converter

Abstract

The invention relates to a converter, which can operate as an AC / DC converter or as a DC / AC converter, for converting an electrical voltage, the converter having: a transformer (T); semiconductor switches (S) which are interconnected in a full-bridge circuit (VBS) comprising two half-bridges (HB1, HB2) which each have semiconductorsemiconductor switches (S) connected in series at a central node (MK1; MK2), wherein the transformer windings of the transformer (T) are connected in series with a capacitor (Ca) between the two central nodes (MK1; MK2) of the full-bridge circuit (VBS); and a controller for controlling the semiconductorsemiconductor switches (S) of the full-bridge circuit (VBS).

Description

[0001] Description

[0002] title

[0003] converter

[0004] The invention provides an isolated converter that can operate as an AC / DC converter or as a DC / AC converter, particularly for an electric vehicle, with a full bridge circuit whose semiconductor switches are subject to a low voltage load and can perform a zero-point switching operation.

[0005] State of the art

[0006] Low voltage (LV) is defined as a direct current (DC) voltage below 60 V. The most common voltage levels used for low voltage are 14 V, 24 V, or 48 V. High voltage (HV) lies between the minimum and maximum operating voltage of the vehicle battery. Most battery technologies used have nominal voltages of 400 V or 800 V, with a wide range of voltage fluctuations depending on the battery's state of charge.

[0007] Conventional full-bridge converters, which operate on a transformer basis, can be used in electric vehicles to reduce a high-voltage battery voltage to lower voltages for various systems in the vehicle.

[0008] DE 102018221 195 A1 describes a bidirectional converter for energy transfer between a primary side (HV) and a secondary side (LV) with connections for a primary energy storage device (UHV) and a secondary energy storage device (UNV), with one or more transformers for galvanic isolation of the primary side (HV) from the secondary side (LV), with switching elements for connecting and reversing the windings of the transformer on the primary and secondary sides, with a control device for controlling the switching elements, and with a secondary-side series inductance and with a blocking switching element which is connected in parallel to the secondary-side series inductance.

[0009] Conventional full-bridge converters, which operate on a transformer basis, can be used in electric vehicles to reduce a high-voltage battery voltage to lower voltages for various systems in the vehicle.

[0010] Fig. 1 (a) to 1 (c) show different possible circuit topologies for conventional converters.

[0011] The transformer T of the converter shown in Fig. 1 comprises a primary and a secondary winding. The primary winding of transformer T receives the input voltage from nodes A and B of the input-side full-bridge circuit, while the secondary winding of transformer T supplies the output voltage. By adjusting the number of turns of transformer T, the voltage can be converted according to the requirements of the electric vehicle.

[0012] A control circuit continuously monitors the output voltage and adjusts the switching frequency and duty cycle of the power switches or transistors in the converter's full-bridge circuit accordingly to achieve and maintain a desired output voltage. The transformed and regulated output voltage can then be distributed to various electrical systems within the electric vehicle, such as a 12-volt electrical system for vehicle lighting, on-board electronics, etc. An electric vehicle's high-voltage battery provides a relatively high DC voltage, depending on the vehicle design and battery configuration. The converter's full-bridge circuit consists of a series of power switches configured to control the electric current I through the primary winding of a transformer T in a specific manner.This circuit allows the electrical voltage V to be regulated and transformed.

[0013] The electrical voltage V applied to the power switches of an inverter or converter has a negative impact on the performance.

[0014] The converter's performance is affected, particularly with regard to losses and stress, which increase with increasing voltage V. Furthermore, circuit breakers with a higher rated voltage are inferior in terms of cost and efficiency to those with similar technology and a lower rated voltage.

[0015] On the other hand, a high-voltage to low-voltage converter must provide a large step-down voltage gain as well as sufficient electrical isolation. This is usually achieved by a magnetic transformer T and a corresponding adjustment of the turns ratio of its windings, so that a higher turns ratio between the primary and secondary sides results in a higher voltage conversion ratio.

[0016] Step-down voltage gain in converters means that the converter's output voltage is lower than the input voltage. In other words, the converter reduces the voltage instead of increasing it. This is often done to provide a specific voltage for various applications, for example, to regulate a higher-voltage battery to the lower operating voltage of a device. However, as the number of turns increases, the copper cross-sectional area of ​​each winding decreases for the same area, resulting in a higher DC resistance of the windings.

[0017] Some conventional converter topologies achieve high conversion ratios and isolation through coupled inductors instead of a transformer, such as the flyback converter. However, this approach is not optimal for magnetic components, which typically pose a significant challenge and concern in power converter design, as they can account for a large portion of the power losses and converter volume. This is because, in a coupled inductor, electrical power is not immediately transferred to the load but is stored in the inductor during one operating cycle and released in the next. This energy storage requirement comes at the expense of efficiency and inductor size compared to a transformer-based solution.

[0018] On the other hand, asymmetric pulse-width modulation (PWM) is a simple and robust control method widely used for controlling DC-DC converters. However, in transformer-based DC-DC converters, this type of modulation has some disadvantages for common rectifier topologies, where asymmetric PWM techniques can cause high voltage loads on the rectifier switches as well as a significant DC offset in the transformer's magnetizing current.

[0019] This leads to increased core losses and magnetic saturation, as well as an overall less optimal transformer design. Furthermore, it results in a larger magnetizing current ripple to achieve zero-voltage switching (ZVS), provided the circuit topology allows it.

[0020] These technical disadvantages become more pronounced as the voltage and load operating ranges increase, since a greater pulse width asymmetry is required. This is the case with isolated DC / DC converters for electric vehicles due to the large voltage fluctuations in the battery and the vehicle's load requirements.

[0021] For these reasons, modulation techniques other than frequency modulation are preferred for transformer-based circuits. However, these modulation techniques have disadvantages with regard to the associated higher circuit complexity of the control and the necessary use of more complex and larger filters for electromagnetic interference (EMI).

[0022] Disclosure of the invention

[0023] According to a first aspect, the invention provides a converter, which can operate as a DC / AC converter or as an AC / DC converter, for converting an electrical voltage with a transformer having transformer windings and with semiconductor switches connected together in a full bridge circuit, which comprises two half-bridges, each having semiconductor switches connected in series at a central node, wherein the transformer windings of the transformer are connected in series with a capacitor between the two central nodes of the full bridge circuit, and with a controller for controlling the semiconductor switches of the full bridge circuit.

[0024] The circuit according to the invention makes it possible to drastically reduce the voltage load on the semiconductor switches compared to other conventional rectifier topologies, especially when asymmetric PWM modulation is used in transformer-based solutions. This has significant advantages in terms of cost, efficiency, and reliability of the semiconductor switches.

[0025] With the aid of the circuit according to the invention, the DC offset in the magnetizing current of the transformer can be significantly reduced, particularly when using asymmetrical PWM modulation in the transformer-based circuit, which can lead to an optimized transformer design and improved ZVS ranges.

[0026] For the aforementioned reasons, it is more attractive to use asymmetric PWM modulation methods, which offer advantages in terms of cost and performance of the control system as well as in EMI filter design compared to other modulation methods.

[0027] In one possible embodiment of the converter for converting an electrical voltage, connections for connecting a voltage source or a load are provided on one of the semiconductor switches of the full bridge circuit.

[0028] This allows the converter according to the invention to be operated as an inverter or as a rectifier.

[0029] The converter according to the invention has the capability to operate as a rectifier, i.e., as an AC / DC converter, or as an inverter, i.e., as a DC / AC converter. This property of the converter according to the invention can also be referred to as the converter's bidirectionality.

[0030] In one possible embodiment of the converter for converting an electrical voltage, an inductive component is provided at one of the two terminals of the full bridge circuit.

[0031] The inductive component helps to filter the electrical currents I, control the converter and achieve a zero-voltage switching (ZVS).

[0032] In one possible embodiment of the converter for converting an electrical voltage, the full-bridge circuit includes an additional capacitor as an intermediate circuit. In this embodiment, the electrical voltages applied to the capacitors of the full-bridge circuit constitute state variables that are adjusted depending on the switching states of the controllable semiconductor switches of the full-bridge circuit.

[0033] In one possible embodiment of the converter for converting an electrical voltage, the full bridge circuit has a potential-free (floating) reference potential.

[0034] In one possible embodiment of the converter for converting an electrical voltage, the full bridge circuit is connected to a bootstrap circuit.

[0035] In one possible embodiment of the converter for converting an electrical voltage, the converter's control system applies a modulated control signal to the control terminals of the semiconductor switches of the full bridge circuit.

[0036] In one possible embodiment of the converter for converting an electrical voltage, the modulated control signal applied by the converter's control unit to the control terminals of the semiconductor switches of the full bridge circuit has a pulse-width modulated (PWM) signal.

[0037] In one possible embodiment of the converter for converting an electrical voltage, the modulated control signal applied by the converter's control unit to the control terminals of the semiconductor switches of the full bridge circuit has a frequency-modulated FM signal.

[0038] In one possible embodiment of the converter for converting an electrical voltage, the modulated control signal applied by the converter's control unit to the control terminals of the semiconductor switches of the full-bridge circuit is a phase-modulated PM signal. In another possible embodiment of the converter for converting an electrical voltage, the controllable semiconductor switches of the full-bridge circuit comprise field-effect transistors (FETs).

[0039] In one possible embodiment of the converter for converting an electrical voltage, the controllable semiconductor switches of the full bridge circuit are controlled by the controller in such a way that they perform a zero-voltage switching operation.

[0040] This avoids power losses and increases the converter's performance. Zero-voltage switching (ZVS) also minimizes wear on the switching components, thus increasing the system's reliability.

[0041] In one possible embodiment of the converter for converting an electrical voltage, the converter is designed to be operated bidirectionally.

[0042] The invention further provides an electric vehicle (EV) with a vehicle battery having a high voltage (HV), and with at least one electrical system having a low voltage (NV), and with a converter according to the first aspect of the invention for converting the high voltage (HV) into the low voltage (NV).

[0043] Preferred embodiments of the converter according to the invention are described in more detail below with reference to the attached figures.

[0044] They show:

[0045] Fig. 1 (a) to Fig. 1 (c) Circuit topologies of conventional converters; Fig. 2(a) to Fig. 2(c) Exemplary embodiments of a converter according to the invention;

[0046] Fig. 3(a) to Fig. 3(d) switching sequences in a possible embodiment of the converter according to the invention;

[0047] Fig. 4 shows a further embodiment of a converter according to the invention;

[0048] Figs. 5 to 12 are diagrams illustrating the operation of the converter according to the invention.

[0049] In one possible embodiment, the circuit according to the invention comprises four semiconductor switches S1 to S4, two capacitors C, a transformer T, and an inductor L, as shown in Fig. 2(a). Fig. 2(a) shows the turns ratio n between the primary and secondary windings of the transformer T.

[0050] The circuit according to the invention enables bidirectional operation.

[0051] A voltage source or a load can be interchangeably connected between the terminals A1 and A2 of a connection port of the full bridge circuit VBS of the converter, depending on whether it is used as an inverter or as a rectifier, as shown in the embodiments according to Fig. 2 (b) and Fig. 2 (c).

[0052] Fig. 2(b) shows the operation of the converter according to the invention as an inverter, i.e. as a DC / AC converter.

[0053] Fig. 2(c) shows the operation of the converter according to the invention as a rectifier, i.e., as an AC / DC converter. The AC circuit part of the circuit according to the invention is formed by the transformer T, and the DC circuit part is formed by the capacitors C. Depending on whether the electrical energy (power) flows from the AC circuit part (transformer T) to the DC circuit part (capacitors C) or from the DC circuit part (capacitors C) to the AC circuit part (transformer T), the converter operates as an AC / DC converter (rectifier) ​​or as a DC / AC converter (inverter).

[0054] According to a first aspect, the invention provides a converter for converting an electrical voltage with a transformer T, which has transformer windings on the secondary side. Several semiconductor switches S1, S2, S3, S4 are interconnected in a full bridge circuit VBS. The full bridge circuit VBS has two half bridges HB1, HB2, each containing two semiconductor switches connected in series at a central node MK1, MK2.

[0055] The secondary windings of transformer T are connected in series with at least one capacitor (Ca) between the two center nodes MK1 and Mk2 of the full-bridge circuit VBS, as shown in Fig. 2. The capacitor Ca has a specific capacitance suitable for the application. In one possible implementation, the capacitance of capacitor Ca is adjustable, for example, by adding or removing further capacitors connected in parallel.

[0056] The converter also has a (not shown) control unit for controlling the semiconductor switches S1, S2, S3, S4 of the full bridge circuit VBS.

[0057] In one possible embodiment of the converter for converting an electrical voltage, terminals A1 and A2 of a connection port for connecting a voltage source or a load are provided on one of the semiconductor switches of the full-bridge circuit VBS. In the embodiment shown in Fig. 2, the connection port with terminals A1 and A2 is provided on the third semiconductor switch S3 of the full-bridge circuit VBS.

[0058] In one possible embodiment of the converter for converting an electrical voltage, an inductive component (L0) is provided at one of the two terminals A1, A2 of the connection port of the full bridge circuit VBS, as shown in Fig. 2.

[0059] In one possible embodiment of the converter for converting an electrical voltage, the full bridge circuit VBS has an additional capacitor (Cb) as an intermediate circuit ZK.

[0060] In one possible embodiment of the converter for converting an electrical voltage, the electrical voltages (V) applied to the capacitors Ca, Cb form state variables that are set depending on the switching states of the controllable semiconductor switches S1, S2, S3, S4 of the full bridge circuit VBS.

[0061] In one possible embodiment of the converter for converting an electrical voltage, the full bridge circuit VBS has a potential-free reference potential.

[0062] In one possible embodiment of the converter for transforming an electrical voltage, the full-bridge circuit VBS is connected to a bootstrap circuit, as shown in Fig. 4. The bootstrap circuit comprises a bootstrap diode Dboot and bootstrap capacitors Cboot, as shown in Fig. 4. Bootstrapping refers to an electrical circuit in which a potential change in one part of the circuit also has an immediate effect in another. This utilizes the effect that capacitors change their voltage only slightly at low currents. The bootstrap circuit shown in Fig. 4 comprises the bootstrap diode Dboot connected to the transformer T and two bootstrap capacitors (Cboot) arranged in series with it, which are connected to the full-bridge circuit VBS.In one possible embodiment of the converter for converting an electrical voltage, the converter's control unit applies modulated control signals to the control terminals of the semiconductor switches S1, S2, S3, and S4 of the full-bridge circuit VBS. In one possible implementation, the converter's control unit may have a control and data interface and / or a user interface for setting the modulation type and / or for setting parameters (e.g., switching frequency and duty cycle) of the control signals applied to the control terminals of the semiconductor switches.

[0063] In one possible embodiment of the converter for converting an electrical voltage, the modulated control signal applied by the converter's control unit to the control terminals of the semiconductor switches of the full bridge circuit VBS has a pulse width modulated PWM signal.

[0064] In another alternative possible embodiment of the converter for converting an electrical voltage, the modulated control signal applied by the control of the converter to the control terminals of the semiconductor switches of the full bridge circuit VBS has a frequency-modulated FM signal.

[0065] In another alternative possible embodiment of the converter for converting an electrical voltage, the modulated control signal applied by the control of the converter to the control terminals of the semiconductor switches of the full bridge circuit VBS has a phase-modulated PM signal.

[0066] In one possible embodiment of the converter for converting an electrical voltage, the controllable semiconductor switches S1, S2, S3, S4 of the full bridge circuit VBS feature field-effect transistors (FETs).

[0067] In one possible embodiment of the converter for converting an electrical voltage, the controllable semiconductor switches S1, S2, S3, S4 of the full bridge circuit VBS are controlled by the controller in such a way that they perform a zero voltage switching (ZVS).

[0068] In one possible embodiment of the converter for converting an electrical voltage, the converter is designed to be operated bidirectionally.

[0069] The invention further provides an electric vehicle (EV) with a vehicle battery having a high voltage (HV), and with at least one electrical system having a low voltage (NV), and with a converter according to the first aspect of the invention for converting the high voltage (HV) into the low voltage (NV).

[0070] The converter according to the invention can also be used in other vehicles, systems or devices in which an electrical DC voltage is converted from a first voltage level to a second voltage level.

[0071] The four semiconductor switches S1, S2, S3, S4 of the converter shown in Fig. 2 form a full bridge structure, in which a capacitor Cb acts as the intermediate circuit ZK and determines the electrical voltage V to which the semiconductor switches S1, S2, S3, S4 are subjected. Another capacitor Ca is connected in series with the transformer windings of transformer T, so that the average current I on the secondary side of transformer T becomes zero. The inductive component L0 shown in Fig. 2 helps to filter the electrical currents I, control the converter, and achieve a zero-voltage circuit (ZVS).

[0072] Zero-voltage switching (ZVS) is a desirable feature of power converters because it drastically reduces switching energy losses, allowing the converter to operate at higher frequencies. Since the volume of passive components depends on the energy stored in them during a switching cycle, this volume can be reduced by shortening the switching cycle by increasing the switching frequency. Furthermore, switching losses are a source of power losses, leading to increased heat generation and a greater need for cooling, resulting in larger and more expensive cooling systems.

[0073] Zero Voltage Switching (ZVS) refers to a switching technique used in power switches to reduce energy loss and the generation of electromagnetic interference. With ZVS, the switching process is synchronized so that the semiconductor switch closes or opens precisely when the voltage at its output or input is close to zero. This prevents the semiconductor switch from being switched at a high voltage, which would result in current flow through the semiconductor switch and power loss.

[0074] ZVS technology is particularly useful in switching regulators and inverters, where it helps to improve efficiency and reduce electromagnetic interference. ZVS also minimizes wear on switching components, thus increasing system reliability.

[0075] The semiconductor switches S1, S2, S3, S4 of the full-bridge circuit VBS of the converter shown in Figures 2 and 3 are preferably switchable power switches, which are preferably formed by controllable semiconductor switches or FET transistors, in particular MOSFETs. The semiconductor switches S1, S2, S3, S4 are controlled by a (not shown) controller of the DC / DC converter.

[0076] The operating principle of the inverter / converter is to control the capacitors Ca and Cb by operating the semiconductor switches S1, S2, S3, and S4, adjusting the on and off times of the semiconductor switches S. The capacitors C store energy in the form of electrical charge, so the voltage V represents a state variable that can be set or controlled by the semiconductor switches S. This results in different steady-state voltages V being achieved for the semiconductor switches S1, S2, S3, and S4, depending on the modulation method. This approach is extremely efficient due to the superior properties of capacitors C as energy storage elements, as capacitors C exhibit good characteristics in terms of power density and loss factor compared to other technologies.

[0077] In circuit topologies with switched capacitors C, it is advantageous to connect an inductor L in series with the capacitors C in at least one of the inverter's operating states. This is because magnetic components store energy in the form of magnetic fields, and the current I is their state variable, allowing them to act as a current source and significantly reduce charge sharing losses in the capacitors C. On the other hand, in applications requiring isolation or a high gain ratio, transformer-based approaches are preferred. A transformer T can be used in conjunction with a switched capacitor topology as a current source to leverage the advantages of both solutions and the synergies they offer. This represents a core concept of the invention.

[0078] The embodiment shown in Fig. 2 is a circuit topology with switched capacitors C, in which the semiconductor switches S1, S2, S3, S4 of the full-bridge circuit VBS are subjected to a voltage V corresponding to the voltage V across capacitor Cb. This voltage can be controlled by operating the semiconductor switches S1, S2, S3, S4, achieving a good compromise between the number of semiconductor switches and the voltage load V acting on them. The transformer T offers great flexibility with regard to the achievable conversion ratios, insulation, reduction of charge division losses, and the possibility of zero-voltage switching (ZVS) across all semiconductor switches. The circuit topology shown in Fig. 2 is compatible with many modulation techniques, e.g., frequency modulation (FM), pulse-width modulation (PWM), phase-shifted modulation (PM), etc.The topology is also compatible with various switching sequences of the semiconductor switches S1, S2, S3, S4 of the full bridge circuit VBS.

[0079] For the sake of simplicity and because this is the application in which the technical advantages of the circuit topology according to the invention are most readily apparent, an embodiment for operating the converter as a rectifier with a fixed switch-off time, which is referred to as "dead time", is presented.

[0080] This operating mode is graphically illustrated in Figs. 3a to 3d.

[0081] Fig. 3a shows the transformer magnetization.

[0082] Fig. 3b schematically shows the switching-on dead times of the Haibi conductor switches S2, S3.

[0083] Fig. 3c shows the transformer demagnetization.

[0084] Fig. 3d schematically represents the switching-on dead times of the semiconductor switches S1, S4.

[0085] During the dead-time periods, the transformer T and the inductor Lo act as current sources and force a current flow in the semiconductor switches. This can help achieve a zero-voltage switching (ZVS) at them by charging / discharging the parasitic capacitances Cp of the semiconductor switches S before the semiconductor switches S are turned on. This results in a zero-voltage switching at the semiconductor switches at the time of turn-on.

[0086] S1, S2, S3, S4 have no or a very low voltage V applied. This is a condition to reduce the energy loss when switching on the semiconductor switches S1, S2, S3, S4 of the full-bridge circuit VBS. Conventional bridge rectifiers typically also use the output as a power supply for the rectifier gate drivers, since it is a controlled low-voltage source referenced to the same ground as the rectifier bridge. A special feature of the circuit according to the invention is that the full-bridge circuit VBS is referenced to a different ground or a different floating reference ground potential than that of the load / source.

[0087] For the embodiment shown in Figs. 2 and 3, the steady-state voltages V generated across the capacitors C can be specified using the following equations, assuming ideal circuit elements:

[0088] VCb = Vtransformer * 1 / (2 (1 - D))

[0089] VCa = VCb *(2D - 1)

[0090] VCO = VCb * D

[0091] The duty cycle (D) is the switching-on time of switches S1 and S4 divided by the total period or switching cycle time.

[0092] The results of the output voltage gain are summarized graphically in the diagrams according to Figs. 5a and 5b. It can be seen in Figs. 5a and 5b that an exponential voltage gain VG (Voltage Gain) is achieved with respect to the voltage V on the secondary side of the transformer T during the switch-on time.

[0093] These results relate to the voltage V across the transformer T on the secondary side, which is determined by the circuit topology used on the primary side, and should be placed in a context where this element is included in the analysis.

[0094] For this reason, a comparison is made below between the most common rectifier topologies, using a popular topology for the primary side, namely the HB half-bridge.AI's control method uses asymmetric PWM modulation with synchronous rectification, also known as asymmetric half-bridge (AHB), since this is a procedure in which the technical advantages of the converter according to the invention become apparent.

[0095] Fig. 6 shows the amplification of the DC / DC converter with different rectifiers.

[0096] The operating range is extended to the entire duty cycle range, effectively doubling it compared to the conventional solution. Furthermore, the resulting voltage gain (VG) is highly linear, which can lead to a more robust control strategy.

[0097] The duty cycle D refers to the ratio of the time during which a semiconductor switch is switched on (active) to the total cycle time.

[0098] The voltage rise across capacitor Cb with respect to the voltage V on the secondary side of transformer T is shown graphically in Fig. 6 as a function of the duty cycle D. The voltage V across capacitor Cb determines the voltage load in the semiconductor switches. Since a voltage-second equilibrium must exist in transformer T, the voltages V applied to it are symmetrical at a duty cycle D of approximately 0.5, as can be seen in Fig. 6.

[0099] Figures 7a and 7b show the voltage V across the capacitor Cb in relation to the voltage V on the secondary side of the transformer T (VCb / Vsec) as a function of the duty cycle D or the complementary duty cycle D' of the control signal applied to the semiconductor switches.

[0100] Fig. 7(b) serves to better illustrate the results. It shows that the maximum relative voltage gain VG compared to the maximum voltage Vmax in the transformer T is one and is given when the duty cycle D is 0.5. Furthermore, it can be seen in Fig. 7(b) that the maximum relative voltage gain decreases when the duty cycle D is reduced to half the maximum voltage Vmax in the transformer T.

[0101] For an asymmetric half-bridge (AHB), the maximum relative voltage stress in transformer T is shown in Fig. 8. Fig. 8 shows the maximum voltage stress in transformer T relative to the input voltage Vin (with AHB on the primary side). As can be seen in the diagram of Fig. 8, the maximum relative voltage stress reaches its maxima when the duty cycle D is at its minimum (D=0) and maximum (D=1). The minimum voltage stress is reached when the duty cycle D = 0.5.

[0102] In conventional rectifier topologies, the maximum voltage load to which the semiconductor switches are subjected is equal to the voltage load in the transformer T or twice as high as in the case of the center-tapped half-bridge rectifier (CT).

[0103] In the proposed invention, however, the relative voltage load of the semiconductor switches S1, S2, S3, S4 of the full bridge circuit VBS is inversely proportional to the transformer T, which results in the voltage load of the semiconductor switches S1, S2, S3, S4 always being equal to half of the maximum voltage Vmax in the transformer T.

[0104] A typical application of EV high-voltage DC / DC converters is to convert the high-voltage (HV) voltage of the high-voltage battery, which can fluctuate greatly, into a low-voltage (NV) voltage, for example, a DC voltage of 14 V.

[0105] An example of this application with the topologies used for comparison is shown in Fig. 9, where the voltage stress of the rectifier semiconductor switches (normalized to the number of turns) is depicted. Fig. 9 shows the voltage stress at the semiconductor switches (drain-source voltage) in the circuit according to the invention (I) and in conventional circuits (II Full-Bridge / Half-Wave; III CT-Half-Bridge). Since both the maximum absolute and the relative voltage stress in the transformer T are greatest when the required gain is minimal and thus the duty cycle D is far from 0.5, the stress on the semiconductor switches is high in conventional rectifier topologies (II, III).

[0106] In the converter according to the invention, however, the relative voltage load of the semiconductor switches S1, S2, S3, S4 of the full bridge circuit VBS is always only half as large as that of the transformer T, regardless of the duty cycle D and linear with the input voltage, which leads to a significantly lower load, as can be seen in the diagram of Fig. 9.

[0107] Advantages of the circuit according to the invention result from a series of simulations using the modulation type of the embodiment described above with the tool PLECS, the results of which with regard to the voltage load are shown in Fig. 10.

[0108] In Fig. 11, the voltages VS1, VS2, VS3 (Fig. 11 c) and currents IS1, IS2, IS3 (Fig. 11 b) in the semiconductor switches S during the switching process are shown as proof of the ZVS capability of the circuit according to the invention.

[0109] Figure 12 shows the currents in transformer T for a given inverter on the primary side, illustrating the magnetization-demagnetization dynamics, instantaneous power transfer capability, and low DC offset in the magnetization current for all duty cycle conditions. Depending on the chosen inverter topology on the primary side and the circuit parameters used, a wide variety of behaviors regarding the magnitude and distribution of the currents I and voltages V in transformer T can occur. The converter can be used with any type of circuit topology on the primary side to form an isolated DC-DC converter and allows bidirectional operation, enabling it to function as both an inverter and a rectifier.

Claims

Claims 1. Converter for converting an electrical voltage with a transformer (T) and with semiconductor switches (S1 ,S2,S3,S4) which are interconnected in a full bridge circuit (FBC), which comprises two half bridges (HB1, HB2) each having semiconductor switches (S1 ,S2;S3,S4) connected in series at a central node (MK1; MK2), wherein the transformer windings of the transformer (T) are connected in series with a capacitor (Ca) between the two center nodes (MK1; Mk2) of the full bridge circuit (VBS); and with a control system for controlling the semiconductor switches (S1,S2;S3,S4) of the full bridge circuit (VBS).

2. Converter according to claim 1, wherein the converter operates as an AC / DC converter or as a DC / AC converter.

3. Converter for converting an electrical voltage according to claim 1 or 2, wherein one of the semiconductor switches of the full bridge circuit (FBC) has connections (A1, A2) for connecting a voltage source or a load.

4. Converter for converting an electrical voltage according to one of claims 1 to 3, wherein an inductive component (LO) is provided at one of the two terminals (A1, A2) of the full bridge circuit (VBS).

5. Converter for converting an electrical voltage according to one of claims 1 to 4, wherein the full bridge circuit (VBS) has a further capacitor (Cb) as an intermediate circuit.

6. Converter for converting an electrical voltage according to one of claims 1 to 5, wherein the capacitors (Ca, Cb) are connected to the In a full bridge circuit (VBS), the applied electrical voltages (V) form state variables that are set depending on the switching states of the controllable semiconductor switches (S1, S2, S3, S4) of the full bridge circuit (VBS).

7. Converter for converting an electrical voltage according to one of claims 1 to 6, wherein the full bridge circuit (FBC) has a potential-free reference potential.

8. Converter for converting an electrical voltage according to any one of claims 1 to 7, wherein the full bridge circuit (VBS) is connected to a bootstrap circuit comprising a bootstrap diode (Dboot) and boot capacitors (Cboot).

9. Converter for converting an electrical voltage according to any one of claims 1 to 8, wherein the control of the DC / DC converter applies a modulated control signal to control terminals of the semiconductor switches (S1 ,S2,S3,S4) of the full bridge circuit (FBC).

10. Converter for converting an electrical voltage according to claim 9, wherein the modulated control signal applied by the control of the DC / DC converter to the control terminals of the semiconductor switches (S1,S2,S3,S4) of the full bridge circuit (VBS) comprises a pulse width modulated PWM signal.

11. Converter for converting an electrical voltage according to claim 9, wherein the modulated control signal applied by the control of the converter to the control terminals of the semiconductor switches (S1,S2,S3,S4) of the full bridge circuit (FBS) comprises a frequency-modulated FM signal.

12. Converter for converting an electrical voltage according to claim 9, wherein the modulated control signal applied by the control of the DC / DC converter to the control terminals of the semiconductor switches (S1,S2,S3,S4) of the full bridge circuit (FBC) comprises a phase-modulated PM signal.

13. Converter for converting an electrical voltage according to any one of the preceding claims 1 to 12, wherein the controllable semiconductor switches (S1 ,S2,S3,S4) of the full bridge circuit (FBC) comprise field-effect transistors (FETs).

14. Converter for converting an electrical voltage according to one of claims 1 to 13, wherein the controllable semiconductor switches (S1,S2,S3,S4) of the full bridge circuit (FBC) are controlled by the controller in such a way that they perform a zero voltage circuit.

15. Converter for converting an electrical voltage according to any one of claims 1 to 14, wherein the converter is designed to be operated bidirectionally.

16. Electric vehicle (EV) comprising a vehicle battery having a high voltage (HV), and comprising at least one electrical system having a low voltage (NV), and comprising a converter according to any of the preceding claims 1 to 15 for converting the high voltage (HV) into the low voltage (NV).

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