Isolated power converters

Abstract

An example isolated power converter includes an isolated switching stage including a first semiconductor bridge on a primary side of the switching stage and a second semiconductor bridge on a secondary side of the isolated switching stage and isolated by a transformer, the first semiconductor bridge connected to an input of the isolated power converter and configured to switch in combination with the second semiconductor bridge to provide an isolated voltage and current; and an auxiliary switching stage coupled to the output of the second bridge of the isolated switching stage, where the auxiliary switch is in series with the auxiliary capacitor and then in parallel with the second bridge output, and an auxiliary inductor is connected between the auxiliary switch and the output of the isolated power converter.

Description

Attorney Docket No. 10046-640W018440 HUAISOLATED POWER CONVERTERSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. provisional patent application No. 63 / 725.766, filed on November 27, 2024, and titled "ISOLATED POWER CONVERTERS.” the disclosure of which is expressly incorporated herein by reference in its entirety.BACKGROUND

[0002] AC-DC and DC-AC converters can convert alternating current (AC) from a source (e.g., mains, a generator, etc.) into direct current (DC) and vice-versa and are employed in many applications. Single phase and three phase AC-DC converters are both widely used.

[0003] While a diode bridge can be used to obtain a simple AC-DC converter, modem AC- DC converters require precise control of the AC current waveform, hence a power factor correction (PFC) AC-DC converter is needed which uses active power switches to control the current waveform. A popular AC-DC PFC converter is a boost rectifier which converts the AC voltage to a higher DC voltage. A boost rectifier is formed typically by a diode rectifier followed by a boost converter with one switch and one diode. If bidirectional power flow is needed, the boost rectifier can be replaced by a H-bridge converter with four active power switches.

[0004] The H-bridge converter can operate as a totem-pole PFC converter with two switches switching at high frequency and two switches switching at the line frequency (i.e. 60 Hz). H- bridge converter can obtain unity power factor (i.e., PF=1) or any arbitrary pow er factor. Nonunity power factor case is useful to provide controlled reactive power to the grid.

[0005] Similarly, an AC-DC converter includes three half-bridges, each half bridge including two active switches and associated antiparallel diode, can be used to form a three phase AC-DC converter or rectifier with controlled pow er factor. AC-DC converters described above typically are hard switching converters, meaning that the switching losses are high, hence limiting the converter power conversion efficiency and size reduction.

[0006] To obtain isolated DC output, the non-isolated AC-DC converter is typically followed by another isolated DC-DC converter. The isolated DC-DC converter can also provide additional voltage step up or step-down function. A classic example is to generate 48V isolated DC output for computer motherboard. The single-phase AC voltage of 240V or 270V is first converted to 400V DC voltage, and a second DC-DC converter is used to convert the 400V DC to 12V DC.

[0007] Due to these multiple stages of power conversion, especially for applications that require isolated output voltage control, the power conversion efficiency is further low ered.Attorney Docket No. 10046-640W01 8440 HUA

[0008] Therefore there is a benefit to improving power conversion efficiency for AC-DC converters with isolated DC outputs.

[0009] In addition, high-voltage direct current (HVDC) grids have become increasingly prevalent on a global scale due to its improved efficiency in power transmission over longer distances. A high voltage AC / DC converter is generally needed. One of the most successful ones is the modular multilevel converter (MMC) which uses many low voltage submodules (SM) to obtain the high voltage. MMC converter does not provide isolated DC output. Therefore there is a benefit to creating isolated DC output in MMC converter.SUMMARY

[0010] An exemplary' system and method are disclosed for the interconnection and integration of medium-voltage-grid and high-voltage-grid infrastructure directly with lower voltage DC-based resources, e.g., for hyperscale data centers, high-power DC fast chargers, solar farm, battery storage system and large industrial power plants. The exemplary system can also be implemented for lower voltage single or three phase grids.

[0011] The exemplary' sy stem and method may be implemented as an isolated converter having galvanic isolation through a high-frequency transformer for an isolated DC / DC converter, isolated DC / AC converter, or isolated AC / DC converter. Power flow can be unidirectional or bidirectional.

[0012] If applied to DC / DC, the exemplary' system is a converter. If applied to the DC / AC with power flow from DC to AC, the exemplary system is an inverter. If applied to AC / DC with power flow from AC to DC, the exemplary system is a rectifier. If applied to DC / AC with bidirectional power flow, the exemplary system is a converter.

[0013] In another aspect, the exemplary system is configured as a multiport modular multilevel converter based solid state transformer system.

[0014] Isolated DC / DC converters are widely used in the power electronics field to provide galvanic isolation and voltage gain. A high-frequency transformer (HFT) is typically used together with several power semiconductor switches and diodes. The three most popular DC / DC converters are dual active bridge (DAB) converter, inductor-inductor-capacitor (LLC) converter (and its CLLC variant), and Phase-shift full bridge (PSFB) converter. Among these and others, LLC and CLLC can achieve higher efficiency due to their ability to maintain zero voltage switching (ZVS) which minimizes the switching loss. However, their drawback is narrower input and / or output voltage range. On the other hand, DAB and FSFB etc. have lower efficiency but larger input and / or output voltage range.Attorney Docket No. 10046-640W01 8440 HUA

[0015] The exemplary system can provide a high-efficiency operation similar to LLC / CLLC from the soft switching point of view, wide output voltage range operation similar to PSFB and DAB with a wide output voltage range including voltage down to zero, and reverse power flow like the CLLC or DAB.

[0016] In some aspects, implementations of the present disclosure include a system including: an isolated power converter including: an isolated switching stage including a first semiconductor bridge (e.g., full or half bridge) on a primary side of the switching stage and a second semiconductor bridge (e.g., full or half bridge) on a secondary side of the isolated switching stage and isolated by a transformer, the first semiconductor bridge connected to an input of the isolated power converter and configured to switch in combination with the second semiconductor bridge to provide an isolated voltage and current at an output in a first power flow direction from the primary side to the secondary side; and an auxiliary switching stage including at least one auxiliary switch, an auxiliary capacitor, and an auxiliary inductor coupled to the output of the second bridge of the isolated switching stage, wherein the auxiliary switch is in series with the auxiliary capacitor and then in parallel with the second bridge output, and said auxiliary inductor is connected between the auxiliary sw itch and the output of the isolated pow er converter, and wherein said auxiliary switch is operating in coordination with the first semiconductor bridge and the second semiconductor bridge to charge the auxiliary inductor in a first switching frequency cycle and discharge the auxiliary inductor in a complementary cycle of the first switching frequency cycle to affect the output voltage of the isolated power converter.

[0017] In some aspects, implementations of the present disclosure include a system, wherein the isolated switching stage is a resonant converter, with one or more additional resonant capacitors and inductors connected between the switching stages and the isolation transformers, and said resonant converter is operating at a second switching frequency much higher than the said first switching frequency.

[0018] In some aspects, implementations of the present disclosure include a system, wherein the isolated switching stage is configured as an Inductor-Inductor-Capacitor (LLC) resonant converter.

[0019] In some aspects, implementations of the present disclosure include a system, wherein the isolated switching stage is configured as a Capacitor-Inductor-Inductor-Capacitor (CLLC) resonant converter.

[0020] In some aspects, implementations of the present disclosure include a system, wherein the isolated switching stage is a dual active bridge (DAB) converter, with additional inductors connected between the switching stages and the isolation transformers, and said dual activeAttorney Docket No. 10046-640W01 8440 HUA bridge converter is operating at a second switching frequency much higher than said first switching frequency.

[0021] In some aspects, implementations of the present disclosure include a system, wherein the isolated power converter is configured as a single-stage isolated DC / DC converter.

[0022] In some aspects, implementations of the present disclosure include a system, wherein the isolated power converter is configured as a single-stage AC / DC converter where the power flows from an AC side at the first semiconductor bridge to a DC side at the second semiconductor bridge.

[0023] In some aspects, implementations of the present disclosure include a system, wherein the isolated power converter is configured as a single-stage DC / AC converter, wherein power is configured to flow from a DC side at the first semiconductor bridge to an AC side at the second semiconductor bridge.

[0024] In some aspects, implementations of the present disclosure include a system, wherein the isolated power converter is configured as a single-stage bidirectional DC / AC converter where power is configured to flow (i) from an AC side at the second semiconductor bridge to a DC side at the first semiconductor bridge or (ii) from the DC side to the AC side.

[0025] In some aspects, implementations of the present disclosure include a system further including: an isolated power converter comprising: an isolated switching stage comprising a first semiconductor bridge on a primary’ side of the isolated switching stage and a second semiconductor bridge on a secondary side of the isolated switching stage and isolated from the first semiconductor bridge by a transformer having a primary winding and a secondary winding; a resonant network including at least one resonant inductor and at least one resonant capacitor coupled between the first semiconductor bridge and the primary w inding of the transformer; and an output filter capacitor coupled to an output of the second semiconductor bridge to provide a DC output voltage across a load; and an AC grid connection coupled to an AC side of the isolated power converter; and an input filter inductor connected in series between the AC grid connection and an AC -side node of the first semiconductor bridge such that an AC input current drawn by the first semiconductor bridge from the AC grid connection flows through the input filter inductor; wherein the first semiconductor bridge is controlled to switch at a switching frequency above a line frequency of the AC grid connection and to transfer power through the resonant netw ork, the transformer, and the second semiconductor bridge to the DC output voltage.

[0026] In some aspects, implementations of the present disclosure include a system further including: a controller operatively coupled to the first semiconductor bridge, the secondAttorney Docket No. 10046-640W018440 HUA semiconductor bridge, and the auxiliary switch, wherein the controller is configured (i) to turn on the auxiliary switch and enable the first semiconductor bridge and the second semiconductor bridge to charge the auxiliary capacitor and inductor in the first switching cycle of the first switching frequency, wherein the enabled first semiconductor bridge and the second semiconductor bridge are operating at a second switching frequency that is typically much higher than the first switching frequency, and (ii) turn off the auxiliary switch and disable the first semiconductor bridge and second semiconductor bridge in the second switching cycle which is complementary of the first switching cycle of the first switching frequency to discharge the inductor, and wherein the disabled first semiconductor bridge stops switching and the second semiconductor bridge provide a short circuit of a winding of the high frequency transformer.

[0027] In some aspects, implementations of the present disclosure include a system, wherein the controller is configured to control the charge of the inductor by duty cycle control at a first frequency, wherein the first frequency is less than the second frequency of the isolated switching stage, and wherein the duty cycle control defines (i) a duty cycle value D when the auxiliary switch and the isolated switching stage are turned on and enabled (ii) a 1-D duty cycle value when the auxiliary switch and the isolated switching stage are turned off and disabled, and the said disable function includes turning on all switches in the second switching bridge.

[0028] In some aspects, implementations of the present disclosure include a system, wherein the controller is configured to control the charge of the inductor by (i) a constant ON time and (ii) a varying OFF time, to discharge the inductor through a feedback control of the controller, where the constant ON time is defined by an integral N times a switching period of the second switching frequency of the isolated switching stage.

[0029] In some aspects, implementations of the present disclosure include a system, wherein the auxiliary switching stage includes a second auxiliary switch connected in series with a second auxiliary capacitor and in parallel to the first auxiliary capacitor and auxiliary switch, wherein the first auxiliary switch together with its antiparallel diode is configured for a first capacitor charging current direction, the wherein the second auxiliary switch together with its antiparallel diode is configured for a second capacitor charging current direction.

[0030] In some aspects, the secondary side of the isolated power converter includes an unfolding bridge coupled to the output of the second switching stage, the unfolding bridge being connected to an AC grid connection through a filter inductor.

[0031] In some aspects, the secondary side of the isolated power converter includes an unfolding bridge coupled to the output of the second switching stage, through a filter inductor, and the unfolding bridge being connected to an AC grid.Attorney Docket No. 10046-640W01 8440 HUA

[0032] In some aspects, implementations of the present disclosure include a system, wherein the second switching stage of the isolated switching stage includes a diode bridge.

[0033] In some aspects, implementations of the present disclosure include a system, wherein the first switching stage of the isolated switching stage includes a diode bridge, and wherein the unfolding bridge is another diode bridge connected to the AC grid.

[0034] In some aspects, implementations of the present disclosure include a system further including: a set of isolated power converters, each having two input terminals and two output terminals, connected in a stack configuration among these terminals, wherein each of the isolated power converters includes: an isolated switching stage including a first semiconductor bridge (full or hall) on a primary’ side of the switching stage and a second semiconductor bridge (full or hall) on a secondary side of the isolated switching stage and isolated by a transformer, the first semiconductor bridge connected to the input of the isolated power converter and configured to switch in combination with the second semiconductor bridge to provide an isolated voltage and current at an output in a first power flow direction from the primary side to the secondary' side; and an auxiliary switching stage including at least one auxiliary switch, an auxiliary capacitor, and an auxiliary inductor coupled to the output of the second semiconductor bridge of the isolated switching stage, wherein the auxiliary7swatch is both in series with the auxiliary capacitor and in parallel with the second bridge output, wherein said auxiliary7inductor is connected between the auxiliary switch and the output of the isolated power converter, and wherein said auxiliary switch is operating in coordination with the first semiconductor bridge and the second semiconductor bridge to charge the auxiliary7inductor in a first switching frequency cycle and discharge the auxiliary7inductor in the complementary cycle of the first switching frequency cycle to affect the output voltage of the isolated power converter.

[0035] In some aspects, implementations of the present disclosure include a system, wherein the isolated power converters are configured in an input parallel and output parallel (IPOP) configuration.

[0036] In some aspects, implementations of the present disclosure include a system, wherein the isolated power converters are configured in an input parallel and output series (IPOS) configuration.

[0037] In some aspects, implementations of the present disclosure include a system, wherein the isolated power converters are configured in an input series and output parallel (ISOP) configuration.Attorney Docket No. 10046-640W01 8440 HUA

[0038] In some aspects, implementations of the present disclosure include a system, wherein the isolated power converters are configured in an input series and output series (ISOS) configuration.

[0039] In some aspects, implementations of the present disclosure include a system, wherein the stack configuration facilitates a medium voltage connection at one of the terminals.

[0040] In some aspects, implementations of the present disclosure include a half bridge converter system including: an isolated power converter cell including two input terminals and two output terminals; a semiconductor switch in series connection with the isolated power converter cell, a capacitor in parallel with the series connection of said switch and isolated converter cell, a controller that is configured to control / enable the isolated converter cell and the switch in complementary in a first switching frequency period so that a PWM voltage is generated at a midpoint (e.g., where the switch is connected to the isolated converter cell) of said half bridge, wherein when the isolated converter cell is enabled, its switching stages are operating at a second switching frequency which is normally much higher than the first switching frequency.

[0041] In some aspects, implementations of the present disclosure include a system wherein (i) a series connection position of the switch corresponding to the series connection and (ii) the isolated converter cell is exchanged.

[0042] In some aspects, implementations of the present disclosure include a system where the series connected switch is replaced by another isolated converter cell (i.e. two isolated converter cells are in series), and the controller is configured to operate the first isolated converter cell and the second isolated converter cell in complementary during the first switching frequency period.

[0043] In some aspects, implementations of the present disclosure include a system, wherein the isolated converter cell can be a resonant converter such as a CLLC cell, an LLC cell, or any isolated converter cell, and said resonant converter or converter does not have a filter capacitor at the input terminals.

[0044] In some aspects, implementations of the present disclosure include a system further including: a filter (e.g., L filter or LC filter) is connected to the midpoint of said half-bridge, wherein the filter output forms a second input / output while the output terminals of said isolated converter cell form the first input / output.

[0045] In some aspects, implementations of the present disclosure include a system, wherein the first half-bridge formed by two isolated converter cells in series with the first midpoint is further connected to: a second half bridge formed by two additional switches in series with aAttorney Docket No. 10046-640W018440 HUA second midpoint; wherein the first midpoint is connected to a filter inductor and the first terminal of a single phase grid, wherein the second midpoint is connected to the second terminal of the single phase grid, and wherein the output terminals of the first isolated converter cell are in parallel with the output terminals of the second isolated converter cell.

[0046] In some aspects, implementations of the present disclosure include a system, where the two additional switches include two additional isolated converter cells, wherein the output terminals of all four isolated converter cells are connected in parallel.

[0047] In some aspects, implementations of the present disclosure include a system, further including: a three-parallel half-bridge configuration, wherein each half-bridge includes two isolated converter cells in series, wherein each half-bridge has a midpoint, wherein said midpoints are connected to a three-phase grid through a filter inductor, wherein the second terminals of these isolated converter cells are typically connected in parallel to form the input or output of the converter system.

[0048] In some aspects, implementations of the present disclosure include a system, wherein each of the isolated converter cells in the each half-bridge includes: two or more isolated converter cells in series connection configuration, wherein the second terminals of these isolated converter cells are typically connected in parallel to form the input or output of the converter system.

[0049] In some aspects, implementations of the present disclosure include a system, wherein the isolated converter cells are non-resonant converters (e.g., Dual Active Bridge (DAB) converters), wherein the input terminal of each isolated converter cell has a filter capacitor, and wherein the controller is configured, according to the feedback control requirements of the three- phase AC current, to generate (i) a first sinusoidal voltage plus a DC offset voltage on the filter capacitors of the isolated converter cells in an upper arm portion of the respective non-resonant converter, and (ii) a second sinusoidal voltage plus a DC offset voltage on the filter capacitors of the isolated converter cells in a lower arm portion of the respective non-resonant converter.

[0050] In some aspects, implementations of the present disclosure include a system, wherein each isolated converter cell includes an additional half bridge coupled through the said filter capacitor.

[0051] In some aspects, implementations of the present disclosure include a multiport modular multilevel converter including a plurality of submodules, each submodule comprising a system as described herein, and configured to perform single-stage DC-to-AC power conversion to provide a galvanically isolated low-voltage DC (LVDC) port; wherein two or more of the submodules are connected together at their LVDC ports to form a subgroup; and wherein theAttorney Docket No. 10046-640W01 8440 HUA LVDC port of each subgroup is configured for connection to an independent DC resource, including at least one of a photovoltaic source, a battery, or another DC source or DC load.

[0052] Additional features will be set forth in part in the description, which follows or may be learned by practice. The features described herein will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive, as claimed.BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Various objects, aspects, features, and advantages of the disclosure will become more apparent and better understood by referring to the detailed description taken in conjunction with the accompanying drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and / or structurally similar elements.

[0054] Fig. 1 A illustrates an isolated DC / DC converter topology, according to an illustrative embodiment.

[0055] Fig. IB illustrates an example method of operating power converters according to various implementations of the present disclosure, including the converter disclosed in FIG. 1A.

[0056] Fig. 2 illustrates a bidirectional DC / AC converter topology, according to an illustrative embodiment.

[0057] Fig. 3 illustrates a bidirectional DC / AC converter topology including a center tapped HFT (high frequency transformer), according to an illustrative embodiment.

[0058] Fig. 4 illustrates the bidirectional DC / AC converter topology of FIG. 3, including a half bridge on the input side.

[0059] Fig. 5 illustrates a bidirectional DC / AC converter topology, according to an illustrative embodiment.

[0060] Fig. 6 illustrates a unidirectional DC / AC converter topology, according to an illustrative embodiment.

[0061] Fig. 7 illustrates a unidirectional AC / DC converter topology7, according to an illustrative embodiment.

[0062] Fig. 8 illustrates a unidirectional AC / DC converter topology, according to an illustrative embodiment.

[0063] Fig. 9A illustrates an input parallel output series converter according to embodiments of the present disclosure used as converter cells for unidirectional DC / AC power flow.Attorney Docket No. 10046-640W01 8440 HUA

[0064] Fig. 9B illustrates an input parallel output series converter according to embodiments of the present disclosure used as converter cells for bidirectional AC / DC power flow.

[0065] Fig. 10 illustrates a converter topology including an inductor fdter, according to an illustrative embodiment.

[0066] Fig. 11 illustrates the converter topology of Fig. 10 configured as cells in a three- phase converter system.

[0067] Fig. 12 illustrates an example embodiment of the present disclosure including a switch and LLC or CLLC cell to provide an isolated input / output.

[0068] Fig. 13 illustrates an example embodiment of the converter of Fig. 12, including an LLC cell without the input capacitor filter.

[0069] Fig. 14 illustrates a bidirectional DC / DC converter example embodiment of Fig. 12, including an LC filter.

[0070] Fig. 15 illustrates an example embodiment of the present disclosure including a switch and LLC or CLLC cell to provide an isolated input / output.

[0071] Fig. 16 illustrates an example embodiment of the present disclosure including two LLC and / or CLLC cells to provide an isolated input / output.

[0072] Fig. 17 illustrates an example bidirectional AC / DC converter embodiment of the present disclosure that can be controlled according to the methods described herein.

[0073] Fig. 18 illustrates an example three phase converter with multiple converter cells, according to embodiments of the present disclosure.

[0074] Fig. 19 illustrates an example method of using multiple converter cells, according to implementations of the present disclosure.

[0075] Fig. 20 illustrates an example controller for an example AC / DC converter according to embodiments of the present disclosure.

[0076] Fig. 21 illustrates a simulation result for implementations of the present disclosure.

[0077] Fig. 22 illustrates an example multiport modular multilevel converter based solid state transformer system (M?-SST) according to embodiments of the present disclosure.

[0078] Fig. 23A illustrates an example M3-SST system, according to embodiments of the present disclosure, utilizing multiple converter cells (e.g. Dual Active Bridge (DAB)) with a filter in the upper and lower arm. The second terminals of the converter cells are connected in parallel to form a single DC port.

[0079] Fig. 23B illustrates an example M3-SST system, according to embodiments of the present disclosure, including converter cells, diodes, and half bridges.Attorney Docket No. 10046-640W01 8440 HUA

[0080] Fig. 23C illustrates an example M3-SST system, according to embodiments of the present disclosure, including converter cells, diodes, and half bridges.

[0081] Fig. 23D illustrates an example M3-SST system, according to embodiments of the present disclosure, including converter cells and diodes.

[0082] Fig. 23E illustrates an example M3-SST system, according to embodiments of the present disclosure, including converter cells and diodes.

[0083] Fig. 24 illustrates an example system including M3-SSTs for photovoltaic arrays.

[0084] Fig. 25A illustrates simulation results of M3-SSTs showing transformer current, output voltage, phase-to-neutral voltage, and inverter currents.

[0085] Fig. 25B illustrates simulation results of M3-SST according to the embodiments of the present disclosure illustrated in Figs. 23B-23E.

[0086] Fig. 26 illustrates an example system including M3-SSTs and energy storage, according to implementations of the present disclosure.

[0087] Fig. 27 illustrates integration of different DC resources into a system including M3- SSTs, according to implementations of the present disclosure.

[0088] Fig. 28 illustrates an example distributed control system for a system including M3- SSTs, according to implementations of the present disclosure.

[0089] Fig. 29 illustrates an example hybrid energy system including multiple M3-SSTs, according to implementations of the present disclosure.DETAILED DESCRIPTION

[0090] Each and every feature described herein, and each and every’ combination of two or more of such features, is included within the scope of the present invention, provided that the features included in such a combination are not mutually inconsistent.

[0091] Some references, which may include various patents, patent applications, and publications, are cited in a reference list and discussed in the disclosure provided herein. The citation and / or discussion of such references is provided merely7to clarify the description of the present disclosure and is not an admission that any such reference is “prior art” to any aspects of the present disclosure described herein. In terms of notation, “[n]” corresponds to the nthreference in the list. All references cited and discussed in this specification are incorporated herein by reference in their entirety and to the same extent as if each reference was individually incorporated by reference.

[0092] Embodiments of the present disclosure include isolated power converters and methods for operating isolated power converters. Embodiments of the present disclosure includeAttorney Docket No. 10046-640W018440 HUA isolated power converters that include an auxiliary switch (referred to herein as “Qaux”) that can be configured for soft switching of various converter topologies.

[0093] When referring to switching, ON means the switching is conducting current with negligible voltage across it. OFF means the switching does not conduct current and the voltage across the switch is determined by other circuit condition.

[0094] Inductor-Inductor-Capacitor (LLC) is a popular resonant converter widely understood in the field. Capacitor-Inductor-Inductor-Capacitor (CLLC) is a bidirectional version of the LLC. When discussing LLC operation, one familiar with the art will understand that it also applies to the CLLC.

[0095] The ability of a converter to switch the inputs and outputs is referred to as a “bidirectional” power converter herein. For example, a building may include a source of energy that sometimes produces more electrical power than is consumed by the building, and other times produces less electrical energy' than is consumed by the building. Thus the building can sometimes draw power from the grid, and other times supply power to the grid. Bidirectional converters that can be configured for AC / DC loads and sources of power can therefore enable different configurations of electrical grids, loads, and renewable sources of power.

[0096] Example Isolated DC / DC Power Converter with reference to Fig. 1 A, embodiments of the present disclosure include power converters that include an input 102 and a load 108. As described with reference to the various example embodiments, the input 102 is a DC source of electrical power, and the load 108 can be a DC load. Additionally, as also described herein, embodiments of the present disclosure can include bidirectional power converters, so that the load 108 and input 102 can be switched by controlling the power converter.

[0097] Still with reference to FIG. 1 A, an example power converter can include an isolated switching stage 104a and an auxiliary switching stage 106a. The isolated switching stage 104a can be configured using any type of half or full bridge topologies. FIG. 1 A illustrates a primary half bridge 115a on a primary side of a high frequency transformer 120, and a second full bridge 115b on the secondary side of the high frequency transformer 120. In the example of FIG. 1A, switches 114a and 114b form a half bridge, and switches 122a, 122b, 122c. 122d form a full bridge. The primary side further includes a pair of input capacitors 112a, 112b connected between the input terminals to define a midpoint node coupled to the switching node of the primary half bridge 115a. A resonant capacitor 116 and resonant inductor 118 are also used on the primary side of the high frequency transformer 120 (high frequency transformer).

[0098] The auxiliary switching stage 106a can be configured, in combination with the switching stage 104a to allow operation of the power converter with the highest efficiency. AsAttorney Docket No. 10046-640W01 8440 HUA shown in FIG. 1A, Qaux 124 can be switched ON and OFF by the controllers and control methods described herein. Qaux is in series with an auxiliary capacitor Caux 126. The auxiliary switching stage 106a can optionally include a fdter inductor 130 and filter capacitor 128 to filter the output of the secondary' bridge 115b.

[0099] Fig. IB illustrates an example method of operating power converters according to various implementations of the present disclosure, including the converter disclosed in FIG. 1 A. As shown in Fig. IB, the control signal of Qaux can be used in combination with the switching stage 104a to control the Vaux seen across the secondary bridge 115b of the power converter.

[0100]

[0101] In the example of FIG. IB, the primary side is a half bridge but the operation with a full bridge is the same. There are two frequencies associated with the converter operation: 1) fr: LLC resonant frequency which determines the switching frequency of Qa, Qb, Q1-Q4; and 2) PWM frequency fpwm, which determines the switching frequency of Qaux and the ON and OFF operation of the LLC converter. Normally fr»fpwm. Example: fr=100 kHz, fpwm=20 khz.

[0102] An example method of operating the converter is show n in Fig. IB that includes controlling the voltage Vaux to have two values like a PWM converter:

[0103] During duty cycle 1-D, Vaux=0V. During this time, Qaux is OFF and switches QI to Q4 are ON and the primary' bridge 115a is disabled (switch Qa and Qb are OFF).

[0104] During D. Vaux=nVdc / 2. (if primary side is full bridge, it will be nVdc. where n is the HFT turns ratio). During this time, Qaux is ON, and the switching stage 104a is operating like an LLC at its resonant frequency' fr.

[0105] Accordingly, duty' cycle D can change to control the inductor current II.

[0106] The plot of FIG. IB illustrates example switching cycles. Before To, Vaux=0V. inductor current IL is non-zero and flows into Vout direction. Qa, Qb are OFF, QI, Q2, Q3, Q4 are ON, Qaux is OFF, so Vaux is zero voltage, inductor current II is discharging and flowing through the parallel path formed by QI, Q3, Q2, Q4. Caux voltage is at its previous state which is nVdc / 2.

[0107] In To— T), Duty cycle D period, Vaux=nVdc / 2.

[0108] Q — Q4are turned off first then the output inductor current flows in the body' diodes of Q1-Q4, after a short deadtime td, Qauxis turned on, and the inductor current is commuted from Q1-Q4 path to the Qaux path. The switching voltage is the voltage on Cauxwhich is approximately nVDC / 2. Meanwhile, the resonant switching signals are applied to Qaand Qbstarts with 25% duty cycle and continue with 50% duty cycle. QI to Q4 are also switching at the resonant frequency with 50% duty' cycle as in a normal LLC resonant converter. In this state, theAttorney Docket No. 10046-640W01 8440 HUA LLC transfers power to the output. Inductor current II is being charged since the Vaux voltage is nVdc / 2. The LLC operates normally, so all high frequency Qa-Qb, Q1-Q4 switches can achieve ZVS turn-on.

[0109] Ti — T2'. Qaux is turned off first, then Q, — Q4are all turned ON. Output inductor current commutes to Q1— Q4. Qa and Qbare also turned off at T . The LLC stops transmitting power. In this state, the output inductor current 1Lis freewheeling through all secondary switches of LLC. so the= 0 7. The voltage on Cn,.xholds at the value at T, until the next start of the LLC. Finally, T2-T0=T=l / fpwm.

[0110] Example DC / AC Converter

[0111] The present disclosure contemplates that the isolated switching stage 104a can be formed using any combination of resonant converter topologies and bridge topologies. The present disclosure also contemplates that the isolated switching stage 104a and auxiliary switching stage 106a can be controlled to achieve unidirectional and / or bidirectional power flow, and that the power flow can be any combination of AC and DC inputs / outputs. In the case of AC, e.g., shown in Figs. 2 - 6, additional circuit element will be added according to various embodiments.

[0112] Bidirectional DC / AC. Fig. 2 illustrates an example bidirectional DC / AC converter topology, according to an illustrative embodiment. The isolated switching stage 104b includes a CLLC cell and the auxiliary switching stage 106b further includes an unfolding bridge 107.

[0113] The present disclosure further contemplates different configurations of HFT and Qaux. Figs. 3 and 4 illustrate additional example bidirectional DC / AC converter topologies including an isolated switching stage 104c, 104d where the high frequency transformer!20 is center-tapped. The auxiliary switching stage 106c, 106d is configured to include two Qaux switchesl24a, 124b connected in series. FIG. 3 includes a full-bridge on the primary side, FIG. 4 includes a half-bridge on the primary side.

[0114] Fig. 5 illustrates a DC / AC full-bridge converter topology where the auxiliary7switching stage 106e includes two Qaux switches 124a and 124b in parallel and the isolated switching stage 104e includes a center-tapped transformer.

[0115] Fig. 6 illustrates a unidirectional DC / AC converter topology7, according to an illustrative embodiment. The second bridge 115b of an isolated switching stage 104f is configured using diodes 109 and the auxiliary switching stage 106f is configured for AC output with an unfolding bridge.

[0116] Unidirectional AC / DC. Fig. 7 illustrates a unidirectional AC / DC converter topology, where the primary7bridge 115a of the isolated switching stage 104g is configured using diodesAttorney Docket No. 10046-640W01 8440 HUA 109 for a unidirectional power flow. Diodes 109 are also included in the auxiliary switching stage 106g. A filter Lf is included between diodes 109 and the grid.

[0117] Unidirectional AC / DC. Fig. 8 illustrates a variation of the unidirectional AC / DC converter topology of Fig. 7, wherein the filter inductor 130 is placed between the diodes 109 and Qaux and Caux.

[0118] Multiple stage AC / DC. Embodiments of the present disclosure further include converters including multiple power converters (e.g., any number of the multiple isolated switching stages 104a and auxiliary switching stages 106a describe herein) connected in series or in parallel. Fig. 9A illustrates multiple converters used as converter cells for unidirectional DC / AC power flow. Fig. 9B illustrates multiple converters used as converter cells for bidirectional AC / DC power flow.

[0119] Filter configuration. FIGS. 10-11 illustrate yet another example embodiment of the present disclosure including a filter inductor 130 between the primary' bridge 115a and the grid as shown in FIG. 10. As shown in FIG. 11, the topology of FIG. 10 can be combined in multiple cells and / or configured as a three-phase power system. It should be understood that the topology of FIG. 10 can be further configured as any number of phases in different power systems (e.g., single phase).

[0120] Example Isolated LLC or CLLC-based system

[0121] With reference to FIGS. 12-21. embodiments of the present disclosure include topologies combining switches with resonant cells (e.g. LLC or CLLC cells).

[0122] Fig. 12 illustrates an example embodiment of the present disclosure including a switch 1202 and resonant cell 1204 (e.g., LLC or CLLC cell) that can provide an isolated input / output power converter. Fig. 13 illustrates an example LLC topology that can be used to implement Fig. 12. and Fig. 14 illustrates the example embodiment of Fig. 12, including an LC filter and a second Vin or Vout.

[0123] As shown in FIGS. 15-18, different topologies of resonant cells 1204 and switches 1202 can be used to implement various embodiments of the present disclosure. FIG. 15 illustrates an additional example arrangement of switch 1202 and resonant cell 1204 (e.g., LLC or CLLC cell) to provide an isolated input / output.

[0124] Fig. 1 illustrates another example topology including two resonant cells 1204.

[0125] Fig. 17 illustrates another example topology7including two switches 1202 and two resonant cells 1204. An L filter is connected between the single phase AC grid and the two midpoints formed by the two switches 1202 and two resonant cells 1204.Attorney Docket No. 10046-640W01 8440 HUA

[0126] Again, the present disclosure also contemplates that any number of converter cells can be combined in series and / or parallel to output AC or DC power in any number of outputs / phases. For example, Fig. 18 illustrates operating multiple converter cells (e.g., the converters illustrated in FIGS. 12-17) to connect to a three-phase grid Va, Vb, and Vc. FIG. 19 illustrates another example topology of multiple converter cells in input series and output in parallel and connected to a three-phase grid Va, Vb. and Vc.

[0127] Again, it should be understood that any of the switches described herein can be controlled to operate the converters described herein in different modes (e.g., different output voltages, different power flow directions). An example controller 2000 is illustrated in FIG. 20 for a single phase unidirectional AC / DC embodiment in which multiple converter cells are connected in input series, output parallel configuration. The controller includes sensors 2002 and PWM modules 2004 for controlling the input current waveforms. Fig. 21 illustrates a simulation result for the control the converter of FIG. 20 in which multilevel voltage steps are obtained at the input of the series connected converter cells.

[0128] Embodiments of the present disclosure can be used to implement a multiport modular multilevel solid state transformer system (M3-SST). While the example of Fig. 22-29 is described with reference to specific energy' sources and systems, it should be understood that these examples are non-limiting, and embodiments of the present disclosure can be used with any type of energy system.

[0129] FIG. 22 illustrates an overall configuration of an M3-SST which interfaces a medium voltage DC (MVDC) line coming from offshore wind and an MV AC line linking the system to the main AC grid. Additionally, this converter provides multiple isolated DC links that can be utilized for various purposes such as distributed PV or energy storage integration. The multiport nature of the proposed M3-SST is shown. The multiple power flow directions and the AC and DC nature of the electricity form the core of the hybrid AC / DC energy router platform that can be expanded to any AC and DC hybrid system configurations, with or without wind interconnection. The MV AC and MVDC ports could also be high voltage AC (HVAC) and high voltage DC (HVDC).

[0130] FIG. 23 A shows an embodiment of M3-SST in which many converter cells 2300 are used to accommodate the required MV AC and MVDC voltages. The converter cells 2300 (also referred to herein as submodules (SMs)) can be a dual active bridge (DAB) converter with a capacitor filter on the AC side and a low voltage DC (LVDC) port on the other side. Other converter topology can also be used. The AC side of the converter cells are connected in series to generate arm voltages Vau, Vai, Vbu, Vbl, Vcu, Vcl. The capacitor voltage is controlled toAttorney Docket No. 10046-640W01 8440 HUA include a DC offset voltage plus an AC voltage. The said capacitor can be substantially smaller than those used in the conventional MMC or MMC based SSTs. This is a key advantage of the M3-SST. Due to the fluctuating nature of the arm instantaneous power, large capacitors are required in the MMC or conventional MMC-SST to keep the low-frequency ripple of capacitor voltages within an acceptable range. About 60% of the SM volume in megawatt-scale MMC- SSTs is occupied by capacitors alone. Moreover, the MMC-SST contains a large number of switching devices, which contributes to the high implementation cost and complexify of the system. Consequently, the size of the capacitors and the number of switching devices are among the primary obstacles to enhancing the power densify and reducing the implementation costs of MMC-SSTs.

[0131] In the MMC-SSTs, the isolated DC-DC converter provides more flexibility compared with the MMC since it can transfer either constant or fluctuating powers from the LVDC terminal to the SM capacitors. In the latter case, the fluctuating power injected by the DAB offsets the arm instantaneous power fluctuations and effectively suppresses the SM capacitor voltage ripple. Since the fluctuating powers in different arms are complementary, the accumulated power at the LVDC port automatically cancels out the power fluctuations. This strategy7can suppress the capacitor voltage ripple, and the results show that the capacitor size is reduced to one-tenth of the normal size. Further improvement in power densify and implementation cost is achieved by employing quad active bridge (QAB) technology. In this technique, the primary-side H-bridges of three different phases are integrated into a single H- bridge. Additionally, instead of three separate HFTs, a single HFT with one primary winding and three secondary windings is utilized. It is worth noting that transferring fluctuating power increases the peak and RMS currents of the DAB, which necessitates the redesign of the SM including the high-frequency transformer, heatsink, and switching devices. Considering the redesigned parameters, this strategy can improve the power densify of the SM by over 50%. However, reduced system efficiency, complexify of two-stage power conversion, and the large number of switching devices are among the main drawbacks of these MMC-SSTs.

[0132] FIG. 23A demonstrates the structure of an example M3-SST with a single isolated LVDC terminal, a medium voltage DC (MVDC) terminal and a three phase medium voltage AC (MV AC) terminal. As can be seen, each arm of the converter includes an inductor for current regulation, a resistor for power loss modeling, and N SMs that are connected in series. The primary sides of SMs are connected in parallel to create an isolated LVDC terminal, and the secondary sides are cascaded to create an MVDC and MV AC terminals. Fig. 23A also demonstrates the structure of SMs. Although the M3-SST is inspired by the MMC topology7, itAttorney Docket No. 10046-640W01 8440 HUA introduces several key innovations. In this proposed structure, half-bridge modules are eliminated, enabling single-stage DC-to-AC power conversion, which significantly enhances the converter’s efficiency and power density. Furthermore, the bulky capacitors used in conventional MMC-SSTs are replaced with smaller capacitors (<200 pF), resulting in substantial reductions in both size and implementation costs. The combination of SM capacitors with the arm and grid inductors forms distributed LCL filters, which significantly improves the power quality’ and reduces the size of elements.

[0133] As shown in FIG. 23B-23E, implementations of the present disclosure can include any number of cells 2300, in combination with half bridges 2302 and diodes 2304. FIG. 23B illustrates an alternative embodiment of the example M3-SST shown in FIG. 23 A, wherein the number of cells 2300 is reduced by half, and the removed cells 2300 are replaced by three diodes 2304. FIG. 23C illustrates an alternative arrangement of diodes and cells to the arrangement illustrated in FIG. 23B. FIG. 23D illustrates the converter of FIG. 23B, configured without the half bridges 2302 shown in FIG. 23B. FIG. 23E illustrates the converter of FIG. 23C, configured without the half bridges 2302 shown in FIG. 23C.

[0134] The example M3-SST shown in FIG. 23A can be further modified as shown in FIG. 24 to provide multiple galvanically isolated low-voltage DC (LVDC) terminals. These LVDC ports can be utilized for connecting distributed DC resources, such as PV arrays. Each PV array is grouped and grounded to meet installation safety requirements. The example M3-SST can include several groups, each of which includes six SMs located in the converter’s six arms (three upper arms for phases A, B, and C, and three lower arms for phases A, B, and C). This subgrouping can be used to provide the flexibility7to connect different DC resources to the LVDC ports. The primary side of each subgroup is connected to an isolated DC link, which can be used for various purposes such as PV or Battery Energy Storage System (BESS) integration.

[0135] The distributed arrangement of PV arrays or BESS, compared to a large single PV array or large BESS, offers numerous benefits in utility-scale solar or BESS applications, particularly when combined with distributed maximum power point tracking (DMPPT) systems or individual battery management system (BMS) for battery strings. DMPPT technology enables each PV array to operate independently at its maximum power point, significantly reducing the impacts of partial shading and enhancing overall energy7harvesting. By' allowing individual arrays to optimize their output despite shading or other factors affecting certain areas of the array. DMPPT maximizes the overall energy7yield of the PV system. Similarly, individual BMS allows each battery string to operate optimally.Attorney Docket No. 10046-640W01 8440 HUA

[0136] The example M3-SST features a symmetric design that eliminates power imbalances and enhances the modularity of the control system. In conventional MMC-based PV systems, DMPPT leads to inter-module, inter-arm, and inter-phase power imbalances. Several studies have addressed this issue by transferring power within the converter. However, transferring power from SMs with higher output to those with lower output increases the differential current within the MMC. To mitigate inter-phase power imbalances, a DC differential current is circulated among phases, while inter-arm imbalances are managed by increasing the AC component of differential currents. Nonetheless, increasing differential current in MMCs leads to higher power losses and larger capacitor voltage ripple, and the effectiveness of these methods is limited by device current ratings.

[0137] The example M3-SST introduces multiple isolated DC links for distributed PV integration. Despite potential variations in power generation from PV arrays connected to these links, the system maintains power balance due to its symmetric design. Each PV array is connected to a subgroup comprising six SMs distributed across six arms. This symmetric design ensures even power sharing among the SMs, promoting homogeneous power distribution throughout the converter and making it a highly efficient and flexible solution for integrating distributed energy resources.

[0138] To demonstrate the feasibility and performance of the proposed M3-SST, a 4.2 kV, 1 MW M3-SST is simulated in PLECS software. In this simulation, the proposed M3-SST includes three SMs per arm and provides three isolated LVDC ports for the integration of three PV arrays. The detailed system parameters are listed in Table I. Notably, the capacitor size in the M3-SST is significantly reduced, and both the arm inductor and grid filter inductor are very small. FIG. 25A shows the current of the high-frequency transformer and the voltages of the SMs in the upper and lower arms. As observed, the SM voltage is a sinusoidal waveform with a DC offset, with no PWM voltage present. FIG. 25 A further illustrates the inverter voltage and current at the MV AC terminal. Despite the small size of the filter used, the current THD is below 1%, demonstrating the excellent power quality of the M3-SST.The embodiments of FIG. 23B-23E were also simulated in the study, as shown in the results of FIG. 25B.

[0139] TABLE I Comparison of Proposed and ZSI-Based MethodsParameter ValueMVAC Grid Voltage 4.2 kVMVDC Grid Voltage 10 kVNominal Power 1 MWDC-Link Voltage 10 kVAttorney Docket No. 10046-640W018440 HUACapacitor Size 100 uFArm Inductor Size 100 pHGrid Filter Inductance 100 uHSwitching Frequency 20 kHzNumber of Modules per 3Arm

[0140] Table II provides a comparative overview of various factors influencing the performance of the traditional MMC-SST, QAB-based MMC-SST, and the proposed M3-SST. Key elements such as efficiency, number of switching devices, and component sizes (capacitors and grid filters) are considered to illustrate the advancements offered by the M3-SST. Notably, the M3-SST demonstrates superior efficiency and power density, thanks to single-stage power conversion, along with excellent power quality and reduced component sizes. This comparison highlights the proposed M3-SST’s potential for improved performance and reliability in applications requiring high power density7and quality.

[0141] TABLE II Comparison of Proposed M3-SST with Existing MMC-based SSTs

[0142] M3-SST For Utility Scale BESS Integration

[0143] Energy storage is becoming essential to enable the integration of the intermittent wind or PV power into the electric energy system. By incorporating battery energy storage systems (BESS), such as lithium-ion batteries, into these systems, operators can effectively smooth out the power delivery curve, ensuring a consistent and reliable energy supply to theAttorney Docket No. 10046-640W01 8440 HUA grid. Energy storage helps mitigate the variability and intermittency inherent in wind power generation by storing excess energy during periods of high wind production and releasing it during low wind periods or high demand periods, thus balancing the supply-demand equation. This improves the overall stability of the system, reducing the need for backup power sources and enhancing grid resilience. Real-world examples of projects integrating wind power with battery energy storage include the Homsdale Power Reserve in South Australia, which utilizes Tesla's Powerpacks to stabilize the grid, and the Kauai Island Utility Cooperative project in Hawaii, which combines wind turbines with lithium-ion batteries to provide reliable renewable energy7to the island's grid. These proj ects demonstrate the effectiveness of energy7storage in enhancing the integration of wind power and ensuring a stable and reliable power supply.

[0144] In the example M3-SST configuration, the energy storage units can be integrated into the LVDC link connecting to one or more subgroups, as shown in FIG. 26. This substantially reduces the number of power conversion stages and results in a more efficient system. This streamlined integration not only enhances system efficiency but also reduces costs associated with additional conversion equipment.

[0145] M3-SST for Hybrid Energy Hub

[0146] The example embodiment of an M3-SST can provide multiple isolated DC links, offering a versatile platform for distributed PV arrays or energy storage systems. However, the potential applications of these DC links extend beyond PV arrays and energy storage, presenting opportunities for diverse applications such as electric vehicle (EV) charging stations and data centers, as depicted in FIG. 27. This versatility7positions the M3-SST as a multifaceted energy hub capable of meeting various energy7demands.

[0147] In this scenario, each subgroup of the M3-SST operates in different modes to accommodate the specific requirements of each application. For instance, the data center may represent a constant load, while the EV charging station constitutes a variable load. Moreover, the exchange of power between the PV generator and the EV charging station further underscores the adaptability7of the M3-SST in facilitating energy flow7across different applications.

[0148] Since the sub-modules of the M3-SST are connected in series, they share the same current. How ever, due to variations in powder flow7direction generation, the voltage levels of individual sub-modules may differ, resulting in opposing directions in the phasor diagram. Consequently, the design of converters and control systems must adhere to stringent stability criteria to ensure reliable operation under diverse load conditions.Attorney Docket No. 10046-640W018440 HUA

[0149] Distributed Control System for the M3-SST

[0150] As can be seen from above discussions, central to the success of the proposed M3- SST is a robust controller that not only needs to perform various power conversion functions, but also addresses the potential power imbalances created by various energy resources connected to the LVDC ports. The complexity of the controller grows exponentially as the number of SMs increases. This is a fundamental and grand challenge in all modular converters including the M3- SST and poses a significant challenge for the real-world implementation of a multi -megawatt M3-SST. In a multi-megawatt M3-SST, managing numerous voltages and currents, executing multiple control algorithms, and generating hundreds of control commands become formidable tasks. Centralized control architectures, while effective, demand substantial computational power for processing and wide communication bandwidths for data exchange between the central controller and the SMs. Moreover, the centralized approach introduces wiring complexity and compromises reliability due to the risk of a single point of failure.

[0151] To address these challenges, distributed control is emerging as a viable alternative, offering a solution to alleviate the processing burden on the central controller and simplify the communication system's complexity. This architecture delegates lower-level control tasks, such as Maximum Power Point Tracking (MPPT), capacitor voltage balancing, and modulation, to local controllers within each sub-module. Previous studies have explored SM-based distributed control systems to offload tasks like capacitor voltage balancing and differential current control to local controllers. However, these approaches require advanced synchronization techniques to mitigate harmonics, voltage waveform distortions, and control conflicts effectively.

[0152] Furthermore, existing studies have categorized SMs into multiple clusters within each arm, with dedicated control modules handling capacitor voltage balancing and modulation for each cluster. While these methods effectively reduce the processing load on the central controller, they may not achieve optimal performance in smaller systems due to the impact of reducing the number of SMs in the clusters on both capacitor voltage balancing and voltage waveform quality.

[0153] In FIG. 28, the exemplar}’ distributed control architecture for the M3-SST assigns each subgroup with a local control system responsible for managing lower-level control tasks. The local controller, typically implemented by a single processor, operates within the isolated secondary side of the SMs. This isolation necessitates the transmission of communication signals between the primary and secondary sides to be isolated, posing challenges in terms of cost and vulnerability to noise.Attorney Docket No. 10046-640W01 8440 HUA

[0154] To address these challenges, a cost-effective approach involves utilizing a Digital Signal Processor (DSP) processor for the primary side of the subgroup and a more economical Microcontroller (MSP) for the secondary side of each SM. The primary-side controller receives communication signals from the central controller and executes lower-level control tasks, while the MSP controllers receive the phase shift references and perform modulation. By doing so, gate signals required for converter operation can be generated locally within each sub-module, eliminating the need for isolated communication signals and reducing complexity and cost associated with communication interfaces.

Claims

Attorney Docket No. 10046-640W018440 HUAWHAT IS CLAIMED IS:

1. A system comprising: an isolated power converter comprising: an isolated switching stage comprising a first semiconductor bridge (e.g., full or half) on a primary side of the switching stage and a second semiconductor bridge (e.g., full or half) on a secondary side of the isolated switching stage and isolated by a transformer, the first semiconductor bridge connected to an input of the isolated power converter and configured to switch in combination with the second semiconductor bridge to provide an isolated voltage and current at an output in a first power flow direction from the primary side to the secondary side; and an auxiliary switching stage comprising at least one auxiliary switch, an auxiliary capacitor, and an auxiliary inductor coupled to the output of the second bridge of the isolated switching stage, wherein the auxiliary switch is in series with the auxiliary capacitor and then in parallel with the second bridge output, and said auxiliary' inductor is connected between the auxiliary switch and the output of the isolated pow er converter, and wherein said auxiliary switch is operating in coordination with the first semiconductor bridge and the second semiconductor bridge to charge the auxiliary inductor in a first switching frequency cycle and discharge the auxiliary inductor in a complementary cycle of the first switching frequency cycle to affect the output voltage of the isolated power converter.

2. The system of claim 1, wherein the isolated switching stage is a resonant converter, with one or more additional resonant capacitors and inductors connected between the switching stages and the isolation transformers, and said resonant converter is operating at a second switching frequency much higher than said first switching frequency.

3. The system of claim 2, wherein the isolated switching stage is configured as an Inductor- Inductor-Capacitor (LLC) resonant converter.

4. The system of claim 2, wherein the isolated switching stage is configured as a Capacitor- Inductor-Inductor-Capacitor (CLLC) resonant converter.

5. The system of any one of claims 1 or 2, wherein the isolated switching stage is a dual active bridge (DAB) converter, with additional inductors connected between the switching stagesAttorney Docket No. 10046-640W01 8440 HUA and the isolation transformers, and said dual active bridge converter is operating at a second switching frequency much higher than said first switching frequency.

6. The system of any one of claims 1 to 5, wherein the isolated power converter is configured as a single-stage isolated DC / DC converter.

7. The system of any one of claims 1 to 5, wherein the isolated power converter is configured as a single-stage AC / DC converter where the power flows from an AC side at the first semiconductor bridge to a DC side at the second semiconductor bridge.

8. The system of any one of claims 1 to 5, wherein the isolated power converter is configured as a single-stage DC / AC converter, wherein power is configured to flow from a DC side at the second semiconductor bridge to an AC side at the first semiconductor bridge.

9. The system of any one of claims 1 to 5, wherein the isolated power converter is configured as a single-stage bidirectional DC / AC converter where power is configured to flow (i) from a DC side at the second semiconductor bridge to an AC side at the first semiconductor bridge or (ii) from the AC side to the DC side.

10. The system of any one of claims 1 to 9 further comprising: a controller operatively coupled to the first semiconductor bridge, the second semiconductor bridge, and the auxiliary switch, wherein the controller is configured (i) to turn on the auxiliary switch and enable the first semiconductor bridge and the second semiconductor bridge to charge the auxiliary capacitor and inductor in the first switching cycle of the first switching frequency, wherein the enabled first semiconductor bridge and the second semiconductor bridge are operating at a second switching frequency that is typically much higher than the first switching frequency, and (ii) turn off the auxiliary switch and disable the first semiconductor bridge and second semiconductor bridge in the second switching cycle which is complementary of the first switching cycle of the first switching frequency to discharge the inductor, and wherein the disabled first semiconductor bridge stops switching and the second semiconductor bridge provide a short circuit of a winding of the transformer.

11. The system of claim 10, wherein the controller is configured to control the charge of the inductor by duty cycle control at a first frequency, wherein the first frequency is less than theAttorney Docket No. 10046-640W01 8440 HUA second frequency of the isolated switching stage, and wherein the duty cycle control defines (i) a duty cycle value D when the auxiliary switch and the isolated switching stage are turned on and enabled (ii) a 1-D duty cycle value when the auxiliary switch and the isolated switching stage are turned off and disabled12. The system of claim 10, wherein the controller is configured to control the charge of the inductor by (i) a constant ON time and (ii) a varying OFF time, to discharge the inductor through a feedback control of the controller, where the constant ON time is defined by an integral N times a switching period of the second switching frequency of the isolated switching stage.

13. The system of claim 12, wherein the auxiliary switching stage includes a second auxiliary switch connected in series with a second auxiliary capacitor and in parallel to the first auxiliary capacitor and auxiliary switch, wherein the first auxiliary switch together with its antiparallel diode is configured for a first capacitor charging current direction, wherein the second auxiliary' switch together with its antiparallel diode is configured for a second capacitor charging current direction.

14. The system of any one of claims 1 to 12, wherein the secondary side of the isolated power converter comprises an unfolding bridge, wherein said unfolding bridge is coupled to the output of the second switching stage and connected with an AC grid connection through a filter inductor.

15. The system of claim 14, where the unfolding bridge is coupled through a filter inductor to the output of the second switching stage and connected to the AC grid connection.

16. The system of any one of claims 1 to 12 and 14 to 15, wherein the second switching stage of the isolated switching stage comprises a diode bridge.

17. The system of any one of claims 14 to 15, wherein the first switching stage of the isolated switching stage comprises a diode bridge, and wherein the unfolding bridge is another diode bridge.

18. A system comprising: an isolated power converter comprising: an isolated switching stage comprising a first semiconductor bridge on a primary' side of the isolated switching stage and a secondAttorney Docket No. 10046-640W01 8440 HUA semiconductor bridge on a secondary' side of the isolated switching stage and isolated from the first semiconductor bridge by a transformer having a primary winding and a secondary winding; a resonant network including at least one resonant inductor and at least one resonant capacitor couple between the first semiconductor bridge and the ri ary winding of the transformer; and an output filter capacitor coupled to an output of the second semiconductor bridge to provide a DC output voltage across a load; and an AC grid connection coupled to an AC side of the isolated power converter; and an input filter inductor connected in series between the AC grid connection and an AC- side node of the first semiconductor bridge such that an AC input cunent drawn by the first semiconductor bridge from the AC grid connection flows through he input filter inductor; wherein the first semiconductor bridge is controlled to switch at a switching frequency above a line frequency of the AC grid connection and to transfer power through the resonant network, the transformer, and the second semiconductor bridge to the DC output voltage.

19. A converter system comprising: a set of isolated power converters, each comprising a system according to any one of claims 1 to 18, each isolated power converter having two input terminals and two output terminals and connected in a stack configuration; and an auxiliary switching stage associated with each isolated power converter and comprising at least one auxiliary switch, an auxiliary capacitor, and an auxiliary inductor coupled to an output of an isolated converter cell of the system according to claims 1 to 18, wherein the auxiliary switch is connected in series with the auxiliary' capacitor and in parallel with the output of said isolated converter cell. wherein the auxiliary inductor is connected between the auxiliary switch and a DC-side output node of the isolated converter cell upstream of any unfolding bridge or diode bridge, and wherein the auxiliary switch is operated in coordination with switching elements of the system according to any one of claims 1 to 18 to charge the auxiliary inductor during a first switching-frequency cycle and to discharge the auxiliary inductor during a complementary switching-frequency cycle to affect an output voltage of the isolated power converter.

20. The system of claim 19, wherein the isolated power converters are configured in an input parallel and output parallel (1POP) configuration.Attorney Docket No. 10046-640W01 8440 HUA21. The system of claim 19, wherein the isolated power converters are configured in an input parallel and output series (IPOS) configuration.

22. The system of claim 19, wherein the isolated power converters are configured in an input series and output parallel (ISOP) configuration.

23. The system of claim 19, wherein the isolated power converters are configured in an input series and output series (ISOS) configuration.

24. The system of claim 21, 22 or 23, wherein the stack configuration provides a mediumvoltage connection at a terminal of the system.

25. A converter system comprising:• an isolated power converter cell having first and second input terminals and first and second output terminals;• a semiconductor switch connected in series with the isolated power converter cell;• a capacitor connected in parallel with the series connection of the semiconductor switch and the isolated power converter cell;• a midpoint node formed between the semiconductor switch and the isolated power converter cell;• a controller configured to control the isolated power converter cell and the semiconductor switch in complementary operation during a first switching-frequency period such that a pulse-width-modulated voltage is generated at the midpoint node;• wherein, when the isolated pow er converter cell is enabled, switching stages of the isolated power converter cell operate at a second switching frequency.

26. The system of claim 25 wherein (i) a series connection position of the switch corresponding to the series connection and (ii) the isolated converter cell are exchanged.

27. The system of any one of claims 25 to 26, wherein the isolated converter cell can be a resonant converter such as a CLLC cell, an LLC cell, or an isolated converter cell, and said resonant converter or converter does not have a filter capacitor at the input terminals.Attorney Docket No. 10046-640W01 8440 HUA28. The system of any one of claims 25 to 27, further comprising a filter connected to the midpoint of the half bridge, wherein an output of the filter forms a second input / output of the system, and wherein the output terminals of the isolated power converter cell form a first input / output of the system.

29. The system of any one of claims 25 to 28, wherein said switch comprises a second isolated converter cell, and the controller is configured to operate the first isolated converter cell and the second isolated converter cell in complementary during the first switching frequency period.

30. The system of any one of claims 25 to 29, wherein a first half bridge is formed by two isolated converter cells connected in series to define a first midpoint, and further comprising:• a second half bridge formed by two additional switches connected in series to define a second midpoint;• wherein the second half bridge is connected in parallel with the first half bridge;• wherein the first midpoint is connected to a filter inductor and to a first terminal of a single-phase grid;• wherein the second midpoint is connected to a second terminal of the single-phase grid; and• wherein output terminals of the first isolated converter cell are connected in parallel with output terminals of the second isolated converter cell.

31. The system of claim 30, where the two additional switches comprise two additional isolated converter cells, wherein the output terminals of all four isolated converter cells are connected in parallel.

32. The system of any one of claims 24 to 31, further comprising:• a three-phase bridge configuration comprising three half bridges, each half bridge including two isolated converter cells connected in series to define a corresponding midpoint;• wherein the midpoints of the three half bridges are respectively connected to a three- phase grid through corresponding filter inductors; andAttorney Docket No. 10046-640W01 8440 HUA • wherein output terminals of the isolated converter cells of the three half bridges are connected in parallel to form a common input / output of the converter system.

33. The system of any one of claims 24 to 32, wherein each isolated converter cell comprises a plurality of sub-converter cells, each sub-converter cell having a first terminal and a second terminal; wherein the first terminal of each sub-converter cell is connected in series with the first terminal of an adjacent sub-converter cell to form a series string; and wherein the second terminals of the plurality of sub-converter cells are connected in parallel with one another to form an input / output port of the converter system.

34. The system of claim 33, wherein the isolated converter cells are non-resonant converters, each having an input terminal coupled to a filter capacitor; wherein an inductor is optionally connected in series with the series-connected isolated converter cells; and wherein the controller is configured, based on feedback control of three-phase AC current, to generate (i) a first voltage waveform comprising a sinusoidal component and a DC offset on the filter capacitors of isolated converter cells in upper-arm positions of the non-resonant converters, and (ii) a second voltage waveform comprising a sinusoidal component and a DC offset on the filter capacitors of isolated converter cells in lower-arm positions of the non-resonant converters.

35. The system of claim 34. wherein the isolated converter cells of all three upper arms, or alternatively the isolated converter cells of all three lower arms, are replaced by a diode device to form a diode arm.

36. The system of claim 34, wherein each isolated converter cell comprises an additional half bridge coupled through the said filter capacitor.

37. A multiport modular multilevel converter comprising: a plurality' of submodules, each submodule comprising a system according to any one of claims 1 to 34 and configured to perform single-stage DC-to-AC power conversion to provide a galvanically isolated low-voltage DC (LVDC) port; wherein two or more of the submodules are connected together at their LVDC ports to form a subgroup; andAttorney Docket No. 10046-640W018440 HUA wherein the LVDC port of each subgroup is configured for connection to an independentDC resource, including at least one of a photovoltaic source, a battery, or another DC source or DC load.

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