Topologies for multiple energy sources

[0001] 1. Field of the Invention

[0002] The present invention relates, in general, to electric power supplies employing either or both DC-DC power converters and DC-AC power inverters.

[0003] 2. Description of the Related Art

[0004] Often, the electrical requirements of a load are incompatible with the electrical characteristics of an available electrical power source. For example, the voltage of the load may be incompatible with the voltage of the power source (e.g., a 24 volt load and a 12 volt source), or the current type of the power source may be incompatible with the current type of the load (e.g., an AC load and a DC source or a DC load and an AC source). In such situations, an electric power supply may remedy the incompatibility.

[0005] An electric power supply is a translation circuit that typically provides power with the characteristics required by the load from an electric power source with characteristics which may be incompatible with the characteristics required by the load. To perform this function, a power supply typically uses one or more instances of two types of circuits in some combination. One type of circuit is a DC-DC converter, which converts between different DC voltage levels (i.e., conversion). The other type of circuit is an inverter, which changes DC power from a DC power source to AC power (i.e., inversion), or changes AC power from an AC power source to DC power (i.e., rectification). A combination of DC-DC converters and DC-AC inverters/rectifiers are frequently used to supply power to an AC load from a DC power source.

[0006] Those skilled in the art will appreciate that in certain applications, such as when the load is an electric motor of an electric drive automobile, the draw on the power supply can change relatively quickly in time (e.g., the amount of current drawn at the rated voltage may increase relatively quickly). For example, when the driver of the automobile needs maximum acceleration, such as to merge with traffic, the current drawn by the AC motor is likely to increase rapidly.

[0007] Those skilled in the art will appreciate that certain types of DC power sources, such as fuel cells, are relatively good at supplying steady output power, but may not have as suitable a short term response. Accordingly, a need exists in the art for a method and system to augment a DC power source when power drawn from that power source changes at a faster rate than that at which the power source can adequately respond.

[0008] In one embodiment, a system includes but is not limited to: a first power source having a first-source positive terminal and a first-source negative terminal; a second power source having a second-source positive terminal and a second-source negative terminal; an output capacitor having an output-capacitor positive terminal operably coupled with the second-source positive terminal and an output-capacitor negative terminal operably coupled with the first-source negative terminal; a series electrical connection between the first-source positive terminal and the second-source negative terminal; a boost converter having a boost-converter input operably coupled with the series electrical connection and the first-source negative terminal, and a boost-converter output operably coupled with the output-capacitor positive and negative terminals; and a buck-boost converter having a buck-boost-converter input operably coupled with the series electrical connection and the second-source positive terminal, and a buck-boost-converter output operably coupled with the output-capacitor positive and negative terminals.

[0009] In a second embodiment, an electrical power supply to supply electrical power between a first power source, a second power source, and a load, includes but is not limited to: a first node couplable to the first power source; a second node couplable to the second power source, the second node in series connection with the first node; a third node couplable to the load; a boost converter coupled between the first node and the third node; and a buck-boost converter coupled between the second node and the third node, wherein the buck-boost converter is operable to provide power from the second node to the third node at a first time, and the boost converter is operable to provide power from the first node to the third node at a second time.

[0010] In a third embodiment, a system includes but is not limited to: a boost converter having a boost-converter-input positive terminal and a boost-converter-input negative terminal and boost-converter-output terminals operably coupled with an output capacitor having output-capacitor positive and negative terminals; a buck-boost converter having a buck-boost-converter-input positive terminal and a buck-boost-converter-input negative terminal and buck-boost-converter-output terminals operably coupled with the output-capacitor positive and negative terminals; and a series electrical connection between the boost-converter-input positive terminal and the buck-boost-converter-input negative terminal.

[0011] In a fourth embodiment, a method for use with an electrical power supply includes but is not limited to: coupling a first-power-source negative terminal with an output-capacitor negative terminal; coupling a first-power-source positive terminal with a second-power-source negative terminal; coupling a second-power-source positive terminal with an output-capacitor positive terminal; coupling a buck-boost converter input with the second-power-source negative terminal and the second-power-source positive terminal; coupling a buck-boost converter output with the output-capacitor positive and negative terminals; coupling a boost converter input with the first-power-source positive terminal and the first-power-source negative terminal; and coupling a boost converter output with the output-capacitor positive and negative terminals.

[0012] In a fifth embodiment, a method of providing power includes but is not limited to: charging an output capacitor by boosting a voltage of a series connection between a first-power-source positive terminal and a second-power-source negative terminal when a first power source has output voltage within a defined tolerance; and charging the output capacitor by boosting and reversing the polarity of the voltage of the series connection between the first-power-source positive terminal and the second-power-source negative terminal when the first power source has output voltage outside of a defined tolerance.

[0013] In a sixth embodiment, a method of providing power includes but is not limited to: charging an output capacitor by boosting and reversing a polarity of a voltage of a series connection between a first-power-source positive terminal and a second-power-source negative terminal when a second power source has output voltage within a defined tolerance; and charging the output capacitor by boosting the voltage of the series connection between the first-power-source positive terminal and the second-power-source negative terminal when the second power source has output voltage outside of a defined tolerance.

[0014] In a seventh embodiment, a system includes but is not limited to: a first power source node having a first-node positive terminal and a first-node negative terminal; a second power source node having a second-node positive terminal and a second-node negative terminal; an output capacitor having an output-capacitor positive terminal operably coupled with the second-node positive terminal and an output-capacitor negative terminal operably coupled with the first-node negative terminal; a series electrical connection between the first-node positive terminal and the second-node negative terminal; and an inductor electrically connected to the series electrical connection, the inductor forming a part of a boost converter and the inductor forming a part of a buck-boost converter.

[0015] The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. Other aspects, inventive features, and advantages of the devices and/or processes described herein, as defined solely by the claims, will become apparent in the non-limiting detailed description set forth herein.

[0016] FIG. 1 is a schematic diagram of an electric power supply employing both DC-DC converters and DC-AC inverters.

[0017] FIG. 2A illustrates an example of a method and system that utilize a boost converter with the primary power source, and a buck-boost converter with the secondary power source.

[0018] FIG. 2B illustrates the operation of a boost converter portion of the method and system of FIG. 3A.

[0019] FIG. 2C illustrates the operation of a buck-boost converter portion of the method and system of FIG. 3A.

[0020] FIG. 3 shows the converter voltage as a function of the boost converter duty cycle, D.

[0021] FIG. 4A illustrates an example of a method and system that utilize a buck-boost converter with the primary power source, and a boost converter with the secondary power source.

[0022] FIG. 4B illustrates the operation of a buck-boost converter portion of the method and system of FIG. 5A.

[0023] FIG. 4C illustrates the operation of a boost converter portion of the method and system of FIG. 5A.

[0024] FIG. 5 shows the converter voltage as a function of the buck-boost converter duty cycle, D.

[0025] The use of the same symbols in different drawings typically indicates similar or identical items.

[0026] FIG. 1 illustrates a related-art circuit configuration. A primary power source 200 (e.g., a fuel cell) having voltage V₁, and a secondary power source 202 (e.g., a battery) having voltage V₂ supply the power. Other examples of primary power source 200 include photovoltaic power sources, biomass power sources, geothermal power sources, etc. Other examples of secondary power source 202 include inertial devices or ultra capacitors. The output voltage V_dc across capacitor bank 204 supplies a three-phase inverter 206, which inverts the DC input power to AC output power, and supplies the AC output power to a load such as a utility 208. In other applications, the voltage V_dc can supply a DC load (not shown) or an inverter with any phase numbers such as single-phase and four-phase inverters (not shown).

[0027] Inductor L_d1, and switch-diode pairs S_1u and S_1d, serve as a bi-directional dc-dc converter. With S_1u, inductor L_d1, and the anti-paralleled diode of S_1d, the circuit serves as a buck converter that can charge the voltage V₂ of the secondary power source 202 to a desired voltage. With S_1d, inductor L_d1, and the anti-paralleled diode of S_1u, the circuit serves as a boost converter that takes the secondary power source 202 voltage V₂ and boosts V₂ to the dc bus voltage V_dc.

[0028] Switch-diode pairs S_2u-S_2d in conjunction with inductor L_d2, and switch-diode pairs S_3u-S_3d in conjunction with inductor L_d3, serve as interleaved dc-dc boost converters of the power of primary power source 200. When switch-diode pairs S_2u-S_2d in conjunction with inductor L_d2, and switch-diode pairs S_3u-S_3d in conjunction with inductor L_d3 serve as interleaved dc-dc boost converters, only the lower switches S_2d and S_3d and anti-paralleled diodes of upper switches S_2u and S_3u are used. The interleaved boost converters take the power from primary power source 200 at voltage V₁ and boost the power to the dc bus voltage V_dc. If the power level is not high, and the ripple current of primary power source 200 is not strictly regulated, the two interleaved branches can be combined to one to reduce the device count. The advantages of using such an interleaved structure may include (1) input ripple current reduction, (2) individual component power rating reduction, and (3) fast current-loop control response.

[0029] The dc bus voltage V_dc is a function of source voltage and duty cycle. Assuming that the duty cycle of the bottom switch is D, the expression of V_dc is simply V_dcV₁/(1-D) or V_dcV₂/(1-D) depending on which voltage is larger. In a typical control system design, the primary power source 200 is normally designated as the steady-state power source that supplies energy constantly to establish the dc bus voltage, and the secondary power source 202 stabilizes the dc bus voltage fluctuation during dynamic load change conditions. Because D is always less than 1, V_dc must be larger than V₁ and V₂. Ideally, the difference between V_dc and V₁, or V_dc and V₂ is small, because the devices need to handle a large current at low voltage side (V₁ or V₂) and withstand a high voltage of V_dc. However, the voltage difference is determined by the availability of the power sources and the utility voltage level, so in real-world systems this is not always possible.

[0030] FIG. 1 illustrates that the switches and diodes utilized by the interleaved boost converters are those of a commercially available standard six-pack module. From the foregoing description of the interleaved boost converters it can be seen that two switches and two diodes of the commercially available standard six-pack modules go unused, making the unused devices redundant. Such redundant and unused components constitute unneeded cost and complexity that are undesirable in virtually any system. However, in systems where the voltage level of either of the power sources 200, 202 driving the capacitor bank 204 is much lower than the dc bus voltage V_dc (i.e., the voltage of capacitor bank 204) but the power is quite high, such as in most utility inter-tie systems, the unneeded cost and complexity are much more significant given the switch ratings and associated costs of high power electronics.

[0031] The present methods and systems may potentially save the cost of devices and components by reducing the required voltage and current ratings of such devices. In addition, the present methods and systems may further improve electric power supply controllability and reliability with less voltage and current stresses. Non-exhaustive examples of such methods and systems will now be discussed.

[0032] A. Method and System Using Boost Converter with the Primary Power Source and Buck-Boost Converter with the Secondary Power Source, Where the Primary and Secondary Power Sources Are Connected in Electrical Series

[0033] FIG. 2A illustrates an example of a method and system that utilize a boost converter with the primary power source, and a buck-boost converter with the secondary power source. The boost converter and the buck-boost converter are not indicated in isolation because in the example shown the boost converter and the buck-boost converter share components and connections. This is an unexpected structure and result, in that the functioning of the boost converter and the buck-boost converter are basically the obverse of each other. The functional operation of the main circuit in FIG. 2A can be divided into two modes: (1) boost converter operation and (2) buck-boost converter operation.

[0034] 1. Boost Converter Operation

[0035] FIG. 2B illustrates the operation of a boost converter portion of the method and system of FIG. 2A. Specifically, while a first boost converter composed of inductor L_d2 in conjunction with switch S_2d and diode D_2u, and a second boost converter composed of inductor L_d3 in conjunction with switch S_3d and diode D_3u are shown in FIG. 2B, only the operation of the first boost converter is described herein for sake of clarity. The operation of the second boost converter can be understood by straightforward application of the discussion of the first boost converter, and it is to be understood that in one embodiment the first and the second boost converters operate in tandem.

[0036] Shown is that the input voltage of the first boost converterwhere the first boost converter is composed of L_d2 in conjunction with switch S_2d and diode D_2uis what has been denoted lower-half voltage V_d. As used herein, the term lower-half voltage V_d is used relationally with the term upper-half voltage V_u to further emphasize certain unique and unexpected aspects of the methods and systems described herein. It is to be understood that where switches, voltages, or other components and parameters are referred to or subscripted herein as upper, lower, up, down, etc., such references are for convenience only, without regard to any orientation in space or location.

[0037] FIG. 2B also shows that in one embodiment the lower-half voltage V_d is supplied by primary power source 200, and thus is the same as the voltage level V, of the primary power source. Depicted is that, in some implementations, an optional energy storage device 300 (e.g., an ultra capacitor) is also present at the input of the first boost converter. In operation, switch S_2d is switched on and off by a control unit (not shown) using conventional control techniques so long as primary power source 200 is able to supply power sufficient such that the output voltage of high frequency capacitor 302 remains at V_dc.

[0038] When the first boost converter and the second boost converter which as described above consist of inductors L_d2 and L_d3, switches S_2d and S_3d and diodes D_2u and D_3uare operating to produce a high dc bus voltage V_dc, the upper-half voltage, V_u, is the difference between V_dc and V_d. Defining the duty cycle of the lower-half switch as D, the dc bus voltage can be obtained as V_dcV_d/(1D)V₁/(1D).

[0039] Notice that the input of the first boost converter has a specific polarity denoted by the polarities of primary power source 200, and (optional) energy storage device 300. Consequently, the input of the first boost converter can be viewed as having a positive terminal and a negative terminal. As will become apparent shortly, the buck-boost converter can also be viewed as having a positive and a negative terminal. When the boost converter and the buck-boost converter are so viewed, it can be seen that, unlike the related art methods and systems, the positive terminal of the first boost converter is series connected with a negative terminal of a buck-boost converter in an unexpected and unique fashion.

[0040] 2. Buck-Boost Converter Operation

[0041] FIG. 2C illustrates the operation of a buck-boost converter portion of the method and system of FIG. 2A. Specifically, while a first buck-boost converter composed of inductor L_d2 in conjunction with switch S_2u and diode D_2d, and a second boost converter composed of inductor L_d3 in conjunction with switch S_3u and diode D_3d are shown in FIG. 2C, only the operation of the first buck-boost converter is described herein for sake of clarity. The operation of the second buck-boost converter can be understood by straightforward application of the discussion of the first boost converter, and it is to be understood that in one embodiment the first and the second buck-boost converters operate in tandem.

[0042] Shown is that the input voltage of the first buck-boost converterwhere the first buck-boost converter is composed of inductor L_d2 in conjunction with switch S_2u and diode D_2dis what has been denoted upper-half voltage V_u. As used herein, the term upper-half voltage V_u is used to relationally with the term lower-half voltage V_d to further emphasize certain unique and unexpected aspects of the methods and system described herein.

[0043] As will become apparent below, the first buck-boost converter in conjunction with the upper-half voltage V_u, can provide several services. One useful service provided by the first buck-boost converter is that it can provide power output in a complementary cycle when the primary power source 200 is not supplying power. One useful service provided by the upper-half voltage V_u is that it stabilizes the lower-half voltage when the primary power source 200 supply has a time delay during dynamic load changes (e.g., as with a fuel cell used in an automobile, as described in the background section above).

[0044] Continuing to refer to FIG. 2C, in one embodiment, the voltage level V₂ of the secondary power source 202 is normally less than V_u and can be boosted to V_u so that the sum of V_d and V_u equals the dc bus voltage, V_dc. In one embodiment, switches S_1U and S_1d form bi-directional dc-dc converter 306 that utilizes S_1d and the anti-paralleled diode of S_1u as a boost converter and S_1u and the anti-paralleled diode of S_1d as a buck converter. In one embodiment, the buck converter allows the secondary power source, V₂, be charged by V_u so that the state of charge of the secondary power source 202 can be maintained.

[0045] Continuing to refer to FIG. 2C, in one embodiment the upper-half voltage V_u is supplied by a boosted version of the voltage V₂ of secondary power source 202. In some implementations, an optional energy storage device 304 (e.g., an ultra capacitor) is also present at the input of the first buck-boost converter. In operation, bi-directional dc-dc converter 306 is controlled by a control unit (not shown) using conventional techniques such that the boosted voltage VU of the voltage V₂ of secondary power source 202 is sufficient to supply the output voltage of high frequency capacitor 302 at V_dc.

[0046] Energy storage component 304, in one embodiment, is a large electrolytic capacitor or ultra capacitor. In one embodiment, an ultra capacitor is used with sufficient energy storage such that secondary power source 202 and associated bidirectional dc-dc converter 306 may not be necessary because V_u can be charged during boost mode operation. Without secondary power source 202, the buck-boost converter operation is suitable for dc bus voltage conditioning during dynamic load changes.

[0047] Notice that the input of the first buck-boost converter has a specific polarity denoted by the polarities of secondary power source 202, and (optional) energy storage device 304. Consequently, the input of the first buck-boost converter can be viewed as having a positive terminal and a negative terminal. In addition, the positive terminal of the first boost converter input is series connected with a negative terminal of the buck-boost converter. This is a very counterintuitive connection, and is the exact obverse of that taught in the related art.

[0048] 3. Boost And Buck-Boost Converter Duty Cycles

[0049] As can be seen in FIGS. 2A-2C, any upper switch and series connected lower switch are preferably not turned on simultaneously (e.g., switches S_2u and S_2d) since neither the buck-boost nor boost converter will operate in such a configuration. Consequently, the duty cycle of boost converter operation becomes the maximum available complementary duty cycle of buck-boost converter operation, and vice versa.

[0050] FIG. 3 shows the converter voltage as a function of the boost converter duty cycle, D. If the primary power source is constrained such that D<0.5, then V_dc2V_d and V_u<V_d. If V_dc>2V_d is preferred, then the buck-boost converter preferably has an input voltage higher than V_d, i.e., V_u>V_d.

[0051] B. Method and System Using Buck-Boost Converter with the Primary Power Source and Boost Converter with the Secondary Power Source, Where the Primary and Secondary Power Sources are Connected in Electrical Series

[0052] FIG. 4A shows an example of a method and system that utilize a buck-boost converter with the primary power source, and a boost converter with the secondary power source. The boost converter and the buck-boost converter are not indicated in isolation because in the example shown the boost converter and the buck-boost converter share components and connections. This is an unexpected structure and result, in that the functioning of the boost converter and the buck-boost converter are basically the obverse of each other. The functional operation of the method and system in FIG. 4A can be divided in two modes: (1) buck-boost converter operation and (2) boost converter operation.

[0053] 1. Buck-Boost Converter Operation

[0054] FIG. 4B illustrates the operation of a buck-boost converter portion of the method and system of FIG. 4A. Specifically, while a first buck-boost converter composed of inductor L_d2 in conjunction with switch S_2u and diode D_2d, and a second boost converter composed of inductor L_d3 in conjunction with switch S_3u and diode D_3d are shown in FIG. 4B, only the operation of the first buck-boost converter is described herein for sake of clarity. The operation of the second buck-boost converter can be understood by straightforward application of the discussion of the first boost converter, and it is to be understood that in one embodiment the first and the second buck-boost converters operate in tandem.

[0055] The input voltage of the first buck-boost converter (i.e., inductor L_d2 in conjunction with switch S_2u and diode D_2d)is what has been denoted upper-half voltage V_u. As used herein, the term upper-half voltage V_u is used relationally with the term lower-half voltage V_d to further emphasize certain unique and unexpected aspects of the methods and system described herein.

[0056] FIG. 4B also shows that in one embodiment the upper-half voltage V_u is supplied by primary power source 200, and thus is the same as the voltage level V₁ of the primary power source. In some implementations, an optional energy storage device 300 (e.g., an ultra capacitor) is also present at the input of the first boost converter. In operation, switch S_2d is switched on and off by a control unit (not shown) using conventional control techniques so long as primary power source 200 is able to supply power sufficient such that the output voltage can be maintained at V_dc.

[0057] Notice that the input of the first buck-boost converter has a specific polarity denoted by the polarities of primary power source 200, and (optional) energy storage device 300. Consequently, the input of the first buck-boost converter can be viewed as having a positive terminal and a negative terminal. Moreover, the negative terminal of the first boost converter input (described below) is series connected with a negative terminal of the buck-boost converter. This is a very counterintuitive connection, and is the exact obverse of that taught in the related art.

[0058] When the first boost converter and the second boost converter (i.e., inductors L_d2 and L_d3, switches S_2d and S_3d and diodes D_2u and D_3u) are operating to produce a high dc bus voltage V_dc, the upper-half voltage V_u is the difference between V_dc and V_d. Defining the duty cycle of the lower-half switch as D, the dc bus voltage can be obtained as V_dcV_d/(1D)V₁/(1D).

[0059] When the first buck-boost converter and the second buck-boost converter (i.e., inductors L_d2 and L_d3, switches S_2u and S_3u and diodes D_2d and D_3d) are operating, a consequence of their operation is that they produce lower-half voltage V_d. The sum of V_d and V_u is the dc bus voltage V_dc. Defining the duty cycle of the upper-half switch as D, the lower-half and dc bus voltages can be obtained as V_dV_uD/(1D)V₁D/(1D) and V_dcV_uV_d.

[0060] 2. Boost Converter Operation

[0061] FIG. 4C illustrates the operation of a boost converter portion of the method and system of FIG. 4A. Specifically, while a first boost converter composed of inductor L_d2 in conjunction with switch S_2d and diode D_2u, and a second boost converter composed of inductor L_d3 in conjunction with switch S_3d and diode D_3u are shown in FIG. 4C, only the operation of the first boost converter is described herein for sake of clarity. The operation of the second boost converter can be understood by straightforward application of the discussion of the first boost converter, and it is to be understood that in one embodiment the first and the second boost converters operate in tandem.

[0062] The input voltage of the first boost converter (i.e., inductor L_d2 in conjunction with switch S_2d and diode D_2u) is what has been denoted lower-half voltage V_d. As used herein, the term upper-half voltage V_u is used to relationally with the term lower-half voltage V_d to further emphasize certain unique and unexpected aspects of the methods and system described herein. The difference of V_dc and V_u becomes V_d.

[0063] As will become apparent below, the first boost converter in conjunction with V_d can provide several services. The first boost converter can provide power output in a complementary cycle when the primary power source 200 is not supplying power. Additionally, or alternatively, the voltage V_d stabilizes the upper-half voltage V_u when the primary power source 200 supply has a time delay during dynamic load changes, such as was outlined in the background section above.

[0064] FIG. 4C also shows that in one embodiment, the voltage level V₂ of the secondary power source 202 normally operates at less than V_d and can be boosted to V_d so that the sum of V_d and V_u equals the desired dc bus voltage V_dc. In one embodiment, switches S_1u and S_1d form bidirectional dc-dc converter 306 that utilizes S_1d and the anti-paralleled diode of S_1u as a boost converter and S_1u and the anti-paralleled diode of S_1d as a buck converter. The buck converter allows the secondary power source V₂ be charged by V_d so that the state of charge of the secondary power source 202 can be maintained.

[0065] In one embodiment the lower-half voltage V_d is supplied by a boosted version of the voltage V₂ of secondary power source 202. In some implementations, an optional energy storage device 304 (e.g., an ultra capacitor) is also present at the input of the first buck-boost converter. In operation, bi-directional dc-dc converter 306 is controlled by a control unit (not shown) such that the boosted version V_d of the voltage V₂ of primary power source 202 voltage is sufficient such that the buck-boost converter can boost V_d enough to supply the output voltage of high frequency capacitor 302 at V_dc.

[0066] Notice that the input of the first buck-boost converter has a specific polarity denoted by the polarities of secondary power source 202, and (optional) energy storage device 304. Consequently, the input of the first buck-boost converter can be viewed as having a positive terminal and a negative terminal. Furthermore, the positive terminal of the first boost converter input is series connected with a negative terminal of the first buck-boost converter. This is a very counterintuitive connection, and is the exact obverse of that taught in the related art.

[0067] In addition, the input of the first boost converter has a specific polarity denoted by the polarities of secondary power source 202, and (optional) energy storage device 304. Consequently, the input of the first boost converter can be viewed as having a positive terminal and a negative terminal. As will become apparent, the buck-boost converter can also be viewed as having a positive terminal and a negative terminal, where the positive terminal of the first boost converter is series connected with a negative terminal of a buck-boost converter.

[0068] 3. Buck-Boost Converter and Boost Converter Duty Cycles

[0069] FIGS. 4A-4C show that any upper switch and lower series connected switch are preferably not turned on simultaneously (e.g., switches S_2u, and S_2d) since neither the buck-boost nor boost converter will operate in such a configuration. Because the upper switch and lower switch cannot be turned on simultaneously, the duty cycle of buck-boost mode operation becomes the maximum available complementary duty cycle of boost mode operation, and vice versa.

[0070] FIG. 5 shows the converter voltage as a function of the buck-boost converter duty cycle, D. If the primary power source is constrained to D<0.5, then V_u<V_d and V_dc<2V_d. If V_dc>2V_d is preferred, then the boost converter is to have an input voltage higher than the primary power source voltage, V_u, i.e., V_d>V_u.

[0071] From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. For example, additional applications of the subject matter described herein include but are not limited to distributed generation converters, microturbine power converters, utility interactive load-banks, photovoltaic source supplied inverters, systems using fuel cells or batteries or other storage devices UPS (uninterruptable power supplies) implementations using power converters, as well as electric, fuel and hybrid electric powertrains. Accordingly, the invention is not limited except as by the appended claims.