Bidirectional DC / DC converter with high efficiency under wide voltage conversion range

Abstract

An example of a bidirectional DC / DC converter includes an energy source side, an energy storage side, a resonant tank comprising a plurality of resonant capacitors, a bidirectional switch connected to the energy source side by a pair of commutation capacitors and a pair of capacitors of the resonant tank, and a transformer. energy source side is configured to be connected to an energy source and includes a first switching arrangement. The energy storage side is configured to be connected to an energy storage system and includes a second switching arrangement. The energy source side is connected to a primary side of the transformer, and the energy storage side is connected to a secondary side of the transformer.

Description

BIDIRECTIONAL DC / DC CONVERTER WITH HIGH EFFICIENCY UNDER WIDE VOLTAGE CONVERSION RANGE CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims the benefit of U. S. Provisional Application Serial No. 63 / 673,490, filed on July 19, 2024, entitled BIDIRECTIONAL DC / DC CONVERTER WITH HIGH EFFICIENCY UNDER WIDE VOLTAGE CONVERSION RANGE, the entire disclosure of which is incorporated herein by reference in its entirety.GOVERNMENT LICENSE RIGHTS

[0002] This invention was made with government support under G03697 awarded by the National Institute of Standards and Technology. The government has certain rights in the invention.TECHNICAL FIELD

[0003] The present application relates generally to power electronics and in particular to DC-to-DC converters for use in systems with a wide voltage conversion range.BACKGROUND

[0004] Renewable energy, such as wind energy, solar energy, etc. has been widely used to reduce reliance on fossil fuels and address the problem of global climate change in many applications, such as, for example, energy distributed systems, microgrid systems, and telecom power systems. Unlike fossil fuel, nuclear, and other power systems, renewable energy generation systems (REGS) are inherently unstable power sources because the renewable energy generation system output depends on environmental factors that are outside human control. For example, the output power of a solar panel is affected by the season of the year, the time of day, local cloud cover, precipitation, air temperature, and other factors.Therefore, energy storage systems, such as batteries, are usually integrated into the renewable energy generation systemto stabilize the power output of the renewable energy generation system.

[0005] Looking beyond renewable energy systems, energy storage systems are increasingly used in a wide variety of power systems to store and discharge electrical power on vehicles and on the energy grid. Converters are used with energy storage systems because energy storage systems operate a different voltage levels from connected energy sources and loads. The voltage levels on the storage and source / load sides of the converter can also vary significantly during operation, increasing the voltage range over which the converters operate. In some energy systems, a load and energy source are arranged on each side of a converter so that the converter is required to work and provide high efficiency under a wide range of voltage conversions.SUMMARY

[0006] This summary is meant to provide examples and is not intended to be limiting of the scope of the invention in any way. For example, any feature included in an example of this summary is not required by the claims, unless the claims explicitly recite the feature.

[0007] An example of a bidirectional DC / DC converter includes an energy source side, an energy storage side, a resonant tank comprising a plurality of resonant capacitors, a bidirectional switch connected to the energy source side by a pair of commutation capacitors and a pair of capacitors of the resonant tank, and a transformer, energy source side is configured to be connected to an energy source and includes a first switching arrangement. The energy storage side is configured to be connected to an energy storage system and includes a second switching arrangement. The energy source side is connected to a primary side of the transformer, and the energy storage side is connected to a secondary side of the transformer.

[0008] In some implementations, during a step-down mode of the bidirectional DC / DC converter the first switching arrangement operates as an inverter, and the second switching arrangement operates as a rectifier; and during a step-up mode of the bidirectional DC / DC converter the second switching arrangement operates as an inverter, and the first switching arrangement operates as a rectifier.

[0009] In some implementations, when the bidirectional DC / DC converter is in a step-down mode, two switches of the first switching arrangement and a split-resonant capacitor of the energy source side form a half-bridge inverter; and when the bidirectional DC / DC converter is in a step-up mode, the two switches of the first switching arrangement and the split- resonant capacitor of the energy source side form a full-wave voltage double rectifier.

[0010] In some implementations, when the bidirectional DC / DC converter is in a step-down mode, four switches of the second switching arrangement are turned on to form a full-bridge rectifier; and when the bidirectional DC / DC converter is in a step-up mode, two switches of the second switching arrangement are turned off to form an inverter.

[0011] In some implementations, the energy source side comprises a low-pass filter that is operable when the bidirectional DC / DC converter is in a step-up mode; and the energy storage side comprises a low-pass filter that is operable when the bidirectional DC / DC converter is in a step-down mode.

[0012] In some implementations, the second switching arrangement comprises two switches that form a half-bridge rectifier when the bidirectional DC / DC converter is in a step-down mode.

[0013] An example of a renewable electric generation system includes any of the examples of the bidirectional DC / DC converter described above, a renewable power source connected to the energy source side of the bidirectional DC / DC converter, andan energy storage system connected to the energy storage side of the bidirectional DC / DC converter.

[0014] In some implementations, the renewable electric generation system includes a first sensor for detecting at least one of a first voltage and a first current flowing between the renewable power source and the bidirectional DC / DC converter; a second sensor for detecting at least one of a second voltage and a second current flowing between the energy storage system and the bidirectional DC / DC converter; and a controller connected to the bidirectional DC / DC converter, the first sensor, and the second sensor, wherein the controller controls the first switching arrangement, the second switching arrangement, and the bidirectional switch based on at least one of the first voltage, the first current, the second voltage, and the second current.

[0015] In some implementations, when the bidirectional DC / DC converter is in a step-down mode: the controller employs pulse-frequency modulation control of the bidirectional DC / DC converter when the second voltage is between a maximum value and an intermediate value; and the controller employs pulse-width modulation control of the bidirectional DC / DC converter when the second voltage is between the intermediate value and a minimum value; wherein a switching frequency of the bidirectional DC / DC converter is fixed at a maximum value during the pulse-width modulation control of the bidirectional DC / DC converter.

[0016] In some implementations, when the bidirectional DC / DC converter is in a step-up mode: the controller employs pulsefrequency modulation control of the bidirectional DC / DC converter when the second voltage is at a minimum until the second voltage is at an intermediate value; and the controller employs pulse-width modulation control of the bidirectional DC / DC converter when the second voltage is at the intermediate value until the second voltage is at a maximumvalue; wherein a switching frequency of the bidirectional DC / DC converter is fixed at a maximum value during the pulse-width modulation control of the bidirectional DC / DC converter.

[0017] In some implementations, the controller employs both pulsewidth modulation control and pulse-frequency modulation control simultaneously.

[0018] An example method of operating any of the example bidirectional DC / DC converters described herein in a stepdown mode or a step-up mode includes: operating, in the stepdown mode, at least one switch of the first switching arrangement to form an inverter; operating, in the step-down mode, at least one switch of the second switching arrangement to form a rectifier; operating, in the step-up mode, at least one switch of the first switching arrangement to form a rectifier; operating, in the step-up mode, at least one switch of the second switching arrangement to form an inverter; operating the bidirectional switch using pulse-frequency modulation for a first voltage range of a voltage of the energy storage system; and operating the bidirectional switch using pulse-width modulation for a second voltage range of the voltage of the energy storage system.

[0019] In some implementations, the bidirectional switch is operated using pulse-width modulation and pulse-frequency modulation simultaneously.BRIEF DESCRIPTION OF THE DRAWINGS

[0020] To further clarify various aspects of embodiments of the present disclosure, a more particular description of the certain embodiments will be made by reference to various aspects of the appended drawings. It is appreciated that these drawings depict only typical embodiments of the present disclosure and are therefore not to be considered limiting of the scope of the disclosure. Moreover, while the figures can be drawn to scalefor some embodiments, the figures are not necessarily drawn to scale for all embodiments.

[0021] Figure 1 shows a block diagram of an example renewable energy generation system;

[0022] Figure 2 shows a block diagram of an example bidirectional DC / DC converter of the renewable energy generation system of Figure 1;

[0023] Figure 3 shows a schematic of an example bidirectional DC / DC converter;

[0024] Figure 4 shows a schematic of an example bidirectional DC / DC converter;

[0025] Figure 5 shows a timing diagram illustrating pulse-frequency modulation control during operation of an example bidirectional DC / DC converter in a step-down mode;

[0026] Figure 6 shows a timing diagram illustrating pulse-width modulation control during operation of an example bidirectional DC / DC converter in a step-down mode;

[0027] Figure 7 shows a plot over time of voltages and currents measured during operation of an example bidirectional DC / DC converter in a step-down mode;

[0028] Figure 8 is a schematic indicating current flow through an example bidirectional DC / DC converter during a first state of the step-down mode;

[0029] Figure 9 is a schematic indicating current flow through an example bidirectional DC / DC converter during a second state of the step-down mode;

[0030] Figure 10 is a schematic indicating current flow through an example bidirectional DC / DC converter during a third state of the step-down mode;

[0031] Figure 11 is a schematic indicating current flow through an example bidirectional DC / DC converter during a fourth state of the step-down mode;

[0032] Figure 12 is a schematic indicating current flow through an example bidirectional DC / DC converter during a fifth state of the step-down mode;

[0033] Figure 13 is a schematic indicating current flow through an example bidirectional DC / DC converter during a sixth state of the step-down mode;

[0034] Figure 14 is a schematic indicating current flow through an example bidirectional DC / DC converter during a seventh state of the step-down mode;

[0035] Figure 15 shows a timing diagram illustrating pulse-frequency modulation control during operation of an example bidirectional DC / DC converter in a step-up mode;

[0036] Figure 16 shows a timing diagram illustrating pulse-width modulation control during operation of an example bidirectional DC / DC converter in a step-up mode;

[0037] Figure 17 shows a plot over time of voltages and currents measured during operation of an example bidirectional DC / DC converter in a step-up mode;

[0038] Figure 18 is a schematic indicating current flow through an example bidirectional DC / DC converter during a first state of the step-up mode;

[0039] Figure 19 is a schematic indicating current flow through an example bidirectional DC / DC converter during a second state of the step-up mode;

[0040] Figure 20 is a schematic indicating current flow through an example bidirectional DC / DC converter during a third state of the step-up mode;

[0041] Figure 21 is a schematic indicating current flow through an example bidirectional DC / DC converter during a fourth state of the step-up mode;

[0042] Figure 22 is a schematic indicating current flow through an example bidirectional DC / DC converter during a fifth state of the step-up mode;

[0043] Figure 23 is a schematic indicating current flow through an example bidirectional DC / DC converter during a sixth state of the step-up mode;

[0044] Figure 24 is a block diagram showing a method of operating an example bidirectional DC / DC converter;

[0045] Figure 25 shows a schematic of an example bidirectional DC / DC converter;

[0046] Figure 26 shows a timing diagram illustrating pulse-frequency and pulse-width modulation control during operation of an example bidirectional DC / DC converter in a step-down mode;

[0047] Figure 27 is a schematic indicating current flow through an example bidirectional DC / DC converter during a first state of the step-down mode;

[0048] Figure 28 is a schematic indicating current flow through an example bidirectional DC / DC converter during a second state of the step-down mode;

[0049] Figure 29 is a schematic indicating current flow through an example bidirectional DC / DC converter during a third state of the step-down mode;

[0050] Figure 30 is a schematic indicating current flow through an example bidirectional DC / DC converter during a fourth state of the step-down mode;

[0051] Figure 31 is a schematic indicating current flow through an example bidirectional DC / DC converter during a fifth state of the step-down mode;

[0052] Figure 32 is a schematic indicating current flow through an example bidirectional DC / DC converter during a sixth state of the step-down mode;

[0053] Figure 33 is a schematic indicating current flow through an example bidirectional DC / DC converter during a seventh state of the step-down mode;

[0054] Figure 34 shows a timing diagram illustrating pulse-frequency and pulse-width modulation control during operation of an example bidirectional DC / DC converter in a step-up mode

[0055] Figure 35 is a schematic indicating current flow through an example bidirectional DC / DC converter during a first state of the step-up mode;

[0056] Figure 36 is a schematic indicating current flow through an example bidirectional DC / DC converter during a second state of the step-up mode;

[0057] Figure 37 is a schematic indicating current flow through an example bidirectional DC / DC converter during a third state of the step-up mode;

[0058] Figure 38 is a schematic indicating current flow through an example bidirectional DC / DC converter during a fourth state of the step-up mode;

[0059] Figure 39 is a schematic indicating current flow through an example bidirectional DC / DC converter during a fifth state of the step-up mode; and

[0060] Figure 40 is a schematic indicating current flow through an example bidirectional DC / DC converter during a sixth state of the step-up mode.DETAILED DESCRIPTION

[0061] The following description refers to the accompanying drawings, which illustrate specific embodiments of the present disclosure. Other embodiments having different structures and operation do not depart from the scope of the present disclosure.

[0062] As described herein, when one or more components are described as being connected, joined, affixed, coupled, attached, or otherwise interconnected, such interconnection maybe direct as between the components or maybe indirect such as through the use of one or more intermediary components. Also as described herein, reference to a "member," “component,” or “portion” shall not be limited to a single structural member, component, or element but can include an assembly of components, members, or elements. Additionally, as used herein, the term “exemplary” is intended to mean serving as an illustration or example of something and is not intended to indicate a preference.

[0063] Numerical values or ranges stated herein are understood to encompass values at or near the stated value and / or above or below the stated range. For this application, the stated value can encompass plus or minus 5% of the value and the stated range can encompass plus or minus 5% of the extent of the range. In addition, the stated value or range can include a margin of error for the value or range typical in the art for the property being measured. The stated value or range can also encompass those values and ranges that would be considered equivalent to the stated value or range by one of ordinary skill in the art. As an example, a voltage expressed as a range of 300 to 400 volts is understood to include voltages above and below the ends of the range by 5% of the extent of the 100-volt range— e.g., 295 volts to 405 volts. As another example, an electrical current expressed as a value of 30 amps includes values above and below 30 amps that are within the margin of error of a tool typically used to electrical current of that magnitude. As yet another example, an output power expressedas a value of 2 kilowatts includes values above and below 2 kilowatts that would be considered equivalent by one of ordinary skill in the art.

[0064] “Component” and “system” as used herein are intended to encompass computer-readable data storage that is configured with computer-executable instructions that cause certain functionality to be performed when executed by a processor. The computer-executable instructions may include a routine, a function, or the like. It is also to be understood that a component or system maybe localized on a single device or distributed across several devices. Moreover, the acts described herein may be computer-executable instructions that can be implemented by one or more processors and / or stored on a computer-readable medium or media. The computerexecutable instructions can include a routine, a sub-routine, programs, a thread of execution, and / or the like. Still further, results of acts of the methodologies can be stored in a computer-readable medium, displayed on a display device, and / or the like.

[0065] “Computer” or “processor” as used herein includes, but is not limited to, any programmed or programmable electronic device or coordinated devices that can store, retrieve, and process data and may be a processing unit or in a distributed processing configuration. Examples of processors include microprocessors, microcontrollers, graphics processing units (GPUs), floating point units (FPUs), reduced instruction set computing (RISC) processors, digital signal processors (DSPs), field programmable gate arrays (FPGAs), etc. One or more cores of a single microprocessor and / or multiple microprocessor each having one or more cores can be used to perform the operations described as being executed by a processor herein. The processor can also be a processor dedicated to the operation or training of neural networks and other artificial intelligence (Al) systems. The processor or processors can be locally installed on the interface and can beprovided in a remote location that can be accessed via a network interface.

[0066] “Data storage device” as used herein means a device or devices for non-transitory storage of code or data, e.g., a device with a non-transitory computer readable medium. As used herein, “non-transitory computer readable medium” mean any suitable non-transitory computer readable medium for storing code or data, such as a magnetic medium, e.g., fixed disks in external hard drives, fixed disks in internal hard drives, and flexible disks; an optical medium, e.g., CD disk, DVD disk, and other media, e.g., ROM, PROM, EPROM, EEPROM, flash PROM, external flash memory drives, etc.

[0067] “Network interface” or “data interface” as used herein includes, but is not limited to, any interface or protocol for transmitting and receiving data between electronic devices. The network or data interface can refer to a connection to a computer via a local network or through the internet and can also refer to a connection to a portable device— e.g., a mobile device or a USB thumb drive— via a wired or wireless connection. A network interface can be used to form networks of computers to facilitate distributed and / or remote computing (i.e., cloudbased computing). “Cloud-based computing” means computing that is implemented on a network of computing devices that are remotely connected to the interface via a network interface.

[0068] “Logic,” synonymous with “circuit” as used herein includes, but is not limited to, hardware, firmware, software and / or combinations of each to perform one or more functions or actions. For example, based on a desired application or needs, logic may include a software-controlled processor, discrete logic such as an application specific integrated circuit (ASIC), programmed logic device, or another processor. Logic may also be fully embodied as software.

[0069] “Software,” as used herein, includes but is not limited to one or more computer readable and / or executable instructions thatcause a processor or other electronic device to perform functions, actions, processes, and / or behave in a desired manner. The instructions may be embodied in various forms such as routines, algorithms, modules, or programs including separate applications or code from dynamically linked libraries (DLLs). Software may also be implemented in various forms such as a stand-alone program, a web-based program, a function call, a subroutine, a servlet, an application, an app, an applet (e.g., a Java applet), a plug-in, instructions stored in a memory, part of an operating system, or other type of executable instructions or interpreted instructions from which executable instructions are created.

[0070] The bidirectional DC / DC converters of the present disclosure provide high efficiency under a wide range of voltage conversions and can be employed in a wide variety of applications. The voltages on either side of the bidirectional DC / DC converters of the present disclosure can vary from 1.4 times to 2 times and the power levels can be up to 100 kW. Example energy systems that benefit from the use of the bidirectional DC / DC converters of the present disclosure include, for example, distributed microgrids, automotive power distribution, electrical vehicle charging (onboard or vehicle-to- grid), aerospace power distribution, server racks and uninterruptable power supplies (UPS) for datacenter and telecom systems, and power distribution on naval vessels. Bidirectional DC / DC converters of the present disclosure operate in these environments at high efficiency levels that help maintain the lifetime of batteries used in energy storage systems and provide a cost-effective solution for manufacturers of these energy systems to compete in a competitive marketplace.

[0071] As has already been noted, the voltage conversion ranges can vary widely in applications where example bidirectional DC / DC converters are employed. Example voltage conversion ranges for various applications are provided below asindications of the types of systems for which the example bidirectional DC / DC converters described herein can be used. In an example of a distributed microgrid system, voltage conversion ranges can include 12V to 400 V, 12V to 200V, 24V to 400V, 24V to 200V, 48V to 400V, 48V to 200V, or the like. In an example of an automotive system, voltage conversion ranges can include 12V to 400 V, 12V to 200V, 24V to 400V, 24V to 200V, 48V to 400V, 48V to 200V, or other similar ranges. In an example electrical vehicle charging system (onboard charging or vehicle-to-grid charging), voltage conversion ranges can include 200V to 400V, 200V to variable 400V, 400V to variable 400V, or other similar ranges. In an example of an aerospace system, voltage conversion ranges can include 28V to 800V (e.g., in emerging applications), 28V to 540V (e.g., on military aircraft and the Boeing 787), 28V to 270V (e.g., on most aircraft), 28V to 120V (e.g., on the international space station), or other similar ranges. In an example of a telecommunication or datacenter system, voltage conversion ranges can include 48V to 800V, 48V to 400V, or other similar ranges. In an example of a naval system, voltage conversion ranges can include 24V to 600 V, 48V to 600V, or other similar ranges. While the above ranges may be typical for the industries cited above, voltage conversion ranges typical for the above industries (or new industries) can vary over time and can be applicable across industries.

[0072] Two circuit topologies have been used as bidirectional DC / DC converters: the CLLC converter and the Resonant Converter with Pulse Width Modulation (i.e., the Alexander topology). The traditional CLLC converter does not perform well when the voltage changes by more than 1.3 times. That is, its full-load efficiency can suffer by as much as 2% when there are wide- voltage variations. The CLLC converter also loses the ability to perform soft switching which limits the range of commutation frequencies. The bidirectional Alexander Topology features soft-switching capability and high efficiency throughout the entire operating voltage range. However, the AlexanderTopology is not suitable for power levels exceeding 3 kW because the peak current on its bidirectional switch is high. The high peak current increases stress on the semiconductors of the converter leading to a loss in efficiency when the commutation frequency is over 100 kHz. The disadvantages presented by the above converter topologies can be mitigated, at least in part, through the use of more costly components. Increased costs, however, undermines the ability of manufacturers to remain cost-competitive in the market.

[0073] Applicant has developed a bidirectional DC / DC converter that incorporates the advantages of the CLLC converter topology and the Alexander topology without the disadvantages associated with either existing converter topology. An example of the bidirectional DC / DC converter described herein is capable of soft switching across the entire voltage range and at all power levels. Additionally, the bidirectional DC / DC converter described herein does not change in efficiency more than 0.4% under maximum load for the entire energy storage system voltage range or the entire renewable energy generation system voltage range. The bidirectional DC / DC converter described herein also does not have high current stress so that wide bandgap devices can be used to increase the commutation frequencies up to as high as 700 kHz.

[0074] Referring now to Figure 1, a block diagram of an energy system 100 is shown. The energy system 100 includes a load or energy source 102 that is connected to an energy storage system 104 (e.g. a battery, a super-capacitor, or the like) via a bidirectional DC / DC converter 106. Sensors 108 for measuring current and voltage are arranged on each side of the bidirectional DC / DC converter 106. A voltage Vi is detected by the sensors 108 on a load / energy source or high-voltage (HV) side 112 and a voltage V2is detected by the sensors 108 on an energy storage or low- voltage (LV) side 114. (See, e.g., Figures 3-4.) A controller 110 is connected to the bidirectional DC / DC converter 106 and to the sensors 108. The controller 110 controls the bidirectionalDC / DC converter 106— and in particular, a plurality of switches of the bidirectional DC / DC converter 106— to charge and discharge the energy storage system 104.

[0075] Referring now to Figure 2, a block diagram of the bidirectional DC / DC converter 106 is shown. The components of the bidirectional DC / DC converter 106 extend from theload / energy source or high-voltage side 112 to the energy storage or low-voltage side 114. During a step-down or charging mode of the bidirectional DC / DC converter 106, power flows from the high-voltage side 112 to the low-voltage side 114.During a step-up or discharging mode of the bidirectional DC / DC converter 106, power flows from the low-voltage side 114 to the high-voltage side 112. Starting at the high-voltage side 112, the components of the bidirectional DC / DC converter 106 include an optional first low-pass filter 116, a first switching arrangement 118, a resonant tank 120 that includes a transformer 122, a second switching arrangement 124, and an optional second low-pass filter 126. As is described below, the first low-pass filter 116 and the second low-pass filter 126 stabilize the output of the bidirectional DC / DC converter 106 in step-up mode and in step-down mode, respectively, but are optional components that are not required for the bidirectional DC / DC converter 106 to operate as described herein.

[0076] The first switching arrangement 118 and the second switching arrangement 124 each operate as an inverter or a rectifier depending on the operating mode of the bidirectional DC / DC converter 106. When the bidirectional DC / DC converter 106 is in the step-down mode, the first switching arrangement 118 acts as an inverter and the second switching arrangement 124 acts as a rectifier. When the bidirectional DC / DC converter 106 is in the step-up mode, the first switching arrangement 118 acts as a rectifier and the second switching arrangement 124 acts as an inverter. The input DC signal is first converted by an inverter (i.e., the first switching arrangement 118 when in stepdown mode and the second switching arrangement 124 whenin step-up mode) into a square wave signal that is fed into the resonant tank 120. The square wave signal is converted by the resonant tank 120 into an AC signal that is adjusted based on the turn ratio of the transformer 122 of the resonant tank 120. The AC signal is then converted by a rectifier (i.e., the first switching arrangement 118 when in step-up mode and the second switching arrangement 124 when in step-down mode) back into an output DC signal. The output DC signal passes through the low-pass filter (i.e., the first low-pass filter 116 when in step-up mode and the second low-pass filter 126 when in step-down mode) to reduce voltage ripple and provide a constant DC voltage across a load.

[0077] During operation of the bidirectional DC / DC converter 106, the controller 110 employs pulse-width modulation and pulsefrequency modulation hybrid control to regulate the output that is monitored via feedback received at the controller 110 from the sensors 108. Both types of control— i.e., pulse-width modulation and pulse-frequency modulation— are used during the step-down and the step-up modes of operating the bidirectional DC / DC converter 106. In particular, pulsefrequency modulation control is used to provide a normalized gain greater than or equal to one and pulse-width modulation control is used to provide a normalized gain less than one.

[0078] In some implementations, pulse-width modulation and pulsefrequency modulation are employed simultaneously. For example, when the commutation frequency is not at a maximum and pulse-width modulation control is active, pulsefrequency modulation control is active so that the converter behaves like a CLLC converter while pulse-width modulation control is also being employed. This mode of operation can be a protection mechanism that is triggered by rapid and significant changes in load levels or overload conditions during, for example, step-up mode.

[0079] The controller no is used to operate the bidirectional DC / DC converter 106 in the step-down mode as follows. Pulsefrequency modulation control of the bidirectional DC / DC converter 106 is used when the voltage (V2in Figures 3 and 4) of the energy storage system 104 is at a maximum until the voltage of the energy storage system 104 is at an intermediate level. During this time the switching frequency is increased from a minimum value to a maximum value. As the voltage of the energy storage system 104 transitions from the intermediate level to a minimum level, the switching frequency is fixed at a maximum value. The first switching arrangement 118 acting as an inverter and the second switching arrangement 124 acting as a rectifier continue to operate as normal, while a bidirectional switch 128 of the bidirectional DC / DC converter 106 is controlled by the controller 110 using pulse-width modulation. Under pulse-width modulation control by the controller 110, the topology of the bidirectional DC / DC converter 106 behaves like a converter having the Alexander topology.

[0080] The controller 110 is used to operate the bidirectional DC / DC converter 106 in the step-up mode as follows. Pulse-frequency modulation control of the bidirectional DC / DC converter 106 is used when the voltage (V2) of the energy storage system 104 is between a minimum until the voltage of the energy storage system 104 is at an intermediate level. The difference between the minimum and intermediate voltages of the energy storage system 104 can be up to 1.25 times. During this time the switching frequency is increased from a minimum value to a maximum value. Under pulse-frequency modulation control by the controller no, the bidirectional switch 128 remains on and the topology of the bidirectional DC / DC converter 106 behaves like a converter having the CLLC converter topology. As the voltage of the energy storage system 104 transitions from the intermediate level to a maximum level, the switching frequency is fixed at a maximum value. The first switching arrangement 118 acting as a rectifier and the second switching arrangement124 acting as an inverter continue to operate as normal, while the bidirectional switch 128 is controlled by the controller no using pulse-width modulation. Under pulse-width modulation control by the controller 110, the topology of the bidirectional DC / DC converter 106 behaves like the Alexander topology.

[0081] Referring now to Figure 3 and Figure 4, examples of topologies of the bidirectional DC / DC converter 106 are shown. The first low pass filter 116 includes an output capacitor Ci arranged across the high-voltage side 112 which is operable during the step-up mode of the bidirectional DC / DC converter 106 to reduce voltage ripple and provide a constant DC voltage across the high-voltage side 112. The second low-pass filter 126 includes an output capacitor C2arranged across the low- voltage side 114 which is operable during the step-down mode of the bidirectional DC / DC converter 106 to reduce voltage ripple and provide a constant DC voltage across the low-voltage side 114.

[0082] The first switching arrangement 118 includes switching devices Q1and Q2that act as an inverter during charging and as a rectifier during discharging of the energy storage system 104. As can be seen in Figure 3, the second switching arrangement 124 includes switching devices Q3, Q4, Q5, and Q6that operate as a full-bridge rectifier during charging and as an inverter during discharging.

[0083] The resonant tank 120 arranged between the first switching arrangement 118 and the second switching arrangement 124 includes resonant inductors Lr1and Lr2, and resonant capacitors Cr1a, Cr1b, and Cr2. Commutation capacitors Cr3and Cr4also participate in the resonance but to a lesser degree than the resonant capacitors Cr1aand Cr1bbecause they are small compared to Cr1aand Cr1b. In some implementations, the ratio between the capacitance of the commutation capacitors to the resonant capacitors is 1:40, the ratio being a result of the materials selected (e.g., Si, GaN, SiC, or the like) and the type(MOSFET, IGBT, HEMT, or the like). The resonant tank 120 also includes the transformer 122. The commutation capacitors Cr3and Cr4enable zero-voltage switching for the bidirectional switch 128.

[0084] In the alternate topology shown in Figure 4, the switching devices Q5and Q6of the second switching arrangement 124 of Figure 3 are replaced by two resonant capacitors— that is, resonant capacitors Cr2aand Cr2b— to so that the switching devices Q3and Q4form a half-bridge rectifier. Consequently, the resonant tank 120 of Figure 4 includes resonant inductors Lr1and Lr2and resonant capacitors Cr1a, Cr1b, Cr2a, and Cr2b. Like with the topology of Figure 3, commutation capacitors Cr3and Cr4also participate in the resonant process but to a lesser degree than Cr1aand Cr1b.

[0085] Referring again to the topologies illustrated in Figure 3 and in Figure 4, it can be seen that the bidirectional DC / DC converter 106 includes a half-bridge inverter that introduces a factor of one-half to one of the gain of the bidirectional DC / DC converter 106 in the step-down mode and a full-wave voltage doubler rectifier that introduces a factor of two to the gain of the bidirectional DC / DC converter 106 in the step-up mode. The half-bridge inverter and full-wave voltage doubler rectifier are formed from switching devices— e.g., two field-effect transistors, voltage-controlled devices, current controlled devices, or the like— and a split resonant capacitor.

[0086] The split resonant capacitor is formed from two capacitors— i.e., Cr1aand Cr1b— that act as a single resonant capacitor. Using two capacitors reduces the stress in each individual capacitor, thereby reducing the size and cost of the components.Additionally, because the two capacitors are in parallel with a bulk capacitor (capacitor Ci) at a DC bus, charging time of both capacitors is reduced as the two capacitors are charged simultaneously. When power flows from the energy source side 114 of the bidirectional DC / DC converter 106 to the high-voltage side 112 of the bidirectional DC / DC converter 106, the two capacitors Cna and Crib act as a voltage doubler and reduce the number of turns required in the transformer 122 to increase the power density of the entire system.

[0087] As can be seen in Figures 3-4 and Figure 25, the bidirectional switch 128 of the bidirectional DC / DC converter 106 includes switching devices Q7and Qs that regulate the output voltage and also ensure high efficiency in light load conditions. The switching devices Q7and Qs can be any suitable switching devices such as, for example, two field-effect transistors, two voltage-controlled devices, two current-controlled devices, or the like. The bidirectional switch 128 enables the bidirectional DC / DC converter 106 to stay below the resonance region where soft-switching is achieved at both the energy source side 114 and the high-voltage side 112. The bidirectional switch 128 also cuts down the power transfer to meet the gain requirement by switching at a certain duty cycle. The bidirectional switch 128 is arranged between a pair of resonant capacitors Cna and Cnb and a pair of commutation capacitors Cr3and Cr4so that the bidirectional switch 128 is capable of soft-commutation and generates no switching loss.

[0088] The example bidirectional DC / DC converters described herein can include a wide variety of components and can operate under a wide variety of parameters. In an example of bidirectional DC / DC converter, the voltage (Vi) on the energy source side 112 is in a voltage range of 380V to 420V or can be a nominal voltage of 400V. In an example of bidirectional DC / DC converter, the voltage (V2) on the low-voltage side 114 is in a voltage range of 42V to 58V or can be a nominal voltage of 50V. An example bidirectional DC / DC converter can operate at a frequency in a range of 100 kHz to 700 kHz, or at a frequency of 320 kHz. A maximum power of an example bidirectional DC / DC converter can be in a range from 3 kW to 100 kW or can be 4 kW. The capacitance of the resonant capacitors Cna and Cnb of an example bidirectional DC / DCconverter can be 120 nF and the capacitance of the resonant capacitor Cr2can be 2 pF. In an example bidirectional DC / DC converter, the inductance Lmcan be 12 pH, the inductance Lncan be 2.8 pH, and the inductance Lr2can be 5 nH. In an example bidirectional DC / DC converter, the capacitance of capacitor Ci can be 3 pF and the capacitance of capacitor C2can be 200 pF. A turn ratio of the transformer of an example bidirectional DC / DC converter can be 4:1. Switch Qi and switch Q2of an example bidirectional DC / DC converter can be field effect transistors such as the UJ4SC075011B7S transistor made by OnSemi™. Switch Q3, switch Q4, switch Q5, and switch Qc> of an example bidirectional DC / DC converter can be field effect transistors such as the FDBL86065 transistor made by OnSemi™. Switch Q7and switch Qs of an example bidirectional DC / DC converter can be field effect transistors such as the UJ4SC0750009B7S transistor made by OnSemi™.

[0089] Referring now to Figures 5-7, timing diagrams illustrating the operation of the bidirectional DC / DC converter 106 in the stepdown mode are shown. The top horizontal axis of the timing diagrams in Figure 5 and Figure 6 illustrates the operation of the switching devices forming the first switching arrangement 118 (i.e., switching devices Q1and Q2). The bottom horizontal axis of the timing diagrams in Figure 5 and Figure 6 illustrates the operation of the switching devices forming the bidirectional switch 128 (i.e., switching devices Q7and Qs). Figure 7 illustrates certain voltage and current levels of the bidirectional DC / DC converter 106 in relation to the voltage of the switching devices forming the first switching arrangement 118 and the bidirectional switch 128.

[0090] Referring now to Figures 8-14, the current path through the bidirectional DC / DC converter 106 is shown in each of the first seven charging or step-down states extending from time to to time t7indicated on the time axis in Figure 7. The bidirectional DC / DC converter 106 goes through fourteen total states during step-down mode and the second seven states are similar to thefirst seven states, so only the first seven states are shown and described herein. That is, the operation of the bidirectional DC / DC converter 106 in the eighth through the fourteenth step-down states is the same as the operation during the first through the seventh step-down states, except that the currents have their polarity reversed. The components of the bidirectional DC / DC converter 106 topology drawn in black are active during the particular step-down state and the components illustrated in gray are inactive in the particular step-down state.

[0091] Referring now to Figure 8, the bidirectional DC / DC converter 106 is illustrated in a first step-down state extending from time to to time ti. At time to, switch Q2of the first switching arrangement 118 turns ON with Zero Voltage Switching (ZVS). During the previous charging cycle (i.e., the seventh step-down state described below, or the fourteenth total step-down state), a primary resonant current iLn of the resonant tank 120 was negative and switch Q2’s parallel capacitance (e.g. Coss) was charged. Switch Q7is already ON from the previous charging cycle. In the first step-down state, the primary resonant current iLn is negative and flows through a first diode Di because switch Qs is reverse biased. Capacitors Cna, Cnb, Cr3, and Cr4disconnect from the resonant tank 120. Switches Q3and Qo of the second switching arrangement 124 conduct and the secondary resonant current iLr2is rectified by the full-bridge rectifier. Thus, energy is transferred to the energy storage system 104 connected to the low-voltage side 114 of the bidirectional DC / DC converter 106. The first step-down state ends when the primary resonant current iLn becomes positive.

[0092] During the first step-down state, the primary resonant current iLn is equal to the sum of a magnetizing current iLm and the secondary resonant current ILK referred to a primary side 130 of the transformer 122. The secondary resonant current ILK and the magnetizing current iLm increase sinusoidally though the magnetizing current iLm resonates much slower than thesecondary resonant current ZLK because a magnetizing inductance Lmof the transformer 122 is designed to be larger than the resonant inductors Lnand Lr2. Consequently, the magnetizing current ZLm increases almost linearly while the secondary resonant current ILK increases sinusoidally. Because current flows into the primary side 130 of the transformer 122 and out of a secondary side 132 of the transformer 122, power transfers from the high-voltage side 112 to the low-voltage side 114 of the of the bidirectional DC / DC converter 106.

[0093] Referring now to Figure 9, the bidirectional DC / DC converter 106 is illustrated in a second step-down state extending from time ti to time t2. The second step-down state begins when the primary resonant current zln becomes positive, thereby charging capacitor Cr3and discharging capacitor Cr4until a body diode (i.e., anti-parallel diode in the case of an IGBT) of switch Qs of the bidirectional switch 128 is forward-biased. The second step-down state ends when capacitor Cr3is charged and capacitor Cr4is discharged, each to the voltage level of the first resonant capacitor Cn.

[0094] Referring now to Figure 10, the bidirectional DC / DC converter 106 is illustrated in a third step-down state extending from time tz to time t3. At time t2, current begins flowing through switch Q7and the body of switch Qs. The resonant capacitors, Cna and Cnb, also become part of the resonant tank 120 in the third step-down state. The secondary resonant current ir2and the magnetizing current ZLm continue to resonate with the resonant tank 120 and power continues to transfer from the load / energy source side 112 to the low-voltage side 114 of the bidirectional DC / DC converter 106. The controller no determines when the third step-down state ends to regulate the output voltage at the low-voltage side 114. The controller no receives voltage signals and current signals from the sensors 108 on the high-voltage side 112 and the low-voltage side 114 of the bidirectional DC / DC converter 106. Based on the voltage and current detected by the sensors 108, the controller 110employs pulse-frequency modulation or pulse-width modulation to regulate the output at the low-voltage side 114 to be a current source, voltage source, or power source.

[0095] Referring now to Figure 11, the bidirectional DC / DC converter 106 is illustrated in a fourth step-down state extending from time t3to time t4. At time t3switch Q7of the bidirectional switch 128 turns OFF and switch Qs of the bidirectional switch 128 simultaneously turns ON to prepare for the next cycle. Switch Q7turns OFF and switch Qs turns ON with soft- commutation because switch Q7and switch Qs are placed between the split-resonant capacitors Cna and Crib and the commutation capacitors Cr3and Cr4. The primary resonant current iLn charges capacitor Cr3and discharges capacitor Cr4until diode D2becomes forward biased.

[0096] Referring now to Figure 12, the bidirectional DC / DC converter 106 is illustrated in a fifth step-down state extending from time t4to time t5. The fifth state starts when capacitor Cr3is fully charged and capacitor Cr4is fully discharged. The primary resonant current iLn circulates through diode D2, Lm, Ln, and switch Q2of the first switching arrangement 118. In the fifth step-down state, the load or energy source 102 is disconnected from the rest of the circuit though the energy in the resonant inductor, kn, continues to discharge to the low-voltage side 114 of the bidirectional DC / DC converter 106. Also, the magnetizing current iLm continues increasing almost linearly so that the voltage across inductor Lmis positive and almost constant and the voltage across inductor Lnis negative and almost constant. Thus, the primary resonant current decreases almost linearly. The fifth step-down state ends when the primary resonant current kn is equal to the magnetizing current km.

[0097] Referring now to Figure 13, the bidirectional DC / DC converter 106 is illustrated in a sixth step-down state extending from time t5to time te. When the primary resonant current kn isequal to the magnetizing current iLm, the rectifier on the low- voltage side 114 of the bidirectional DC / DC converter 106 formed by the second switching arrangement 124 enters a reverse-biased state and no current flows in or out of the transformer 122. Inductor Lmjoins the resonance with inductor Ln in the sixth step-down state and no power transfers from the high-voltage side 112 to the low-voltage side 114 of the bidirectional DC / DC converter 106. At this time the output capacitor CLV (i.e., output capacitor C2of the second low-pass filter 126 of Figure 3 and Figure 4) discharges to maintain a constant DC voltage across the low-voltage side 114.

[0098] Referring now to Figure 14, the bidirectional DC / DC converter 106 is illustrated in a seventh step-down state extending from time te to time t7. During the seventh step-down state switch Q2of the first switching arrangement 118 turns OFF, leading to a dead time when both switch Qi and switch Q2of the first switching arrangement 118 are OFF. The primary resonant current iLn discharges the output capacitor of switch Qi and charges the output capacitor (Coss,2) of switch Q2so that switch Q2is prepared to switch ON with ZVS in the next charging cycle.

[0099] Referring now to Figures 15-17, timing diagrams illustrating the operation of the bidirectional DC / DC converter 106 in the step-up mode. The top horizontal axis of the timing diagrams in Figure 15 and Figure 16 illustrates the operation of the second switching arrangement 124 (i.e., switching devices Q3, Q4, Q5, and Qe). The bottom horizontal axis of the timing diagrams in Figure 15 and Figure 16 illustrates the operation of the switching devices forming the bidirectional switch 128 (i.e., switching devices Q7and Qs). Figure 17 illustrates certain voltage and current levels of the bidirectional DC / DC converter 106 in relation to the voltage of the switching devices forming the first switching arrangement 118 and the bidirectional switch 128.

[0100] Referring now to Figures 18-23, the current path through the bidirectional DC / DC converter 106 is shown in each of the six discharging or step-up states extending from time to to time te indicated in Figure 17. The components of the bidirectional DC / DC converter 106 topology drawn in black are active during the particular step-up state and the components illustrated in gray are inactive in the particular step-up state.

[0101] Referring now to Figure 18, the bidirectional DC / DC converter 106 is illustrated in a first step-up state extending from time to to time ti. At time to, switch Q3and switch Qc> turn ON under ZVS while switch Q4and switch Q5are OFF. On theload / energy source side 122 the primary resonant current iLn increases sinusoidally during the first step-up state. The magnetizing current iLm also increases and is more linear as the magnetizing inductance Lmof the transformer 122 is designed to be larger than the resonant inductors Lnand Lr2. The secondary resonant current ILK is equal to the sum of a magnetizing current iLm and the primary resonant current iLn referred to the secondary side 132 of the transformer 122. During the first step-up state, power transfers from the low- voltage side 114 of the bidirectional DC / DC converter 106 to the high-voltage side 112 of the bidirectional DC / DC converter 106 so that capacitor Cr3is discharged and capacitor Cr4is charged until the body diode of switch Q7is forward biased.

[0102] Referring now to Figure 19, the bidirectional DC / DC converter 106 is illustrated in a second step-up state extending from time ti to time t2. At time ti, the anti-parallel diode of switch Q7becomes forward biased and the bidirectional switch 128 on the high-voltage side 112 begins conducting. The resonant capacitors Cna and Cnb participate in resonance with Ln, Cr3, and Cr4so that resonant capacitor Cna begins to discharge and resonant capacitor Cnb begins to charge. Power transfers from the low-voltage side 114 of the bidirectional DC / DC converter 106 to the high-voltage side 112 of the bidirectional DC / DCconverter 106. The resonant current on both sides resonates until switch Qs of the bidirectional switch 128 is turned OFF.

[0103] Referring now to Figure 20, the bidirectional DC / DC converter 106 is illustrated in a third step-up state extending from time t2to time t3. At time t2, switch Qs of the bidirectional switch 128 turns OFF under ZVS and switch Q7of the bidirectional switch 128 simultaneously turns ON to prepare for the next cycle. Switch Q7turns ON and switch Qs turns OFF with soft commutation because switch Q7and switch Qs are placed the resonant capacitors Cna and Cnb and the commutation capacitors Cr3and Cr4. Power continues to transfer from the low-voltage side 114 of the bidirectional DC / DC converter 106 to the high-voltage side 112 of the bidirectional DC / DC converter 106. Capacitor Cr3is being discharged and capacitor Cr4is being charged during the third step-up state. The third step-up state ends when capacitor Cr3is fully discharged and capacitor Cr4is fully charged.

[0104] Referring now to Figure 21, the bidirectional DC / DC converter 106 is illustrated in a fourth step-up state extending from time t3to time t4. In the fourth step-up state, diode Di conducts once capacitor Cr3is fully discharged and capacitor Cr4is fully charged. The resonant tank 120 on the high-voltage side 112 of the bidirectional DC / DC converter 106 is clamped by the voltage Vi of the load / energy source 102 and the voltage across the inductor Lnswitches its polarity. Consequently, the resonant current zln on the high-voltage side 112 of the bidirectional DC / DC converter 106 begins to decrease. The decrease of the resonant current zln is close to linear because the voltage across inductor Lmremains constant during the fourth step-up state. Power continues to transfer from the low- voltage side 114 of the bidirectional DC / DC converter 106 to the high-voltage side 112 of the bidirectional DC / DC converter 106. The fourth step-up state ends when the secondary resonant current lire is equal to the magnetizing current zlm.

[0105] Referring now to Figure 22, the bidirectional DC / DC converter 106 is illustrated in a fifth step-up state extending from time t4to time t5. The fifth step-up state begins when the secondary resonant current ILK is equal to the magnetizing current iLm. During this state, no power transfers from the low-voltage side 114 of the bidirectional DC / DC converter 106 to the high- voltage side 112 of the bidirectional DC / DC converter 106; that is, the secondary side 132 disconnects from the primary side 130.

[0106] Referring now to Figure 23, the bidirectional DC / DC converter 106 is illustrated in a sixth step-up state extending from time t5to time te. At time t5, switch Q3and switch Qo of the second switching arrangement 124 both turn OFF, leading to a dead time when all of the switches of the second switching arrangement 124— i.e., switch Q3switch Q4, switch Q5, and Qo— are turned OFF. Current continues to flow on the low-voltage side 114 of the bidirectional DC / DC converter 106 to charge the output capacitor Coss,3of switch Q3, charge the output capacitor Coss, 6 of switch Qo, discharge the output capacitor Coss, 4 of switch Q4, and discharge the output capacitor Coss, 5 of switch Q5. AS a result, switch Q4and switch Q5are prepared to turn ON with ZVS in the next discharging cycle.

[0107] Referring now to Figure 24, a method 200 that facilitates the charging of the energy storage system 104 via the bidirectional DC / DC converter 106 is shown. The method 200 can be performed with any of the bidirectional DC / DC converters described herein and can include any steps described above for operating or otherwise interacting with the bidirectional DC / DC converters described above. The method 200 can also be used to transfer power between two electrical systems in a step-down mode in situations other than charging an energy storage device. The method 200 begins at 202 wherein, in a step-down mode, at least one switch of the first switching arrangement is operated to form an inverter. At 204, continuing in the step-down mode, at least one switch of thesecond switching arrangement to form a rectifier. In a step-up mode, at 206, at least one switch of the first switching arrangement to form a rectifier. Also in the step-up mode, at least one switch of the second switching arrangement to form an inverter at 208. During operation of the bidirectional DC / DC converter, the bidirectional switch 128 is operated using pulse-frequency modulation at 210 for a first voltage range of a voltage of the energy storage system and using pulsewidth modulation at 212 for a second voltage range of the voltage of the energy storage system. In some implementations, pulse-frequency modulation and pulse-width modulation are used simultaneously, such as when the first voltage range and the second voltage range overlap.

[0108] Referring now to Figures 26-40, timing and circuit diagrams are shown to illustrate an alternate method of operating the bidirectional DC / DC converter 106 in step-down mode (Figures 26-33) and in step-up mode (Figures 34-40).

[0109] Referring now to Figure 26, timing diagrams illustrating the operation of the bidirectional DC / DC converter 106 in the stepdown mode are shown. The top horizontal axis of the timing diagrams illustrates the operation of the switching devices forming the first switching arrangement 118 (i.e., switching devices Q1and Q2). The middle horizontal axis of the timing diagrams in Figure 26 illustrates the operation of the switching devices forming the second switching arrangement 124 (i.e., switching devices Q3, Q4, Q5, and Qe). The bottom axis of the timing diagrams in Figure 26 illustrates the operation of the switching devices forming the bidirectional switch 128 (i.e., switching devices Q7and Qs).

[0110] Referring now to Figures 27-33, the current path through the bidirectional DC / DC converter 106 is shown in each of the first seven charging or step-down states extending from time to to time t7indicated on the time axis in Figure 7. The bidirectional DC / DC converter 106 goes through fourteen total states duringstep-down mode and the second seven states are similar to the first seven states, so only the first seven states are shown and described herein. That is, the operation of the bidirectional DC / DC converter 106 in the eighth through the fourteenth step-down states is the same as the operation during the first through the seventh step-down states, except that the currents have their polarity reversed. The components of the bidirectional DC / DC converter 106 topology drawn in black are active during the particular step-down state and the components illustrated in gray are inactive in the particular step-down state.[oom] Referring now to Figure 27, the bidirectional DC / DC converter 106 is illustrated in a first step-down state extending from time to to time ti. At time to, switch Q2of the first switching arrangement 118 turns ON with Zero Voltage Switching (ZVS). During the previous charging cycle (i.e., the seventh step-down state described below, or the fourteenth total step-down state), a primary resonant current iLn of the resonant tank 120 was negative and switch Q2’s parallel capacitance (e.g. Coss) was charged. Switch Q7is already ON from the previous charging cycle. In the first step-down state, the primary resonant current iLn is negative and flows through a first diode Di because switch Qs is reverse biased. Capacitors Cna, Cnb, Cr3, and Cr4disconnect from the resonant tank 120. Switches Q3and Qo of the second switching arrangement 124 conduct and the secondary resonant current iLr2is rectified by the full-bridge rectifier. Thus, energy is transferred to the energy storage system 104 connected to the low-voltage side 114 of the bidirectional DC / DC converter 106. The first step-down state ends when the primary resonant current iLn becomes positive.

[0112] During the first step-down state, the primary resonant current iLn is equal to the sum of a magnetizing current iLm and the secondary resonant current ILK referred to a primary side 130 of the transformer 122. The secondary resonant current ILK and the magnetizing current iLm increase sinusoidally though themagnetizing current ZLm resonates much slower than the secondary resonant current ILK because a magnetizing inductance Lmof the transformer 122 is designed to be larger than the resonant inductors Lnand Lr2. Consequently, the magnetizing current ZLm increases almost linearly while the secondary resonant current ILK increases sinusoidally. Because current flows into the primary side 130 of the transformer 122 and out of a secondary side 132 of the transformer 122, power transfers from the high-voltage side 112 to the low-voltage side 114 of the of the bidirectional DC / DC converter 106.

[0113] Referring now to Figure 28, the bidirectional DC / DC converter 106 is illustrated in a second step-down state extending from time ti to time t2. The second step-down state begins when the primary resonant current zln becomes positive and increases sinusoidally, thereby charging capacitors Cr3and Cr4until their voltages equal the voltage of capacitors Cha and Crib, which happens quickly because capacitors Cr3and Cr4are small. Currents zln and ZLm increase sinusoidally but current i m resonates much slower because Lmis designed to be much larger than Ln. Therefore, ZLm increases almost linearly while ZLn increases as a sinusoid. Switches Q3and Qc> of the second switching arrangement 124 conduct, and power transfers from the primary side to the secondary side. The second step-down state ends when capacitor Cr3is charged to the voltage level of Cna and capacitor Cr4is discharged to the voltage level of Crib.

[0114] Referring now to Figure 29, the bidirectional DC / DC converter 106 is illustrated in a third step-down state extending from time tz to time t3. The bidirectional switch 128 conducts during this state, and capacitors Cna and Cnb join the resonant tank 120. Resonant current zln keeps increasing sinusoidally but slower because the resonant frequency gets smaller. Power transfers from the primary to the secondary side.

[0115] Referring now to Figure 30, the bidirectional DC / DC converter 106 is illustrated in a fourth step-down state extending fromtime t3to time t4. At time t3switch Q7of the bidirectional switch 128 turns OFF and switch Qs of the bidirectional switch 128 simultaneously turns ON to prepare for the next cycle. Switch Q7turns OFF and switch Qs turns ON with soft- commutation because switch Q7and switch Qs are placed between the split-resonant capacitors Cna and Crib and the commutation capacitors Cr3and Cr4. The primary resonant current iLn charges capacitor Cr3and discharges capacitor Cr4. The values of capacitors Cr3and Cr4are small relative to the value of capacitors Cna and Cnb, so this mode happens quickly. The power transfers from the primary side to the secondary side.

[0116] Referring now to Figure 31, the bidirectional DC / DC converter 106 is illustrated in a fifth step-down state extending from time t4to time t5. The fifth step-down state starts when capacitor Cr3is fully charged and capacitor Cr4is fully discharged. The primary resonant current iLn circulates through diode D2, the resonant tank 120, and the channel of switch Q2of the first switching arrangement 118. In the fifth step-down state, the load or energy source 102 is disconnected from the rest of the circuit. The magnetizing current iLm continues to increase and the voltage across Lmis positive and almost constant. As a result, the voltage across Lnis negative and almost constant. The primary resonant current decreases almost linearly. The fifth step-down state ends when the primary resonant current iLn is equal to the magnetizing current iLm.

[0117] Referring now to Figure 32, the bidirectional DC / DC converter 106 is illustrated in a sixth step-down state extending from time t5to time te. When the primary resonant current iLn is equal to the magnetizing current iLm, the rectifier switches (i.e., switching devices Q3, Q4, Q5, and Qc>) are all OFF and the body diodes of the rectifier switches are reversed-biased.Consequently, no current flows in and out of the transformer 122. Lmjoins the resonance with inductor Lnin the sixth step-down state and no power transfers from the primary side to the secondary side.

[0118] Referring now to Figure 33, the bidirectional DC / DC converter 106 is illustrated in a seventh step-down state extending from time te to time t7. This is the dead time when both switches Qi and Q2 of the first switching arrangement 118 are OFF. The primary resonant current iLn discharges the output capacitor of switch Qi and charges the output capacitor of switch Q2so that switch Qi is prepared to switch ON under ZVS in the next time interval.

[0119] Referring now to Figure 34 timing diagrams illustrating the operation of the bidirectional DC / DC converter 106 in the step- up mode. The top horizonal axis of the timing diagrams in Figure 34 illustrates the operation of the second switching arrangement 124 (i.e., switching devices Q3, Q4, Q5, and Qe). The middle horizontal axis of the timing diagrams in Figure 34 illustrates the operation of the switching devices forming the first switching arrangement 118 (i.e., switching devices Q1and Q2). The bottom horizontal axis of the timing diagrams in Figure 34 illustrates the operation of the switching devices forming the bidirectional switch 128 (i.e., switching devices Q7and Qs).

[0120] Referring now to Figures 35-40, the current path through the bidirectional DC / DC converter 106 is shown in each of the six discharging or step-up states extending from time to to time te indicated in Figure 17. The components of the bidirectional DC / DC converter 106 topology drawn in black are active during the particular step-down state and the components illustrated in gray are inactive in the particular step-down state.

[0121] Referring now to Figure 35, the bidirectional DC / DC converter 106 is illustrated in a first step-up state extending from time to to time ti. At time to, switch Q3and switch Qc> turn ON under ZVS while switch Q4and switch Q5are OFF. In the low voltageside, the secondary resonant current ILK increases sinusoidally. The magnetizing current ZLm also increases but almost linearly as Lmis relatively large compared to the resonant inductors. Power transfers from the low-voltage side to the high-voltage side, the capacitor Cr3is being discharged, and the capacitor Cr4is being charged until D7, the body diode of switch Q7, is forward biased. This state happens quickly because Cr3and Cr4are small relative to Cna and Crib.

[0122] Referring now to Figure 36, the bidirectional DC / DC converter 106 is illustrated in a second step-up state extending from time ti to time t2. The bidirectional switch 128 conducts during this state. Capacitors Cna and Cnb join the resonance with Ln, Cr3, and Cr4. Power transfers from the primary side to the secondary side. The resonant current on both sides resonates until Qs turns OFF.

[0123] Referring now to Figure 37, the bidirectional DC / DC converter 106 is illustrated in a third step-up state extending from time t2to time t3. At the time t2, switch Qs turns OFF under ZVS because the current diverts through the capacitors Cr3and Cr4and maintains the zero difference between the middle points of the two capacitors Cna and Cnb and the commutation capacitors Cr3and Cr4. This allows more time for the bidirectional switch 128 to turn OFF without generating significant losses. Switch Q7also soft turns ON simultaneously because there is no current flowing via the bidirectional switch at the time it turns ON. This state ends when capacitor Cr3is fully discharged and capacitor Cr4is fully charged.

[0124] Referring now to Figure 38, the bidirectional DC / DC converter 106 is illustrated in a fourth step-up state extending from time t3to time t4. Once capacitor Cr3is fully discharged and capacitor Cr4is fully charged, diode Di conducts. The resonant tank 120 in the high voltage side is capped by the output voltage, and the voltage across Lnswitches its polarity.Therefore, the resonant current in the high voltage side startsto decrease. As the voltage across Lmis almost constant due to the relatively large value of Lm, the primary resonant current iLn decreases almost linearly as well. The resonant current in the low voltage side also decreases almost linearly. The state ends when the secondary resonant current ZLK equals the magnetizing current ZLm.

[0125] Referring now to Figure 39, the bidirectional DC / DC converter 106 is illustrated in a fifth step-up state extending from time t4to time t5. During this state, the secondary resonant current ILK is equal to the magnetizing current ZLm and no power transfers from the low voltage side to the high voltage side. The secondary side disconnects from the primary side.

[0126] Referring now to Figure 40, the bidirectional DC / DC converter 106 is illustrated in a sixth step-up state extending from time t5to time te. At time t5, switches Q3and Qc> turn OFF. This is the dead time when all of the switches of the second switching arrangement 124— i.e., switch Q3switch Q4, switch Q5, and Qo— are turned OFF. Current still flows in the low voltage side to charge the capacitor of switch Q3and the capacitor of switch Qc> and to discharge the capacitor of switch Q4and the capacitor of switch Q5. This prepares switch Q4and switch Q5to turn ON under ZVS in the next state.

[0127] While various inventive aspects, concepts and features of the disclosures maybe described and illustrated herein as embodied in combination in the exemplary embodiments, these various aspects, concepts, and features may be used in many alternative embodiments, either individually or in various combinations and sub-combinations thereof. Unless expressly excluded herein all such combinations and subcombinations are intended to be within the scope of the present application. Still further, while various alternative embodiments as to the various aspects, concepts, and features of the disclosures— such as alternative materials, structures, configurations, methods, devices, and components, alternativesas to form, fit, and function, and so on— may be described herein, such descriptions are not intended to be a complete or exhaustive list of available alternative embodiments, whether presently known or later developed. Those skilled in the art may readily adopt one or more of the inventive aspects, concepts, or features into additional embodiments and uses within the scope of the present application even if such embodiments are not expressly disclosed herein.

[0128] Additionally, even though some features, concepts, or aspects of the disclosures maybe described herein as being a preferred arrangement or method, such description is not intended to suggest that such feature is required or necessary unless expressly so stated. Still further, exemplary, or representative values and ranges may be included to assist in understanding the present application, however, such values and ranges are not to be construed in a limiting sense and are intended to be critical values or ranges only if so expressly stated.

[0129] Moreover, while various aspects, features and concepts may be expressly identified herein as being inventive or forming part of a disclosure, such identification is not intended to be exclusive, but rather there may be inventive aspects, concepts, and features that are fully described herein without being expressly identified as such or as part of a specific disclosure, the disclosures instead being set forth in the appended claims. Descriptions of exemplary methods or processes are not limited to inclusion of all steps as being required in all cases, nor is the order that the steps are presented to be construed as required or necessary unless expressly so stated. The words used in the claims have their full ordinary meanings and are not limited in any way by the description of the embodiments in the specification.

Claims

CLAIMSWhat is claimed is:

1. A bidirectional DC / DC converter comprising:an energy source side, wherein the energy source side is configured to be connected to an energy source, the energy source side comprising a first switching arrangement;an energy storage side, wherein the energy storage side is configured to be connected to an energy storage system, the energy storage side comprising a second switching arrangement;a resonant tank comprising a plurality of resonant capacitors;a bidirectional switch connected to the energy source side by a pair of commutation capacitors and a pair of capacitors of the resonant tank; anda transformer, wherein the energy source side is connected to a primary side of the transformer, and wherein the energy storage side is connected to a secondary side of the transformer.

2. The bidirectional DC / DC converter of claim 1, wherein:during a step-down mode of the bidirectional DC / DC converter the first switching arrangement operates as an inverter, and the second switching arrangement operates as a rectifier; andduring a step-up mode of the bidirectional DC / DC converter the second switching arrangement operates as an inverter, and the first switching arrangement operates as a rectifier.

3. The bidirectional DC / DC converter of any of the above claims, wherein:when the bidirectional DC / DC converter is in a step-down mode, two switches of the first switching arrangement and a split-resonant capacitor of the energy source side form a half-bridge inverter; andwhen the bidirectional DC / DC converter is in a step-up mode, the two switches of the first switching arrangement and the split-resonant capacitor of the energy source side form a full-wave voltage double rectifier.

4. The bidirectional DC / DC converter of any of the above claims, wherein:when the bidirectional DC / DC converter is in a step-down mode, four switches of the second switching arrangement are turned on to form a full-bridge rectifier; andwhen the bidirectional DC / DC converter is in a step-up mode, two switches of the second switching arrangement are turned off to form an inverter.

5. The bidirectional DC / DC converter of any of the above claims, wherein:the energy source side comprises a low-pass filter that is operable when the bidirectional DC / DC converter is in a step-up mode; andthe energy storage side comprises a low-pass filter that is operable when the bidirectional DC / DC converter is in a step-down mode.

6. The bidirectional DC / DC converter of any of the above claims, wherein the second switching arrangement comprises two switches that form a half-bridge rectifier when the bidirectional DC / DC converter is in a step-down mode.

7. An electric generation system comprising:the bidirectional DC / DC converter of any of the above claims;a power source connected to the energy source side of the bidirectional DC / DC converter; andan energy storage system connected to the energy storage side of the bidirectional DC / DC converter.

8. The electric generation system of claim 7, further comprising:a first sensor for detecting at least one of a first voltage and a first current flowing between the energy source and the bidirectional DC / DC converter;a second sensor for detecting at least one of a second voltage and a second current flowing between the energy storage system and the bidirectional DC / DC converter; and a controller connected to the bidirectional DC / DC converter, the first sensor, and the second sensor, wherein the controller controls the first switching arrangement, the second switching arrangement, and the bidirectional switch based on at least one of the first voltage, the first current, the second voltage, and the second current.

9. The electric generation system of claim 8, wherein when the bidirectional DC / DC converter is in a step-down mode:the controller employs pulse-frequency modulation control of the bidirectional DC / DC converter when the second voltage is between a maximum value and an intermediate value; andthe controller employs pulse-width modulation control of the bidirectional DC / DC converter when the second voltage is between the intermediate value and a minimum value; wherein a switching frequency of the bidirectional DC / DC converter is fixed at a maximum value during the pulse-width modulation control of the bidirectional DC / DC converter.

10. The electric generation system of claim 8 or claim 9, wherein when the bidirectional DC / DC converter is in a step-up mode:the controller employs pulse-frequency modulation control of the bidirectional DC / DC converter when the second voltage is at a minimum until the second voltage is at an intermediate value; andthe controller employs pulse-width modulation control of the bidirectional DC / DC converter when the second voltage is at the intermediate value until the second voltage is at a maximum value;wherein a switching frequency of the bidirectional DC / DC converter is fixed at a maximum value during the pulsewidth modulation control of the bidirectional DC / DC converter.

11. The electric generation system of claim 9 or claim 10, wherein the controller employs both pulse-width modulation control and pulse-frequency modulation control simultaneously.

12. A method of operating a bidirectional DC / DC converter in a step-down mode or a step-up mode, wherein bidirectional DC / DC converter comprises an energy source side connected to a power source, an energy storage side connected to an energy storage system, a bidirectional switch of the energy source side, a first switching arrangement of the energy source side, and a second switching arrangement of the energy storage side, the method comprising:operating, in the step-down mode, at least one switch of the first switching arrangement to form an inverter;operating, in the step-down mode, at least one switch of the second switching arrangement to form a rectifier;operating, in the step-up mode, at least one switch of the first switching arrangement to form a rectifier;operating, in the step-up mode, at least one switch of the second switching arrangement to form an inverter;operating the bidirectional switch using pulse-frequency modulation for a first voltage range of a voltage of the energy storage system; andoperating the bidirectional switch using pulse-width modulation for a second voltage range of the voltage of the energy storage system.

13. The method of claim 12, wherein the bidirectional DC / DC converter comprises the bidirectional DC / DC converter of any of claims 1-11.

14. The method of claim 12 or claim 13, wherein the bidirectional switch is operated using pulse-width modulation and pulsefrequency modulation simultaneously.

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