacitor) can be used.
DC-to-DC converters can be used to interface the elements in the electric powertrain by boosting or chopping the voltage levels as required by the load in different regimes of the EV operations. By introducing the DC-to-DC converters, one can select the voltage variations of the devices, and the power of each device can be controlled.
Different configurations of EV power supplies dictates that at least one DC-to-DC converter is necessary to interface the fuel cell, the battery, or the super-capacitor's module and the DC Link. The DC-to-DC converter structure has to be reliable and lightweight, small in volume, operate with high efficiency, and low electromagnetic interference, and low current/voltage ripple.
A DC-to-DC converter is a category of power converters having an electric circuit which converts a source of direct current (DC) from one voltage level to another, by storing the input energy temporary and subsequently releasing the energy to the output at a different voltage level.
The storage of the input energy may be in either magnetic field storage components (inductors, transformers) or electric field storage components (capacitors). DC-to-DC converters can be designed to transfer power in only one direction, from the input to the output. However many all DC-to-DC converter topologies can be made bi-directional which can move power in either direction which is useful in applications requiring regenerative braking.
The amount of power flow between the input and the output of the DC-to-DC converter can be controlled by adjusting the duty cycle (which is identified as a ratio of on/off time of a switch in the DC-to-DC converter). Usually, this is done to control the output voltage, the input current, the output current, or to maintain a constant power.
It is desirable to use DC-to-DC converters with minimized switching losses, lower inductor current ripple and low switch voltage ratings, as well as reduced output voltage ripples, and enhanced reliability and autonomity of the operation in plug-in electric vehicles.
It is also desirable to provide various configurations of power electronic interfaces between the Energy Storage Sub-Systems and the Propulsion Machines (including electric traction system), in the plug-in electric vehicles, with efficient dynamic splitting of power output from the Energy Storage Sub-Systems to power the Propulsion Machines in accordance with their operational power requirements in a dynamic fashion in order to attain an extended ESS usability and lifetime and to provide the power flow control flexibility.