Abstract:
A multi-directional power conversion system for providing power to a motor of a hybrid vehicle includes an electric control unit; an electric motor which provides a driving force to the vehicle; a primary energy storage device; and a secondary energy storage device. An energy management system communicates parameters of the energy storage devices to the electronic control unit. A power manipulating device is coupled between the primary energy storage device and the secondary energy storage device that is configured to manage power between the energy sources and deliver electricity external the vehicle.

Description:
This application claims benefit of co-pending U.S. Provisional Application Nos. 60/888,991 and 60/889,022, both filed Feb. 9, 2007, entitled “Control System for Hybrid Vehicle With Plurality of Energy Sources” and “Reconfigurable Multi-Function Power Converter for Electric Vehicle Applications” respectively. The disclosures of the applications are hereby incorporated by reference in their entirety. 
    
    
     FIELD OF THE NON-LIMITING EMBODIMENTS OF THE INVENTION 
     An exemplary embodiment of the present invention is directed generally to a control system, and more particularly, to a control system for use in a hybrid vehicle that includes at least two energy storage devices connected to a power converter. 
     BACKGROUND OF THE NON-LIMITING EMBODIMENTS OF THE INVENTION 
     In a typical hybrid electric vehicle (HEV), a high powered battery supplies power to the hybrid drive train containing electric motors. Under heavy load conditions, the high powered battery can supply additional energy to augment the energy supplied through the vehicle&#39;s internal combustion engine, thereby improving performance. The battery can then be recharged during lighter load conditions, or charged from energy generated during vehicle deceleration. Due to the high power restraints placed on the battery, it must maintain a state of charge (SOC) within a narrow operating range to ensure long life. Also, a buffer below full charge of the battery must remain to allow energy to be recaptured when the vehicle decelerates while the battery is “fully charged.” Consequently, a shortcoming of this typical HEV system is that the maximum capacity of this battery can never be fully utilized for vehicle propulsion. 
     More than one power source may also be used to provide power to the vehicle. For example, a high power battery pack may be used in unison with a high capacity battery pack in order to provide both high capacity and high power. Also, a solar panel may be used alongside a battery pack in order to capture the energy production capabilities of the solar panel as well as the storage capability of the battery pack. This integration of multiple energy sources in conjunction with the load of the vehicle itself can be achieved if energy can freely and efficiently be exchanged between such sources to the vehicle or any other load. 
     SUMMARY OF THE NON-LIMITING EMBODIMENTS OF THE INVENTION 
     An illustrative, non-limiting embodiment of the present invention is a system of operation and control of energy flow in a vehicle having a plurality of energy storage devices. An example of such vehicle would be a hybrid vehicle with at least two energy storage devices, a primary energy storage device such as a nickel metal hydride (NiMH) battery, lithium ion (LI) battery or super/ultra capacitor bank, coupled to a secondary energy storage device such as either of those used as a primary device. The energy devices can be recharged, either individually or collectively, through an external energy source such as the energy supplied through an ordinary electrical socket powered by the electrical grid. The two energy storage devices can be directly coupled to the drivetrain of the vehicle. Alternatively, it can be coupled through a multi-function power converter which allows for a buck, boost or isolated mode of operation, in which energy is transferred in a bi-directional manner between the energy storage devices. 
     An additional battery enables the system to supply additional electrical energy, as well as absorb increased amounts of regenerated power supplied during deceleration of the vehicle. The external energy source interface allows for the recharging of either the primary or secondary battery, individually or collectively. The external energy provided optimizes operation of the vehicle by propulsion of the vehicle from the external energy stored in the batteries, propulsion from the internal combustion engine, or a combination of the two. 
     An aspect of a non-limiting embodiment of the invention may provide for a DC to AC and AC to DC conversion in order to allow for the free exchange of energy between the hybrid vehicle and an outside power source or load. For example, it may be desirable for a user to utilize a common household appliance when there is no other AC outlet is available. An illustrative, non-limiting embodiment of the present invention provides for an AC electrical outlet within the system and the ability for the control system to switch to a mode that will allow energy from the energy sources to be transferred to the drivetrain of the hybrid vehicle. Accordingly, a hybrid vehicle user has the ability to supply AC power to devices while energy is not being supplied to the drivetrain of the vehicle. In an exemplary embodiment, an ordinary AC outlet may be affixed to the secondary energy source. 
     Furthermore, in accordance with an aspect of an exemplary embodiment, the energy could be provided to the electricity grid if the control system determines this to be necessary. Periodically, electrical grids, which provide electricity, experience power failures. If this were to occur, an embodiment of the invention, while plugged into an ordinary electrical outlet could switch to a mode that would enable energy to be taken from either primary or secondary energy batteries and supplied to the electrical grid. 
     Aspects of other, non-limiting embodiment may also optimize energy flow between multiple devices, e.g., the primary and secondary energy storage device, the external energy source and the hybrid vehicle itself. The two energy storage devices may be coupled through a power converter. In accordance with an exemplary embodiment of the present invention, a ontroller communicates with each energy storage device, the vehicle, a reconfigurable multi-function power converter and an energy management system which supplies vehicle and energy storage device data to the controller. The controller regulates the energy flow between the three devices based on vehicle operating parameters. Specifically, the controller communicates with the power converter that either enables or disables a series of electronic switches to produce the desired mode of operation in accordance with information received from the energy management system. 
     In accordance with an exemplary embodiment, a hybrid drive system is provided that comprises an engine and an electric motor which provide power to the vehicle. A primary energy storage device stores electrical energy, in addition to a secondary energy storage device. An electronic control unit (ECU) monitors a state of charge of the primary energy storage device and the secondary energy storage device. A power converter manipulates power transfer between the primary energy source and the secondary energy source. 
     In accordance with a further exemplary embodiment, a multi-directional power conversion system for providing power to a motor of a hybrid vehicle is provided, which comprises an electric control unit; an electric motor which provides a driving force to the vehicle; a primary energy storage device; and a secondary energy storage device. An energy management system communicates parameters of the energy storage devices to the ECU. A power manipulating device is coupled between the primary energy storage device and the secondary energy storage device and is configured to manage power sent to the electric motor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other aspects of non-limiting embodiments of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings: 
         FIG. 1  is a block diagram showing a general structure of an illustrative, non-limiting embodiment of the present invention; 
         FIG. 2  is a flowchart showing the determination of control parameters according to an illustrative non-limiting embodiment of the present invention; 
         FIG. 3  is a graph based on a secondary battery charge limit table according to an illustrative non-limiting embodiment of the present invention; 
         FIG. 4  is a graph representing the secondary charge limit as a function of temperature according to an illustrative non-limiting embodiment of the present invention; 
         FIG. 5  is a graph representing the secondary battery charge limit according to an illustrative embodiment of the present invention; 
         FIG. 6  is a graph based on a primary battery current as a function of the primary state of charge according to an illustrative non-limiting embodiment of the present invention; 
         FIG. 7  is a graph based on a primary battery charge limit as a function of temperature according to an illustrative non-limiting embodiment of the present invention; 
         FIG. 8  is a three-dimensional graph representing the secondary battery current according to an illustrative embodiment of the present invention; 
         FIG. 9  is a flow chart representing a secondary charge limit according to an illustrative non-limiting embodiment of the present invention; 
         FIG. 10  is an illustrative embodiment of a bidirectional power converter in a non-manipulated state; 
         FIG. 11  is an illustrative embodiment of a bi-directional power converter in charging, grid connect or power generation modes; 
         FIG. 12  is an illustrative embodiment of the bi-directional power converter in power a transfer mode; 
         FIG. 13  is an illustrative embodiment of the bi-directional power converter in DC-to-AC (Electrical Grid Tie) mode demonstrating switching components and phases; 
         FIG. 14  is an illustrative embodiment of the bi-directional power converter in DC-to-DC (Buck/Boost) mode demonstrating different switching components and phases; and 
         FIG. 15  is an illustrative embodiment of the bi-directional power converter in DC-to-AC (AC Generator) mode demonstrating the different switching components and phases. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is directed to certain embodiments of the invention. However, other embodiments can be embodied in a multitude of different systems and methods. 
       FIG. 1  is a block diagram schematically illustrating two energy storage devices based in a hybrid vehicle architecture. A primary energy storage (e.g., a battery)  7  is coupled to a secondary energy storage  11  through a reconfigurable multi-function power converter  6 . A controller or ECU  1  communicates with battery management systems (BMS)  2  and BMS  3  and monitors current sensors  4  and  5 . The ECU  1  also communicates with elements of the vehicle  8 , including a combustion engine, electric motor, electric motor drive, controls for the engine, the vehicle&#39;s internal ECU, and the like through a communications bus, such that a controller area network (CAN) is created. The ECU  1  is connected to the power converter  6 , which manipulates power in accordance with communications from the ECU  1 . For instance, power transfer from the secondary energy storage  11  to the primary energy storage  7  or vehicle  8 , as well as power transfer from an electrical grid  12  is controlled by the ECU  1 . An AC/DC power converter  10  is connected to the charging inlet of the vehicle  8  and provides auxiliary power to the ECU  1  when connected to the electrical grid  12  and indicates the presence of grid power to the ECU  1 . 
     The secondary energy storage  11  may contain a plurality of battery modules connected in a series arrangement or other arrangement. Each module can be comprised of a parallel, serial or combination thereof battery cell configuration. The BMS  2  is connected to the secondary energy storage  11  and monitors various battery parameters, which it then communicates to the ECU  1  and to the vehicle  8  through the ECU  1 . The ECU  1  also monitors the state of charge of the secondary energy storage or module (Ssoc)  11  based on the parameters it receives from the BMS  2 , including information, such as cell voltages, temperatures or current sensor  4  data. 
     The reconfigurable power converter  6  may be connected to an external energy source such as the electrical grid  12 . The ECU  1  controls the converter  6  and may disable the hybrid vehicle or load when it senses that the electrical grid  12  is present. Upon sensing the presence of an electrical grid  12 , energy sources  7  and  11  can be charged or alternatively, can supply AC power to the electrical grid  12  via the reconfigurable power converter  6 . 
       FIG. 2  is a flow diagram of the controller or ECU  1 . When the ECU  1  determines that the electrical grid  12  is present (operation S 20 : Yes), the system is in a charging mode. As such, the power converter  6  is set to a charge enabled status (operation S 21 ), and a disable vehicle drive (DVD) signal is also set to prevent vehicle movement while connected to the electrical grid  12  (operation S 22 ). The ECU  1  compares a state of charge of the secondary battery  11  to a target set-point of the secondary battery  11  (SsocTrgt) (operation S 23 ). If the Ssoc is less than SsocTrgt (operation S 23 : Yes), charging of the secondary battery  11  is enabled (SchEn) (operation S 24 ). In one implementation, the value of a charge limit (SchargLmt) is determined based on a charge limit table which identifies a relationship between the voltages of the electrical grid  12  and the SchrgLmt ( FIG. 3 ). The SchrgLmt is further modified based on the high (StmpMax) or low (StmpMin) temperatures as shown in  FIG. 4  (operation S 25 ). The SchargLmt is further modified based on a battery cell voltage which is determined by a table that identifies the relationship between voltage and current (operation: S 25 ) ( FIG. 5 ). The power controller  6  is set to maintain a charge current (Scrnt) at SchrgLmt by setting a secondary state of charge set-point (SPset) (operation S 26 ) or an optimal state of charge according to current conditions. 
     After the Ssoc rises above the SsocTrgt, the state of the charge of the primary energy storage or battery  7  (Psoc) is checked (operation S 27 ). If Psoc is less than the primary state of charge target (PsocTc) (operation S 27 : Yes), then the primary battery charge set-point is retrieved from the Pchrg table ( FIG. 6 ) (operation S 28 ), which demonstrates a relationship between current and the minimum and maximum state of charge. Also, PsocTc is further modified based on the charge temperature Limit (PchrgLmt) which is determined by a relationship between temperature and minimum or maximum charge ( FIG. 7 ). Then the primary energy storage  7  is enabled (PchEn) (operation S 29 ), and the power converter  6  is set to maintain the charge current to the PchrgLmt by setting PPset. 
     Then, the power converter  6  stops its charging operation (operation S 30 ), and the flow returns to operation S 22 . In operation S 27 , when Psoc becomes greater than PsocTc (operation S 27 : No), the Ssoc is again verified, and the charging of the power converter  6  is disabled upon the completion of charging of both energy storage  7  and  11 . 
     If the ECU  1  determines that the grid  12  is not preset (operation S 20 : No), the DVD signal is set to allow vehicle movement (operation S 31 ). The ECU  1  waits for a vehicle bus to be pre-charged (BusPrChrg) (operation S 32 : Yes), and the secondary bus pre-charge is initiated by setting SpreChrg to True (operation S 33 ). Once the secondary battery bus is pre-charged, the ECU  1  waits for the primary battery  7  to connect to the vehicle bus by determining if a primary battery connect (PbattConnect) signal is present (operation S 34 : Yes). At this point, the secondary battery  11  can also be connected (Sconnect), by setting Sconnect to true (operation S 35 ). Next, if Ssoc is above the minimum set point (SsocMin) (operation S 36 : Yes) the ECU  1  retrieves the secondary battery current set-point (Scrnt) from the table shown in  FIG. 8 . This value is determined based on vehicle information such as vehicle speed (VS), vehicle load (VL), the temperature of the internal combustion engine (Etmp), and primary current (Pcrnt) (operation S 37 ). 
     The resultant Scrnt is evaluated, and if Scrnt is negative (operation S 38 : Yes), Ssoc is checked to determine if it is below a secondary state of charge maximum (SsocMax) (operation S 39 ). If Ssoc is less than SsocMax (operation S 39 : No), SPset is set to Scrnt (operation S 40 ). Otherwise (operation S 39 : Yes), Spset is set to 0 (operation S 41 ). 
     On the other hand, if Scrnt is positive (operation S 38 : No), SPset is set to Scrnt (operation S 40 ). 
     If the Ssoc is below SsocMin (operation S 36 : No), the system is shut down by setting Sprechrg and Sconnect to false and SPset to 0 (operations S 42 -S 44 ). 
       FIG. 9  is a flow diagram of the ECU  1  to determine whether or not the vehicle  8  is in a vehicle enabled (EVenbl) mode, which allows battery power from the primary battery  7  and/or the secondary battery  11  to be supplied to the vehicle  8 . As shown, if the ECU  1  determines that Ssoc is greater than SsocMin (operation S 90 : Yes), the value Psoc is checked (operation S 91 ). If Psoc is above a primary state of charge minimum (PsocMin) (operation S 91 : Yes), the ECU  1  determines if Pcrnt is less than the primary current set point maximum (PcmtMax) (operation S 92 ). If so (operation S 92 : Yes), a user selectable by pass (EVbypass) input, which sets the ECU  1  to run only off battery power without warming up the engine and catalyst, is verified (operation S 93 ). A non-limiting embodiment may include an user operable push button located within the interior of the vehicle  8 , which would allow manipulation of the ECU  1 , including toggling the EVbypass function. If the user selects the bypass (operation S 93 : Yes), EVenabl is set to true (operation S 97 ). Otherwise (operation S 93 : No), the ECU  1  will check to see if the engine temperature (Etmp) is above its target (EtempTrgt) (operation S 94 ), the catalyst temperature (Ctmp) is between a maximum catalyst temperature (CtempMax) and a target temperature (CtmpTrgt) (operation S 95 ), and if the vehicle set point (VS) is below a maximum vehicle speed set-point (operation S 96 ). If so (operations S 94 : Yes, S 95 : Yes, and S 96 : Yes), the vehicle is placed in the vehicle enabled mode (operation S 97 ). On the other hand, as shown in  FIG. 9 , if various conditions are not met (operation S 90 : No, S 91 : No, S 92 : No, S 94 : No, S 95 : No, or S 96 : No), the vehicle is not placed in the vehicle enabled mode (operation S 98 ). 
       FIG. 10  is an example of the structure of the power converter  6 . It is composed of a generic three phase H bridge electronic circuit and six contactors K 1 -K 6 . The device has various modes of operation based on the activation of the contactors K 1 -K 6  and the switching scheme employed by the ECU  1 . According to the manipulation of the various switches S 1 -S 6 , as a result of various switching algorithms, a DC bus voltage can be transferred between the primary energy storage  7  and the secondary energy storage  11 , or alternatively an AC waveform  3  can be generated by the power converter  6 . 
       FIG. 11  is an example of an illustrative non-limiting embodiment of the power converter  6 . In this configuration, the contactors K 1 , K 3  and K 6  are on, the contactors K 2 , K 4  and K 5  are off, and the converter can operate in a battery charging mode, a grid connect mode and a power generation mode. 
     In the battery charging mode, switches S 1 , S 2 , S 3  and S 4  of the H bridge are used to rectify the AC waveform  40  and generate a current profile to ensure that a unity power factor is maintained. This current waveform is sampled by the controller  1  in order to ensure that the unity power factor is maintained. Switch S 6  is used to buck the DC bus voltage down to a voltage level for the secondary energy storage  11  and charge the storage  11 . 
     In the grid connect mode, switches S 1 ,S 2 ,S 3  and S 4  are used to generate the required current profile. Switches S 5  and S 6  is used as a bi-directional DC-to-DC converter that can either regulate the DC bus voltage to a pre-determined value or control power flow to and from the secondary energy storage  11 . 
     The power generation mode is similar to the battery charging or grid connect mode. However, the converter  6  receives DC power from the storage  11 , and the switches S 1 , S 2 , S 3  and S 4  are used as voltage sources to generate an AC waveform  40 . Switch S 6  is operated as a boost converter to step up the voltage of the secondary energy storage device to the required level for generating the AC waveform. 
       FIG. 12  is an example of the converter  6  when it operates in a multiphase bi-directional power converter mode, which allows DC-to-DC transfer between energy storage  7  and secondary energy storage  11 . In this mode of operation, the contactors K 1 , K 2 , K 4  and K 5  are on, and the contactors K 3  and K 6  are off. Also, the switches S 3 , S 4 , S 5  and S 6  are used to control power flow between the primary energy storage  7  and the secondary energy storage  11 . In the case of light loads, there is an option of enabling only one phase in order to improve efficiency. The use of only one phase reduces switching loss, which is the main contributor to power loss within the system. This allows the system to achieve maximum power from the energy storage devices. Optionally, another inductor/contactor can be added, such that switch S 1  and S 2  can be used as well as shown in  FIG. 14 . 
       FIG. 13  is an example of the power converter operating in the DC-to-AC grid connect mode of the invention. In this mode, AC power can be supplied directly from (or to) the hybrid vehicle&#39;s secondary energy storage  11  (SEC) to (or from) the electrical grid  12 . In this illustrative embodiment, the secondary energy source  11  is coupled to an external energy source, such as the electrical grid  12  by means of the bi-directional power converter  6 . In one implementation, diodes are coupled with six switching devices S 1 -S 6  in a parallel relationship. The grid or a charge port  12  is connected to an electromagnetic interference (EMI) filter  15 , which can optionally be connected to the power converter  6  via inductors  14 , which regulate energy or current flow. 
       FIGS. 13(   a )- 13 ( g ) show an AC waveform and on/off switching states of the switches S 1 -S 6  when the converter  6  inputs AC power and converts it into DC power for charging the secondary energy source  11 . As shown in the figures, when the converter  6  receives AC power during a first phase I 1  of the conversion, when the AC power is in the “positive” portion of the waveform ( FIG. 13(   a )), the switch S 1  opens and closes multiple times ( FIG. 13(   b )). In another implementation, the switch S 1  may open and close at non-periodic intervals. In addition, during the first phase I 1 , the switches S 2 , S 4 , and S 5  are off, the switch S 3  is on, and the switch S 6  opens and closes multiple times (but less than the switch S 1 ) ( FIGS. 13(   c )- 13 ( g )). 
     During a second phase I 2  of the conversion, when the AC power is in the “negative” portion of the waveform ( FIG. 13(   a )), the switch S 3  opens and closes multiple times ( FIG. 13(   d )). In addition, during the second phase I 2 , the switches S 2 , S 4 , and S 5  are off, the switch S 1  is on, and the switch S 6  opens and closes multiple times (but less than the switch S 3 ) ( FIGS. 13(   b ),  13 ( c ), and  13 ( e )- 13 ( g )). 
     During a third phase I 3  of the conversion, when the AC power is in the “positive” portion of the waveform ( FIG. 13(   a )), the switch S 4  opens and closes multiple times ( FIG. 13(   e )). In addition, during the third phase I 3 , the switches S 1 , S 3 , and S 5  are off, the switch S 2  is on, and the switch S 6  opens and closes multiple times (but less than the switch S 3 ) ( FIGS. 13(   b )- 13 ( d ),  13 ( f ), and  13 ( g )). 
     Finally, during a fourth phase I 4  of the conversion, when the AC power is in the “negative” portion of the waveform ( FIG. 13(   a )), the switch S 2  opens and closes multiple times at periodic intervals ( FIG. 13(   c )). In addition, during the fourth phase I 4 , the switches S 1 , S 3 , and S 5  are off, the switch S 4  is on, and the switch S 6  opens and closes multiple times (but less than the switch S 3 ) ( FIGS. 13(   b ) and  13 ( d )- 13 ( g )). 
     After phase I 4 , the first through fourth phases I 1  to I 4  are repeated. 
       FIGS. 13(   h )- 13 ( n ) show an AC waveform and switching states of the switches S 1 -S 6  when the converter  6  converts DC power from the source  11  into output AC power. The operation of the converter  6  is similar to the operation described in conjunction with  FIGS. 13(   a )- 13 ( g ), except that the on/off switching states of the switches S 1 -S 6  are different. 
       FIG. 14  is a schematic diagram demonstrating the on/off states of the switches S 1 -S 6  during two DC-to-DC transfer modes of operation.  FIGS. 14(   a )-( c ) and ( g )-( i ) (or (g′)-(i′)) show the switching states during the first mode, or boost mode, in which energy is transferred from the secondary energy storage  11  to the primary energy storage  7 . Also, if the system implements a synchronous rectification technique, the switches S 1 -S 6  are implemented via Metal Oxide Semi-Conductor Field Effect Transistors (MOSFETs) and diodes, which are respectively connected in parallel, as shown in the figure. This technique is used to improve efficiency during power conversion.  FIGS. 14(   g )-( i ) show the states of the switches S 2 , S 4 , and S 6  when the system does not use the synchronous rectification technique in the boost mode, and FIGS.  14 (g′)-(i′) show the states of the switches S 2 , S 4 , and S 6  when the system uses the technique. 
       FIGS. 14(   d )-( f ) and ( j )-( l ) (or ( j ′)-( l ′)) show the switching states during the second mode, or buck mode, in which energy is transferred from the primary energy source  7  to the secondary energy source  11 .  FIGS. 14(   j )-( l ) show the states of the switches S 1 , S 3 , and S 5  when the system does not use the synchronous rectification technique in the buck mode, and  FIGS. 14  ( j ′)-( l ′) show the states of the switches S 1 , S 3 , and S 5  when the system uses the technique. 
     In the boost mode, when the system is not implementing the synchronous rectification technique, the controller or ECU  1  turns the switch S 1  on during a first phase I 1 , off during a second phase I 2 , on during a third phase I 3 , and off during a fourth phase I 4  ( FIG. 14(   a )). Also, the switch S 3  is turned on between the starting times of the first and second phases I 1  and I 2 , is turned off between the starting times of the second and third phases I 2  and I 3 , and is turned on between the starting times of the third and fourth phases I 3  and I 4  ( FIG. 14(   b )). As in the case of the switch S 3 , the switch S 5  is turned on and off between the starting times of the various phases I 1 -I 4 , except that the on and off times of the switch S 5  occur after the corresponding on and off times of the switch S 3  ( FIG. 14(   c )). By staggering the on and off times (or interleaving the phases of the on and off times) of the switches S 1 , S 3 , and S 5 , the ripple in the output is reduced. Also, when the system is not implementing the synchronous rectification technique in the boost mode, the switches S 2 , S 4 , and S 6  are off during the phases I 1 -I 4  ( FIGS. 14(   g )-( i )). On the other hand, when the system is implementing the technique, the on and off times of the switches S 2 , S 4 , and S 6  are staggered during the intervals I 1 -I 4  as shown in FIGS.  14 (g′)-(i′). 
     In the buck mode, when the system is not implementing the synchronous rectification technique, the controller or ECU  1  turns the switch S 2  on during a first phase I 1 , off during a second phase I 2 , on during a third phase I 3 , and off during a fourth phase I 4  ( FIG. 14(   d )). Also, the switch S 4  is turned on between the starting times of the first and second phases I 1  and I 2 , is turned off between the starting times of the second and third phases I 2  and I 3 , and is turned on between the starting times of the third and fourth phases I 3  and I 4  ( FIG. 14(   e )). As in the case of the switch S 4 , the switch S 6  is turned on and off between the starting times of the various phases I 1 -I 4 , except that the on and off times of the switch S 6  occur after the corresponding on and off times of the switch S 4  ( FIG. 14(   f )). Also, when the system is not implementing the synchronous rectification technique in the buck mode, the switches S 1 , S 3 , and S 5  are off during the phases I 1 -I 4  ( FIGS. 14(   j )-( l )). On the other hand, when the system is implementing the technique, the on and off times of the switches S 1 , S 3 , and S 5  are staggered during the intervals I 1 -I 4  as shown in  FIGS. 14  ( j ′)-( l ′). 
       FIG. 15  is a schematic diagram demonstrating the on/off states of the switches S 1 -S 6  during a DC-to-AC transfer modes of operation in which the internal combustion engine (ICE) of the vehicle  8  is used to generated output AC power.  FIG. 15(   a ) shows the output AC power, and  FIGS. 15(   b )-( g ) show the switching states of the switches S 1 -S 6  when the AC power is generated from the ICE. As shown in the figure, the controller  1  is coupled to the primary battery  7  and the secondary battery  11  via the power converter  6 , and the AC power is supplied to an AC outlet  16 , which may be located physically on the secondary energy source  11 . Also, the power may be filtered through an EMI filter  15  before being output via the outlet  16 , and inductors  14  may be installed to regulate the current flow to the AC outlet  16 . 
       FIG. 15(   a ) shows a waveform of the AC power signal after it has passed through the power converter  6  as a result of kinetic energy received from the ICE of the vehicle  8 . To generate the AC power, during a first phase I 1 , the controller  1  instructs the converter  6  to open and close the switch S 2  multiple times ( FIG. 15(   c )). In addition, during the first phase I 1 , the switches S 1  and S 4 -S 6  are off, and the switch S 3  is on ( FIGS. 15(   b ) and  15 ( d )- 15 ( g )). 
     During a second phase I 2 , the switch S 1  opens and closes multiple times ( FIG. 15(   b )). Also, during the second phase I 2 , the switches S 2 , S 3 , S 5 , and S 6  are off, and the switch S 4  is on ( FIGS. 15(   c )- 15 ( g )). 
     During a third phase I 3 , the switch S 3  opens and closes multiple times ( FIG. 15(   d )). Furthermore, during the third phase I 3 , the switches S 1  and S 4 -S 6  are off, and the switch S 2  is on ( FIGS. 15(   b ),  15 ( c ), and  15 ( e )- 15 ( g )). 
     During a fourth phase I 4 , the switch S 4  opens and closes multiple times ( FIG. 15(   e )). Also, during the fourth phase I 4 , the switches S 2 , S 3 , S 5 , and S 6  are off, and the switch S 1  is on ( FIGS. 15(   b )- 15 ( d ),  15 ( f ), and  15 ( g )). 
     After phase I 4 , the first through fourth phases I 1  to I 4  are repeated. 
     In addition, the operation of the power converter  6  can be modified to convert both the energy from the ICE of the vehicle  8  and the power from the secondary energy storage  11  into AC power. The operation is the same as the previous operation, except that the controller  1  instructs the converter  6  to turn the switch S 5  on and off, as shown in  FIG. 13(   i ). 
     Numerous additional advantages or modifications may be realized by those having ordinary skill in the art. Accordingly, it is intended that the invention not be limited to the disclosed non-limiting embodiments but be interpreted within the full spirit and scope of the appended claims.