Patent Publication Number: US-9413271-B2

Title: Power conversion system with a DC to DC boost converter

Description:
RELATED APPLICATION DATA 
     This application claims the benefit of priority of U.S. Provisional Patent Application No. 61/781,965, filed Mar. 14, 2013, and titled Power Conversion System with a DC to DC Boost Converter which is incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to the field of power electronics. In particular, the present invention is directed to a Power Conversion System with a DC to DC Boost Converter for variable low voltage DC power sources and high voltage DC loads. 
     BACKGROUND 
     There are many devices that either produce variable low voltage DC power (below 100 VDC) or require low voltage DC power. A common example of a low voltage DC power source is a fuel cell, which is an electrochemical device which reacts hydrogen with oxygen to produce electricity and water. The basic process is highly efficient, and fuel cells fueled directly by hydrogen are substantially pollution free. Yet, the output of fuel cells (variable low voltage, high current DC power) makes efficient engineering solutions difficult, especially in residential and light commercial applications, where the power output demands of a fuel cell are not as significant. While sophisticated balance-of-plant systems are used for optimizing and maintaining relatively low power capacity applications, they do not effectively meet residential and light commercial power needs and at least a portion of this failure is attributable to the lack of effective power electronics to pair with low voltage DC power sources and loads. 
     For some low voltage DC sources such as fuel cells and batteries, the power conversion efficiency degrades over time as the sources are depleted. For example, for fuel cells, the fuel cell stack produces variable low-voltage DC based on power demand and the stack&#39;s average voltage degrades over time based on catalyst loss and energy conversion efficiency degradation. Increasing the number of fuel cells running in parallel will increase the output stack voltage, but degradation will still render the system inefficient and possibly unusable in a relatively short amount of time, requiring that the fuel cell stack be replaced. 
     SUMMARY 
     In a first exemplary aspect, a DC to DC boost converter (DDBC) for converting power received from a variable, low voltage DC source, the DDBC comprises a plurality of interleaved, isolated, full-bridge DC-DC converters arranged in a Delta-Wye configuration and a multi-leg bridge. 
     In another exemplary aspect, a power conversion system has a power rating, and the power conversion system comprises a low voltage power source having an initial output voltage, a DC-DC boost converter (DDBC) coupled to the power source, and an inverter coupled to the DC-DC boost converter, wherein the DDBC is designed and configured to provide a continuous output current, a voltage ratio less than one, and to receive a discontinuous input current. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For the purpose of illustrating the invention, the drawings show aspects of one or more embodiments of the invention. However, it should be understood that the present invention is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein: 
         FIG. 1  is a graph of commercial inverter efficiency versus load percentage for DC sources; 
         FIG. 2  is a block diagram of a power conversion system according to an embodiment of the present invention; 
         FIG. 3  is a block diagram of another power conversion system according to an embodiment of the present invention; 
         FIG. 4  is a block diagram of another power conversion system according to an embodiment of the present invention; 
         FIG. 5  is an electrical schematic of a DC to DC (DC-DC) boost converter according to an embodiment of the present invention; 
         FIG. 6  is a process diagram of providing high voltage DC power to a load from a low voltage DC power source; and 
         FIG. 7  is a block diagram of a computing system suitable for use with a DC-DC boost converter according to embodiments of the present invention. 
     
    
    
     DESCRIPTION OF THE DISCLOSURE 
     There are many DC power sources that produce low voltage, high current, unregulated DC power (“LVDC source”). However, many of these LVDC sources are relatively incompatible with commercially available inverters insofar as their efficiencies and usefulness are impeded by the high input voltage required for efficient operation of these types of inverters. As an example of this phenomenon,  FIG. 1  shows a graph  10  of inverter efficiency versus load percentage for a solar power source, i.e., a high voltage source, and a LVDC source, where a traditional inverter is used, i.e., one typically designed to receive high voltage inputs. As shown, the power conversion efficiency of power coming from the solar source varies as a function of load and is represented by solar voltage input  14 . Solar voltage input  14  reaches a maximum, in this example, of about 93% conversion at between about 50 and 75 percent of maximum load. In comparison, the power conversion efficiency of the LVDC source at its beginning of life (BOL) (represented by BOL voltage input  18 ) is considerably less. At its peak, the LVDC source obtains an efficiency of about 85% at between about 65 and 75 percent of maximum load. Moreover, as the LVDC source ages, there is an even greater disparity in power conversion when compared to the solar source. At the end of life (EOL) of LVDC sources, the power conversion efficiency peaks at about 70% at 70% maximum load. 
     The differences between the voltage inputs are designated as loss areas  26 , i.e., loss area  26 A and B, and represent inefficiencies in conversion of power. Loss area  26 A is the difference between solar voltage input  14  and BOL voltage input  18  and loss area  26 B is the difference between solar voltage input  14  and EOL voltage input  22 . Loss area  26 B is greater than loss area  26 A because while solar voltage input  14  may decrease over time, the longevity of the solar source far surpasses that of LVDC power sources, such as batteries and fuel cells. The difference between BOL voltage input  18  and EOL voltage input  22  is largely due to the decreased ability for the LVDC source to generate sufficiently high output voltages to reduce inefficiencies related to the use of inverters designed for high voltage power sources. 
     A variable low voltage DC-DC converter according to present disclosure allows for improved and increased use of numerous power sources, such as, but not limited to, fuel cells, batteries, solar cells, wind turbines, and hydro-turbines. The variable low voltage DC-DC converter allows for optimization of a distributed generation system including a LVDC source and an energy storage device for efficiency, life expectancy and cost without being limited to high voltage outputs from the LVDC source. LVDC generation systems employing a variable low voltage DC-DC converter of the present disclosure may be used without a power inverter in applications requiring high voltage DC inputs, such as a vehicle or other battery charger, a heater, a welder, a motor starter, a motor, a high voltage DC (HVDC) utility application, a telecommunications equipment, a vehicle, a tractor, a marine auxiliary power, and a material handling equipment. A variable low voltage DC-DC converter according to the present disclosure can also allow for the employment of common, low cost, reliable, low voltage energy storage chemistries (operating in the 12-48 VDC range) while continuing to employ the use of traditional inverters designed for high voltage power supplies. The variable low voltage DC-DC converter also can simplify the design (by reducing components required) and increase the useful life of the LVDC sources while allowing for efficient charging and discharging to a high voltage DC application. 
     Additionally, incorporating LVDC power storage with highly efficient generating sources such as fuel cells, solar or wind increases the economic viability of the generating device by capturing more useable energy and allowing for the use of low cost LVDC power storage employing batteries technologies, such as lead acid batteries. For example, vehicle and residential energy storage systems are dominated by battery chemistries allowing for high voltage storage topologies. Lithium-ion and nickel-metal-hydride are two examples of batteries that operate in the 400-600 VDC range so as to ensure high voltage DC output to commercially available inverters for high efficiency conversion. High voltage storage topologies are expensive and can only accept power at certain voltages. Use of a variable low voltage DC-DC converter allows for use of produced power at lower voltages to be stored within the batteries as well as the use of lower cost batteries, such as lead acid. 
     Referring now to  FIG. 2 , a high-level block diagram of a power conversion system  100  is shown that converts a variable low DC voltage from a DC source  104  into an AC voltage for an AC load  108 . Examples of a DC source  104  can include a variety of variable DC power sources that are typically subject to low voltage cutoffs, such as, but not limited to, a solar array, a wind turbine, a fuel cell, a water turbine, a battery, and a capacitor. Examples of AC load  108  include, but are not limited to, an electric power grid, a vehicle, and a residence. 
     As shown in  FIG. 2 , power conversion system  100  includes a DC-DC boost converter (DDBC)  120 , an inverter  124 , and a control system  128 . As described in more detail below with reference to  FIG. 3 , DDBC  120  converts variable, low voltage DC to constant, high voltage DC using the high current output of the DC source  104 . Once converted to fixed, high voltage DC, the voltage is delivered to inverter  124 , which converts the DC voltage to an AC voltage suitable for distribution to AC load  108 . The operation of DDBC  120  is regulated by control system  128  that, at a high level controls, among other things, the input current level and a set-point for inverter  124  so as to draw power from the DC source. Control system  128  is described in more detail below with respect to  FIG. 4 . 
       FIG. 3  shows an alternative embodiment of power conversion system  200  employing a DDBC  120 . In this embodiment, power conversion system  200  is designed and configured so as to receive power from both a DC source, i.e., DC source  104 , and an AC power source  208  (e.g., the electrical grid). DDBC  120  receives inputs from DC source  104  and AC power source  208  (via inverter  212 ) and converts the inputs to a low, high current, DC voltage for a low DC voltage storage/load  204 . Examples of low DC voltage storage/load  208  include, but are not limited to, an electric vehicle, a material handling equipment (e.g., electric fork-lifts), a tractor, a marine auxiliary power system, and a telecommunications equipment. 
     As is well known, a DC to DC converter is a species of power converter. Moreover, many power converters can be configured to convert DC into AC (typically called inverters), as well as many other power conversion functions. Thus, DDBC  120  can, in some embodiments, be implemented by a multi-function power converter that is configured to operate as a converter, i.e., convert DC into DC, or inverter, i.e. to convert DC into AC, or rectifier, i.e. to convert AC into DC. However, in some embodiments, DDBC  120  is implemented by a device that can only convert DC into DC, DC into AC or AC into DC. 
     Turning now to  FIG. 4 , there is shown a more detailed embodiment of power conversion system  100 . In this embodiment, power conversion system  100  includes input terminals  132 , allowing mechanical and electrical connection to DC source  104  and the other components of the power conversion system. At the point of power input to DDBC  120  is a sensor  136 , which measures an input current  140  coming from DC source  104  and transmits a signal  144 , representative of the input current, to control system  128 . 
     Input current  140  is then fed into an input current filter  148 . In an exemplary embodiment, input current filter  148  is an inductor/capacitor combination that filters input current  140  to protect DC source  104  from the effects of AC ripple caused by switching of transistors, e.g., MOSFETS (high frequency), and feedback from the grid (low frequency). This AC ripple can cause localized voltage changes in electrodes, which results in increased fuel consumption and loss of efficiency in fuel cells, Faradaic heating in batteries and disruptions in peak power tracking algorithms, thereby leading to loss of efficiency in distributed generation devices. 
     In an exemplary embodiment, when DDBC  120  is configured as a pulse width modulated (PWM) boost converter, the DDBC receives the filtered current input from input current filter  148  and converts the low voltage DC to high voltage DC in three steps. Power conversion system  100  converts the low voltage DC signal to a low voltage AC signal, then converts the low voltage AC signal to high voltage AC signal, then converts the high voltage AC signal to a high voltage DC signal. Two, interacting control loops are used in conjunction with DDBC  120 . 
     Regulation of the voltage boost provided by DDBC  120  can be accomplished using one or more feedback or control loops. For example, and as shown in  FIG. 4 , a feedback loop  152  is electronically coupled to DDBC  120  and control system  128 . Feedback loop  152 , is an inner, fast loop of power conversion system  100 , which regulates the voltage boost of the transformers by receiving, as an input, a fixed input voltage signal, representative of the voltage on inverter  124 , and adjusting the phase angles of the voltages of the power sent from the DC source on the transformer primary windings (discussed and shown in  FIG. 5 ) by using, for example, phase-shifting modulation technique whose algorithms reside in the microprocessor. The DC source voltage will vary based on several variables including: output current, level of degradation, and control methods such as peak power tracking (PPT) employed by the control system. In one embodiment, feedback loop  152  transmits power at a reference voltage from a power supply (not shown) to the transformer primary windings to adjust the voltage phase angles seen by the transformers; the higher the applied voltage, the lower the duty ratio of the transformers and the lower the voltage boost and vice versa: the lower the applied voltage, the higher the duty ratio and the higher the boost. In another embodiment, feedback loop  152  receives reference voltage power from a dedicated circuit (not shown) that draws power from DC source  104  and fixes its voltage through an additional dedicated transformer arrangement designed especially for this task, such as one with primary windings connected in the delta configuration and secondary windings connected in wye to produce a desired phase-shift. 
     An output current sensor  156  is coupled between DDBC  120  and control system  128 . Output current sensor  156  measures the current exiting DDBC  120  and sends a signal  160  representative of the current to control system  128 . 
     The high DC voltage power generated by DDBC  120  is transmitted to inverter  124  (through an output current filter, such as output filter  316  shown in  FIG. 5 ) for conversion into high voltage AC suitable for introduction to the AC load  108 . The amount of power available to AC load  108  is dependent upon, among other things, the available output current of DC source  104 . Control over the power draw to AC load  108  can be accomplished using control loop  154 , which is electrically coupled to control system  128  and output current sensor  156 . In an exemplary embodiment, control loop  154  provides a power draw command to inverter  124  based upon the available output current of DC source  104  (determined from a signal received from output current sensor  156 ). If DC source  104  cannot meet the power draw command, control system  128  de-rates power conversion system  100 . In another exemplary embodiment, control system  128  can sense power demands of AC load  108  (via voltage drop or other indication not shown) and responsively increase demand on DC source  104 . In this embodiment, control loop  154  monitors output current sensor  156  to ensure voltage demand is met using DC link capacitors to maintain voltage at a level set by the microprocessor. If the voltage level is not reached, control loop  154  signals to control system  128  that power conversion system  100  needs to be derated. 
       FIG. 5  shows an embodiment of a DC-DC boost converter, DDBC  300 , suitable for use in the power conversion systems described herein. The topology of DDBC  300  can be described as three interleaved, isolated, full-bridge DC-DC converters in a Delta-Wye configuration followed by a synchronous, three-leg bridge. In this embodiment, three phases are used. However, more or fewer phases may be implemented. Splitting the conversion into three phases allows the high current to be spread across multiple legs thereby increasing efficiency and heat dissipation. The figure shows, first, a plurality of MOSFET-based DC-AC converters  304 , e.g.,  304 A-C, that are electrically coupled to an input current filter, such as input current filter  148 . 
     A plurality of transformers  308 , e.g., transformers  308 A-C, are electrically coupled to the DC-AC converters, such that each transformer is coupled to one or more of the DC-AC converters. Thus, for example, transformer  308 A is coupled to DC-AC converter  304 A and  304 B Transformers  308  convert the low voltage AC generated by converters to a relatively higher voltage AC. Typically, the number of transformers  308  used in DDBC  300  corresponds to the number of converters  304 . The amount of boost provided by DDBC  300  is regulated by the turns ratio of the transformers  308 : the higher the turns ratio, the higher the boost. An undesirable side effect of increasing the turns ratio of the transformers is higher the flux losses. However, by increasing the number of legs or phases, the duty on each transformer  308  is reduced, and the turns ratio and the flux losses are both lower. Moreover, by interleaving, or summing, the legs after transformers  308 , DDBC  300  achieves a higher overall effective boost with minimal flux losses. Also, since the MOSFET-based topology is inherently high switching frequency, the high-frequency transformers  308  have the advantages of being small in size and highly efficient. 
     Additionally, in grid-tied applications, transformers  3080  fulfill the requirement of grid isolation. Isolation requirements take two forms: power and signal. Power isolation is the minimization of DC current injected into the grid and is a stringent requirement of most utilities. Transformerless inverters are being introduced in Europe, which while smaller and lighter than their transformer-based counterparts, these devices suffer from lower efficiencies and additional components to prevent DC leakage onto the grid. Signal isolation is the separation of generation-side measurements from the grid to ensure critical parameter measurement accuracy. The integration of small, highly efficient high-frequency transformers, such as transformer  308 , fulfills all isolation requirements with a topology solution whose complexity and cost is less than a transformerless topology. 
     The high voltage AC output by transformers  308  is fed to a corresponding number of synchronous rectifiers  312 , e.g.,  312 A-C. Synchronous rectifiers  312  convert the high voltage AC from transformers  308  to high voltage DC. In an exemplary embodiment, synchronous rectifiers  312  are MOSFET-based rectifying devices. Typically, the number of synchronous rectifiers  312  included with DDBC  300  is the same as the number of transformers; however, more or fewer may be used. As shown in  FIG. 5 , three MOSFET-based synchronous rectifiers are electrically coupled to transformers  308 . Before the output of DDBC  300  is provided to an inverter, such as inverter  124 . it can pass through an output filter  316 . Output filter  316  protects the power conversion system from AC ripple from the electrical grid (if so coupled). AC ripple from the electrical grid will affect the performance of the power conversion system inductors, capacitors, switches and the low voltage DC source, e.g., fuel cell. 
     Overall, a power conversion system including DDCB  300  is based on a boost topology which features continuous output current, a voltage ratio less than one, and discontinuous input current. AC-DC converters  304  are operated in parallel to help distribute dissipated heat, lower the switch RMS current, and enable soft switching features. AC-DC converters  304  are also interleaved in such fashion that when one phase stops conducting current and its phase transformer freewheels, another phase starts or already is conducting current. Current is thereby spread across all phases. Input current is always pulsed. A noted benefit of this topology is that power conversion efficiency increases as the input voltage decreases. This is counter to most power electronics behavior which is just the opposite, conversion efficiency increases as voltage increases due to a reduction in resistive losses. However, the topology described herein requires higher performance, and therefore an increased duty ratio on the switching devices, leading to reduced switching losses that outweigh an increase in resistive losses due to lower input voltage. 
     For DDBC  300 , the input currents of all three phases sum, thus, depending on the operational mode the total input current could be found either between zero and one phase current (which is n×the output current) or between one and two phase currents; hence, the total input current resembles that of a multilevel converter. The system will include at least two control loops, as described above, which will regulate both the output current and output voltage. 
     Benefits of the average current control method are infinite DC gain, better control of inductor current, and immunity to noise. A lack of inherent peak current capability is a disadvantage of this control method, along with a limited current loop bandwidth characteristic to linear control systems. 
     Power conversion system  100  as described herein greatly extends the range over which a DC source, such as DC source  104 , can operate. For example, using power conversion system  100  enables a fuel cell system to continue to provide power to a utility grid closer as the output voltage of the fuel cell system decreases over time. 
     Control system  128  is designed and configured to manage the components of power conversion systems  100  ( FIGS. 2 and 4 ) or  200  ( FIG. 3 ) by collecting information from inputs internal and external to the system, such as, but not limited to sensed input voltage, sensed input current, output voltage set point, and heat sink temperature. Information collected by control system  128  is input into programmed algorithms, set points, or lookup tables so as to determine operating parameters for power conversion system  100 , components, such as DDCB  120 , control signals, feedback loops, and/or to generate external data for use in evaluating the efficiency, lifespan, or diagnosing problems with the power conversion system. Although control system  128  is presently described as a separate component of power conversion system  100 , it is understood that control system  128  can be dispersed among the various components described herein without affecting the function of the power conversion system. 
     Turning now to an exemplary operation  400  of a power conversion system, such as power conversion system  100 , and with reference to exemplary embodiments shown in  FIGS. 1-4  and in addition with reference to  FIG. 5 . 
     At step  404 , measure the output voltage of the DC source, such as DC source  104 . Measurement of the output voltage may be via a sensor, such as input current sensor  204 , which is capable of transmitting a signal representative of the voltage to a control system, such as control system  128 . 
     At step  408 , the measurement from step  404  is compared to prior measurement of the output voltage, an initial voltage output value (at initial commissioning, the DC source, such as a fuel cell stack, will have its highest beginning of life (BOL) voltage curve for power output), a maximum power output value (as determined in step  420  discussed below) or a predetermined set-point or threshold value. If the output voltage measured in step  404  has decreased below a certain percentage (when compared to a prior measurement or the initial voltage output value) or has fallen below the predetermined set-point or threshold value, method  400  proceeds to step  412 ; otherwise, the method proceeds to step  416 . 
     At step  412 , a determination is made as to whether the DC source can increase its current output so as to meet the rated power output of the DC source. If the DC source can increase its current output, the method proceeds to step  416 , if not, the method proceeds to step  420 . 
     At step  416 , current is drawn from a DC source into the power conversion system at a value sufficient to supply the inverter, such as inverter  124 , with sufficiently high DC voltage to efficiently operate the inverter. Process  400  then returns to step  404  to remeasure the output voltage of the DC source. Notably, use of a power conversion system, such as power conversion system  100 , and especially DDBC&#39;s described herein, can allow for a significant drop in DC source output voltage (in comparison to an initial voltage output value) and still provide sufficient power to the inverter. In an exemplary embodiment, the voltage drop from a commissioned value of the DC power source can be as much as 80%. 
     At step  420 , if sufficient current cannot be drawn from the DC source to effectively operate the inverter, it is determined whether or not the power conversion system can be de-rated such that the maximum power output of the system is based on maximum voltage sustainable by the DC source. Sufficient current to maintain the voltage may be unavailable because other balance-of plant-components of the DC source, such as the stack air supply blower or the waste heat rejection loop of a fuel cell power system, limit the output of the DC source. If the power conversion system cannot be de-rated, the system is removed from service. Otherwise, the power conversion system is de-rated at step  424  such that a maximum power output of the DC source is transmitted to step  408  for further comparisons between the actual output voltage measured in step  404 . 
     At step  428 , a determination is made as to whether or not the DC source can continue to supply power to the inverter in an amount suitable to efficiently run the inverter. If not, the DC source is taken out of service at step  432  and the DC source is replaced or recharged. If the DC source can provide a suitable power output to efficiently run the inverter, process  400  returns to step  404 . 
     A power conversion system as described herein allows for: optimization of a fuel cell, energy storage, and distributed generation system for efficiency, life, and cost instead of voltage output; recovery, use, and/or storage of additional power from these devices that until now would be unobtainable; increasing the useable life of the DC source portion of the system, e.g., a fuel cell stack, a battery string, a solar panel, a wind turbine, or a water turbine; allowing DC sources to be used without an inverter stage in applications requiring high voltage DC inputs, e.g., a vehicle or other battery charger, a heater, a welder, a motor starter, a motor, a high voltage DC (HVDC) utility application, telecommunication equipment, a vehicle, a tractor and/or a marine auxiliary power, a material handling equipment; or providing additional power and life to improve the economics of the distributed power generation systems. Additionally, the bi-directional capability of the device allows for the implementation of low voltage battery storage, charged by relatively high power sources allowing for the employment of readily available, low cost, reliable energy storage systems such as lead acid battery. 
       FIG. 6  shows a diagrammatic representation of one implementation of a machine/computing device  500  that can be used to implement a set of instructions for causing one or more control systems of power conversion system  100 , for example, control system  128 , to perform any one or more of the aspects and/or methodologies of the present disclosure. Device  500  includes a processor  505  and a memory  510  that communicate with each other, and with other components, such as DDBC  120  and inverter  124 , via a bus  515 . Bus  515  may include any of several types of communication structures including, but not limited to, a memory bus, a memory controller, a peripheral bus, a local bus, and any combinations thereof, using any of a variety of architectures. 
     Memory  510  may include various components (e.g., machine-readable media) including, but not limited to, a random access memory component (e.g, a static RAM “SRAM”, a dynamic RAM “DRAM”, etc.), a read-only component, and any combinations thereof. In one example, a basic input/output system  520  (BIOS), including basic routines that help to transfer information between elements within device  500 , such as during start-up, may be stored in memory  510 . Memory  510  may also include (e.g., stored on one or more machine-readable media) instructions (e.g., software)  525  embodying any one or more of the aspects and/or methodologies of the present disclosure. In another example, memory  510  may further include any number of program modules including, but not limited to, an operating system, one or more application programs, other program modules, program data, and any combinations thereof. 
     Device  500  may also include a storage device  530 . Examples of a storage device (e.g., storage device  530 ) include, but are not limited to, a hard disk drive for reading from and/or writing to a hard disk, a magnetic disk drive for reading from and/or writing to a removable magnetic disk, an optical disk drive for reading from and/or writing to an optical media (e.g., a CD, a DVD, etc.), a solid-state memory device, and any combinations thereof. Storage device  530  may be connected to bus  515  by an appropriate interface (not shown). Example interfaces include, but are not limited to, SCSI, advanced technology attachment (ATA), serial ATA, universal serial bus (USB), IEEE 1395 (FIREWIRE), and any combinations thereof. In one example, storage device  530  may be removably interfaced with device  500  (e.g., via an external port connector (not shown)). Particularly, storage device  530  and an associated machine-readable medium  535  may provide nonvolatile and/or volatile storage of machine-readable instructions, data structures, program modules, and/or other data for control system  128 . In one example, instructions  525  may reside, completely or partially, within machine-readable medium  535 . In another example, instructions  525  may reside, completely or partially, within processor  505 . 
     Device  500  may also include a connection to one or more sensors, such as sensor  208  and/or output current sensor  240 . Sensors may be interfaced to bus  515  via any of a variety of interfaces (not shown) including, but not limited to, a serial interface, a parallel interface, a game port, a USB interface, a FIREWIRE interface, a direct connection to bus  515 , and any combinations thereof. Alternatively, in one example, a user of device  500  may enter commands and/or other information into device  500  via an input device (not shown). Examples of an input device include, but are not limited to, an alpha-numeric input device (e.g., a keyboard), a pointing device, a joystick, a gamepad, an audio input device (e.g., a microphone, a voice response system, etc.), a cursor control device (e.g., a mouse), a touchpad, an optical scanner, a video capture device (e.g., a still camera, a video camera), touchscreen, and any combinations thereof. 
     A user may also input commands and/or other information to device  500  via storage device  530  (e.g., a removable disk drive, a flash drive, etc.) and/or a network interface device  545 . A network interface device, such as network interface device  545 , may be utilized for connecting device  500  to one or more of a variety of networks, such as network  550 , and one or more remote devices  555  connected thereto. Examples of a network interface device include, but are not limited to, a network interface card, a modem, and any combination thereof. Examples of a network include, but are not limited to, a wide area network (e.g., the Internet, an enterprise network), a local area network (e.g., a network associated with an office, a building, a campus or other relatively small geographic space), a telephone network, a direct connection between two computing devices, and any combinations thereof. A network, such as network  550 , may employ a wired and/or a wireless mode of communication. In general, any network topology may be used. Information (e.g., data, instructions  525 , etc.) may be communicated to and/or from device  500  via network interface device  555 . 
     Device  500  may further include a video display adapter  560  for communicating a displayable image to a display device  565 . Examples of a display device  565  include, but are not limited to, a liquid crystal display (LCD), a cathode ray tube (CRT), a plasma display, and any combinations thereof. 
     In addition to display device  565 , device  500  may include a connection to one or more other peripheral output devices including, but not limited to, an audio speaker, a printer, and any combinations thereof. Peripheral output devices may be connected to bus  515  via a peripheral interface  570 . Examples of a peripheral interface include, but are not limited to, a serial port, a USB connection, a FIREWIRE connection, a parallel connection, a wireless connection, and any combinations thereof. 
     A digitizer (not shown) and an accompanying pen/stylus, if needed, may be included in order to digitally capture freehand input. A pen digitizer may be separately configured or coextensive with a display area of display device  565 . Accordingly, a digitizer may be integrated with display device  565 , or may exist as a separate device overlaying or otherwise appended to display device  565 . 
     Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions, and additions may be made to that which is specifically disclosed herein without departing from the spirit and scope of the present invention.