A series resonant converter having an increased overall efficiency, as well as a related arrangement and method of operating the series resonant converter

Abstract

A series resonant converter, wherein said series resonant converter comprises a first and a second switch, connected in series, and connected between a first port and a second port of the series resonant converter, a third and a fourth switch, connected in series, and connected between said second port and a third port of the series resonant converter, a main resonant tank connected in parallel over said second and third switch, wherein said main resonant tank has a main resonant frequency, wherein the series resonant converter further comprises an auxiliary resonant tank connected in parallel over said main resonant tank, wherein said auxiliary resonant tank has an auxiliary resonant frequency being different from said main resonant frequency, and a controller for controlling said switches, such that said main resonant tank is operated in an impedance region being one of an inductive and capacitive region and said auxiliary resonant tank is operated in different impedance region being opposite to the impedance region of the main resonant tank.

Description

[0001] Title

[0002] A series resonant converter having an increased overall efficiency, as well as a related arrangement and method of operating the series resonant converter.

[0003] Technical field

[0004] The present disclosure is directed to a series resonant converter and, more specifically, to a new topology for a series resonant converter which has an increased overall efficiency.

[0005] Background

[0006] Series Resonant Converters, SRCs, are often utilized in power electronics, showcasing their efficacy in various applications, and in high voltage DC, HVDC, lines where power balancing is often an important requirement.

[0007] SRCs operate on the principle of resonance, capitalizing on the efficient energy transfer that occurs at a specific frequency within a resonant tank circuit. This circuit, comprising an inductor, L, and a capacitor, C, in series, is the backbone of SRCs. Integrated with a switching network and a load, usually composed of power semiconductor devices, SRCs achieve Zero Voltage Switching, ZVS, or Zero Current Switching, ZCS, during switching transitions, thereby minimizing losses and enhancing overall efficiency.

[0008] SRCs may find a stronghold in induction heating systems where precise control of resonant frequency is crucial for optimal energy transfer to the load. ZVS capability proves advantageous, reducing switching losses and augmenting the efficiency of the induction heating process. In Wireless Power Transfer, WPT, applications, SRCs facilitate resonant coupling, making them useful in wireless charging systems for electric vehicles, consumer electronics, and medical devices.

[0009] In the medical field, SRCs may be deployed in medical imaging systems like MRI machines, leveraging their high-frequency operation and resonant characteristics. Additionally, SRCs contribute to certain high-frequency power supplies, meeting the demands of applications such as telecommunications equipment and high-performance computing systems.

[0010] SRCs are also used in ultrasonic applications, driving ultrasonic cleaning and welding machines with high-frequency power efficiently. In energy storage systems, SRCs aid in the transfer of energy to and from storage devices, making them pertinent to renewable energy systems and grid-tied applications.

[0011] Expanding their reach, SRCs have found a new application in the realm of High Voltage DC, HVDC, lines for power balancing. In HVDC transmission systems, power imbalances between the sending and receiving ends can arise due to fluctuations in generation or consumption. SRCs, with their ability to efficiently transfer power and manage resonance, offer a potential solution for HVDC line power balancing. By integrating SRCs into the HVDC system, power flow can be dynamically controlled, ensuring optimal utilization of the transmission infrastructure and minimizing losses associated with power imbalances.

[0012] The efficiency of a series resonant converter is, to an extent, related to the switching losses that occur during operation. ZVS is beneficial as this may decrease potential switching losses. However, obtaining ZVS is not always feasible, especially for different kinds of loads.

[0013] Summary

[0014] It is an object of the present disclosure to provide for a series resonant converter that has an increased overall efficiency. Further objects of the present disclosure include corresponding methods and an arrangement for such a series resonant converter.

[0015] In a first aspect of the present disclosure, there is provided series resonant converter, wherein said series resonant converter comprises: a first and a second switch, connected in series, and connected between a first port and a second port of the series resonant converter; a third and a fourth switch, connected in series, and connected between said second port and a third port of the series resonant converter; a main resonant tank connected in parallel over said second and third switch, wherein said main resonant tank has a main resonant frequency; wherein the series resonant converter further comprises: an auxiliary resonant tank connected in parallel over said main resonant tank, wherein said auxiliary resonant tank has an auxiliary resonant frequency being different from said main resonant frequency, a controller for controlling said switches, such that said main resonant tank is operated in an impedance region being one of an inductive and capacitive region and said auxiliary resonant tank is operated in different impedance region being opposite to the impedance region of the main resonant tank.

[0016] It was the insight of the inventors that it may be beneficial to include an auxiliary resonant tank in parallel over the main resonant tank. The switches may then be controlled such that the auxiliary resonant tank is operated in an impedance region that is opposite to the impedance region in which the main resonant tank is operated.

[0017] This may be beneficial for the following reasons.

[0018] It was first recognized that situations arise in which the converter may have difficulties to obtain Zero Voltage Switching, ZVS. This is especially the case when the load is light, i.e. a low load is connected to the series resonant converter.

[0019] Zero Voltage Switching, ZVS, is a concept in power electronics that plays a role in minimizing switching losses in switches, such as transistors or thyristors. The idea behind ZVS is to synchronize the switching operation of these switches with the natural zero-crossing points of the voltage waveform. By ensuring that the switch transitions occur when the voltage across the device is zero, ZVS mitigates the energy dissipation associated with the abrupt changes in current during switching.

[0020] In a ZVS scenario, the switching device turns on or off when the voltage across it is close to zero, allowing the current to start or stop flowing with minimal resistance. This synchronization results in reduced switching losses, as the energy stored in the parasitic capacitance of the device is lower when the transition occurs near zero voltage. Lower switching losses lead to higher efficiency in power conversion applications, contributing to energy savings and improved overall performance of power electronic systems.

[0021] By introducing such an auxiliary resonant tank, the current at, or around, the switching point of the corresponding switch is increased. The benefit is that the parasitic capacitance of the corresponding switch is depleted more quickly, thereby reducing the voltage of that switch.

[0022] The above is especially the case for low load situation as, in these situations, the flow of current through the circuit is relatively low. The result thereof is that the parasitic capacitance over a switch may not yet be depleted before the switch is activated, such that a ZVS situation is difficult to obtain. By utilizing the auxiliary resonant tank circuit in accordance with the present disclosure, the current is increased and thus also the discharging of the parasitic capacitance of the switch is increased, thereby reducing the voltage of the switch when the switch is to be activated.

[0023] In an example, the controller is arranged for controlling said switches such that said main resonant tank is operated in the inductive region and such that said auxiliary resonant tank is operated in the capacitive region.

[0024] The converter operates by utilizing the main resonant frequency of the main resonant tank and the auxiliary resonant frequency of the auxiliary resonant tank to efficiently transfer energy from the input to the output. The resonant frequency is determined by the values of the inductor and capacitor of the respective resonant tank.

[0025] Whether the resonant tank operates in the inductive or capacitive region depends on the specific design and operating conditions of the converter. The converter can be designed to operate in either the inductive or capacitive region of the tank's impedance profile, depending on the application requirements.

[0026] In the inductive region, the inductor dominates the impedance, and the converter operates with a net inductive reactance. In the capacitive region, the capacitor dominates the impedance, and the converter operates with a net capacitive reactance.

[0027] The choice of operating region depends on factors such as efficiency, control strategy, and specific application requirements. Designers aim to optimize the converter's performance based on factors like power efficiency, voltage regulation, and overall system stability.

[0028] In a further example, the said main resonant tank comprises a main inductor connected in series with a main capacitor; said auxiliary resonant tank comprises an auxiliary inductor connected in series with an auxiliary capacitor.

[0029] The above describes specific examples of resonant tanks. It is noted that other configurations that provide a resonance at a particular frequency may also be used in connection with the present disclosure.

[0030] In another example, said main resonant tank and said auxiliary resonant tank are designed such that both: said auxiliary resonance frequency is at most two times said switching frequency, and said main resonance frequency is at most two times said switching frequency.

[0031] The inventors have found that it may be beneficial to keep the auxiliary resonance frequency and / or the main resonance frequency at most two times said switching frequency for correct operation of the series resonant converter.

[0032] In other words, at most two times the switching frequency may entail that both the main resonant frequency and the auxiliary resonant frequency are around the switching. That is, in close proximity to the switching frequency. Preferably, the switching frequency is about 0,8-1 , 2 times any of the main and auxiliary resonant frequency.

[0033] In yet another example, any of said first, second, third and fourth switch comprise Metal Oxide Semiconductor, MOS, Field Effect Transistor, FETs.

[0034] MOSFETs are preferred in many power electronic applications, including resonant converters, due to their fast switching speeds, high efficiency, and good power-handling capabilities.

[0035] The MOSFETs are used to control the flow of current in the circuit by switching on and off at a high frequency. This switching action is of importance for the operation of the resonant converter, allowing energy to be transferred efficiently between the input and output.

[0036] The choice of MOSFETs for the switches is based on their ability to handle the high-frequency switching demands of resonant converters and their overall suitability for power electronic applications. Other types of power switches, such as Insulated Gate Bipolar Transistors, IGBTs, could also be used depending on the specific requirements of the application. In a further example, the main resonant tank and said auxiliary resonant tank are designed such that said auxiliary resonant frequency is at least two times higher compared to said main resonant frequency.

[0037] In yet another example, the controller is arranged for: controlling said switches using a switching frequency, wherein said switching frequency is higher than the main resonant frequency and lower than the auxiliary resonant frequency.

[0038] In a further example, the controller is arranged for: controlling said switches using a switching frequency, wherein said switching frequency is higher than the auxiliary resonant frequency and lower than the main resonant frequency.

[0039] In a second aspect of the present disclosure, there is provided a series resonant converter in accordance with any of the previous examples and a bipolar Direct Current, DC, power grid, wherein: said first and second port are connected between a first DC terminal of said bipolar DC power grid and a neutral terminal of said bipolar DC power grid; said second and said third port are connected between said neutral terminal of said bipolar DC power grid and a second DC terminal of said bipolar DC power grid.

[0040] It is noted that the advantages as explained with reference to the first aspect of the present disclosure, being the series resonant converter, also apply to the second aspect of the present disclosure, being the arrangement of a series resonant converter and the bipolar DC power grid.

[0041] Typically, a DC distribution system may be construed as a unipolar of a bipolar system. A unipolar distribution system only has one single voltage level that is distributed using two wires I terminals. A bipolar distribution system may be considered as a two-phase system having three wires I terminals. The bipolar architectures is an energy efficient DC system architecture that uses a neutral terminal for power distribution purposes. It allows for two times the power distribution with only half the additional installation costs.

[0042] Further, a bipolar architecture has a lower line-to-ground safety risk because the interfacing neutral point is grounded and this halves the maximum DC line voltage with respect to ground. Other advantages of the two phase DC bipolar architecture include the flexible selection of multiple DC voltage level for efficient operation and higher system reliability.

[0043] Bipolar DC power grid may be advantageous over regular three phase AC and unipolar DC grid for the following reasons.

[0044] First, the power transfer capability of bipolar grid is higher compared to three phase AC and unipolar DC grids, when regular voltages and cable installations are considered. Second, the pole to ground voltage is lower in bipolar DC power grids. This makes the costs for the converter much lower as the lower rated semiconductor switches may be used. Third, more voltage levels are available in a bipolar DC power grid compared to unipolar DC power grid due to the use of a neutral line.

[0045] In an example, the bipolar DC power grid is arranged to provide at least 600Volts DC between said first and said second terminal of said bipolar DC power grid.

[0046] In a third aspect of the present disclosure, there is provided a method of operating a series resonant converter in accordance with any of the previous examples, wherein said method comprises the step of: controlling, by said controller, the switches, such that said main resonant tank is operated in an impedance region being one of an inductive and capacitive region and said auxiliary resonant tank is operated in different impedance region being opposite to the impedance region of the main resonant tank.

[0047] It is noted that the advantages as explained with reference to the first aspect of the present disclosure, being the series resonant converter, also apply to the third aspect of the present disclosure, being the method of operating the series resonant converter.

[0048] In an example, said auxiliary resonant tank further comprises an activation switch for enabling said auxiliary resonant tank.

[0049] The controller may be arranged for activating said activation switch such that said auxiliary resonant tank is activated.

[0050] In an example, the step of controlling comprises: controlling, by said controller, said switches such that said main resonant tank is operated in the inductive region and such that said auxiliary resonant tank is operated in the capacitive region. In a further example: said main resonant tank comprises a main inductor connected in series with a main capacitor; said auxiliary resonant tank comprises an auxiliary inductor connected in series with an auxiliary capacitor.

[0051] In yet another example, the step of controlling comprises: controlling, by the controller, said switches using a switching frequency, wherein said switching frequency is higher than the main resonant frequency and lower than the auxiliary resonant frequency.

[0052] In an example, the step of controlling comprises: controlling, by the controller, said switches using a switching frequency, wherein said switching frequency is higher than the auxiliary resonant frequency and lower than the main resonant frequency.

[0053] In a fourth aspect of the present disclosure, there is provided a computer program product comprising a computer readable medium having instructions stored thereon which, when executed by a controller of a series resonant converter, cause said controller to implement a method in accordance with any of the previous examples.

[0054] In the appended figures, similar components and / or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label by a dash and a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label.

[0055] The above and other aspects of the disclosure will be apparent from and elucidated with reference to the examples described hereinafter.

[0056] Brief description of the figures

[0057] Fig. 1 shows a series resonant converter in accordance with the present disclosure;

[0058] Fig. 2 discloses illustrations of various possible power flow directions in the converter; Fig. 3 shows a series resonant converter in accordance with the present disclosure;

[0059] Fig. 4 shows an example of converter operation with main resonant tank working in capacitive region and auxiliary resonant tank working in inductive region;

[0060] Fig. 5 shows an example of converter operation with main resonant tank working in inductive region and auxiliary tank working in capacitive region.

[0061] Detailed description

[0062] It is noted that in the description of the figures, same reference numerals refer to the same of similar components performing a same of essentially similar function.

[0063] A more detailed description is made with reference to particular examples, some of which are illustrated in the appended drawings, such that the features of the present disclosure may be understood in more detail. It is noted that the drawings only illustrate typical examples and are therefore not to be considered to limit the scope of the subject matter of the claims. The drawings are incorporated for facilitating an understanding of the disclosure and are thus not necessarily drawn to scale. Advantages of the subject matter as claimed will become apparent to those skilled in the art upon reading the description in conjunction with the accompanying drawings.

[0064] The ensuing description above provides preferred exemplary embodiment(s) only, and is not intended to limit the scope, applicability, or configuration of the disclosure. Rather, the ensuing description of the preferred exemplary embodiment(s) will provide those skilled in the art with an enabling description for implementing a preferred exemplary embodiment of the disclosure, it being understood that various changes may be made in the function and arrangement of elements, including combinations of features from different embodiments, without departing from the scope of the disclosure.

[0065] Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise," "comprising," and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to." As used herein, the terms "connected," "coupled," or any variant thereof means any connection or coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, logical, electromagnetic, or a combination thereof. Additionally, the words "herein," "above," "below," and words of similar import, when used in this application, refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word "or" in reference to a list of two or more items, covers all the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list.

[0066] These and other changes can be made to the technology considering the following detailed description. While the description describes certain examples of the technology, and describes the best mode contemplated, no matter how detailed the description appears, the technology can be practiced in many ways. Details of the system may vary considerably in its specific implementation, while still being encompassed by the technology disclosed herein. As noted above, particular terminology used when describing certain features or aspects of the technology should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the technology with which that terminology is associated. In general, the terms used in the following claims should not be construed to limit the technology to the specific examples disclosed in the specification, unless the Detailed Description section explicitly defines such terms. Accordingly, the actual scope of the technology encompasses not only the disclosed examples, but also all equivalent ways of practicing or implementing the technology under the claims.

[0067] Fig. 1 shows a series resonant converter in accordance with the present disclosure.

[0068] The series resonant converter comprises four switches 2, 3, 4, 5 all connected in series. A controller is present (not shown) for controlling the switches 2, 3, 4, 5. Controlling the switches means that the controller is arranged to drive the switches by providing control signals on the respective gates of the switches 2, 3, 4, 5. In this particular case, the switches 2, 3, 4, 5 are embodied as so-called NPN Metal Oxide Semiconductor, MOS, Field Effect Transistor, FETs, MOSFETs. The controller is arranged to either activate or disactivate each of the MOSFETs. Activating an MOSFET means that the MOSFET is able to conduct current from its drain to its source terminal, or vice versa. Deactivating an MOSFET means that there is no conductive part between the drain terminal and the source terminal of the MOSFET.

[0069] The series resonant converter 1 further comprises a main resonant tank

[0070] 6 for operation. In this particular case, the main resonant tank 6 consists of an inductor

[0071] 7 that is connected in series with a capacitor 8. The resonant frequency of the series resonant converter depends on the value choices of the inductor 7 and the capacitor

[0072] 8.

[0073] The series resonant converter 1 may be used for a plurality of applications. One of these applications is directed to balancing a bipolar Direct Current, DC, power grid. The DC power grid has three terminals, i.e. a first DC terminal

[0074] 9, a neutral terminal 10 and a second DC terminal 11.

[0075] Fig. 2 discloses illustrations 101 of various possible power flow directions in the converter.

[0076] Series resonant converters with inverted output may be used in applications where voltage and power balancing are required. Furthermore, the converter can be also utilized as a voltage doubler to output nearly double voltage of the input voltage. Some of the example applications are capacitor balancing in neutral point clamped, NPC, converter, battery cell balancing, bipolar DC grid voltage balancing, interfacing a lower voltage with higher voltage, and so on. Due to its resonant nature, it is expected that the size of passive components and losses are lower than other inverting type topologies like buck-boost converter, Cuk converter, etc.

[0077] The converter has three ports namely upper, middle, and lower. These ports can be configured as inputs and output by suitable connection of sources and loads. Any source / load connected from the middle port to either upper or lower is termed as lower voltage, LV; any source / load connected from the upper to lower port is termed as higher voltage, HV. It should be noted that this terminology is relative and is not related with the voltage levels of LV and HV in various industrial standards.

[0078] Fig. 3 shows a series resonant converter in accordance with the present disclosure. The topology shown in figure 1 can be operated either in inductive or capacitive region with respect to the resonant frequency of the main resonant tank. The converter can be operated in the inductive region or in the capacitive region. The main reason for operating the converter in the inductive region, is that the zero-voltage switching, ZVS, turn-on range for the switches is limited.

[0079] At low power output, the converter may not be able to achieve ZVS turn on for all the switches. Whereas, in capacitive region, there is a much higher ZVS turnon range for the converter. Regardless, due to the parasitic resistances in the circuit, the ZVS range is further reduced no matter in which region the converter is operated.

[0080] The present disclosure is directed is to add a parallel auxiliary resonant tank “Auxiliary Resonant Tank” to the main resonant tank “Main Resonant Tank” as shown in figure 3.

[0081] The values of the inductors and capacitors comprised by auxiliary (Lr,aux, Cr,aux) and main (Lr.main, Cr.main) resonant tanks are selected such that the control remains approximately the same. The auxiliary resonant tank, in this example comprising Lr,aux and Cr,aux, is designed such that it operates in an impedance region opposite to the main resonant tank, i.e. comprising of Lr.main and Cr.main.

[0082] This means that if the main resonant tank is designed to operate in capacitive region then the auxiliary resonant tank is designed to operate in inductive region for the designed power range and frequency range of the converter; if the main resonant tank is designed to operate in inductive region then the auxiliary resonant tank is designed to operate in capacitive region for the designed power range and frequency range of the converter.

[0083] With the auxiliary resonant tank addition, the ZVS turn on range of the converter can be extended without adding any complex control methods. This is because the auxiliary resonant tank increases the current at the switching instances.

[0084] When the converter is operated in capacitive mode, the auxiliary resonant tank aids in increasing the current at the switching instance. As shown in Figure 4, at the switching instants (trt?) the current seen by the switches is the sum of the two resonant tanks (io, main + io, aux). This makes it easier for the switches to achieve ZVS turn on. When the converter is operated in inductive mode, then with the addition of the auxiliary resonant tank, has two benefits. Firstly, as shown in Figure 5, like the capacitive region, the currents at the switching instants are increased. Furthermore, with proper design of the auxiliary LC tank, the converter can achieve ZVS turn-on at very low output powers.

[0085] In other words, the problem of a prior art converter is that it can hardly reach ZVS when the load is light. Light load means the average power from one pole to the other is low.

[0086] This means the average value of i_main in the duration t_0 ~ t_2 is low, given that the pole voltage is fixed - so it has no impact on power. Meanwhile, insufficient ZVS means l_2 is not big enough. So you want the auxiliary resonant tank to increase l_2, while do not increase the average value of i_main, so you need the auxiliary resonant tank to operate in the other region. If they are in the same region, l_2 will be increased, but the average current of i_main will also be increased, which means the load is not light any more, in the other word, when the load is light, you may still have difficulty to reach ZVS.

[0087] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “Comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0088] Any reference signs in the claims should not be construed as limiting the scope thereof

Claims

CLAIMS1. A series resonant converter, wherein said series resonant converter comprises: a first and a second switch, connected in series, and connected between a first port and a second port of the series resonant converter; a third and a fourth switch, connected in series, and connected between said second port and a third port of the series resonant converter; a main resonant tank connected in parallel over said second and third switch, wherein said main resonant tank comprises a main inductor connected in series with a main capacitor, wherein said main resonant tank has a main resonant frequency; wherein the series resonant converter further comprises: an auxiliary resonant tank connected in parallel over said main resonant tank, wherein said auxiliary resonant tank comprises an auxiliary inductor connected in series with an auxiliary capacitor, wherein said auxiliary resonant tank has an auxiliary resonant frequency being different from said main resonant frequency, a controller for controlling said switches, such that said main resonant tank is operated in an impedance region being one of an inductive and capacitive region and said auxiliary resonant tank is operated in different impedance region being opposite to the impedance region of the main resonant tank.

2. A series resonant converter in accordance with claim 1 , wherein said controller is arranged for controlling said switches such that said main resonant tank is operated in the inductive region and such that said auxiliary resonant tank is operated in the capacitive region.

3. A series resonant converter in accordance with any of the previous claims, wherein said main resonant tank and said auxiliary resonant tank are designed such that both: said auxiliary resonance frequency is at most two times said switching frequency, andsaid main resonance frequency is at most two times said switching frequency.

4. A series resonant converter in accordance with any of the previous claims, wherein any of said first, second, third and fourth switch comprise Metal Oxide Semiconductor, MOS, Field Effect Transistor, FETs or Insulated Gate Bipolar Transistors, IGBTs.

5. A series resonant converter in accordance with any of the previous claims, wherein said main resonant tank and said auxiliary resonant tank are designed such that one of: said auxiliary resonant frequency is at least two times higher compared to said main resonant frequency. said main resonant frequency is at least two times higher compared to said auxiliary resonant frequency.

6. A series resonant converter in accordance with any of the previous claims, wherein said controller is arranged for: controlling said switches using a switching frequency, wherein said switching frequency is higher than the main resonant frequency and lower than the auxiliary resonant frequency.

7. A series resonant converter in accordance with any of the claims 1 - 4, wherein said controller is arranged for: controlling said switches using a switching frequency, wherein said switching frequency is higher than the auxiliary resonant frequency and lower than the main resonant frequency.

8. A series resonant converter in accordance with any of the previous claims, wherein said auxiliary resonant tank further comprises an activation switch for enabling said auxiliary resonant tank.

9. A series resonant converter in accordance with claim 7, wherein said controller is arranged for activating said activation switch such that said auxiliary resonant tank is activated.

10. An arrangement of a series resonant converter in accordance with any of the previous claims and a bipolar Direct Current, DC, power grid, wherein: said first and second port are connected between a first DC terminal of said bipolar DC power grid and a neutral terminal of said bipolar DC power grid; said second and said third port are connected between said neutral terminal of said bipolar DC power grid and a second DC terminal of said bipolar DC power grid.

11. An arrangement in accordance with claim 10, wherein said bipolar DC power grid is arranged to provide at least 600Volts DC between said first and said second terminal of said bipolar DC power grid.

12. A method of operating a series resonant converter in accordance with any of the claims 1 - 9, wherein said method comprises the step of: controlling, by said controller, the switches, such that said main resonant tank is operated in an impedance region being one of an inductive and capacitive region and said auxiliary resonant tank is operated in different impedance region being opposite to the impedance region of the main resonant tank.

13. A method in accordance with claim 12, wherein the step of controlling comprises: controlling, by said controller, said switches such that said main resonant tank is operated in the inductive region and such that said auxiliary resonant tank is operated in the capacitive region.

14. A method in accordance with any of the claims 12 - 13, wherein: said main resonant tank comprises a main inductor connected in series with a main capacitor;said auxiliary resonant tank comprises an auxiliary inductor connected in series with an auxiliary capacitor.

15. A method in accordance with any of the claims 12 - 14, wherein said step of controlling comprises: controlling, by the controller, said switches using a switching frequency, wherein said switching frequency is higher than the main resonant frequency and lower than the auxiliary resonant frequency.

16. A method in accordance with any of the claims 12 - 14, wherein said step of controlling comprises: controlling, by the controller, said switches using a switching frequency, wherein said switching frequency is higher than the auxiliary resonant frequency and lower than the main resonant frequency.

17. A computer program product comprising a computer readable medium having instructions stored thereon which, when executed by a controller of a series resonant converter, cause said controller to implement a method in accordance with any of the claims 12 - 16.

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