Bi-directional power conversion apparatus for combination of energy sources

A bi-directional power converter apparatus and method of conversion is disclosed which comprises three active elements (output, primary, and auxiliary switches) and two passive components (primary and auxiliary energy storage elements) coupled so as to enable operation in any one of several different power conversion modes, including: a Non-Assisted Generation mode, a Super-Assisted Generation mode, a Super-Assisted Regeneration mode, an Auxiliary Generation mode, and an Output Regeneration mode. The individual components operate in a synergistic fashion to enable the free, bi-directional, transfer of power from primary and auxiliary energy sources to a load, and from the load to the primary and auxiliary energy sources. Further, the invention operates in such a way as to dramatically reduce the effective boost/buck ratios required for low voltage energy sources connected to a high voltage output power bus.

BACKGROUND OF THE INVENTION
 1. Technical Field
 This invention relates generally to methods and apparatus for power
 conversion, and more particularly, to an apparatus and method for
 combining the energy available from multiple sources, including those with
 low total energy content and high power delivery capability, as well as
 those having high total energy content and low power delivery capability,
 to supply a single load, and conversely, to transfer power from a single
 load to multiple, varied energy sources.
 2. History of Related Art
 Batteries remain the most widely accepted electrical storage devices
 despite their volume, weight, and life-cycle usage limitations. Designs of
 both all-electric and hybrid vehicles rely significantly on the
 availability of advanced battery technology for energy storage. Since
 battery life depends upon the charge and discharge rates employed during
 operation, it can be enhanced significantly by maintaining an optimum
 usage profile. Of course, this also means that the life of a battery can
 be shortened considerably if the use requirements are such that the
 battery is charged and discharged rapidly.
 To augment battery power, auxiliary energy systems such as ultracapacitors
 and flywheels are used to deliver (source) or absorb (sink) power in
 electric vehicle applications, for example, during rapid acceleration and
 deceleration. Thus, the basic capability of a battery-powered electric
 vehicle can be enhanced significantly by supplying vehicle peak-power
 requirements using such auxiliary energy storage systems. Auxiliary
 systems obviate the need for excessive electrical currents forced into or
 drawn from the batteries, as may be required during sudden acceleration or
 deceleration. Further, due to the reduced energy demand on the batteries
 in these circumstances, auxiliary energy systems bring with them the
 potential for significant volume and weight reduction, which translates
 into higher overall energy system efficiency.
 Using a battery pack and an auxiliary energy storage system does not solve
 all of the problems engendered when such power systems are combined to
 provide power to an electric drive system, for example. If batteries are
 used as the primary energy source, and ultracapacitor banks are used for
 peak-power requirements, size and volume constraints typically prohibit
 the direct connection of the ultracapacitors to the high-voltage vehicle
 propulsion bus. Further, it may be prohibitively expensive to size an
 ultracapacitor bank to match the main bus voltage. Hence, some kind of
 interface is required between the auxiliary storage device (e.g.
 ultracapacitors) and the battery-powered bus. While combining multiple
 energy sources can be achieved in a fundamental fashion (i.e., in theory),
 there is no known, efficient DC/DC power converter topology which enables
 such a combination in a practical sense. In addition, in the interests of
 even higher efficiency, is the desired capability to source and sink
 current from the energy sources onto the power bus, and from the power bus
 to the energy sources (i.e., the free exchange of power between multiple
 sources and a load).
 There is also a need to reduce the size of the matching components required
 when interfacing a primary energy source, such as batteries, and a
 secondary energy source, such as ultracapacitors. As a practical example,
 ultracapacitors are currently available in modules of 2,700 F, with a
 maximum allowed working voltage of 2.3 VDC. Considering the typical
 electric drive motor requirement of approximately 350 VDC, many
 ultracapacitors are required for a direct match to the power bus. The
 typical approach to solving this problem includes the use of a DC/DC
 converter. However, even using enough ultracapacitors to provide a 50 VDC
 capability requires a step-up ratio of approximately 7:1. The resulting
 converter requires very large inductors to handle the currents, and large
 step-up voltage ratio. Such a conventional solution is also very
 inefficient. The duty cycle (without enabling free power transfer between
 sources) involves heavy use of the ultracapacitor, or other auxiliary
 energy source within the power converter, as is well known in the art.
 Therefore, there is a need for an efficient, bi-directional DC/DC power
 converter topology which allows the combination of multiple energy sources
 for operation of a single load. Any desired combination of low-peak energy
 delivery capability with a large energy content power source (e.g.
 batteries or fuel cells), or high-peak energy delivery capability with a
 low energy content power source (e.g., ultracapacitors or flywheels)
 should be accommodated. Further, such a topology should embody a design
 which obviates the need for large inductance values in the power
 conversion circuitry so that a lower voltage step-up/step-down ratio can
 be maintained. Such a desired topology inherently increases system
 efficiency by introducing smaller passive components and reducing the
 weight/volume of the overall system. Finally, as noted above, there is a
 need for such a topology which permits the free exchange of power among
 the sources and load, to include the generation and regeneration of power
 in an assisted fashion such that the individual energy sources maintain a
 reduced duty cycle. The step-up (boost) and step-down (buck) capability of
 the needed topology should be maintained irrespective of the direction of
 power flow.
 SUMMARY OF THE INVENTION
 The power conversion apparatus of the present invention features
 bi-directional power flow and step-up/step-down (i.e., boost/buck)
 capability while combining the power from two independent energy sources
 to supply a single load using only three active components (an output
 switch, primary switch, and an auxiliary switch) and two passive
 components (primary and auxiliary energy storage elements), regardless of
 the direction of power flow) in its minimal configuration. The output
 switch is typically coupled to a load of the converter, while the primary
 energy storage element is coupled between the primary switch and a primary
 energy source of the converter. The primary switch, primary energy storage
 element, and primary energy source are in electrical communication with
 the output switch. The auxiliary energy storage element is coupled between
 the primary switch and an auxiliary energy source of the converter, such
 that the auxiliary switch, auxiliary energy storage element, and auxiliary
 energy source are in electrical communication with the output switch. The
 primary and secondary energy storage elements may be inductors for
 coupling voltage sources and capacitors for coupling current sources.
 Another embodiment of the bi-directional power converter of the present
 invention comprises an output switch; at least one of a primary energy
 storage element coupled between a corresponding primary energy source and
 primary switch, each of the primary energy storage elements, primary
 energy sources, and primary switches being in electrical communication
 with the output switch; and at least one of an auxiliary energy storage
 element coupled between an auxiliary energy source and an auxiliary
 switch, each of the auxiliary storage elements, auxiliary energy sources,
 and auxiliary switches also being in electrical communication with the
 output switch.
 The bi-directional power converter apparatus may operate in several
 different modes. These include a Non-Assisted Generation (NAG) mode for
 charging the auxiliary energy source, if required, and capacitances
 associated with the load from the primary energy source, a Super-Assisted
 Generation (SAG) mode for supplying power to the load from the primary and
 auxiliary energy sources, a Super-Assisted Regeneration (SAR) mode for
 charging the primary and auxiliary energy sources from the load, an
 Auxiliary Generation (AUXG) mode, typically used for supplying the load
 from the auxiliary energy source (without substantial assistance from the
 primary energy source), and an Output Regeneration (OUTR) mode, typically
 used for regenerating power from the load to the auxiliary energy source
 without any major assistance from the primary energy source.
 In the NAG mode, the power converter includes a means for preventing
 current flow across the output and auxiliary switches when the primary
 switch is conducting, so as to source power to the load and the auxiliary
 energy source from the primary energy source. Further, the converter may
 include a means for conducting current flow across the output and
 auxiliary switches when the output, primary, and auxiliary switches are
 not conducting in this mode.
 In the SAG mode, the power converter may include a means for preventing
 current flow across the output switch when the primary and auxiliary
 switches are conducting so as to source power from the primary and
 auxiliary energy sources to the load. The converter, when operating in
 this mode, may also include a means for conducting current flow across the
 output and primary switches when the output, primary, and auxiliary
 switches are not conducting.
 When operating in the SAR mode, the power converter includes a means for
 preventing current flow across the auxiliary switch when the output and
 primary switches are conducting so as to source power from the load and
 the primary energy source to the auxiliary energy source. When operating
 in this mode, the power converter also includes a means for conducting
 current flow across the primary and auxiliary switches when the output,
 primary, and auxiliary switches are not conducting.
 In the AUXG mode, the power converter includes a means for preventing
 current flow across the output switch when first the auxiliary switch
 alone, and then both the auxiliary and primary switches are conducting, so
 as to source most of the power from the auxiliary energy source to the
 load and prevent oscillatory current flow through the primary and the
 auxiliary energy storage elements. Further, when operating in this mode,
 the converter may include a means for conducting current flow across the
 output and primary switches when the output, primary, and auxiliary
 switches are not conducting. Finally, the power converter, when operating
 in the OUTR mode, includes a means for preventing current flow across the
 auxiliary switch when first the output switch alone, and then both the
 output and primary switches are conducting so as to source most of the
 power from the load to the auxiliary energy source and prevent oscillatory
 current flow through the primary and auxiliary energy storage elements.
 Further, when operating in this mode, the power converter may include a
 means for conducting current flow across the primary and auxiliary
 switches when the output, primary, and auxiliary switches are not
 conducting.
 The output, primary and auxiliary switches may comprise MOS Field Effect
 Transistors (MOSFETs), or other power switching devices such as Insulated
 Gate Bipolar Transistors (IGBTs). Other switches, such as Bipolar Junction
 Transistors (BJTs) and MOS-Controlled Thyristor (MCTs) can also be used.
 The means to conduct current across the output, primary, and auxiliary
 switches usually comprises diodes which are integral to the semiconductor
 switch construction, such as occurs with an IGBT switch. However, the
 means used to conduct current may also comprise individual diodes.
 The apparatus of the present invention also includes a power conversion
 system comprising a bi-directional power converter as described above, and
 a switch controller means in communication with the output, primary, and
 auxiliary switches. The switch controller means is able to place each of
 these switches in a conducting or nonconducting mode so as to regulate the
 magnitude and direction of the current flow through the load, the primary
 energy source, and the auxiliary energy source.
 The present invention includes a method of operating a bi-directional power
 converter which may comprise the step of sourcing power from an auxiliary
 energy source to the load while preventing oscillatory current from the
 load by using a primary energy source to provide a first counteractive
 current (in the AUXG mode), and may also include the step of sourcing
 power from the load to the auxiliary energy source while preventing
 oscillatory current from the auxiliary energy source by using a primary
 energy source to provide a second counteractive current (in the OUTR
 mode). The method may also comprise the steps of sourcing power from the
 primary energy source and the auxiliary energy source to the load (in the
 SAG mode), sourcing power from the load and the primary energy source to
 the auxiliary energy source (in the SAR mode), and sourcing power from the
 primary energy source to the load and the auxiliary energy source (in the
 NAG mode).

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS
 The invention described hereinbelow is useful in any application which
 requires the maximum, simultaneous utilization of primary and auxiliary
 energy sources to deliver power to a main bus, such as a propulsion bus.
 In addition, the invention accommodates energy regeneration and storage.
 The typical application includes supplying energy to an electric or hybrid
 electric-powered vehicle, wherein the primary source of energy is a bank
 of batteries, the auxiliary source of energy is a bank of ultracapacitors
 or a flywheel, and energy regeneration/storage is obtained through
 recovery braking.
 Turning now to FIG. 1, a schematic diagram of the present invention can be
 seen. Here the power conversion system 10 is shown to comprise a
 bi-directional power converter 15 and a switch controller 20.
 The bi-directional power converter 15 comprises an output switch 30 which
 is coupled to a load 110 of the converter 15. The output switch 30 is
 shown in FIG. 1 as an IGBT 33 combined with a anti-parrallel diode 37.
 While the output switch 30 is shown as comprising an IGBT/diode, it may
 also comprise individual components, such as an IGBT, or bipolar junction
 transistor (BJT) or MOSFET and an anti-parrallel diode in combination, or
 some other means which provides for the switched control of current
 through the switch 30 and unidirectional flow of current across the open
 switch 30, as provided by the anti-parrallel diode 37. A primary energy
 storage element 60, which may be an inductor, or some component capable of
 inductive behavior, is coupled between the output switch 30 and a primary
 energy source 90 of the converter. The power converter 15 also comprises a
 primary switch 40 (also shown herein as an IGBT and an anti-parrallel
 diode, but may also consist of an IGBT or BJT or MOSFET 43 and diode 47,
 as described above), which is coupled to the primary energy storage
 element 60. An auxiliary energy storage element 70, which may also be an
 inductor, or some other inductive circuit element, is coupled between the
 primary switch 40 and an auxiliary energy source 100 of the converter 15.
 An auxiliary switch 50 is coupled to the auxiliary energy storage element
 70. As is the case for the output and primary switches 30 and 40, the
 auxiliary switch 50 is also shown as comprising an IGBT and an
 anti-parrallel diode, but may comprise the separate elements of an IGBT or
 BJT or MOSFET 53 and an anti-parrallel diode 57, or other components as
 described above.
 The primary energy source 90 in a typical application of the power
 conversion system 10 comprises a bank of batteries, but may also be any
 other type of energy source which is typically able to deliver or absorb a
 high total energy content (i.e. a source of high kWh) but is not capable
 of high peak-power sourcing or sinking (i.e., not a source of high kW).
 Other applications may involve a primary energy source of any type, such
 as a bank of ultracapacitors, a flywheel, or any other energy source which
 is capable of supplying power to a load, or receiving power for storage. A
 unidirectional energy source, such as a fuel cell, may be employed as the
 primary energy source but would require additional load leveling
 bi-directional energy devices to accommodate reverse power flow. Thus, the
 invention may be adapted for a uni-directional energy source as the
 primary energy storage. However, it is most preferably implemented with
 bidirectional energy storage system As will be described below, the rate
 of sourcing/sinking power will be determined by the switching activity of
 the output switch 30, primary switch 40, and auxiliary switch 50.
 In a similar fashion, the auxiliary energy source 100 will typically
 comprise a bank of ultracapacitors, or a flywheel, but may also be any
 type of energy source which is capable of supplying power to, and
 receiving power from, a load. For flywheel-based auxiliary energy storage,
 additional circuitry (e.g., an AC/DC converter) is required which converts
 the AC energy of the flywheel system to DC energy. Such a bidirectional
 AC/DC converter is commonly used in the electric drive industry and is
 well known to those skilled in the art. The auxiliary energy source 100 is
 typically one that is capable of supplying high peak-power (i.e., a source
 of high kW), and low energy content storage (i.e., not a source of high
 kWh). Again, the rate of power sourcing/sinking from the auxiliary energy
 source 100 is determined by the switching activity of the output, primary,
 and auxiliary switches 30, 40, and 50.
 An output capacitor, also known as the output energy storage element 80, is
 shown in FIG. 1A as a practical means of sustaining the output voltage
 (V.sub.0) 120 across the load 110.
 The switch controller 20 portion of the power conversion system 10 has
 three outputs and five inputs. The switch controller 20 is in
 communication with the output, primary, and auxiliary switches, 30, 40,
 and 50 by means of the output switch controls 180, 181, the primary switch
 controls 190, 191, and the auxiliary switch controls 200, 201,
 respectively. Each of the switches 30, 40, and 50 is typically controlled
 using Pulse Width Modulation (PWM), as is well known in the art. More
 detail on this type of switching may be found in the book Power
 Electronics: Converters, Applications and Design by Mohan, et al, John
 Wiley & sons, 1995 and U.S. Pat. No. 4, 736,151, issued to Dishner. Each
 of these references is incorporated herein by reference in their entirety.
 As a means of monitoring the power converter 15 performance and controlling
 the switches 30, 40, and 50, the switch controller 20 monitors the voltage
 across the output capacitor V.sub.co 120, the voltage across the primary
 energy source (V.sub.P) 130, and the voltage across the auxiliary energy
 source (V.sub.A) 140. In addition, the load 110 and energy sources 90 and
 100 are protected by monitoring the measured currents through the primary
 energy storage element (i.sub.mLP) 150, and the auxiliary energy storage
 element (i.sub.mLA) 160. Monitoring the measured currents (i.sub.mLP and
 i.sub.mLA) allows derivation of the values for i.sub.L, I.sub.P, and
 i.sub.A using conventional network theory techniques. While the turn-on
 duration of the switches 30, 40, and 50 is based on the error of the
 measured voltages, the measured currents provide additional protection
 from exceeding the design limits of the circuit components and energy
 sources. If the measured currents exceed a predetermined magnitude, the
 appropriate switches may be turned off to prevent further build-up of
 current and potential destruction of the circuit.
 As noted previously, the primary and auxiliary energy storage elements 60
 and 70 may comprise inductors, or other energy storage elements. However,
 the most preferred elements are inductors when the converter 15 is used
 primarily to combine or transfer energy with intermediate current energy
 storage. However, if intermediate voltage energy storage is desired, then
 the most preferred energy storage elements may be capacitors.
 FIGS. 1B and 1C illustrate alternative embodiments of the bi-directional
 power converter 15 of the present invention, which may be employed
 depending upon the desires of the user. FIG. 1B shows the same components
 as illustrated in FIG. 1A, but the ladder network within the converter 15,
 comprised of the switches 40 and 50, the energy storage elements 60 and
 70, and the energy sources 90 and 100, respectively, shows these
 components connected in reverse order from that shown in FIG. 1A. That is,
 while the switches 40 and 50, the energy storage elements 60 and 70, and
 the energy sources 90 and 100 are all in electrical communication with the
 output switch 30, they may be transposed along the ladder network that
 makes up the structure of the converter 15.
 FIG. 1C further illustrates the possible circuit configurations enabled by
 the present invention. The ladder network may also comprise one or more
 primary energy storage elements 60, coupled between a corresponding one or
 more primary energy sources 90 and a corresponding one or more primary
 switches 50. Further, the ladder network may also comprise one or more
 auxiliary energy storage elements 70, coupled between a corresponding one
 or more auxiliary energy sources 100 and a corresponding one or more
 auxiliary switches 50. These "rungs of the ladder" may be arranged in any
 order, and in any number, provided that the current-carrying capabilities
 of individual components (energy sources 90 and 100, energy storage
 elements 60 and 70, and switches 30, 40, and 50) are not exceeded.
 Typically, the further away such components are located along the "ladder"
 from the load, the greater the corresponding currents which must be
 processed.
 Turning now to FIGS. 2A and 2B, the first of several operational modes
 enabled by the present invention can be seen. In this case, the converter
 15 is operating in the Non-Assisted Generation (NAG) mode, wherein the
 primary switch 40 is conducting, and the reverse-biased diodes 37 and 57
 prevent current flow across the output and auxiliary switches 30 and 50,
 as is shown in FIG. 2A. The primary energy source 90 is allowed to source
 energy into the primary and auxiliary energy storage elements 60 and 70
 when the primary switch 40 is conducting. This energy buildup within the
 primary and auxiliary energy storage elements 60 and 70 is illustrated by
 the flow of primary energy element current 310, which continues to flow as
 a primary switch current 320, and as an auxiliary energy element current
 330, originating from and returning to the primary energy source 90.
 When the primary switch 40 is turned off, the anti-parrallel diodes 37 and
 57 allow the stored current energy in the primary and auxiliary energy
 storage elements 60 and 70 to flow across the output and auxiliary
 switches 30 and 50, respectively, so as to transfer power from the primary
 and auxiliary energy storage elements 60 and 70, to the load 110, output
 capacitor 80, and the auxiliary energy source 100. This is indicated by
 the flow of auxiliary energy element current 330, which travels through
 the auxiliary energy storage element 70, the auxiliary energy source 100,
 and the anti-parrallel diode 57 as an auxiliary diode current 350. In
 addition, the primary energy element current 310 now flows through the
 anti-parallel diode 37 as an output diode current 340 and on through the
 load 110 and the output capacitor 80. This mode is typically used for
 start-up operation, for example, and allows the transfer of energy from
 the primary energy source 90 to the auxiliary energy source 100 and the
 load 110 and output capacitor 80 as a prelude to full system power
 operation. The NAG mode of operation may also be utilized during regular
 (i.e., full power) system operation if the power requirement at the load
 can be satisfied by the primary energy source alone, and the energy
 content of the auxiliary source is lower than a predetermined minimum
 level. The energy content for ultracapacitor systems may be determined by
 measuring the capacitor voltage, whereas for flywheel systems it is
 determined by the speed of operation.
 Assuming continuous current conduction through the primary and auxiliary
 storage elements 60 and 70, which are illustrated as inductors, during the
 NAG mode, the various voltages in the circuit are given by:
 ##EQU1##
 Where D.sub.NAG is the switch duty ratio (i.e., the ratio of the switch
 turn-on time to the total switching cycle time during continuous
 conduction operation).
 From equations (1) and (2), it can be shown that:
 ##EQU2##
 Hence, by establishing an upper limit on the current flowing through the
 primary energy source, I.sub.Pmax, while operating the switches at a
 desired frequency, f.sub.sw, it can be shown that:
 ##EQU3##
 substituting for the voltages V.sub.co, V.sub.A, and V.sub.P and the
 required I.sub.Pmax in (3) and (4), the magnitude of the inductances
 L.sub.A, L.sub.P can be determined.
 FIG. 3 illustrates the various conduction states of the circuit in the
 generation mode of operation, that is, when energy is transferred from the
 primary energy source 90 alone, or from the auxiliary energy source 100
 alone, or from both the primary and auxiliary energy sources 90 and 100 to
 the load 110 and the output energy storage element 80. In the NAG mode,
 however, only energy from the primary energy source 90 is transferred to
 the auxiliary energy source 100, output load 110, and the output energy
 storage element 80. For simplicity in FIG. 3, the switches and diodes of
 FIGS. 1 and 2 have been replaced by arrows indicating current flow paths.
 Those skilled in the art may readily determine those switches and diodes
 that are conducting current, and those that are blocking current and
 voltages in this manner.
 During operation in the NAG mode, if equation (3) is satisfied, the typical
 NAG mode operational circuit sequence may be any of the following based
 upon the duty cycle of operation.
 Continuous mode: G7.fwdarw.G6.fwdarw.G7.fwdarw.G6.fwdarw.G7 . . .
 Discontinuous mode:
 G7.fwdarw.G6.fwdarw.G5.fwdarw.G7.fwdarw.G6.fwdarw.G5.fwdarw.G7.fwdarw. . .
 .
 If, however, equation (3) is not satisfied due to the actual magnitudes of
 V.sub.A & V.sub.L, any of the following operational circuit sequences may
 be established based upon the duty cycle of operation.
 Partial discontinuous mode:
 G7.fwdarw.G6.fwdarw.G8.fwdarw.G7.fwdarw.G6.fwdarw.G8.fwdarw.G7.fwdarw. . .
 .
 Discontinuous mode:
 G7.fwdarw.G6.fwdarw.G8.fwdarw.G5.fwdarw.G7.fwdarw.G6.fwdarw.G8.fwdarw.G5.f
 wdarw.G7.fwdarw. . . .
 The Super-Assisted Generation (SAG) mode of operation is illustrated in
 FIGS. 4A and 4B. In this case, the primary and auxiliary switches 40 and
 50 are switched on and off simultaneously. This mode is utilized for
 normal operation of the system, when the auxiliary energy source may also
 be employed for power generation as determined by the controller. The
 total power transferred to the load is controlled by the duty cycle of the
 switches 40 and 50, as is well known to those skilled in the art.
 In the SAG mode, the primary and auxiliary switches 40 and 50 are both
 turned on simultaneously to allow the primary energy element current 310
 and auxiliary energy element current 330 to flow through the primary and
 auxiliary energy storage elements 60 and 70, respectively. As shown in
 FIG. 4A, the primary energy element current 310 is allowed to flow through
 the primary switch 40 (conducting) as a primary switch current 320, and
 the auxiliary energy element current 330 flows through the auxiliary
 switch 50 (conducting) as a part of the auxiliary switch current 370. The
 primary energy storage element current 310 makes up the remaining portion
 of the auxiliary switch current 370. The return current, or auxiliary
 total current 360, is then passed through the auxiliary energy source 100,
 which is capable of receiving the entire summed current. However, the
 primary energy source 90 only receives the current which flows through the
 primary energy storage element 60 (i.e., the primary energy element
 current 310). The reverse-biased diode 37 prevents current flow across the
 output switch 30 when the primary and auxiliary switches 40 and 50 are
 conducting so as to source power from the primary energy source 90 and the
 auxiliary energy source 100 to the load 110.
 As shown in FIG. 4B, when the output, primary, and auxiliary switches 30,
 40, and 50 are not conducting (the primary and auxiliary switches 40 and
 50 are turned off simultaneously), the anti-parallel diodes 37 and 47
 provide a means for conducting current flow across the output and primary
 switches 30 and 40. This permits the primary energy element current 310 to
 flow through the anti-parallel diode 37 as part of the output diode
 current 340 and the auxiliary energy element current 330 to flow through
 the anti-parallel diode 47 as primary diode current 345. The total output
 current 380, flowing into the load 110 and the output capacitor 80, is
 therefore the sum of the primary energy element current 310 and the
 auxiliary energy element current 330. Thus, the SAG mode is capable of
 providing maximum peak-power from the primary and auxiliary energy sources
 90 and 100 if demanded by the load.
 As illustrated in FIG. 3 and in accord with the description of operation in
 the previous paragraphs, the SAG mode may consist of one of the following
 sets of operational circuit state sequences depending upon the duty cycle
 desired:
 Partial discontinuous mode:
 G2.fwdarw.G3.fwdarw.G4.fwdarw.G2.fwdarw.G3.fwdarw.G4.fwdarw.G2.fwdarw. . .
 .
 Discontinuous mode:
 G2.fwdarw.G3.fwdarw.G4.fwdarw.G5.fwdarw.G2.fwdarw.G3.fwdarw.G4.fwdarw.G5.f
 wdarw.G2.fwdarw. . . .
 During normal/full power operation of the circuit, it may be adequate or
 necessary, as determined by the controller, to transfer power to the load
 from the auxiliary source as the predominant element. Under these
 circumstances, the Auxiliary Generation (AUXG) mode is employed, using the
 primary and auxiliary switches 40 and 50 in a sequential,
 non-simultaneous, fashion.
 The AUXG mode is illustrated in FIGS. 5A and 5B. While most of the power
 transferred to the load 110 during the AUXG mode is derived from the
 auxiliary energy source 100, the primary and the auxiliary switches 43, 53
 are operated sequentially to prevent uncontrolled energy build-up in the
 primary and auxiliary energy storage elements 60 and 70, respectively.
 The AUXG mode is initiated by turning on the auxiliary switch 53 alone.
 This initiates an auxiliary energy storage element current 330, which
 flows through the auxiliary energy storage element 70 and the auxiliary
 energy source 100, as shown in FIG. 6A. When the auxiliary switch 53 is
 turned off, the current energy in the auxiliary storage element 70 flows
 so as to forward bias the antiparallel diodes 47 and 37 of the primary
 switch 40 and the output switch 30. As long as the current 340 flows
 through the diode 37, the diode 37 provides a path for a parasitic current
 310 which flows through the output capacitor 80, diode 37, and the primary
 energy storage element 60. This is illustrated in FIG. 6B. When the
 current 330 becomes non-existent due to equalization of volt-time area
 across the auxiliary energy storage element 70, the current 310 supplied
 by the primary energy storage element 60 finds a path through the primary
 energy source 90, auxiliary energy source 100 and the anti-parallel diodes
 57 and 47 of the auxiliary and primary switches 50 and 40, respectively.
 Again, as long as the current 350 flows through the anti-parallel diode 57
 of the auxiliary switch 50, a path for a parasitic current 330 is
 provided, and the current 330 flows through the auxiliary energy source
 100, auxiliary energy storage element 70, and the auxiliary anti-parallel
 diode 57. This is shown in FIG. 6C. Upon depletion of the current 310 due
 to equalization of the volt-time area across the auxiliary energy storage
 element 70, the parasitic current 330 finds a new current path as shown in
 FIG. 6B.
 In a loss-less (i.e., ideal) system, such uncontrolled parasitic currents
 would oscillate indefinitely between the auxiliary energy storage element
 70 and the primary energy storage element 60. However, in a real system,
 the losses associated with real components act to dissipate the
 oscillatory energy. In the disclosed system, using lossy components,
 turning on the auxiliary switch 50 alone leads to uncontrolled parasitic
 currents in the circuit. Hence, the AUXG mode utilizes a particular
 sequence of switching the auxiliary switch 53 and the primary switch 43 to
 prevent the build-up of these parasitic currents. FIGS. 5 and 7 show the
 switching sequence employed during the AUXG mode.
 The AUXG mode controller utilizes predictive duty cycle and turn-OFF time
 estimation to determine the turn-ON activation time 415 for the primary
 switch 43. As shown in FIG. 7, assume that the controller determines the
 turn-ON activation time of the auxiliary switch 53 to be at time 475. The
 determination of the turn-ON activation time 475 and the turn-on duration,
 t.sub.ON,AUXG 555 happens during the turn-OFF period of the previous
 switching event. Hence, as illustrated in FIG. 7, the turn-ON calculations
 occur during the time intervals 565 and 560. Based upon the measured
 magnitude of the auxiliary source voltage, the turn-OFF duration
 (t.sub.OFF,AUXG), 565 of the auxiliary switch 53 can be calculated using
 the anticipated turn-ON duration (t.sub.ON,AUXG), 555 selected by the
 controller. In order to prevent the flow of parasitic currents 330 and 310
 of FIG. 6, the primary energy storage element 60 needs to conduct a
 current such that it counters the parasitic current of FIG. 6B. This
 occurs when element 60 conducts the current 310 shown in FIG. 5A, which
 requires closure of the primary switch to help build a current flow in the
 direction of current 310. However, in order to maximize energy transfer
 from the auxiliary energy source 100 to the load 110, the primary switch
 turn-ON time 415 is selected such that it minimizes the turn-ON duration
 553 of the primary switch 43.
 As shown in FIG. 7, the turn-OFF duration (t.sub.OFF,AUXG) 565 of both the
 switches 43 and 53, are equal. Hence, an estimate of t.sub.OFF,AUXG 565
 may be utilized to calculate the peak primary energy storage element
 current (i.sub.LP,tOFF) 430 using the measured values of the auxiliary
 energy source 100 and primary energy source 90 voltages (that is, V.sub.A
 and V.sub.P). Similarly, using volt-time area considerations across the
 element 60, the turn-ON time 415 of the primary switch 43 can now be
 calculated. While the calculations occur during the turn-OFF period of the
 switches (that is, 565 and 560), the switch activation signals for the
 auxiliary and primary switches 43 and 53 are issued at times 475 and 415,
 respectively.
 The above switching strategy ensures current flow through the primary and
 auxiliary energy storage elements as shown in FIG. 5 and FIG. 7. This
 prevents parasitic currents which would otherwise have been created due to
 operation of the auxiliary switch 53 alone.
 Referring to the graph of i.sub.LA vs. time 540, the auxiliary switch 53 is
 turned ON at time 475, illustrated by the rising slope 480 of the current
 i.sub.LA through the auxiliary energy storage element 70. The primary
 switch 43 is turned ON at time 415, as illustrated by the rising slope 420
 of the current i.sub.LP through the primary energy storage element 60.
 During the time interval 553, when both the primary and the auxiliary
 switches 43 and 53 are conducting, the current 310 through the primary
 energy storage element 60 and the current 330 through the auxiliary energy
 storage element 70 flow in the paths illustrated in FIG. 5A.
 At time 500, both the primary and the auxiliary switches 43 and 53 are
 turned OFF. The currents through the primary energy storage element 60 and
 the auxiliary storage element 70 attain peak magnitudes of i.sub.L,tOFF
 430 and i.sub.LA,tOFF 490, respectively. At turn OFF, the currents through
 the elements 60 and 70 decreases to zero simultaneously. During the switch
 turn-OFF period 565, the currents through the elements 60 and 70 flow in
 the direction indicated in FIG. 5B.
 The inactive time period 560 during which no switching occurs may tend
 toward zero depending upon the selected switching frequency, the turn-ON
 duration t.sub.ON,AUXG 555, and the values of individual circuit elements.
 If the time period 560 is not zero, then the circuit operates in a totally
 discontinuous mode of operation, which is characterized by zero current
 through both the inductors during each switching event. If, on the other
 hand, the time period 560 equals zero, the circuit operates in a partially
 discontinuous mode.
 As a means of increasing switching cycle speed and conserving energy, it is
 preferable to match the time instants, 510 and 460 of the current turn OFF
 times. However, prolonging the conduction time of the primary current
 (time period 565) allows for inaccuracies in the estimation of the turn-on
 time 415 of the primary switch 43, further reducing the possibility of
 allowing any undesirable parasitic currents, such as currents 310 and 330
 of FIGS. 6B and 6C, to propagate through the circuit. This may be
 accomplished by allowing the primary current turn-OFF instant 460 to occur
 beyond time 510, so as to require an earlier turn ON time 415 of the
 primary switch 43 (causing the primary source 90 to generate more energy
 than necessary). It should be noted that delaying the ultimate reduction
 of the primary current to zero at time 460 may result in decreased
 switching cycle speed.
 Turning back to FIG. 3, the preferred circuit operational sequences for the
 AUXG mode may be represented as follows:
 Partial discontinuous
 G1.fwdarw.G2.fwdarw.G3.fwdarw.G1.fwdarw.G2.fwdarw.G3.fwdarw.G1.fwdarw. . .
 .
 Discontinuous
 G1.fwdarw.G2.fwdarw.G3.fwdarw.G5.fwdarw.G1.fwdarw.G2.fwdarw.G3.fwdarw.G5.f
 wdarw.G1.fwdarw. . . .
 Delayed partial discontinuous
 G1.fwdarw.G2.fwdarw.G3.fwdarw.G4.fwdarw.G1.fwdarw.G2.fwdarw.G3.fwdarw.G4.f
 wdarw.G1.fwdarw. . . .
 Delayed Discontinuous
 G1.fwdarw.G2.fwdarw.G3.fwdarw.G4.fwdarw.G5.fwdarw.G1.fwdarw.G2.fwdarw.G3.f
 wdarw.G4.fwdarw.G5.fwdarw.G1.fwdarw. . . .
 The Super-Assisted Regeneration (SAR) mode is illustrated in FIGS. 8A and
 8B. This mode is useful for the efficient recovery of power from the load
 110 (e.g., during braking of an electric vehicle). This mode also allows
 for power recovery when there is more power available for recovery over a
 short time period than can be easily absorbed by the primary energy source
 90 alone.
 Thus, as shown in FIG. 8A, the output and primary switches 30 and 40 are
 turned on simultaneously (they are also shut off simultaneously in the SAR
 mode) so that the output total current 380, coming from the load 110 and
 the output capacitor 80 flows through the output switch 30 as output
 switch current 390, and on through the primary switch 40 as a primary
 switch current 320. Part of the output total current 380 flows into the
 primary storage element 60 as a primary energy element current 310, and
 the remaining portion flows into the auxiliary energy storage element 70
 as auxiliary energy element current 330. The diode 57 prevents current
 flow across the auxiliary switch 50 when the output and primary switches
 30 and 40 are conducting, so as to provide for sourcing power from the
 load 110 to the primary and auxiliary energy sources 90 and 100.
 This process continues when the switches 30, 40, and 50 are placed in a
 non-conducting state (i.e. the output and primary switches 30 and 40 are
 turned off simultaneously), and diodes 47 and 57 provide a means for
 conducting current flow across the primary and auxiliary switches 40 and
 50. The auxiliary total current 360 flows through the diode 57 and the
 auxiliary energy storage element 70 as an auxiliary energy element current
 330 to provide power to, or charge, the auxiliary energy source 100. The
 remaining portion of the auxiliary total current 360 flows through the
 diode 47 as a primary diode current 345, and the primary energy storage
 element 60 as a primary energy element current 310, so as to source power
 to, or charge, the primary energy source 90. In this manner, the power
 exceeding that which can safely be absorbed by the primary energy source
 90 is sent on to the auxiliary energy source 100.
 FIG. 9 illustrates various conduction states of the circuit during
 regeneration modes of operation, that is, when energy is transferred from
 the load 110 and output storage element 80 alone, or the primary energy
 source 90 alone, or from both the load 110 and output energy storage
 element 80 and the primary energy storage element 90 to the auxiliary
 energy source 100. For simplicity, the switches and diodes of FIGS. 1 and
 8 have been replaced in FIG. 9 by arrows indicating current flow paths.
 This representation is similar to the generation modes of operation
 illustrated in FIG. 3. Those skilled in the art may readily determine
 those switches and diodes that are conducting current, and those that
 block currents and voltages, in this manner.
 As illustrated in FIG. 9 and in accord with the description of operation,
 the SAR mode may comprise one of the following sets of operational circuit
 state sequences, depending upon the duty cycle of operation desired:
 Partial discontinuous mode:
 RG2.fwdarw.RG3.fwdarw.RG4.fwdarw.RG2.fwdarw.RG3.fwdarw.RG4.fwdarw.RG2.fwda
 rw. . . .
 Discontinuous mode
 RG2.fwdarw.RG3.fwdarw.RG4.fwdarw.RG5.fwdarw.RG2.fwdarw.RG3.fwdarw.RG5.fwda
 rw.RG2.fwdarw. . . .
 The final mode of operation, which is called the Output Regeneration mode
 (OUTR), is illustrated in FIGS. 10A and 10B. The OUTR mode is most useful
 for power transfer from the load 110 and output energy storage element 80
 into the auxiliary energy source 100. The OUTR mode of operation is the
 electronic conjugate of the AUXR mode. Hence, regenerated energy from the
 load 110 and the output energy storage element 80 is stored in the
 auxiliary energy source 110 without major assistance from the primary
 energy source 90, unlike the SAR mode of operation. However, similar to
 the AUXG mode of operation, the OUTR mode has the potential to develop
 parasitic oscillatory currents in the primary and auxiliary energy storage
 elements 60 and 70, respectively, if the output switch 33 is activated
 alone. Parasitic currents develop in a manner similar to that described
 above for the AUXG mode. In accord with the description of parasitic
 oscillatory current development during the AUXG mode, those skilled in the
 art may readily determine an appropriate method to prevent such parasitic
 oscillatory currents during the OUTR mode. The overall operation and the
 switching sequence during the OUTR mode is discussed here and illustrated
 in FIGS. 10 and 11.
 While most of the energy transferred to the auxiliary energy source 100
 during the OUTR mode is derived from the output load 110 and the output
 energy storage element 80, the output and the primary switches 33 and 43
 are operated sequentially to prevent uncontrolled energy build-up in the
 primary and auxiliary energy storage elements 60 and 70, respectively.
 The OUTR mode is initiated by turning ON the output switch 33 alone. This
 initiates a primary energy storage element current 310, which flows
 through the primary energy storage element 60, the output load 110, and
 the output energy storage element 80, as shown in FIG. 10A. In order to
 prevent oscillatory parasitic currents during the OUTR mode operation, a
 particular sequence of switching the output switch 33 and the primary
 switch 43 is utilized. The OUTR mode controller makes use of predictive
 duty cycle and turn-OFF time estimation to determine the turn-On
 activation time 815 for the primary switch 43.
 Referring now to FIG. 11, assume the controller determines the turn-ON
 activation time of the output switch 33 to be at time 875. The
 determination of the turn-ON activation time 875 and the turn-ON duration
 t.sub.ON,.sub.OUTR 955 happens during the turn-OFF period of the previous
 switching event. Hence, as illustrated in FIG. 11, the turn-ON
 calculations occur during the time intervals 965 and 960. Based upon the
 measured magnitude of the output load or the output energy storage element
 80 voltage, the turn-OFF duration (t.sub.OFF,.sub.OUTR) 965 of the output
 switch 33 can be calculated using the anticipated turn-ON duration
 (t.sub.ON,.sub.OUTR) 955 selected by the controller. In order to prevent
 the flow of parasitic currents, the auxiliary energy storage element 70
 needs to conduct a current such that it counters the parasitic current.
 This occurs when element 70 conducts the current 330 shown in FIG. 10B,
 which requires closure of the primary switch 43 to help build a current
 flow in the direction of current 330. However, in order to maximize energy
 transfer from the output load 110 and the output storage element 80 to the
 auxiliary source 100, the primary switch tum-ON time 815 should be
 selected to minimize the turn-ON time 953 of the primary switch 43.
 As shown in FIG. 11, the turn-OFF duration (t.sub.OFF,.sub.OUTR)965 of both
 the switches 43 and 33 are equal. Hence, an estimate of
 t.sub.OFF,.sub.OUTR 965 may be utilized to calculate the peak auxiliary
 energy storage current (i.sub.LA,t.sub.OFF) 830 using the measured voltage
 values across the primary energy source 90 and the output energy storage
 element 80 (that is, V.sub.P and V.sub.co). Utilizing volt-time area
 considerations across element 70, the turn-ON time 815 of the primary
 switch 43 can now be calculated. While the calculations occur during the
 turn-OFF period of the switches (that is, 965 and 960), the switch
 activation signals for the output and primary switches are issued at times
 975 and 815 respectively.
 This switching strategy ensures current flow through the primary and
 auxiliary storage elements 60 and 70, shown in FIGS. 10 and 11. This
 prevents parasitic currents which may otherwise have been created due to
 operation of the output switch 33 alone.
 Referring to the graph of i.sub.LP vs. time 940, the output switch 33 is
 turned-ON at time 875, illustrated by the rising slope 880 of the current
 i.sub.LP through the primary energy storage element 60. The primary switch
 43 is turned ON at time 815, as illustrated by the rising slope 820 of the
 current i.sub.LA through the auxiliary energy storage element 70. During
 the time interval 953, when both the output and the primary switch 33 and
 43 are conducting, the current 310 through the primary energy storage
 element 60 and the current 330 through the auxiliary energy storage
 element 70 flow in the paths illustrated in FIG. 10B.
 At time 900, both the output and primary switches 33 and 43 are turned OFF.
 The currents through the primary energy storage element 60 and the
 auxiliary energy storage element 70 attain peak magnitudes of
 i.sub.LP,t.sub.OFF 890 and i.sub.LA,t.sub.OFF 830, respectively. At
 turn-OFF, the currents through the elements 60 and 70 decrease to zero
 simultaneously. During the switch turn-OFF period 965, the currents
 through the elements 60 and 70 flow in the direction shown in FIG. 10C.
 As in the AUXG mode, the inactive period 960 during which no switching
 occurs may tend toward zero depending upon the selected switching
 frequency, the turn-ON duration, t.sub.ON,.sub.OUTR 955, and the values of
 individual circuit elements. If the time period 960 is not zero, then the
 circuit operates in a totally discontinuous mode of operation, which is
 characterized by zero current through both energy storage elements 60 and
 70 during each switching event. If, on the other hand, the time period 960
 equals zero, the circuit operates in a partially discontinuous mode.
 As a means of increasing switching cycle speed and conserving energy, it is
 preferable to match the time instants, 910 and 860 of the current turn-OFF
 times. However, prolonging conduction time of the auxiliary current (time
 period 965) allows for inaccuracies in the estimation of the turn-On time
 815 of the primary switch 43, reducing the possibility of allowing
 undesirable parasitic currents to propagate through the circuit. This may
 be accomplished by allowing the auxiliary current turn-OFF instant 860 to
 occur beyond time 910, so as to ensure an earlier turn-ON time 815 of the
 primary switch 43 (causing the primary source 90 to generate more energy
 than necessary). It should be noted that delaying the ultimate reduction
 of auxiliary current to zero at time 960 may result in decreased switching
 cycle speed.
 Referring back to FIG. 9, the circuit operational sequences for the OUTR
 mode may be represented as follows:
 Partial discontinuous:
 RG1.fwdarw.RG2.fwdarw.RG3.fwdarw.RG1.fwdarw.RG2.fwdarw.RG3.fwdarw.RG1.fwda
 rw. . . .
 Discontinuous:
 RG1.fwdarw.RG2.fwdarw.RG3.fwdarw.RG5.fwdarw.RG1.fwdarw.RG2.fwdarw.RG3.fwda
 rw.RG5.fwdarw.RG1.fwdarw. . . .
 Delayed partial discontinuous:
 RG1.fwdarw.RG2.fwdarw.RG3.fwdarw.RG4.fwdarw.RG1.fwdarw.RG2.fwdarw.RG3.fwda
 rw.RG4.fwdarw.RG1.fwdarw. . . .
 Delayed discontinuous:
 RG1.fwdarw.RG2.fwdarw.RG3.fwdarw.RG4.fwdarw.RG5.fwdarw.RG1.fwdarw.RG2.fwda
 rw.RG3.fwdarw.RG4.fwdarw.RG5.fwdarw.RG1.fwdarw. . . .
 Although the invention has been described with reference to specific
 embodiments, this description is not meant to be construed in a limited
 sense. The various modifications of the disclosed embodiments, as well as
 alternative embodiments of the invention, will become apparent to persons
 skilled in the art upon reference to the description of the invention. It
 is, therefore, contemplated that the appended claims will cover such
 modifications that fall within the scope of the invention, or their
 equivalents.