Circuit arrangement with a boost converter and inverter circuit with such a circuit arrangement

Abstract

Circuit arrangement, with a direct current source (10) to provide a direct voltage; a boost converter (12-16) for boosting the DC voltage provided by the DC power source (10); and a DC link capacitor (18) connected between the outputs of the boost converter (12-16), wherein the DC source (10) comprises a first DC source (10a) and a second DC source (10b); wherein the boost converter (12-16) is provided as a common boost converter for boosting both the DC voltage provided by the first DC source (10a) and the DC voltage provided by the second DC source (10b), and wherein the common boost converter (12-16) includes a common switching element (14) which is connected between the first and the second DC sources (10a, 10b), characterized by that the common boost converter (12-16) includes a series circuit of a first inductor (12a) and a first rectifier element (16a) connected between an output of the first DC source (10a) and one side of the intermediate circuit capacitor (18), and a series circuit of a second inductor (12b) and a second rectifier element (16b) connected between an output of the second DC source (10b) and another side of the intermediate circuit capacitor (18); that the common switching element (14) is connected between the connection between the first inductor (12a) and the first rectifier element (16a) and the connection between the second inductor (12b) and the second rectifier element (16b); and that the first and second DC sources (10a, 10b) are two strings or substrings of a photovoltaic generator; wherein the first and second inductances (12a, 12b) of the common boost converter (12-16) are coupled together.

Description

[0001] Circuit arrangement with a boost converter and inverter circuit with such a circuit arrangement.

[0002] The present invention relates to a circuit arrangement with a boost converter for boosting a provided DC voltage, and to an inverter circuit with such a circuit arrangement. In particular, the present invention relates to a circuit arrangement with a boost converter for boosting a DC voltage provided by a photovoltaic generator, and to a solar inverter circuit with such a circuit arrangement.

[0003] An inverter typically requires a specific intermediate circuit voltage to generate an alternating voltage. Optimal efficiency is achieved when this intermediate circuit voltage is precisely matched to the alternating voltage to be generated.

[0004] Photovoltaic generators typically deliver a highly fluctuating DC voltage depending on the light incidence, temperature, and the number of connected modules. The wider the operating range of the DC input voltage that an inverter can handle, the more options there are for finding suitable module combinations during installation. An input voltage range of 1:2 at full load or 1:2.5 from full load to idle is often desirable.

[0005] To adapt the DC voltage provided by the photovoltaic generator to the inverter, a circuit arrangement with a so-called boost converter is generally used, which increases the variable DC voltage to a relatively constant intermediate circuit voltage. The structure of a conventional standard inverter circuit (step-up converter) is shown in Fig. 3 illustrates.

[0006] A photovoltaic generator 10 supplies a variable DC voltage. This is boosted to the intermediate circuit voltage by the boost converter, which comprises a choke 12, a power switch 14, and a diode 16. The intermediate circuit voltage is applied to an intermediate circuit capacitor 18. The downstream inverter 20 then converts the intermediate circuit voltage into an AC voltage.

[0007] To increase the DC voltage of the photovoltaic generator 10, the power switch 14 is periodically switched on and off. The duty cycle is selected via an analog or digital controller to achieve the desired intermediate circuit voltage. When the power switch 14 is closed, current flows from the photovoltaic generator 10 into the choke 12 and is temporarily stored there. When the power switch 14 is then opened, the current flows from the photovoltaic generator 10 via the diode 16 into the intermediate circuit capacitor 18, whereby the energy previously stored in the choke 12 is also transferred to the intermediate circuit capacitor 18.

[0008] Fig. 4 shows a further development of this standard inverter circuit, as known, for example, from DE 10 2004 037 446 A1. Fig. The inverter circuit shown in Figure 4 differs from the standard circuit of Fig. 3 by the symmetrical arrangement of two chokes 12a and 12b and the additional diode 16b, which is necessary for symmetry reasons. The operation of this inverter circuit is the same as that of the standard inverter circuit described above. The symmetrical distribution of the chokes 12a, 12b causes the photovoltaic generator 10 to be at a different voltage level relative to ground, which is desirable for certain types of solar modules.

[0009] US Pat. No. 3,459,957 A shows a circuit arrangement comprising batteries as a direct current source. The batteries can be connected either in series or in parallel via a switch formed by a transistor. A capacitor sums the voltage drop across it. A coil is provided, which is arranged after a junction of the branches formed by one of the batteries and one of the diodes.

[0010] Another circuit arrangement is disclosed in US 6 963 497 B1.

[0011] However, the use of a boost converter in an inverter circuit also leads to losses that reduce the overall efficiency of the inverter circuit. In the modified circuit arrangement of Fig. 4, losses from the second diode 16b are added, further reducing the overall efficiency. Furthermore, the use of a boost converter always involves additional weight, volume, and cost.

[0012] The problems mentioned above with these conventional circuit arrangements also occur with other types of DC power sources and when the provided DC voltage is used directly, i.e. without the use of the inverter. The invention is based on the object of creating an improved circuit arrangement comprising a DC power source and a boost converter. Furthermore, an improved inverter circuit comprising a boost converter is also to be provided.

[0013] This object is achieved by a circuit arrangement having the features of claim 1. Advantageous embodiments and further developments of the invention are the subject of the dependent claims.

[0014] The circuit arrangement includes a DC power source for providing a DC voltage; a boost converter for boosting the DC voltage provided by the DC power source; and an intermediate circuit capacitor connected between the outputs of the boost converter. According to the invention, the DC power source comprises a first DC power source and a second DC power source, and the boost converter is provided as a common boost converter for boosting both the DC voltage provided by the first DC power source and the DC voltage provided by the second DC power source.

[0015] The circuit arrangement of the invention uses a common boost converter for a first DC power source and a second DC power source. This significantly reduces the energy that must be temporarily stored in the boost converter, or the circulating reactive power. As a result, the losses of the boost converter are significantly reduced, thereby increasing the overall efficiency of the circuit arrangement. Further advantages include reduced cooling requirements due to the improved efficiency, a reduction in the load on the semiconductor components of the boost converter, lower requirements for the design of the individual components, and a relatively small number of components required to achieve high efficiency.

[0016] In one embodiment of the invention, the common boost converter of the circuit arrangement contains a series circuit comprising a first inductance (e.g. choke) and a first rectifier element (e.g. freewheeling diode or active power switch), which is connected between an output of the first DC power source and one side of the intermediate circuit capacitor, and a series circuit comprising a second inductance and a second rectifier element, which is connected between an output of the second DC power source and another side of the intermediate circuit capacitor.

[0017] In a further embodiment of the invention, the common boost converter contains a common switching element which is connected between the first and the second direct current source.

[0018] This common switching element is connected, for example, between the connection between the first inductor and the first rectifier element and the connection between the second inductor and the second rectifier element. Furthermore, this common switching element is preferably a power switch that can be switched at a fixed or variable frequency.

[0019] In yet another embodiment of the invention, the first and second direct current sources are partial current sources of a direct current source connected in parallel or in series. For example, the first and second direct current sources are different (sub-)strings of a solar module or a photovoltaic generator.

[0020] Furthermore, the first and second inductances of the common boost converter can be optionally coupled to one another or formed separately from one another.

[0021] The circuit arrangement of the invention described above can be used advantageously in an inverter circuit which, in addition to this circuit arrangement, has an inverter for converting the direct voltage provided by the intermediate circuit capacitor into an alternating voltage.

[0022] In a further development of the invention, two or more boost converters connected in parallel or in series can also be connected to the input terminals of the inverter or to the intermediate circuit capacitor.

[0023] The above and other features and advantages of the invention will become more apparent from the following description of a preferred, non-limiting embodiment with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram of the structure of an inverter circuit according to an embodiment of the present invention; Fig. 2 a diagram illustrating the improvement in the efficiency of the inverter circuit of Fig. 1 compared to conventional circuit arrangements; Fig. 3 is a schematic diagram of the structure of a conventional inverter circuit; and Fig. 4 a schematic representation of the structure of another conventional inverter circuit.

[0024] In Fig. 1 shows the exemplary structure of an inverter circuit according to the present invention. The same or similar components are identified by reference numerals corresponding to those described above with reference to Fig. 1 and Fig. 2 described conventional circuit arrangements.

[0025] At first glance, the inverter circuit of the invention is very similar to the conventional arrangement of Fig. 4 according to DE 10 2004 037 446 A1. However, it differs significantly from this state of the art due to a different connection of the photovoltaic generator to the boost converter. This also results in a significantly modified mode of operation and, consequently, improved properties.

[0026] The photovoltaic generator constituting the DC power source according to the invention is divided into two (sub-)strings 10a, 10b, each providing a DC voltage. The first and second DC power sources 10a, 10b are preferably symmetrical, i.e., generate substantially the same DC voltage for best results. The voltage per string 10a, 10b at full load typically ranges between approximately 50% and 100% of the intermediate circuit voltage. In this way, an input voltage range of approximately 1:2 at full load is achieved.

[0027] The division of a photovoltaic generator 10 into two substrings 10a and 10b does not represent a restriction for inverters with higher power, since several identical substrings are usually connected in parallel anyway.

[0028] Both DC power sources 10a, 10b are connected to a common boost converter 12-16, which is followed by an intermediate circuit capacitor 18 and finally an inverter 20. As shown in Fig. 1, this common boost converter 12-16 includes a series circuit of a first choke 12a and a first diode 16a connected between an output of the first DC power source 10a and one side of the intermediate circuit capacitor 18, and a series circuit of a second choke 12b and a second diode 16b connected between an output of the second DC power source 10b and another side of the intermediate circuit capacitor 18.

[0029] Furthermore, the common boost converter 12-16 includes a common power switch 14, which is connected between the first and second DC power sources 10a, 10b such that it is connected between the connection between the first choke 12a and the first diode 16a and the connection between the second choke 12b and the second diode 16b. In this way, the two DC power sources 10a, 10b are connected in series when the power switch 14 is closed, and in parallel via the first and second diodes 16a, 16b when the switch 14 is open. In contrast to the conventional circuit arrangement of Fig. 4 Therefore, energy is released into the intermediate circuit capacitor 18 both when the circuit breaker 14 is closed and when it is open.

[0030] This significantly reduces the energy that must be temporarily stored in the chokes 12a, 12b of the boost converter 12-16, or the reactive power circulating in the boost converter. As a result, the two chokes 12a, 12b only need to be approximately half as large, and the load on the power switch 14 and other semiconductor components is reduced. Losses in the boost converter are significantly reduced, the overall efficiency of the inverter circuit is higher, and the required cooling capacity is lower.

[0031] If the DC voltage of the two strings 10a, 10b is approximately 50% or approximately 100% of the desired DC link voltage, a static series or parallel connection of the two DC sources 10a, 10b is sufficient to generate the DC link voltage. Therefore, no switching losses occur at the power switch 14. If the DC voltages of the two strings 10a, 10b are each in the range between approximately 50% and 100% of the DC link voltage, the power switch 14 is operated in a pulsed mode. The duty cycle is regulated by a controller (not shown, analog or digital, e.g. a conventional PWM control circuit) so that the desired DC link voltage is generated. The duty cycle can vary between 100% (i.e. static series connection of 10a and 10b) and 0% (i.e. static parallel connection of 10a and 10b).

[0032] To illustrate the improved functionality of the circuit arrangement according to the invention compared to conventional circuit arrangements with boost converters, Fig. 2 shows a comparison of the efficiency of the boost converters compared to the DC voltages provided by the power sources. Under the operating conditions of full load, constant switching frequency of the power switch 14 and an intermediate circuit voltage of 700 volts, the overall efficiencies of the boost converters 12-16 for the conventional circuit arrangement of Fig. 3 (curve ▪) and the circuit arrangement of the invention (curve ▲). The overall efficiency of the modified conventional circuit arrangement of Fig. 4 is still slightly below that of Fig. 3.

[0033] It should be noted that at a DC voltage of 700 volts, the boost converter 12-16 does not cycle, eliminating the switching losses of the power switch 14 in both cases. In the circuit arrangement according to the invention, this is also possible at a DC voltage of 350 volts due to the static series connection of the two substrings 10a, 10b.

[0034] Fig. Figure 2 clearly shows that the efficiency of the boost converter, and thus also the overall efficiency of the inverter circuit, could be significantly improved by the circuit arrangement according to the invention with minimal component expenditure. Furthermore, it can also be seen that the dependence of the boost converter's efficiency or losses on the DC voltage provided by the photovoltaic generator 10 is significantly lower.

[0035] The invention is, of course, not limited to the embodiment described above. Those skilled in the art will immediately recognize various modifications and variations within the scope of protection defined by the appended claims.

[0036] For example, it is conceivable to work with different substrings 10a, 10b, for example, by using different numbers of solar modules in the substrings. The boost converter circuit 12-16 can easily be designed for this case as well, although the efficiency improvement decreases with increasing asymmetry.

[0037] The two DC sources or (partial) strings 10a, 10b can optionally consist of DC sources connected in parallel or in series.

[0038] With regard to the boost converter 12-16, the chokes or inductors 12a, 12b can either be coupled to each other or designed separately. Instead of diodes, active power switches can also be used as the first and second rectifier elements 16a, 16b.

[0039] The boost converter 12-16 can also be constructed from two or more branches operating in parallel and, if necessary, out of phase (multiphase operation). In this case, individual branches can be completely switched off at partial load to increase partial load efficiency.

[0040] To reduce switching losses, oscillation networks can optionally be installed (resonant switching).

[0041] The power switch 14 can be controlled either at a fixed or variable frequency. If the DC voltage of the substrings 10a, 10b is close to 50% or 100% of the intermediate circuit voltage, the switching frequency of the power switch 14 can be reduced, for example, to reduce switching losses. At values ​​of 50% and 100% of the desired intermediate circuit voltage, the boost converter is preferably not switched; instead, the two DC sources 10a, 10b are operated statically in parallel or in series. In such static operation, the semiconductors can be bridged, for example, with a relay, to further increase efficiency.

[0042] Furthermore, two or more boost converters 12-16 can be connected in parallel or in series at the input of the inverter 20.

[0043] Various topologies can be used for the inverter 20 itself. Suitable topologies include half-bridges (for single-phase feed-in) and three-phase bridges.

[0044] Instead of the photovoltaic generators described above, other power sources can also be used for the first and second direct current sources 10a, 10b. For example, fuel cells, thermoelectric generators, electromagnetic generators, accumulators, supercapacitors, and the like are also suitable as direct current sources for the circuit arrangement according to the invention.

[0045] The circuit arrangement according to the invention with direct current sources 10a, 10b, boost converter 12-16 and intermediate circuit capacitor 18 can be used not only in combination with the downstream inverter 20, as in Fig.1. The circuit arrangement of the invention can also be used advantageously, for example, for directly supplying DC loads or for supplying DC intermediate circuits in other devices such as motor frequency converters. REFERENCE NUMBER LIST 10a first DC source 10b second DC source 12a first inductance 12b second inductance 14 common switching element 16a first rectifier element 16b second rectifier element 18 DC link capacitor 20 inverters

Claims

[1] Circuit arrangement, with a DC power source (10) for providing a DC voltage; a boost converter (12-16) for boosting the DC voltage provided by the DC power source (10); and an intermediate circuit capacitor (18) connected between the outputs of the boost converter (12-16), wherein the DC power source (10) comprises a first DC power source (10a) and a second DC power source (10b); wherein the boost converter (12-16) is provided as a common boost converter for boosting both the DC voltage provided by the first DC power source (10a) and the DC voltage provided by the second DC power source (10b), and wherein the common boost converter (12-16) contains a common switching element (14) connected between the first and second DC power sources (10a, 10b), characterized by , that the common boost converter (12-16) contains a series circuit of a first inductance (12a) and a first rectifier element (16a) connected between an output of the first DC power source (10a) and one side of the intermediate circuit capacitor (18), and a series circuit of a second inductance (12b) and a second rectifier element (16b) connected between an output of the second DC power source (10b) and another side of the intermediate circuit capacitor (18); that the common switching element (14) is connected between the connection between the first inductance (12a) and the first rectifier element (16a) and the connection between the second inductance (12b) and the second rectifier element (16b); and that the first and second direct current sources (10a, 10b) are two strings or substrings of a photovoltaic generator; wherein the first and second inductances (12a, 12b) of the common boost converter (12-16) are coupled to one another. [2] Circuit arrangement according to claim 1, characterized by that the first and second direct current sources (10a, 10b) are partial current sources of a direct current source (10) connected in parallel or in series. [3] Circuit arrangement according to one of claims 1 or 2, characterized by that the common switching element (14) is a power switch which can be clocked at a fixed or variable frequency. [4] Inverter circuit, with a circuit arrangement (10-18) according to one of claims 1 to 3 and an inverter (20) for converting the direct voltage provided by the intermediate circuit capacitor (18) into an alternating voltage. [5] Inverter circuit according to claim 4, characterized bythat at least two boost converters (12-16) connected in parallel or in series are connected to the input terminals of the inverter (20) or to the intermediate circuit capacitor (18).

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