Galvanically coupled DC / DC converter and vehicle electrical system

Abstract

A galvanically coupled DC-DC converter with a first side (A1) and a second side (A2), wherein the first side (A1) has a first potential (U+) and a second potential (U-), wherein the DC-DC converter has a first, second and third transistor (S1, S2, S3) which are connected in series via a first and a second junction point (VP1, VP2) and are connected to the potentials (U+, U-) of the first side (A1), wherein a working inductor (L1, L1') is connected to each of the two junction points (VP1, VP2) and the working inductors (L1, L1') are each connected between one of the junction points (VP1, VP2) and one of two potentials (V+, V-) of the second side (A2) of the DC-DC converter, wherein the DC-DC converter is galvanically isolated from a reference potential (M) by means of an insulation that exists between the potentials (U+, U-, V+, V-) of the DC-DC converter and the reference potential (M), wherein the DC-DC converter has a control unit (C) which is connected to the transistors (S1 - S3) and is configured to simultaneously drive the first and third transistors (S1, S3) into an ON state in a converter state and wherein the control (C) is configured to permanently drive the first transistor (S1) in an ON state and to alternately drive the third transistor (S3) and the second transistor (S2) in an ON state in the event of an insulation fault present in the insulation between the first potential (U+) of the first side (A1) and the reference potential (M), and / or wherein the control (C) is configured to permanently drive the third transistor (S3) in an ON state and to alternately drive the second transistor (S2) and the first transistor (S1) in an ON state in the event of an insulation fault present in the insulation between the second potential (U-) of the first side (A1) and the reference potential (M).

Description

[0001] Electric vehicles have an electrical system that uses high voltages, such as 400 V or 800 V, to support high power outputs. In some cases, voltages of 400 V are desirable, for example, for AC charging or driving, while other applications require higher voltages, such as 800 V, for example, when charging with DC or when particularly high power output is needed during driving. Furthermore, some high-voltage components, such as electric heaters, may have a nominal voltage of 400 V, while other components, such as the traction battery, charging electronics, and / or the electric drive system, may have a different nominal high-voltage voltage, such as 800 V.

[0002] To avoid having to equip each of the on-board power supply branches with different nominal voltages with its own battery, DC voltage converters are used to couple on-board power supply branches with different nominal voltages together for power transmission.

[0003] Document US 2017 / 0085170A1 describes a bidirectional converter, for example, for a solar power system. The converter has four series transistors connected in series, with this series connection being connected to a voltage source. The middle connection point of the series connection is connected to a neutral conductor, and the voltage between the remaining connection points is tapped via inductors. When the outer transistors are switched on, the inner transistors are switched on, and vice versa, to achieve voltage symmetry with respect to the neutral conductor through this alternating, synchronous switching. In one embodiment, the two middle transistors are replaced by a single transistor element; the neutral conductor is then not connected to the transistors, but only to a connection point of a series connection of two DC link capacitors to ensure voltage symmetry.

[0004] Document US 7,292,462 B2 describes a similar DC-DC converter with two external switches connected via a series connection of two internal switches. A supply voltage is applied to the two connection points of the internal switches with the external switches. The connection point of the internal switches is connected via an inductor to a connection point of a series connection of two output-side capacitors, the outer terminals of which are connected to the outer ends of the external switches. One internal switch is operated alternately with the other internal switch (or the opposite external switch).

[0005] German patent application DE 10 2014 203 157 A1 describes a two-part symmetrical converter with a grounded center potential. An input voltage is applied via an inductor to two mutually symmetrical T-circuits, each containing switchable inductors in its longitudinal path and a capacitor in its transverse path. The T-circuits are interconnected via their transverse paths, with the connection point being grounded.

[0006] It is an object of the invention to demonstrate a way in which the aforementioned coupling circuits can be implemented in a simple manner.

[0007] This task is accomplished by the DC-DC converter and the vehicle electrical system according to the independent claims. Further properties, features, embodiments, and advantages will become apparent from the dependent claims, the figure, and the description.

[0008] It is proposed to use a galvanically coupled DC-DC converter with three transistors connected in series. The transistors are connected in series, thus forming a series circuit of three switches, unlike conventional half-bridges with two switches. The operating inductors are symmetrically distributed around the middle transistor, namely the second transistor, and connected to the series circuit of the transistors. Due to this symmetrical arrangement, the DC-DC converter can generate a voltage that is symmetrical with respect to the input voltage. In particular, no output voltage potential corresponds to an input potential. This allows for a shift of the output voltage relative to the input voltage, which can be adjusted by controlling the transistors.

[0009] A galvanically coupled DC-DC converter is therefore proposed, comprising a first side and a second side. The first side can be considered the input side and the second side the output side. The first side preferably includes (or can be considered equivalent to) a two-pole connection, and the second side likewise preferably includes (or can be considered equivalent to) a two-pole connection. Thus, the first side has two potentials, and the second side also has two potentials.

[0010] The DC-DC converter comprises three transistors: a first, a second, and a third. These transistors are connected in series via two junctions. The first junction is between the first and second transistors, and the second junction is between the second and third transistors. The ends of the series connection are connected to the two potentials of the first terminal. A load inductor is connected to each of the two junctions. The load inductors are connected in series to each junction, meaning only one end of each load inductor is connected to each junction. The load inductors are connected to different junctions.The operating inductors are each connected between one of the connection points and one of two potentials on the second side of the DC-DC converter. The operating inductors are thus connected in series between the second side (i.e., the second terminal) of the DC-DC converter and the connection points. The first operating inductor is connected in series between the first connection point and one of the two potentials on the second side of the DC-DC converter. The second operating inductor is connected in series between the second connection point and the second potential on the second side of the DC-DC converter. The operating inductors connect different operating points to different potentials on the second side of the DC-DC converter. The operating inductors are therefore connected, or arranged, symmetrically with respect to the second transistor.

[0011] The working inductors are preferably connected in series between the associated connection points and the second side. The connection between the connection points on the one hand and the potentials of the second side of the DC-DC converter on the other hand can be direct, or preferably further includes a filter element (as a series element). The second side can thus be directly connected to those ends of the working inductor that are opposite the connection points. Preferably, however, a common-mode choke is provided between the working inductors and the second side (i.e., the two potentials of the second side). The second side can thus be connected via a common-mode choke to those ends of the working inductor that are opposite the connection points.The common-mode choke can have two (preferably mutually symmetrical) inductors, one inductor being located between the first operating inductor and the first potential of the second side, and a second inductor being located between the second operating inductor and the second potential of the second side. The inductors are preferably magnetically coupled, in particular by being wound around the same magnetic core.

[0012] The first side, the second side, or both sides can each have an intermediate circuit capacitor. This capacitor connects the potentials of the respective side in parallel. A first intermediate circuit capacitor can be connected between the potentials of the first side. Alternatively, or in combination with this, an intermediate circuit capacitor can connect the ends of the load inductors that are opposite the transistors. In particular, an intermediate circuit capacitor can be provided between the load inductors and the common-mode choke. Furthermore, alternatively or in combination with this, an intermediate circuit capacitor can be provided at the potentials of the second side of the DC-DC converter. This can be the case, in particular, if a common-mode choke is provided between the load inductors and the potentials of the second side.Here, one or more intermediate circuit capacitors can be considered part of a filter circuit or filter element, which also includes the common-mode choke. Instead of a (two-part) common-mode choke, a single filter choke can also be used.

[0013] A control circuit is provided that is connected to the transistors for control purposes. The control circuit is preferably configured to prevent all three transistors from being in the ON state simultaneously. This avoids a bridge short circuit. The control circuit is configured to simultaneously drive the first and third transistors to an ON state in a converter state. Furthermore, the control circuit is preferably configured to simultaneously drive the first and third transistors to an OFF state, or to keep the first and third transistors in the same state. In particular, the control circuit is configured to drive the transistors in a clocked manner, preferably with an adjustable duty cycle. This allows the first and third transistors to connect the operating inductors (in the ON state) to the potentials of the first side, or to disconnect them (in the OFF state).

[0014] Furthermore, the control system can be configured to alternately switch the second transistor on and the first and third transistors into an ON state in a converter state, particularly in the aforementioned converter state. The control system is specifically configured to alternately switch the second transistor on and the third and first transistors off. In particular, the control system can be configured to switch the second transistor in the converter state with a switching state that is complementary to the switching states of the first and third transistors. The first and third transistors preferably have the same switching state in the converter state. The control system can switch the second transistor on and off according to a predetermined duty cycle.In the converter state, the operating inductors can thus be connected to the two potentials of the first side via the first and third transistors, then disconnected, whereupon the third transistor is closed to connect the two operating inductors together. The second transistor is then opened again (and the first transistors are closed as described above). This is preferably carried out in a clocked manner, particularly according to a predetermined duty cycle, in order to convert the voltage applied to the first side into a voltage that is applied to the second side of the converter, or that is applied to the ends of the operating inductors facing away from the transistors. If a filter circuit is provided between the operating inductors and the second side, it can filter the converted voltage and output it on the second side.

[0015] The converter state corresponds to a state of the DC-DC converter in which, according to normal operation, it converts a voltage, specifically the voltage on the first side, to output the converted voltage on the second side. The aforementioned converter state is, in particular, a state of the converter that exhibits no insulation fault.

[0016] The DC-DC converter is galvanically isolated from a reference potential such as ground or the vehicle chassis. Insulation exists between the potentials of the DC-DC converter, particularly those of the first side, and the respective reference potential. If this insulation is defective, i.e., if the insulation resistance between a potential of the DC-DC converter and the reference potential falls below a predefined threshold, then an insulation fault occurs. The control system is configured, in the event of an insulation fault between the first potential of the first side and a reference potential (ground or vehicle chassis), to permanently switch the first transistor on, and to alternately switch the third and second transistors on. This corresponds to a converter state with an insulation fault (as opposed to the previously mentioned converter state without an insulation fault).Alternatively, or in combination with this, the control circuit is configured to permanently switch the third transistor to an ON state in the event of an insulation fault between the second potential of the first side and a reference potential, and to alternately switch the second and first transistors to an ON state. This allows voltage conversion despite the insulation fault, whereby, due to the permanently ON transistor, there is no voltage between the relevant potential of the first side and the corresponding load inductor. When the first transistor is permanently ON, the first connection point, and thus the corresponding end of the load inductor, is at the first potential of the first side. When the third transistor is off, the second connection point, or the end of the second load inductor connected to it, is at the second potential of the first side.It can be provided that, in the event of insulation faults between the first potential and the second potential relative to the reference potential, the control system alternately controls a second transistor on the one hand and the first and third transistors on the other hand (in an ON state and an OFF state) for voltage conversion.

[0017] The first and third transistors are preferably connected symmetrically to the second transistor. The first and third transistors are preferably identical (in particular with the same current-carrying capacity and switching characteristics). Preferably, the first, second, and third transistors are identical, especially with regard to current-carrying capacity and switching characteristics. Furthermore, the operating inductors are preferably connected symmetrically to the second transistor. The operating inductors are also preferably identical, in particular with the same inductance value, and preferably also with the same DC resistance or current-carrying capacity.

[0018] The DC-DC converter described here is specifically a DC-DC converter used in a vehicle, particularly within a vehicle's electrical system. It can therefore be considered a vehicle-integrated DC-DC converter. In particular, the DC-DC converter can be part of a vehicle's charging circuit.

[0019] Furthermore, a vehicle electrical system with at least two branches is described. These branches have different nominal voltages, for example, approximately 400 V and approximately 800 V. The vehicle electrical system includes at least one DC-DC converter, as described herein. This converter connects the two electrical systems with the different nominal voltages by converting the voltage. A first electrical system with a first nominal voltage can have two potentials connected to the two potentials of the first side of the DC-DC converter. A second electrical system with a different nominal voltage can have two further potentials connected to the two potentials of the second side. The DC-DC converter described herein is preferably a power DC-DC converter. Preferably, the DC-DC converter has a conversion power of at least 5 kW, 50 kW, or 200 kW.The DC-DC converter is a high-voltage DC-DC converter, wherein both sides are preferably designed for voltages of more than 60 V, of at least 100 V, 200 V, 400 V or 800 V. The Fig. Figure 1 serves to further explain the embodiment described here.

[0020] The Fig. Figure 1 shows a DC-DC converter with a first side A1 and a second side A2. The first side A1 has a first potential U+ and a second potential U-. A DC link capacitor is provided between these potentials, comprising two capacitors C1' and C1. These serve to stabilize the voltage on side A1.

[0021] Between the potentials U+ and U- of the first side A1, three transistors S1 to S3 are connected in series. This results in a first junction point VP1 between the first transistor S1 and the second transistor S2, and a second junction point VP2 between the second transistor S2 and the third transistor S3. Operating inductors L1 and L1' are connected to these junction points. One end of operating inductor L1 is connected to junction point VP1, and one end of operating inductor L1' is connected to junction point VP2. Thus, the two operating inductors L1 and L1' extend in series from junction points VP1 and VP2. The operating inductors and the transistors are located between the sides of the DC-DC converter.

[0022] A filter circuit is connected to the ends of the working inductors L1, L1', which are not connected to the connection points VP1, VP2, and leads to the second side A2. This filter circuit includes a common-mode choke L2, L2', which has two windings magnetically connected via a common core. A DC link capacitor T3 is connected between the common-mode choke L2, L2' and the working inductors L1, L1', and is charged via the working inductors L1, L1'.

[0023] The DC choke L2, L2' is connected to an intermediate circuit capacitor C2, C2', which is formed by two capacitors. These stabilize the voltage between the first potential V+ and the second potential V- of the second side A2 of the DC-DC converter.

[0024] Between the working inductances L1, L1' and the second side A2 there is a filter circuit, to which the common-mode choke L2, L2' belongs and the capacitances C3 can be added.

[0025] A controller C, as symbolically represented, controls transistors S1 to S3. In converter mode, these are controlled such that transistor S2, on the one hand, and transistors S1 and S3, on the other, are alternately switched on and off. This converts the voltage UHV1 on the first side A1 into a voltage UHV2 on the second side A2 of the DC-DC converter.

[0026] The depicted DC-DC converter is installed in a vehicle that has a reference potential M, such as the vehicle chassis. This reference potential is galvanically isolated from the potentials U+, U-, V+, and V-. Therefore, a voltage UHV1P exists between the potential U+ and the reference potential M, and a voltage UHV1M exists between the potential U- and the reference potential M. Furthermore, a voltage UHV2P exists between the potential V+ and the reference potential M, and a voltage UHV2M exists between the potential V- and the reference potential M. If an insulation fault exists, i.e., the insulation resistance between the reference potential M and the potentials U+, U-, V+, or V- is not above a minimum value, then an insulation fault exists. The insulation fault can be detected by voltage measurement or by measuring the insulation resistance. If the insulation fault exists between U- and M, i.e., the voltage UHV1M is too low, then the DC-DC converter is not installed.If an insulation fault is indicated by an insufficient insulation resistance, the control unit is configured to keep switch S3 permanently closed, while switches S1 and S2 are alternately switched on and off to maintain the conversion. If an insulation fault exists between U+ and M, i.e., the voltage UHV1P is too low or indicates an insulation fault between U+ and M, then the control unit C keeps transistor S1 permanently closed and switches transistors S2 and S3 alternately on and off. This switching is preferably performed in a clocked manner, particularly according to a predetermined duty cycle.One embodiment provides that in the event of a double insulation fault (insulation resistance between U+ and M less than the minimum value and insulation resistance between U- and M less than the minimum value), transistor S2 is switched on and off alternately, while transistors S1 and S2 are switched on and off alternately. Alternatively, all switches are permanently opened if a double insulation fault is present.

[0027] In a vehicle electrical system, a first branch can be connected to A1 and a second branch to A2, with the two branches having different operating voltages or nominal voltages. The converter shown is used for voltage-converting power transmission and, in particular, for adapting the different voltage levels on side A1 and side A2.

Claims

[1] Galvanically coupled DC-DC converter with a first side (A1) and a second side (A2), wherein the first side (A1) has a first potential (U+) and a second potential (U-), wherein the DC-DC converter has a first, second and third transistor (S1, S2, S3) which are connected in series via a first and a second junction point (VP1, VP2) and are connected to the potentials (U+, U-) of the first side (A1), wherein a working inductor (L1, L1') is connected to each of the two junction points (VP1, VP2) and the working inductors (L1, L1') are each connected between one of the junction points (VP1, VP2) and one of two potentials (V+, V-) of the second side (A2) of the DC-DC converter, wherein the DC-DC converter is galvanically isolated from a reference potential (M) by means of an insulation that exists between the potentials (U+, U-, V+, V-) of the DC-DC converter and the reference potential (M), wherein the DC-DC converter has a control unit (C) which is connected to the transistors (S1 - S3) and is configured to simultaneously drive the first and third transistors (S1, S3) into an ON state in a converter state and wherein the control (C) is configured to permanently drive the first transistor (S1) in an ON state and to alternately drive the third transistor (S3) and the second transistor (S2) in an ON state in the event of an insulation fault present in the insulation between the first potential (U+) of the first side (A1) and the reference potential (M), and / or wherein the control (C) is configured to permanently drive the third transistor (S3) in an ON state and to alternately drive the second transistor (S2) and the first transistor (S1) in an ON state in the event of an insulation fault present in the insulation between the second potential (U-) of the first side (A1) and the reference potential (M). [2] DC voltage converter according to claim 1, wherein the working inductances (L1, L1') are each connected in series between the connection points (VP1, VP2) connected thereto and the second side (A2). [3] DC-DC converter according to claim 1 or 2, wherein the second side (A2) is connected directly or via a common-mode choke (L2, L2') to those ends of the working inductors (L1, L1') which are opposite the connection points (VP1, VP2). [4] DC-DC converter according to claim 1, 2 or 3, wherein the first and / or the second side (A1, A2) has an intermediate circuit capacitor (C1; C1', C2, C2') which is connected in parallel to the potentials of the respective side. [5] DC-DC converter according to any of the preceding claims, wherein a common-mode choke is connected to the working inductors (L1, L1') and a smoothing capacitor (C3) is connected between the working inductors (L1, L1') and the common-mode choke (L2, L2'). [6] DC voltage converter according to claim 1, wherein the control (C) is configured to alternately drive the second transistor (S2) on the one hand and the first and third transistors (S1, S3) on the other hand into an ON state in a converter state. [7] DC-DC converter according to one of the preceding claims, wherein the first and third transistors (S1, S3) are connected symmetrically to the second transistor (S2) and are of the same design, and wherein the operating inductances (L1, L1') are connected symmetrically to the second transistor (S2) and are of the same design. [8] Vehicle electrical system with at least two branch circuits having different nominal voltages, wherein the vehicle electrical system has at least one DC voltage converter according to one of the preceding claims, which connects the two branch circuits of different nominal voltages to each other in a voltage-converting manner.

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