Cascaded isolated DC / DC converter having two secondary sides with step-down converters for parallel or series operation

Abstract

The invention relates to a stationary DC voltage charging device for electrically driven vehicles, comprising a galvanically isolated DC / DC converter having a transformer for galvanic isolation, the transformer comprising a first and a second winding on the secondary side, a first and a second secondary-side rectifier circuit, a first and a second step-down converter which are connected to the respective rectifier circuits and each have a negative and positive output terminal, switchable connections between the positive output terminal of the second step-down converter and the negative output terminal of the first step-down converter, the positive output terminal of the second step-down converter and the positive output terminal of the first step-down converter, and the negative output terminal of the second step-down converter and the negative output terminal of the first step-down converter, the negative output terminal of the second step-down converter and the positive output terminal of the first step-down converter forming DC output terminals of the charging device.

Description

[0001] 202421817

[0002] 1

[0003] Description

[0004] Stationary DC charging station for electrically powered vehicles

[0005] The invention relates to a stationary DC charging device for electrically powered vehicles.

[0006] It is known that stationary DC chargers for electric vehicles usually consist of a so-called mains frontend, which is designed as an AC-DC converter, and at least one galvanically isolated DC converter, i.e. a DC-DC converter.

[0007] The mains front end converts the AC mains voltage into a first DC voltage, while the DC-DC converter(s) are responsible for generating a galvanically isolated second DC voltage from this first DC voltage, which is used to charge the EV. It is also known that a DC charger is generally designed to charge different battery systems, in particular both 400V and 800V systems. Therefore, the charger must cover a very wide voltage range, especially 200-920V.

[0008] A disadvantage here is that achieving this large voltage range with good efficiency is only possible with considerable effort. This is because the required DC-DC converter topologies, such as LLC converters, phase-shift converters, or dual active bridges, achieve very high efficiency rates at their nominal operating point, but efficiency drops significantly when the converters are operated outside of their nominal voltage range.

[0009] One solution is to use two DC-DC converters or a single DC-DC converter with two outputs to generate two galvanically isolated secondary voltages for charging. In the lower voltage range, the voltages are connected in parallel, and in the upper voltage range, they are connected in series. This narrower voltage range reduces the efficiency drop-off away from the nominal operating point.

[0010] The disadvantage is that under high demands, for example charging at 25 kW or at a voltage of 1250 V, which is intended for MW (megawatt) charging applications, the switches used must be designed for the higher power and voltage levels, which is detrimental in terms of both cost and lifespan. 202421817

[0011] 2

[0012] The object of the invention is to provide a charging device that at least partially overcomes the disadvantages of the prior art. This object is achieved by a DC charging device with the features specified in claim 1.

[0013] The stationary DC charging device for electrically powered vehicles according to the invention comprises a galvanically isolated DC / DC converter with a transformer for galvanic isolation, wherein the transformer comprises a first and a second winding on the secondary side. Advantageously, the transformer comprises a single winding on the primary side, i.e., exactly one winding.

[0014] Furthermore, the charging device comprises a first and second secondary-side rectifier circuit, wherein the first secondary-side rectifier circuit is connected to the first secondary-side winding and the second secondary-side rectifier circuit is connected to the second secondary-side winding.

[0015] Furthermore, the charging device includes a first and second buck converter, which is connected to the respective rectifier circuit and each has a negative and positive output terminal, wherein in each of the buck converters the two output terminals are connected via a respective output capacitor.

[0016] There is a first switchable connection between the positive output terminal of the second buck converter and the negative output terminal of the first buck converter. Furthermore, there is a second switchable connection between the positive output terminal of the second buck converter and the positive output terminal of the first buck converter. Finally, there is a third switchable connection between the negative output terminal of the second buck converter and the negative output terminal of the first buck converter.

[0017] The negative output terminal of the second buck converter and the positive output terminal of the first buck converter form the DC output terminals of the charging device.

[0018] The charging device according to the invention makes it possible to cover a wide voltage range while maintaining high efficiency. In particular, it can cover not only the usual voltage range of 200 V to 920 V, but also the 202421817

[0019] 3

[0020] Voltage range up to 1250 V is covered, which is used for very high charging capacities up to the MW range.

[0021] By using buck converters, the galvanically isolated DC / DC converter does not need to change the voltage, but can operate at its nominal operating point, where it exhibits optimal efficiency, independently of the output voltage of the charging device, i.e., the charging voltage for the vehicle. This results in improved average efficiency across the entire controllable voltage range.

[0022] Charging capacities of, for example, 25 kW at a voltage of 1250 V can be achieved without the need for higher-rated switches. This prevents the reduction in the charging equipment's lifespan caused by such switches.

[0023] Advantageous embodiments of the charging device according to the invention are described in the dependent claims. The embodiment of the independent claims can be combined with the features of one of the dependent claims or, preferably, with those of several dependent claims. Accordingly, the following additional features can be provided:

[0024] The switchable connections can each include a relay, meaning they can be connected and disconnected via the respective relay. Alternatively, the switchable connections can each include a semiconductor switch. Advantageously, the switchable connections, apart from the aforementioned switching element, do not include any other components; they are therefore direct connections via exactly this one switching element.

[0025] In one embodiment of the invention, the first and / or second secondary-side rectifier circuit comprises a half-bridge with two semiconductor switches. Each secondary-side rectifier circuit can be formed by a parallel connection of the half-bridge with a series connection of two capacitors. The first and / or second secondary-side rectifier circuit can also comprise a full bridge.

[0026] Each of the secondary windings is conveniently connected to the center terminals of the half-bridge and the series connection of the capacitors of the respective rectifier circuit. 202421817

[0027] 4

[0028] The first and / or second buck converter preferably comprises a half-bridge with two semiconductor switches, the center terminal of which is connected to the respective positive output terminal via an inductor.

[0029] The DC / DC converter can be connected on the primary side to a single-phase or multi-phase rectifier, which is designed for connection to a supply voltage.

[0030] The charging device expediently includes a control for its switches, which is designed such that the output terminals of the buck converters are connected in parallel in a first area for the output voltage of the charging device by controlling the switchable connections, and in series in a second area for the output voltage.

[0031] The invention will now be described and explained in more detail with reference to the embodiment shown in the single figure. Figure 1 schematically shows a circuit diagram of an embodiment of the arrangement according to the invention in the form of a multi-stage circuit 1 for charging an electrically powered vehicle.

[0032] Circuit 1 includes a DC link capacitor CN, whose terminals also form the terminals of a subsequent galvanically isolated DC / DC converter. Not shown in Circuit 1 is a rectifier circuit, which can be connected to a mains power supply on its input side and is connected to the DC link capacitor CN on its output side. Such a rectifier circuit can, for example, be built in a known manner as an active circuit with three half-bridges, each with two semiconductor switches, and is thus suitable for converting a three-phase supply voltage into a DC voltage.

[0033] The primary side of the galvanically isolated DC / DC converter comprises a full bridge with four MOSFETs P1...4. The center terminals of the full bridge are connected to a primary winding 25 of a transformer TF, with an additional inductor 26 and a capacitor 27 arranged in the current path. The exact elements in the current path depend on the specific design of the DC / DC converter, i.e., whether it is an LLC converter, a phase-shift converter, or another type; this is therefore an exemplary design. 202421817

[0034] 5

[0035] The secondary side of the DC / DC converter and the subsequent part of circuit 1 are divided into two identical sub-circuits 40 and 41. Each sub-circuit 40 and 41 has its own secondary-side winding 30a and 30b. The terminals of these windings are connected to the center terminals of a half-bridge 31a and 31b and a capacitor bank 32a and 31b, consisting of two capacitors. Each half-bridge 31a and 31b, and each capacitor bank 32a and 31b, form a rectifier.

[0036] In each of the sub-circuits 40, 41, a buck converter 35a, b is connected to the rectifier. This buck converter comprises a further half-bridge 36a, b, arranged in parallel to the half-bridge 31a, b. The center terminal of each of these half-bridges is connected to a respective coil 38a, b, the second terminal of which forms the respective positive output terminal of the buck converter 35a, b. The negative output terminal of the sub-circuits 40, 41 is formed by the lower outer terminal of the further half-bridges 36a, b.

[0037] Due to the arrangement of the two secondary-side windings 30a, b, the first of the two sub-circuits 40 is the upper sub-circuit with respect to the voltages, while the second of the sub-circuits 41 is the lower sub-circuit.

[0038] The output terminals of the buck converters 35a, b have three switchable connections. A relay 50a, b, c is arranged in each of the switchable connections, which can be used to open or close the connection. The first of the three connections is between the positive output terminal of the second sub-circuit 41 and the negative output terminal of the first sub-circuit 40. The second connection is between the positive output terminal of the second sub-circuit 41 and the positive output terminal of the first sub-circuit 40. The third connection is between the negative output terminal of the second sub-circuit 41 and the negative output terminal of the first sub-circuit 40.

[0039] The positive output terminal of the first sub-circuit 40 and the negative output terminal of the second sub-circuit 41 form the (DC) output terminals of circuit 1, i.e., the terminals for connection to the electrically operated vehicle.

[0040] Circuit 1 comprises a control device (not shown in Figure 1) for controlling the semiconductor switches and relays of circuit 1. During operation of circuit 1, the switchable connections are used to connect the two sub-circuits 40 and 41 either in series, depending on the required output voltage of circuit 1.

[0041] 6. By closing the second and third switchable connections and simultaneously opening the first connection, the two sub-circuits 40 and 41 are connected in parallel. Closing the first connection and opening the second and third connections results in a series connection, causing the output voltages of sub-circuits 40 and 41 to add up.

[0042] Furthermore, the buck converters 35a, b are operated by modulating the other half-bridges such that the voltage output by the galvanically isolated DC / DC converter is reduced to the required voltage. This allows the galvanically isolated DC / DC converter itself to always operate at its ideal efficiency point, where the output voltage equals the input voltage, taking into account the turns ratio of the transformer 29. In other words, at this point, no voltage change occurs other than that of the transformer 29. If the transformer 29 has a turns ratio of 1, the output voltage of the galvanically isolated DC / DC converter is equal to its input voltage. Due to this fixed turns ratio and the typically high voltage level, which is, for example, in the range of 700–950 V, this DC / DC full-bridge converter is very efficient.

[0043] The switching frequency of the buck converters 35a, b need not be constant, but can be adjusted or adapted such that the respective buck converter 35a, b is always switched on at current = 0 A. This advantageously prevents switch-on losses.

[0044] 202421817

[0045] Reference sign

[0046] CN intermediate circuit capacitor

[0047] P1...4 MOSFETs in the DC / DC converter 25 primary-side winding

[0048] 26 Inductance

[0049] 27 Capacitor

[0050] 29 transformer

[0051] 30a, b secondary winding 31a, b half bridge

[0052] 32a, b Capacitor series

[0053] 35a, b Buck converter

[0054] 36a, b further half-bridge

[0055] 38a, b Coil 41, 42 Sub-circuits

[0056] 50a, b, c relays

Claims

202421817 8 Patent claims 1. Stationary DC charging equipment (1) for electrically powered vehicles, comprising: - a galvanically isolated DC / DC converter with a transformer (29) for galvanic isolation, wherein the transformer (29) comprises a first and second winding (30a, b) on the secondary side, - a first and second secondary-side rectifier circuit, wherein the first secondary-side rectifier circuit is connected to the first secondary-side winding (30a) and the second secondary-side rectifier circuit is connected to the second secondary-side winding (30b), - a first and second buck converter (35a, b) which is connected to the respective rectifier circuit and each has a negative and positive output terminal, wherein in each of the buck converters (35a, b) the two output terminals are connected via a respective output capacitor, - a first switchable connection between the positive output terminal of the second buck converter (35b) and the negative output terminal of the first buck converter (35a), - a second switchable connection between the positive output terminal of the second buck converter (35b) and the positive output terminal of the first buck converter (35a), - a third switchable connection between the negative output terminal of the second buck converter (35b) and the negative output terminal of the first buck converter (35a), wherein the negative output terminal of the second buck converter (35b) and the positive output terminal of the first buck converter (35a) form DC output terminals of the charging device (1).

2. Charging device (1) according to claim 1, wherein the switchable connections each have a relay (50a... c).

3. Charging device (1) according to claim 1, wherein the switchable connections each have a semiconductor switch.

4. Charging device (1) according to one of the preceding claims, wherein the first and / or second secondary-side rectifier circuit comprises a half-bridge (31a, b) with two semiconductor switches. 202421817 9 5. Charging device (1) according to claim 4, wherein a respective secondary-side rectifier circuit is formed by a parallel connection of a half-bridge (31a, b) with a series connection of two capacitors (32a, b).

6. Charging device (1) according to claim 5, wherein a secondary-side winding (30a, b) is connected to the center terminals of the half-bridge (31a, b) and the series connection (32a, b) of the capacitors of the respective rectifier circuit.

7. Charging device (1) according to one of the preceding claims, wherein the first and / or second buck converter (35a, b) each comprises a half-bridge with two semiconductor switches, the center terminal of which is connected via an inductor (38a, b) to the respective positive output terminal.

8. Charging device (1) according to one of the preceding claims, wherein the first and / or second secondary-side rectifier circuit comprises a full bridge.

9. Charging device (1) according to one of the preceding claims, wherein the DC / DC converter is connected on the primary side to a single-phase or three-phase rectifier designed for connection to a supply voltage.

10. Charging device (1) according to one of the preceding claims with a control for the switches of the charging device, which is designed such that the output terminals of the buck converters are connected in parallel in a first area for the output voltage of the charging device by controlling the switchable connections and are connected in series in a second area for the output voltage.

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