Wide voltage range power conversion systems for electric or hybrid vehicles

Abstract

Wide voltage range power conversion system for electric or hybrid vehicles. The present invention relates to a power conversion system for electric and / or hybrid vehicles, the power conversion system comprising: a transformer having at least two windings; at least two power modules, each connected to one winding of the transformer; and a control unit adapted to control the operation of each of the power modules. The transformer is a variable turns ratio transformer, and the control unit is further adapted to dynamically set the transformer turns ratio based on input voltages and output voltages measured at the two power modules of the system, respectively, connected to the transformer windings, so that the expression V o / (V in *N m ) is closest to 1. For a wide range of voltages, the power conversion system of the present invention operates at or near the optimal conversion point to improve power conversion efficiency and reduce reactive power and EMI noise.

Description

Technical Field

[0001] The present invention relates to an electronic power conversion system for converting and / or transmitting electric power between terminals (such as power supplies and / or loads operating at different or the same voltages), and is preferably adapted for integration into electric and / or hybrid vehicles.

[0002] The object of the present invention is to provide a transformer-based power conversion system that can operate at or near an optimal conversion point for a wide voltage variation, thereby improving power conversion efficiency and reducing reactive power and EMI noise.

[0003] Another object of the present invention is to provide a power conversion system capable of multi-directional power conversion and energy transmission between multiple ports of an electric vehicle.

[0004] The power conversion system of the present invention can be manufactured using a reduced number of components, thereby reducing manufacturing cost and size, and improving efficiency and system performance. Background Art

[0005] The vehicle electrical system integrates various devices or subsystems that operate at the same or different DC nominal voltages. For example, electric and hybrid vehicles incorporate a traction battery that operates at a high voltage (typically in the range of 250V to 450V or 350V to 800V) to power the traction system through an inverter; a heating system; and an auxiliary battery that supplies low-voltage power to on-board devices that require lower voltages. The most common nominal low voltages are 12V and 48V, corresponding to devices traditionally used in the automotive environment.

[0006] Therefore, multiple voltage levels and their required charging systems are typically integrated in electric and / or hybrid electric vehicles, i.e., high-voltage battery, low-voltage battery, traction inverter bus, etc. Power is transferred between these different voltage ports with the help of power modules (usually a dc / dc converter and often an ac / dc converter as PFC).

[0007] Figure 1 A conventional electric vehicle system is illustrated, which includes a subsystem for recharging an electric or hybrid vehicle from the grid, as well as several other vehicle electrical subsystems. External charging of the traction battery can be performed using alternative charging systems, such as directly from the grid via an AC / DC converter including power factor correction (PFC), directly from the grid via a DC / DC converter, via a wireless charger, etc. Furthermore, electric and hybrid vehicles can charge an auxiliary battery from the traction battery, or even from an external power source during external charging, and therefore, a dedicated power converter is provided for this power conversion between internal subsystems.

[0008] The low-voltage auxiliary battery can also be used to transfer power to the high-voltage traction battery, and for this power transfer, another DC / DC power converter must be provided. Furthermore, the traction battery can also be used to power the vehicle's onboard electronics, so another DC / DC bidirectional converter, or the same DC / DC bidirectional converter, is incorporated to step down the traction battery voltage to the nominal voltage of the onboard electronics.

[0009] As a result, multiple power converters with different topologies are integrated into vehicles. LLC resonant topology is commonly used to implement converters due to its wide voltage gain, however, these converters only allow unidirectional power flow. Figure 2A 、 Figure 2B A prior art LLC converter is shown that is commonly used to transfer power individually between different power sources / loads in a vehicle.

[0010] The conventional approach is to provide specific converters with different topological configurations for each voltage conversion rate requirement, so that multiple different power converters must be provided to convert and transmit power between different power sources and loads, and therefore, two or more converters are connected to the same power source or load.

[0011] This conventional approach results in a complex circuit with many components (power semiconductors, transformers, microcontrollers, sensors, connector wires, housing, etc.) resulting in many redundancies, thereby increasing manufacturing cost and complexity.

[0012] In most isolated power converters (such as LLC or dual active bridge topologies), when v o / n is equal to v in When , the best working point can be obtained, such as Figure 6A is shown and represented in the following formula:

[0013]

[0014] Among them, v o and v in are the output voltage and input voltage respectively, N is the transformer winding turns ratio, and “′” is the power transfer efficiency.

[0015] However, in the on-board battery charger, v o corresponds to a high battery voltage in a wide range of 250V to 450V, and therefore, the efficiency "η" decreases, as Figure 6B is shown and represented in the following formula:

[0016]

[0017] Therefore, there is a need for a power conversion system for an on-board battery charger having improved power transfer efficiency over a wide operating range. Summary of the Invention

[0018] The present invention satisfactorily addresses the above-mentioned needs of the prior art by providing a power conversion system for electric and / or hybrid vehicles, the power conversion system comprising a transformer having at least two windings and at least two power modules, each of the at least two power modules being connected to one of the windings of the transformer such that the power modules are electrically isolated from each other.

[0019] According to the present invention, the transformer is a variable turns (coil) ratio N having at least one variable turns winding. m Furthermore, the system comprises a control unit adapted to determine the input voltage V in and the output voltage V o To dynamically set the transformer turns ratio N m , so that the expression V o / (V in *N m ) is closest to 1. Specifically, the voltage V in 、V o It is measured at the corresponding input / output terminals of the power modules between which power is transferred.

[0020] In addition, at least one power module includes means for blocking "dc" current circulation. Preferably, the blocking means includes a capacitor, and more preferably, includes one or more capacitors connected in parallel with the power module in a manner that blocks or prevents dc current circulation.

[0021] Conventionally, with a variable turns ratio N m A transformer has at least one winding having two or more winding taps, that is, the winding has a plurality of connection taps connected to different points of the winding, so that by selecting one of the taps for winding connection, more or fewer turns or coils of the winding are connected for current circulation.

[0022] Therefore, the input voltage V is continuously measured at a pair of connected power modules. in and the output voltage V o , and the control unit is adapted to measure the input voltage V in and the output voltage V o By choosing o / (V in *N m ) is closest to 1 to dynamically set the transformer transformation ratio N of the connected converter m .

[0023] In this way, the converter operating point is optimized for wide voltage variations, achieving high efficiency and less reactive power flowing along the circuit, thereby achieving higher power density and reducing EMI noise.

[0024] The system further includes a winding tap selector operable by the control unit to selectively connect one of the winding taps to an input terminal of a power module connected to the winding, thereby changing the transformer turns ratio N to m Set so that the expression V o / (V in *N m ) is the value closest to 1. For example, the control unit may dynamically calculate the expression V for all available winding taps o / (V in *N m ), and select a winding tap that makes this expression closest to 1.

[0025] The winding tap selector may be implemented as a multi-pole switching device, preferably as a multi-pole relay.

[0026] Furthermore, when a full-bridge module is physically placed, it can be operated to behave as a half-bridge. To this end, the control unit is further configured to continuously disconnect one switching device in one branch and continuously connect another switching device in the same branch during a time period, such that the full-bridge power module operates as a half-bridge power module. In this way, the use of a half-bridge module or a full-bridge module can be subtle, allowing a factor of two to be incorporated into the transformation ratio.

[0027] This feature of the invention can be combined with a variable transformer to save winding output. That is, as described above, the turns ratio N can be converted to 2N if operating in full-bridge mode, or to N if operating in half-bridge mode.

[0028] In a preferred embodiment, the power conversion system additionally includes multiple ports having different or the same voltages and is capable of inconspicuously transmitting and converting power in any direction between the ports with a reduced number of modules between the ports.

[0029] The control unit is adapted to control the operation of each of the power modules (i.e., the power conversion process), for example, by means of PWM control. The control unit is additionally adapted to operate the power modules so as to transfer power between two or more of the power modules. The control unit is additionally adapted to select the direction of power flow during power transfer between the power modules. The control unit achieves this function by setting a phase shift between a pair of power modules (between which power is transferred).

[0030] With the architecture specified above, all power modules share only one transformer and one control unit. Compared to the prior art, only one power converter is connected to each power source or load, thus significantly reducing the number of components and system complexity.

[0031] The system includes an inductor connected in series between the pair of power modules (power is transferred between the pair of power modules). This can be achieved by incorporating the inductor into at least one of the power modules. If only one power module incorporates an inductor, to transfer power between power modules without an inductor, the control unit is adapted to transfer power between the two power modules via the power module with the inductor. At high frequencies or when a small L is required, the inductor can also be integrated into the transformer as a leakage inductor.

[0032] In another preferred embodiment, each of the power modules has an incorporated inductor.

[0033] The control unit is further adapted to control or change the duty cycle and / or phase shift and / or frequency of any pair of power modules (power is transmitted between the pair of power modules) and the control unit operates the pair of power modules at the same switching frequency.

[0034] The power module includes: a half-bridge or a full-bridge.

[0035] In other preferred embodiments, the control unit is adapted to operate the full-bridge module as a half-bridge.

[0036] The half-bridge may have two input / output terminals and a first branch and a second branch. The first branch includes two switching devices connected in series between the two terminals, and the second branch includes two capacitors connected in series between the two terminals. Thus, the two branches are connected in parallel. The converted half-bridge power is connected to the two terminals of the transformer winding at a center connection line between the two semiconductor switching devices and a center connection line between the two capacitors, respectively.

[0037] The other half-bridge may be composed of two switches and a capacitor, the capacitor being connected via a first terminal to a center connection line between the two switches. The transformer winding is also connected to a second terminal of the capacitor and one end of the first branch. Other half-bridge topologies are not excluded.

[0038] The full-bridge module has two input / output terminals, a first branch, a second branch, and a third branch. Each of the first and second branches includes two semiconductor switching devices connected in series between the two terminals. The third branch includes a capacitor connected between the two terminals. Thus, the three branches are connected in parallel. The converted full-bridge power is connected to the two terminals of the transformer winding at the center connection line between the two semiconductor switching devices.

[0039] In a preferred embodiment, one power module is adapted to be connected (via its input / output terminals) to a traction battery of an electric and / or hybrid vehicle, while the other power module is adapted to be connected (via its input / output terminals) to a low-voltage battery of an electric and / or hybrid vehicle. In addition, one power module is adapted to be connected (via its input / output terminals) to an external charging system for charging the electric and / or hybrid vehicle. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings, in which:

[0041] Figure 1 A conventional electric vehicle system is shown that includes a subsystem for charging an electric or hybrid vehicle from the electrical grid, as well as several other vehicle electrical subsystems.

[0042] Figure 2A 、 Figure 2B A conventional LLC topology according to the prior art is shown.

[0043] Figure 3 An electrical diagram of a preferred embodiment of the present invention is shown, comprising a variable turns ratio transformer and two power modules.

[0044] Figure 4 An electrical diagram of another preferred embodiment of the present invention is shown, which also includes a variable turns ratio transformer and a plurality of power modules.

[0045] Figure 5A 、 Figure 5B and Figure 5C Three graphs are shown representing the voltage conversion between two power modules and the phase shift control performed during this process.

[0046] Figure 6A and Figure 6B Shows something like Figure 5A 、 Figure 5B 、 Figure 5C The two graphs of the curve are for the converter operating near the optimal operating point ( Figure 6A ) and converters with poor efficiency operating under large differences between input and output voltages ( Figure 6B ) were compared in terms of power transmission efficiency.

[0047] Figure 7 and Figure 8 Two electrical diagrams of an example of a power module system according to the present invention are shown. Figure 7 In FIG, arrows represent different paths for transmitting and / or converting power, taking into account that at least two power modules or even all power modules can operate simultaneously. DETAILED DESCRIPTION

[0048] Figure 3 A power conversion system 1 according to the present invention is shown. The system comprises a first power module 3a and a second power module 3b, in this particular example half-bridge power modules, each coupled to two windings 2a and 2b of a transformer 2, for transferring power from one module to the other. Each power module 3a and 3b has a first branch in which two switching devices are connected in series between terminals a1 and a2 and between terminals b1 and b2, respectively. In this example, the power switching devices are comprised of power transistors Q1 and Q2 and Q3 and Q4, respectively.

[0049] Each power module 3a, 3b also includes a device for blocking the circulation of "dc" current through the module. Preferably, the blocking device comprises at least one capacitor. In this particular example, the device is embodied as a second branch connected in parallel with the first branch between terminals a1, a2 and between terminals b1, b2, and having two capacitors Cdc1, Cdc2 and Cdc3, Cdc4 connected in series, respectively. Each power module 3a, 3b is connected to two terminals of the transformer winding 2a, 2b at the connection line between transistors Q1, Q2, the connection line between transistors Q3, Q4, and the connection line between capacitors Cdc1, Cdc2, Cdc3, and Cdc4.

[0050] like Figure 3 As shown, the first power module 3a is connected to the transformer winding 2a via an inductor L, and the second power module 3b is connected to the transformer winding 2b via a winding tap selector.

[0051] The transformer 2 of this embodiment is a variable turns transformer having at least one winding 2b, wherein the at least one winding 2b is provided with a plurality of winding taps N1, N2, and N3.m+1 、N i By means of said plurality of winding taps, a plurality of points of the winding 2b can be connected externally, thereby selectively connecting a winding with a greater or fewer number of turns in a known manner.

[0052] To this end, the system 1 comprises a winding tap selector which is realized as a multi-pole switching device, such as a multi-pole relay or switch 5 which can be operated by a control unit 4 .

[0053] In addition, the system includes a voltage measuring device for measuring the input voltage / output voltage at the power module, more specifically for measuring the voltage V at the terminals a1, a2 of the first power module 3a. in and the voltage V at the terminals b1 and b2 of the second power module 3b o .

[0054] The control unit 4 is adapted to determine the voltage V in 、V o To select winding taps N1, N m+1 、N i (i.e., transformer turns ratio N m ), so that the expression V o / (V in *N m ) is closest to 1.

[0055] exist Figure 4 In an alternative embodiment, the transformer 2 has two or more conical windings 3b, 3n on the primary side, the secondary side or both sides. The additional conical windings 3n can be arranged in the same manner as described above with respect to Figure 3 The operation is performed in the same manner as described above, by the control unit 4 by means of one or more tap selectors 5n.

[0056] exist Figure 4 In the case of N, if power is transmitted between the power module 3b and the power module 3n, the control unit 4 selects N m and N n The combination of turns ratios is selected so that the ratio V n *N m / (V o *N m ) is the combination closest to 1.

[0057] Figure 3 and Figure 4 A preferred embodiment comprising only half-bridges is shown, but other preferred embodiments of the invention include any combination of full-bridge modules and half-bridge modules and have the same characteristics as those described above for Figure 3 and Figure 4The functions and operations described are the same functions and operations.

[0058] To change the winding tap selector, i.e., to activate / deactivate the relay, the control unit 4 is adapted to first stop the power conversion process by bringing the phase between the modules to zero, thereby stopping the switching transistors Q1-Qn, and then wait for a short time to ensure that there is no recirculating current through the system. When there is no current flowing through the transistors, the control unit 4 operates the relay as described above, thereby setting the desired turns ratio.

[0059] Then, the control unit 4 waits for a short time again to ensure that the relay is switched correctly. Finally, the control unit 4 restarts the power transmission process with the newly selected transformer turns ratio.

[0060] Furthermore, if it is necessary to improve the response of the system with respect to transients, disturbances, accuracy, etc., the configuration of the control unit can be modified during this time to address these issues.

[0061] In addition, the control unit 4 is further adapted to disconnect one switching device of one branch and connect another switching device of the same branch during a period of time in such a way that the full-bridge power module can operate as a half-bridge power module. Figure 7 In the full bridge 3c, the control unit 4 can always turn on the transistor Q7 and always turn off Q5 during a period of time, so that the full bridge will operate as a half bridge.

[0062] Preferably, combined with the above Figure 3 and Figure 4 The variable transformer ratio described above is used to achieve this function, so that if the power module works as a full bridge, the turns ratio N m Can be converted to 2N m , or if operating as a half-bridge mode, it is converted to N m , as described above.

[0063] Figure 7Figure 1 is an example of a multi-port, multi-directional power conversion system 1 according to the present invention. System 1 includes a transformer 2 having at least three windings 2a, 2b, and 2c ("n" windings 2a, 2b, 2c, ..., 2n are shown in the figure), and at least three power modules 3a, 3b, and 3c (n power modules 3a, 3b, 3c, ..., 3n are shown in the figure, each connected to one winding 2a, 2b, 2c, ..., 2n of transformer 2). More specifically, a first power module 3a is connected to the first winding 2a of transformer 2, operates at 400V, and is adapted to be connected to a high-voltage traction battery. A second power module 3b is connected to the second winding 2b of transformer 2, also operates at 400V, and is adapted to be connected to an external charging system. A third power module 3c is connected to the third winding 2c of transformer 2, operates at 12V, and is adapted to be connected to the vehicle's low-voltage battery. Each power module represents a port of system 1. An additional n-module 3n can be connected to the n-winding 2n of the transformer 2 and is adapted to be connected to any electric vehicle system.

[0064] The first and second power modules 3a, 3b are half-bridges having two input / output terminals a1, a2, b1, and b2, and a first branch and a second branch. The first branch includes two semiconductor switching devices Q1, Q2 and Q3, Q4 (in this case, two power transistors (MOSFETs)) connected in series between the two terminals a1, a2 and between the two terminals b1, b2, and the second branch includes two capacitors Cdc1, Cdc2 and Cdc3, Cdc4 connected in series between the two terminals a1, a2 and between the two terminals b1, b2, respectively. The two half-bridge power modules 3a, 3b are connected to the two terminals of the first winding 2a and the second winding 2b of the transformer 2 at the center connecting line between the two semiconductor switching devices Q1, Q2 and between Q3, Q4, and at the center connecting line between the two capacitors Cdc1, Cdc2 and between Cdc3, Cdc4, respectively.

[0065] Inductor L is connected in series between the first winding 2a of transformer 2 and the center connection line between power transistors Q1, Q2 of first power module 3a. In case of high frequency or when small L is required, the leakage inductance of transformer 2 can be regarded as inductor L.

[0066] The third power module 3c is a full-bridge circuit having two input / output terminals C1 and C2, as well as a first branch and a second branch. Each branch includes two power transistors connected in series (Q5 and Q7 in the first branch, and Q6 and Q8 in the second branch), and each branch is connected in parallel between the two terminals C1 and C2. The third power module 3c has a third branch including a capacitor Clv connected between the two terminals C1 and C2. The full-bridge power module is connected to two terminals of the third winding 2c of the transformer 2 at the center connecting line of the two branches, between the power transistor pairs Q5 and Q7 and between the power transistor pairs Q6 and Q8, respectively.

[0067] The capacitor Cdc is connected in series between the winding of the transformer 2c and the center connection line between the power transistors Q6 and Q8 of the same branch.

[0068] In other practical implementations of the present invention, three or more power modules (half-bridge or full-bridge) can be connected to three or more windings of the transformer 2 and operate at N voltages, such as by Figure 7 The power module 3n is represented.

[0069] A control unit 4, such as a programmable electronic controller, is provided (e.g., integrated with the power module 3a), and is adapted to control the switching operation of each of the power modules 3a, 3b, 3c, ..., 3n by switching the power transistors on and off in a manner known to those skilled in the art. The power transistor driving may be performed via an isolated driver, such as an opto-driver.

[0070] Preferably, the isolated voltage and current measurements of each module are provided to the control unit 4 .

[0071] Optionally, relays can be connected at the input of each power module to enable or disable a specific power module. Similarly, relays can be placed between the windings and the power module to completely disconnect the module.

[0072] The control unit 4 is additionally adapted to dynamically select at least one pair of power modules to transfer power between.Depending on the requirements of the specific situation of the vehicle, all power modules 3a, 3b, 3c, 3n may be operated simultaneously.

[0073] Furthermore, the control unit 4 is adapted to set the duty cycle during power transfer and to set the switching frequency of any pair of power modules.The selected pair of power modules operates at the same switching frequency.

[0074] exist Figure 7In the embodiment of the present invention, only the first power module 3 a is provided with the inductor L, so that in order to transfer power, for example, between the second power module 3 b and the third power module 3 c, the control unit 4 is adapted to transfer power between those power modules via the first power module 3 a, so that power is always transferred via at least one inductor L. In this case, power is first transferred from the second power module 3 b to the high-voltage battery via the first power module 3 a, and then finally transferred from the high-voltage battery to the low-voltage battery via the third power module 3 c.

[0075] In other implementations, two or more of the power modules are provided with series inductors L connected in the same manner as described above for the first power module 3 a. To this end, an inductor may be incorporated into one or more of the power modules, or the internal leakage inductance of a transformer may be used if its value is suitable for a particular operating frequency range.

[0076] exist Figure 8 In the embodiment, the power module 3a is provided with an inductor L, and the power module 3n (due to the operation at high frequency or the need to use low inductance to transmit energy, the power module 3n is considered to be suitable for using the leakage inductance L of the transformer) n ) Using the transformer's leakage inductance L n as an inductor, and Figure 8 The arrows in indicate all possible combinations of two power modules for transferring power through at least one inductor.

[0077] Using the above architecture, the control unit 4 dynamically operates the power modules to transfer power between at least two of the power modules as required at any particular moment during vehicle operation, e.g. Figure 8 Indicated by the arrow.

[0078] In other preferred embodiments, Figure 7 or Figure 8 One or more of the windings 2a, 2b, 2c may be provided with a plurality of winding taps, as described above for Figure 3 and Figure 4 described.

[0079] like Figure 5A 、 Figure 5B and Figure 5C As shown, the control unit 4 is additionally adapted to control the direction of power transfer between the two power modules by setting a voltage phase shift between the two power modules for transferring power. The larger the phase shift, the more power is transferred until a maximum available voltage phase shift is reached, which is half the duty cycle.

[0080] By setting the voltage phase shift φ, the direction of power transfer can be selected dynamically and can therefore be reversed as needed.

[0081] Figure 5A The voltage signal V at the terminals a1 and a2 shows the power transfer from the second power module 3b connected to the power factor correction (PFC) device to the first power module 3a connected to the high voltage battery. HVBat Relative to the voltage signal V at terminals b1 and b2 PFC The figure also shows the current i through the inductor. L .

[0082] Similarly, Figure 5B shows the power transmission from the first power module 3a connected to the high voltage battery to the third power module 3c connected to the low voltage battery, such a voltage signal V at the terminals c1, c2 LVBat Relative to the voltage signal V at terminals a1 and a2 HVBat The figure also shows the current i through the inductor. L .

[0083] at last, Figure 5C shows the power transmission from the third power module 3c connected to the low voltage battery to the first power module 3a connected to the high voltage battery, such a voltage signal V at the terminals a1, a2 HVBat Relative to the voltage V at terminals c1 and c2 LVBat Delayed by a phase shift φ3.

Claims

1. A power conversion system (1) adapted to be integrated in an electric or hybrid vehicle, the power conversion system (1) comprising: A transformer (2), the transformer (2) having at least two windings (2a, 2b), at least two power modules (3a, 3b), each power module being connected to one of the windings (2a, 2b) of the transformer (2), a control unit (4) adapted to control the operation of each of the power modules (3a, 3b), The invention is characterized in that the transformer (2) is a variable turns ratio transformer, and wherein the control unit (4) is further adapted to adjust the voltage V of the at least two power modules (3a, 3b) based on the input voltages V measured at the at least two power modules (3a, 3b) respectively. in and the output voltage V o To dynamically set the transformer turns ratio N m , so that the expression V o / (V in *N m ) is closest to 1, and At least one of the windings (2a, 2b) has two or more winding taps (N1, N2). m+1 、N i ), and wherein the power conversion system (1) further comprises a winding tap selector (5), the winding tap selector (5) being operable by the control unit (4) to set the winding taps (N1, N2) to m+1 、N i ) is selectively connected to the corresponding power module (3a, 3b, 3c), so as to set the expression V o / (V in *N m ) The transformer turns ratio N closest to 1 m ,and The control unit (4) is further adapted to provide a voltage regulator for all available winding taps (N1, N m+1 、N i ) dynamically calculates the expression V o / (V in *N m ), and choose to make the expression V o / (V in *N m ) is the one winding tap closest to 1, The power conversion system (1) further comprises a voltage measuring device for measuring a voltage V at the terminals (a1, a2) of the first power module (3a). in and the voltage V at the terminals (b1, b2) of the second power module (3b) o .

2. The power conversion system according to claim 1, wherein: The winding tap selector (5) includes at least one multi-pole switching device.

3. The power conversion system according to claim 2, wherein: The multi-pole switching device is a multi-pole relay.

4. The power conversion system according to claim 1 or 2, wherein: The power modules (3a, 3b, 3c) are selected from the following list: (i) half-bridge or (ii) full-bridge.

5. The power conversion system according to claim 1 or 2, wherein: The control unit (4) is adapted to dynamically select at least two of the at least two power modules to transfer power therebetween, and the control unit (4) is adapted to control the power transfer direction between the at least two power modules (3a, 3b) by setting a voltage phase shift (φ) between the at least two power modules (3a, 3b) to transfer power, and wherein the power modules (3a, 3b, 3c) include switching devices, and the control unit (4) is adapted to control the operation of each of the power modules (3a, 3b, 3c) by turning on and off the switching devices of the power modules, and The control unit (4) is further adapted to control or change the duty cycle and / or phase shift and / or frequency of any pair of power modules between which power is transmitted, so that the control unit (4) operates the pair of power modules at the same switching frequency.

6. The power conversion system according to claim 1, wherein: At least one power module of the at least two power modules (3a, 3b) comprises an inductor (L).

7. The power conversion system according to claim 1 or 2, wherein: The first power module (3a) is adapted to be connected to a traction battery of an electric or hybrid vehicle, and the second power module (3b) is adapted to be connected to a low-voltage battery of an electric or hybrid vehicle, and wherein the third power module (3c) is adapted to be connected to an external charging system to charge the electric or hybrid vehicle.

8. The power conversion system according to claim 6, wherein: The transformer (2) has only three windings (2a, 2b, 2c), and wherein the inductor (L) is connected in series between one terminal of a winding and a power module to which the winding is connected.

9. The power conversion system according to claim 5, wherein: The control unit (4) is additionally adapted to control the operation of the power modules (3a, 3b, 3c) so as to transmit power between the first power module and the second power module via the third power module, and to transmit power from the first power module to the third power module and from the third power module to the second power module.

10. The power conversion system according to claim 1 or 2, wherein: At least one of the power modules (3a) is a half-bridge having two input / output terminals (a1, a2) and a first branch and at least one capacitor (Cdc1-Cdc2), the first branch comprising two switching devices (Q1, Q2) connected in series between the two input / output terminals, the at least one capacitor (Cdc1-Cdc2) being connected for blocking a "dc" current circulation, and wherein the half-bridge is connected to two terminals of a winding of the transformer (2) at a center connection line between the two switching devices (Q1, Q2) and a center connection line between the at least one capacitor (Cdc1-Cdc2), respectively.

11. The power conversion system according to claim 1 or 2, wherein: At least one of the power modules (3a, 3b, 3c) is a half-bridge having a first branch including two switching devices (Q1-Q4) and a capacitor connected for blocking "dc", and wherein a transformer winding (2a, 2b, 2c) is also connected to a second end of the capacitor and one end of the first branch.

12. The power conversion system according to claim 1 or 2, wherein: At least one of the power modules (3a, 3b, 3c) is a full-bridge power module having two input / output terminals, a first branch and a second branch, and at least one capacitor, wherein each of the first branch and the second branch includes two semiconductor switching devices connected in series between the two input / output terminals, and the at least one capacitor is connected to block "dc" current circulation, and wherein the full-bridge power module is connected to two terminals of the winding (2a, 2b, 2c) of the transformer at a center connection line between the two semiconductor switching devices of each branch.

13. The power conversion system according to claim 1, wherein: At least one power module (3a, 3b, 3c) is a full-bridge power module having two input / output terminals, a first branch and a second branch, and a capacitor, each of the first branch and the second branch comprising two semiconductor switching devices (Q1-Q4) connected in series between the two input / output terminals, the capacitor being connected for blocking a "dc" current circulation, and wherein the control unit (4) is further adapted to, during a time period, always disconnect one switching device (Q1-Q4) of one branch and always connect another switching device of the same branch in such a way that the full-bridge power module can operate as a half-bridge power module.

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