DC Converter and Its Operation

Abstract

The present invention relates to a DC converter (1) for converting a DC input voltage (U in ) generates a multi-phase primary AC voltage, transforms the primary AC voltage into a secondary AC voltage with a transformation ratio (n), and generates a DC output voltage (U) from the secondary AC voltage out The DC converter (1) has a dual active bridge topology with a series resonant circuit (9) for each AC voltage phase. out ) and DC input voltage (U in ) is greater than the transformation ratio (n), the phase shift between the primary AC voltage and the corresponding series resonant circuit current is adjusted to zero by changing the clock frequency (f) for timing the AC voltage. out ) and DC input voltage (U in ) is less than the transformation ratio (n), the phase shift between the secondary AC voltage and the corresponding series resonant circuit current is adjusted to zero by changing the clock frequency (f).

Description

Technical Field

[0001] The invention relates to a DC converter and a method for operating such a DC converter, which has a double active bridge topology and a series resonant circuit for each AC voltage phase. Background Art

[0002] The DC converter with a dual active bridge topology has two converter units and a transformer unit connected between the two converter units, each of which has a plurality of semiconductor switches. A converter unit is used to generate a primary AC voltage from a DC input voltage, which is converted into a secondary AC voltage by a transformer unit. A DC output voltage is generated from the secondary AC voltage by another converter unit. The AC voltage can be single-phase or multi-phase and is clocked at a clock frequency, and the semiconductor switches are driven by the clock frequency.

[0003] The present invention relates to a DC converter implemented with a dual active bridge topology, which has a series resonant circuit for each AC voltage phase, the series resonant circuit having at least one capacitor and at least one coil. Usually, the power or DC output voltage of such a DC converter is adjusted by changing the phase offset between the primary and secondary AC voltages. In this case, the efficiency of the DC converter depends on the ratio of the DC output voltage to the DC input voltage. The efficiency is optimal when the ratio is consistent with the transformation ratio of the transformer unit or only slightly deviates from the transformation ratio of the transformer unit. When the ratio deviates greatly from the transformation ratio of the transformer unit, the efficiency decreases.

[0004] FR 3 050 593 A1 discloses a method and a system for controlling a DC converter for converting a first DC voltage into a second DC voltage, the DC converter having a double-bridge topology with two converters. Here, an optimal phase offset angle between two AC voltages is determined, which is generated for converting the first DC voltage into the second DC voltage. Summary of the invention

[0005] The object of the invention is to optimize the efficiency of a DC converter having a double active bridge topology and a series resonant circuit for each AC voltage phase.

[0006] According to the invention, this object is achieved by a method having the features of the invention and a DC converter having the features of the invention.

[0007] Advantageous embodiments of the invention are the subject matter of the individual exemplary embodiments.

[0008] The method according to the invention concerns a method for operating a DC converter for generating a multiphase primary AC voltage from a DC input voltage, transforming the primary AC voltage into a secondary AC voltage with a transformation ratio, and generating a DC output voltage from the secondary AC voltage, wherein the DC converter has a dual active bridge topology with a series resonant circuit for each AC voltage phase. According to the invention, when the ratio of the DC output voltage to the DC input voltage is greater than the transformation ratio, the phase offset between the primary AC voltage and the series resonant circuit current corresponding thereto is adjusted to zero by changing the clock frequency for clocking the AC voltage, and when the ratio of the DC output voltage to the DC input voltage is less than the transformation ratio, the phase offset between the secondary AC voltage and the series resonant circuit current corresponding thereto is adjusted to zero by changing the clock frequency.

[0009] The core concept of the present invention is that the phase offset between the primary or secondary AC voltage of the DC converter and the corresponding series resonant circuit current is adjusted to zero by changing the clock frequency of the AC voltage. Here, whether the phase offset between the primary or secondary AC voltage and the series resonant circuit current is adjusted to zero depends on whether the ratio of the DC output voltage to the DC input voltage is greater than or less than the transformation ratio of the transformer unit. By adjusting, on the one hand, the conduction loss is reduced because the reactive power flowing into the series resonant circuit by the primary side or the secondary side is reduced. In addition, when the bridge is connected, the switching loss is significantly reduced because when the current through the corresponding series resonant circuit is zero or at least almost zero (so-called zero current switch), the bridge of the primary side or the secondary side is just switched. In the case of not adjusting the phase offset between the primary or secondary AC voltage and the series resonant circuit current in this way, the larger deviation of the ratio of the DC output voltage to the DC input voltage and the transformation ratio of the transformer unit leads to the phase offset between the primary or secondary AC voltage and the series resonant circuit current, which improves the conduction loss and the switching loss and reduces the efficiency of the DC converter.

[0010] Here, as a phase offset between the primary or secondary AC voltage and the series resonant circuit current corresponding thereto, the average value of the phase offset between each AC voltage phase and the current of the series resonant circuit flowing through the AC voltage phase is adjusted. Therefore, when the ratio of the DC output voltage to the DC input voltage is greater than the transformation ratio, the average value of the phase offset between each AC voltage phase of the primary AC voltage and the current of the series resonant circuit flowing through the AC voltage phase is adjusted to zero, and when the ratio of the DC output voltage to the DC input voltage is less than the transformation ratio, the average value of the phase offset between each AC voltage phase of the secondary AC voltage and the current of the series resonant circuit flowing through the AC voltage phase is adjusted to zero. Therefore, the average value of the phase offset between each AC voltage phase of the primary or secondary AC voltage and the series resonant circuit current corresponding thereto is determined and the average value is adjusted to zero. This can advantageously compensate for measurement errors when determining each phase offset and small fluctuations in the phase relationship between different AC current phases.

[0011] A design scheme of the present invention proposes that by changing the phase offset between the primary AC voltage and the secondary AC voltage, the power transmitted by the DC converter is adjusted to the power rated value or the DC output voltage is adjusted to the voltage rated value. This design scheme of the present invention proposes a dual parameter control system of the DC converter, in which the phase offset between the primary AC voltage and the secondary AC voltage is also changed in addition to the clock frequency. Thus, the power or DC output voltage of the DC converter can be adjusted with optimized efficiency. Here, "the phase offset between the primary AC voltage and the secondary AC voltage" is understood as the phase offset between the AC voltage phase of the primary AC voltage and the AC voltage phase corresponding to it of the secondary AC voltage, or is understood as the average value of the phase offset between each AC voltage phase of the primary AC voltage and the AC voltage phase corresponding to it of the secondary AC voltage.

[0012] The DC converter according to the invention for generating a multi-phase primary AC voltage from a DC input voltage, transforming the primary AC voltage into a secondary AC voltage with a transformation ratio, and generating a DC output voltage from the secondary AC voltage comprises: a primary-side voltage bridge for each AC voltage phase, which has a plurality of semiconductor switches for generating a primary AC voltage from the DC input voltage; a transformer unit for transforming the primary AC voltage into a secondary AC voltage; a secondary-side voltage bridge for each AC voltage phase, which has a plurality of semiconductor switches for generating a DC output voltage from the secondary AC voltage; a series resonant circuit for each AC voltage phase; and a measuring device arrangement. The measuring device arrangement is designed to detect the DC input voltage, the DC output voltage, the time variation curves of the multiple AC voltage phases of the primary AC voltage, the time variation curves of the AC voltage phases corresponding thereto of the secondary AC voltage, and the time variation curve of the current flowing through the series resonant circuit of the AC voltage phase. In addition, the DC converter includes a regulating unit, which is designed to, when the ratio of the DC output voltage to the DC input voltage is greater than the transformation ratio, adjust the average value of the phase offset between each of the AC voltage phases of the primary AC voltage and the current flowing through the series resonant circuit of the AC voltage phase to zero by changing the clock frequency for timing the AC voltage, and when the ratio of the DC output voltage to the DC input voltage is less than the transformation ratio, adjust the average value of the phase offset between each of the AC voltage phases of the secondary AC voltage and the current flowing through the series resonant circuit of the AC voltage phase to zero by changing the clock frequency.

[0013] The DC converter according to the invention enables the implementation of the method according to the invention. The advantages of the DC converter according to the invention correspond to the above-mentioned advantages of the method according to the invention and are not repeated here.

[0014] In one embodiment of the DC converter according to the present invention, each primary-side voltage bridge has a half-bridge, in whose bridge arms semiconductor switches are respectively arranged and whose bridge branches are connected to the primary winding of the transformer unit, and each secondary-side voltage bridge has a half-bridge, in whose bridge arms semiconductor switches are respectively arranged and whose bridge branches are connected to the secondary winding of the transformer unit. Compared with a voltage bridge configured as a full bridge, a voltage bridge configured as a half-bridge advantageously reduces the number of semiconductor switches and thus reduces the space requirement and material costs for the voltage bridge.

[0015] In another embodiment of the DC converter according to the present invention, the transformer unit is constructed as a multi-phase transformer or has a transformer for each AC voltage phase. Compared to the embodiment with individual transformers (which can also be realized), the multi-phase transformer embodiment of the transformer unit simplifies the construction of the DC converter.

[0016] In another embodiment of the DC converter according to the invention, the regulating unit has at least one first phase detector for determining the phase offset between the AC voltage phase of the primary AC voltage and the current of the series resonant circuit flowing through the AC voltage phase and a second phase detector for determining the phase offset between the AC voltage phase of the secondary AC voltage and the current of the series resonant circuit flowing through the AC voltage phase. The use of different phase detectors for the AC voltage phases of the primary and secondary AC voltages can advantageously achieve a fast switch between the determination of the phase offset between the primary AC voltage phase and the corresponding series resonant circuit current and the determination of the phase offset between the secondary AC voltage phase and the corresponding series resonant circuit current when the ratio of the DC output voltage to the DC input voltage changes.

[0017] In another embodiment of the DC converter according to the invention, the regulating unit is designed to regulate the power delivered by the DC converter to a rated power value or the DC output voltage to a rated voltage value by changing a phase offset between the primary AC voltage and the secondary AC voltage.

[0018] In another embodiment of the DC converter according to the invention, each series resonant circuit is arranged on the secondary side or the primary side of the transformer unit, or the components of each series resonant circuit are distributed on the secondary side and the primary side of the transformer unit. It is not important for the present invention whether the series resonant circuit is constructed on the secondary side or the primary side of the transformer unit and the components of the series resonant circuit are distributed, that is, the present invention can be advantageously applied to various embodiments of the series resonant circuit, because the transformer unit basically only plays the role of impedance change for the current of the series resonant circuit. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The above characteristics, features and advantages of the present invention and the manner and method of achieving them will be more clearly understood in conjunction with the following detailed description of the embodiments with reference to the accompanying drawings. Herein, it is shown:

[0020] Figure 1 is a circuit diagram of a three-phase DC converter having a dual active bridge topology and a series resonant circuit for each AC voltage phase,

[0021] Figure 2 It is a regulating circuit used to regulate the DC converter.

[0022] Figure 3 is a block diagram of a clock frequency regulator for regulating the clock frequency of a DC converter,

[0023] Figure 4is a time variation curve of the AC voltage phase of the primary AC voltage, the AC voltage phase of the secondary AC voltage corresponding thereto, and the current flowing through the series resonant circuit of the AC voltage phase without changing the clock frequency of the AC voltage,

[0024] Figure 5 It is the time variation curve of the AC voltage phase of the primary AC voltage, the AC voltage phase of the secondary AC voltage corresponding thereto, and the current flowing through the series resonant circuit of the AC voltage phase when the clock frequency of the AC voltage is changed.

[0025] Corresponding parts are marked with the same reference numerals in the drawings. DETAILED DESCRIPTION

[0026] Figure 1 shows the DC input voltage U in Converted into DC output voltage U out The circuit diagram of the three-phase DC converter 1 is shown in FIG. The DC converter 1 includes a circuit for converting a DC input voltage U in A first converter unit 3 for generating a primary AC voltage, a transformer unit 5 for converting the primary AC voltage into a secondary AC voltage, and a transformer unit 5 for generating a DC output voltage U from the secondary AC voltage out A second converter unit 7 and a series resonant circuit 9 for each AC voltage phase.

[0027] The DC converter 1 has a dual active bridge topology. The first converter unit 3 provides a DC voltage phase U of the primary AC voltage. P1 , U P2 , U P3 Each has a primary-side voltage bridge 11 . Each primary-side voltage bridge 11 is designed as a half bridge, in whose bridge arms a semiconductor switch 13 and a freewheeling diode 15 connected in parallel with the semiconductor switch 13 are arranged, and whose bridge branches are connected to a primary winding 17 of the transformer unit 5 .

[0028] The second converter unit 5 provides a voltage for each AC voltage phase U of the secondary AC voltage. S1 , U S2 , U S3 Each secondary-side voltage bridge 19 is designed as a half-bridge, in whose bridge arms a semiconductor switch 13 and a freewheeling diode 15 connected in parallel are arranged, and whose bridge branches are connected to the secondary winding 21 of the transformer unit 5 via the series resonant circuit 9 .

[0029] Each series resonant circuit 9 has a series circuit of a capacitor 23 and a coil 25 .

[0030] The transformer unit 5 is designed as a three-phase transformer.

[0031] Furthermore, the DC converter 1 has a measuring device arrangement 27 and a regulating unit 29, which Figure 1 Not shown (see Figure 2 The measuring device arrangement 27 has a plurality of measuring devices for detecting the DC input voltage U in , DC output voltage U out , the power P delivered by the DC converter 1, the AC voltage phase U of the primary AC voltage P1 , U P2 , U P3 The time variation curve of the secondary AC voltage phase U S1 , U S2 , U S3 and the time variation curve of the currents I1, I2, I3 flowing through the series resonant circuit 9 of the AC voltage phase.

[0032] The regulating unit 29 is designed to, when the DC output voltage U out With DC input voltage U in When the ratio is greater than the transformation ratio n of the transformer unit 5, the respective AC voltage phases U of the primary AC voltage are changed by changing the clock frequency f for timing the AC voltage. P1 , U P2 , U P3 The AC voltage U P1 , U P2 , U P3 The average value of the phase shift between the currents I1, I2, and I3 of the series resonant circuit 9 is adjusted to zero, and when the DC output voltage U out With DC input voltage U in When the ratio is less than the transformation ratio n, the secondary AC voltage phases U S1 , U S2 , U S3 The AC voltage U S1 , U S2 , U S3 The average value of the phase shift between the currents I1, I2, I3 of the series resonant circuit 9 is adjusted to zero.

[0033] Furthermore, the control unit 29 is designed to control the power P delivered by the DC converter 1 to a rated power value P by changing the phase shift between the primary AC voltage and the secondary AC voltage. S .

[0034] Figure 2 A method for operating the method according to the present invention is shown Figure 1The control circuit of the DC converter 1 is shown. The control circuit has a control unit 29 , a control device 31 for controlling the semiconductor switch 13 , the DC converter 1 and a measuring device arrangement 27 .

[0035] The regulating unit 29 includes a clock frequency regulator 33 , a voltage controlled oscillator 35 (VCO=Voltage Controlled Oscillator) and a phase shift regulator 37 .

[0036] The DC input voltage U is detected by means of the measuring device arrangement 27 in , DC output voltage U out , the power P delivered by the DC converter 1, the AC voltage phase U of the primary AC voltage P1 , U P2 , U P3 The time variation curve of the secondary AC voltage phase U S1 , U S2 , U S3 and the time variation of currents I1, I2, I3 flowing through the series resonant circuit 9.

[0037] The measuring device arrangement 27 supplies a DC input voltage U 20 to the clock frequency regulator 33 . in , DC output voltage U out , the AC voltage phase U of the primary AC voltage P1 , U P2 , U P3 The time variation curve of the secondary AC voltage phase U S1 , U S2 , U S3 The time variation curve of the current I1, I2, I3 flowing through the series resonant circuit 9. The clock frequency regulator 33 is based on Figure 3 The clock frequency f is determined in a manner described in detail and supplied to the voltage-controlled oscillator 35 .

[0038] The voltage-controlled oscillator 35 generates a square-wave voltage with a clock frequency f, which serves as a clock signal S for the control device 31. f .

[0039] The power P of the DC converter 1 transmitted by the measuring device arrangement 27 is compared with the power rated value P S The deviation is transmitted to the phase offset regulator 37. The phase offset regulator 37 converts the phase offset signal S P The power P is transmitted to the control device 31, and the power P is adjusted to the power rated value P by changing the phase offset between the primary AC voltage and the secondary AC voltage according to the phase offset signal. S The phase shift controller 37 is designed as a PI controller, for example.

[0040] The driving device 31 drives the semiconductor switches 13 of the DC converter 1. For example, each semiconductor switch 13 is an insulated-gate bipolar transistor (IGBT) or a metal-oxide-semiconductor field-effect transistor (MOSFET), and the driving device 31 has a gate driver for each semiconductor switch 13, and transmits a clock signal S to the gate driver. f and phase shift signal S P .

[0041] Figure 3 A block diagram of a clock frequency regulator 33 is shown, wherein the primary AC voltage phase U P1 Evaluation of the corresponding secondary AC voltage phase U S1 and the evaluation of the current I1 flowing through the corresponding series resonant circuit 9. For each parameter U P1 , U S1 , I1 are all continuously measured with digital elements 39 to 41 as digital symbol signals, which illustrate the parameter U P1 , U S1 , the instantaneous sign of I1. Each digitizing element 39 to 41 is designed as a Schmitt trigger, for example.

[0042] The primary AC voltage phase U P1 The sign signal determined with the current I1 is supplied to the first digital phase detector 43, which derives therefrom the primary AC voltage phase U P1 The phase shift between the current I1 and the

[0043] The secondary AC voltage phase U S1 The sign signal determined with the current I1 is supplied to the second digital phase detector 44, which derives therefrom the secondary AC voltage phase U S1 The phase shift between the current I1 and the

[0044] The phase detectors 43 and 44 each have an exclusive OR gate, for example.

[0045] The output signal of the first phase detector 43 and the inverted output signal of the second phase detector 44 are supplied to a selection element 45 which is controlled by a comparison element 47. The comparison element 47 generates a DC output voltage U out With DC input voltage U in The ratio U out / U in And compare this ratio with the transformation ratio n. out / U inWhen the ratio U is greater than the transformation ratio n, the output signal of the first phase detector 43 is forwarded to the averaging element 49 by the selection element 45. out / U in When the transformation ratio n is less than, the inverted output signal of the second phase detector 44 is forwarded by the selection element 45 to the averaging element 49. The output signal of the second phase detector 44 is inverted so as to adjust the clock frequency f to the value of U out / U in If <n, change to out / U in >n in different directions.

[0046] Correspondingly, for the other two primary AC voltage phases U P2 , U P3 , the corresponding secondary AC voltage phase U S2 , U S3 and the currents I2 and I3 flowing through the corresponding series resonant circuit 9 are evaluated, so that when the ratio U out / U in When the transformation ratio is greater than n, for each AC voltage phase of the DC converter 1, the primary AC voltage phase U P1 , U P2 , U P3 The phase shift between the currents I1, I2, I3 flowing through the corresponding series resonant circuit 9 is supplied to the averaging element 49, and when the ratio U out / U in When the transformation ratio is less than n, the secondary AC voltage phase U is transmitted as a reversal signal S1 , U S2 , U S3 and the phase shift between the currents I1 , I2 , and I3 flowing through the corresponding series resonant circuit 9 .

[0047] The averaging element 49 has a low-pass filter, with which the signal supplied to it is filtered and from the filtered signal a mean value is formed, which is sent to the regulating element 51 .

[0048] The control element 51 derives the clock frequency f for controlling the semiconductor switch 13 from the signal supplied to it by the averaging element 49. The control element 51 is designed, for example, as a PI controller.

[0049] Figure 4 and 5 Shows that the ratio U out / U in When the transformation ratio is greater than n, the present invention applies the primary AC voltage phase U P1 , the corresponding secondary AC voltage phase U S1 The influence of the time t-dependent variation curve of the current I1 flowing through the corresponding series resonant circuit 9.

[0050] Figure 4 The current I1 flowing through the series resonant circuit 9 leads the primary AC voltage phase U P1 and the secondary AC voltage phase U S1 This leads to an increase in conduction losses, since reactive power flows from the primary side and the secondary side into the series resonant circuit 9. In addition, the primary AC voltage phase U P1 The primary side voltage bridge 11 and the secondary AC voltage phase U S1 High switching losses occur in the secondary-side voltage bridge 19 of the embodiment, since the instantaneous value of the current I1 is at the AC voltage phase U P1 , U S1 All time points where a sign change occurs are relatively large.

[0051] Figure 5 The time curve of the control according to the invention is shown, wherein U is changed by changing the clock frequency f. P1 The phase offset between I1 and I2 is adjusted to zero. Figure 4 In the case shown, the primary AC voltage phase U P1 The conduction losses of the primary-side voltage bridge 11 are reduced, since no reactive power flows from the voltage bridge 11 into the corresponding series resonant circuit 9. In addition, the switching losses are significantly reduced, since the primary-side voltage bridge 11 is switched exactly when the current I1 is zero (so-called zero current switching). This improves the efficiency. S1 Although the secondary side voltage bridge 19 still causes a certain amount of power loss, it is Figure 4 Compared to the situation shown, the power loss is lower because the maximum value of the current I1 and the instantaneous value of the current I1 at the switching time of the voltage bridge 19 are lower.

[0052] The DC converter 1 according to the invention and the method according to the invention are based on Figures 1 to 5 The described embodiment can be modified in various ways to other embodiments. For example, the DC converter 1 can have a number of AC voltage phases other than three. In particular, it can also be implemented as a single-phase. In the case of a single-phase, the DC converter 1 has, for example, a primary-side voltage bridge 11 and a secondary-side voltage bridge 19, which are each constructed as a full bridge. In addition, in the case of a multi-phase DC converter 1, it is proposed that when the ratio U out / U in When the transformation ratio is greater than n, only the primary AC voltage phase U P1 , U P2 , U P3 The phase shift between one of the currents I1, I2, I3 of the corresponding series resonant circuit 9 and adjusts it to zero, or when the ratio U out / U inWhen the transformation ratio is less than n, the secondary AC voltage phase U S1 , U S2 , U S3 Instead of detecting the phase shift between one of the currents I1, I2, I3 of the corresponding series resonant circuit 9 and adjusting it to zero, the entire primary AC voltage phase U P1 , U P2 , U P3 , secondary AC voltage phase U S1 , U S2 , U S3 and currents I1, I2, I3 and find the average value of the phase offset. In addition, in the case of a multi-phase DC converter 1, it can be proposed that the transformer unit 5 has a transformer for each AC voltage phase instead of a multi-phase transformer. In addition, an analog phase detector can be provided instead of the digital phase detectors 43, 44. In addition, it can be proposed that the regulating unit 29 is designed to adjust the DC output voltage U by changing the phase offset between the primary AC voltage and the secondary AC voltage. out Instead of regulating the transmitted power to the power rating P, the voltage is regulated to the rated value. S .

[0053] Although the present invention has been illustrated and described in detail through preferred embodiments, the present invention is not limited to the disclosed examples, and other variations can be derived by those skilled in the art without departing from the scope of protection of the present invention.

Claims

1. A method for operating a DC converter (1) for converting a DC input voltage (U in ) generates a multi-phase primary AC voltage, transforms the primary AC voltage into a secondary AC voltage with a transformation ratio (n), and generates a DC output voltage (U out ),in, The DC converter (1) has a dual active bridge topology, which has a series resonant circuit (9) for each AC voltage phase, and is characterized in that: - When the DC output voltage (U out ) and the DC input voltage (U in ) is greater than the transformation ratio (n), the average value of the phase shift between each AC voltage phase of the primary AC voltage and the current (I1, I2, I3) of the series resonant circuit (9) flowing through the AC voltage phase is adjusted to zero by changing the clock frequency (f) for timing the AC voltage, - When the DC output voltage (U out ) and the DC input voltage (U in ) is less than the transformation ratio (n), the average value of the phase shift between each AC voltage phase of the secondary AC voltage and the current (I1, I2, I3) of the series resonant circuit (9) flowing through the AC voltage phase is adjusted to zero by changing the clock frequency (f).

2. The method according to claim 1, characterized in that: By changing the phase shift between the primary AC voltage and the secondary AC voltage, the power (P) delivered by the DC converter (1) is adjusted to a rated power value (P S ) or the DC output voltage (U out ) is adjusted to the voltage rating.

3. A DC converter (1) for converting a DC input voltage (U in ) generates a multi-phase primary AC voltage, transforms the primary AC voltage into a secondary AC voltage at a transformation ratio (n), and generates a DC output voltage (U out ), the DC converter (1) comprises: a primary-side voltage bridge (11) having a plurality of semiconductor switches (13) for each AC voltage phase, the primary-side voltage bridge (11) being used to connect the DC input voltage (U in ) generates the primary AC voltage, - a transformer unit (5), configured to transform the primary AC voltage into the secondary AC voltage, a secondary-side voltage bridge (19) having a plurality of semiconductor switches (13) for each AC voltage phase, the secondary-side voltage bridge (19) being used to generate the DC output voltage (U out ), - a series resonant circuit (9) for each AC voltage phase, a measuring device arrangement (27) designed to detect the DC input voltage (U in ), the DC output voltage (U out ), time variation curves of a plurality of AC voltage phases of the primary AC voltage, time variation curves of an AC voltage phase of the secondary AC voltage corresponding to the AC voltage, and time variation curves of currents (I1, I2, I3) of the series resonant circuit (9) flowing through the AC voltage phases, -characterized in that the DC converter (1) further comprises a regulating unit (29), wherein the regulating unit (29) is designed to, when the DC output voltage (U out ) and the DC input voltage (U in ) is greater than the transformation ratio (n), the average value of the phase shift between each of the AC voltage phases of the primary AC voltage and the current (I1, I2, I3) of the series resonant circuit (9) flowing through the AC voltage phase is adjusted to zero by changing the clock frequency (f) for timing the AC voltage, and - When the DC output voltage (U out ) and the DC input voltage (U in ) is less than the transformation ratio (n), the average value of the phase shift between each of the AC voltage phases of the secondary AC voltage and the current (I1, I2, I3) of the series resonant circuit (9) flowing through the AC voltage phase is adjusted to zero by changing the clock frequency (f).

4. The DC converter (1) according to claim 3, characterized in that: Each primary-side voltage bridge (11) has a half-bridge, in the bridge arms of which the semiconductor switches (13) are respectively arranged and the bridge branches of the half-bridge are connected to the primary winding (17) of the transformer unit (5), and each secondary-side voltage bridge (19) has a half-bridge, in the bridge arms of which the semiconductor switches (13) are respectively arranged and the bridge branches of the half-bridge are connected to the secondary winding (21) of the transformer unit (5).

5. The DC converter (1) according to claim 3 or 4, characterized in that: The transformer unit (5) is designed as a multi-phase transformer or has a transformer for each AC voltage phase.

6. The DC converter (1) according to claim 3 or 4, characterized in that: The regulating unit (29) has at least one first phase detector (43) and a second phase detector (44), wherein the first phase detector (43) is used to determine a phase offset between the AC voltage phase of the primary AC voltage and the current (I1, I2, I3) of the series resonant circuit (9) flowing through the AC voltage phase, and the second phase detector (44) is used to determine a phase offset between the AC voltage phase of the secondary AC voltage and the current (I1, I2, I3) of the series resonant circuit (9) flowing through the AC voltage phase.

7. The DC converter (1) according to claim 3 or 4, characterized in that: The regulating unit (29) is designed to regulate the power (P) delivered by the DC converter (1) to a rated power value (P) by changing the phase shift between the primary AC voltage and the secondary AC voltage. S ) or make the DC output voltage (U out ) is adjusted to the voltage rating.

8. The DC converter (1) according to claim 3 or 4, characterized in that: Each series resonant circuit (9) is arranged on the secondary side or the primary side of the transformer unit (5), or components of each series resonant circuit (9) are distributed on the secondary side and the primary side of the transformer unit (5).

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