WO2009039887A1

WO2009039887A1 - Dc control unit for controlling an input voltage in a photovoltaic system

Info

Publication number: WO2009039887A1
Application number: PCT/EP2007/060265
Authority: WO
Inventors: Thomas Lenz
Original assignee: Conergy Ag
Priority date: 2007-09-27
Filing date: 2007-09-27
Publication date: 2009-04-02
Also published as: EP2206229A1

Abstract

The present invention is directed to a DC control unit for coupling an output of a solar generator to an input of an inverter, comprising: a control input for receiving an output signal of a solar generator, a control output for providing an output signal for feeding into the inverter, and a control circuit for limiting the control output voltage to a first predetermined value. The present invention is further directed to an inverter and a solar system comprising a control unit to limit the input voltage of an inverter. The present invention is further related to a method for controlling such a solar system. The invention is further directed to a corresponding inverter.

Description

DC control unit for controlling an input voltage in a photovoltaic system

The present invention is related to a DC control unit for coupling an output of a solar generator to an input of an inverter. The invention is further related to a solar system comprising such a DC control unit. And the invention is still further related to a method for controlling a DC voltage of a solar generator which is to be fed into an inverter for transforming the DC voltage into an AC voltage, by means of a DC control unit.

It is to be noted, that in the meaning of the current application, a single photovoltaic module or a plurality of photovoltaic modules forms a solar generator.

In known solar systems a solar generator generates a DC voltage. For feeding electrical energy into the grid this DC voltage is transformed into an AC voltage and current according to the requirements of the grid with respect to frequency, phase angle and voltage level. If the solar generator generates only few power, e.g. dependent on the available light, the inverter cannot deliver the required level of the AC voltage. As a result, the inverter will stop transforming energy delivered by the solar generator. As a further result, the DC voltage of the solar generator will rise, causing the risk of damaging semi-conductor elements of the inverter.

This risk of damaging the inverter is in particular high because such inverters are usually dimensioned, having few tolerance. The reason for dimensioning with only few tolerance is, that semi-conductors having a higher tolerance cause more power loss, what is to be avoided.

In other words, the maximum output voltage of a solar generator is significantly higher than the usual operating range of the inverter, which is in the range of the MPP (maximum power point). This situation is illustrated in figure 1. Additionally, it is to be noted that the maximum voltage of the solar generator is only reached at low temperatures, dependent on a negative temperature coefficient of the voltage.

To conclude, the components of the inverter are designed with respect to a maximum energy conversion efficiency and the withstand voltage is limited to the maximum voltage of a solar generator, which can be connected. The maximum voltage of the solar generator can only be present at the inverter input, if the inverter is not in a working condition, i.e. if the inverter is not feeding electricity into the grid. If the inverter is in a working condition, it provides a load for the solar generator and accordingly leads to a working point (e.g. the MPP) compris- ing a lower input voltage in comparison to a condition when the inverter is not working.

This problem is generally known in the state of the art. One approach for improving the situation has been made by providing a switch for disconnecting the solar generator from the inverter, when the inverter is not in working condition, i.e. when the converter is not feeding into the grid. However, if such a switch is mechanically operated, disconnecting the solar generator from the inverter might take too long. Accordingly, there is still the risk of damaging the inverter caused by overvoltages. Additionally, when the disconnected inverter is to be connected again to the solar generator because the inverter is intended to start feeding into the grid again, the voltage of the solar generator is still high. Accordingly, in the moment of reconnecting the solar generator to the inverter the input voltage of the inverter will be high, at least for this moment. It is therefore an object of the present invention to provide an efficient solution to protect an inverter against damages caused by overvoltages.

In accordance with this object, the present invention proposes a DC control unit for coupling an output of a solar generator to an input of an inverter, comprising a control input for receiving an output signal of a solar generator, a control output for providing an output signal for feeding into the inverter, and a control circuit for limiting the control output voltage to a first predetermined value comprising a step down converter for controlling the voltage at the control output to a value equal or smaller than the first predetermined value. Such a control unit receives the elec- trical energy from the solar generator as a DC voltage signal, whereby the voltage may vary within a large range. The control circuit limits this received voltage to a first predetermined value. I.e. it controls the voltage to this first predetermined value if the solar generator provides a sufficient voltage. If the solar generator does not provide a voltage with a sufficient level, the output voltage of the control unit may also fall below the first predetermined value. In particular, the output voltage of the control unit will fall down to 0 Volts, if the input voltage received from the solar generator is also 0.

Accordingly, the proposed control unit will usually not disconnect the solar generator from the inverter. The control unit will instead just limit the output voltage which will be fed into the inverter to a first predetermined level. Accordingly, the inverter can be protected against overvoltage in a simple manner. No additional information is needed from the inverter. In particular, there is no need to transmit the current working condition of the inverter to the control unit i.e. the control unit does not need the information whether the inverter is feeding into the grid or if it is not.

If the input voltage received from the solar generator rises above the first predetermined value, the step down converter will reduce this voltage to the first predetermined value. Accordingly, there is no need to disconnect the solar generator from the inverter, if the solar generator output voltage rises above that first prede- termined value.

According to one embodiment the step down converter is adapted to control the voltage at the control output by means of a pulse-width modulation. In this way, a - A -

certain lower voltage at the control output in relation to the voltage at the control input can be provided depending on the pulse-to-width ratio. If the input voltage of the DC control unit equals the output voltage of the DC control unit, the pulse- to-width ratio is 1 . I.e. the step down converter will directly let the current pass through and do not provide any interruption.

With respect to a further embodiment, the control unit comprises a switch, in particular a mechanically operated switch for bypassing the step down converter or part of it. This feature provides a simple connection from the solar generator to the inverter, if the output voltage of the solar generator reaches or falls below a first predetermined value. The output voltage of the solar generator is usually above the first predetermined value, when there is sufficient light feeding the solar generator and when the inverter is not in working condition. This situation usually comes up, when the light conditions have just been changed and the inverter will accordingly be just to be about to start working again and start to feed energy into the grid. In this situation, the control unit already provides energy from the solar generator to the inverter, even though the mechanically operated switch is open. If the inverter has started working, it will soon reach the maximum power point or will at least get close to this point. When this working condition is reached, the inverter will provide a load to the solar generator and the voltage of the solar generator will drop. The system of solar generator and inverter is than in normal working condition. The solar generator provides a DC voltage and the inverter converts this DC voltage into an AC voltage and feeds this into the grid. For this working condition it is not necessary, that the mechanically operated switch of the pre control is closed. However, when the solar system is in such working condition it is easier to bypass the step down converter by closing the mechanically operated switch. This way also any loss occurring in the step down converter is avoided.

It is to be noted, that closing the mechanically operated switch to bypass the step down converter is clearly to be distinguished from connecting a solar generator to an inverter by closing a switch. When the mechanically operated switch according to this embodiment is closed, the solar generator is already connected to the inverter by means of the step down control or possibly another active control, as described above. This clearly avoids any voltage jump when closing the switch. As the mechanically operated switch a relay or connector can be used. The use of a mechanically operated switch is proposed, to avoid any loss at the switch as is expected, if a semi-conductor switch is used. However, for some particular systems or for future semi-conductor switches having less losses than currently known in state of the art, a semi-conductor switch might be used instead of the mechanically operated one.

According to still a further embodiment of the invention the DC control unit comprises a passive limiter for limiting the voltage and the control output to a second predetermined value. Since such a passive limiter does not need any control, it limits the voltage to that second predetermined value irrespectively of any control failure. This passive operation usually has, if at all, a very short response time. This provides a simple and quick protection of the inverter input against overvolt- ages, even when high voltages suddenly occur. In particular, high voltages suddenly occur at the input of the inverter, when the inverter switches from working condition into non-working condition. Such a sudden change in the input voltage requires a short response of the DC control unit, and this short response can be provided by the passive limiter.

The second predetermined value of this passive limiter is in the same range as the first predetermined value, which is relevant for the step down converter. However, when operating the step down converter and the passive limiter the second predetermined value should be slightly larger than the first predetermined value, to avoid any conflict of the step down converter or any other active control and the passive limiter. The passive limiter should not be effective, once the active control, in particular a step down converter takes over the control of the voltage.

According to one embodiment of the invention the limiter comprises a Zener diode. Such a semi-conductor element is known in the state of the art and will be driven in the reverse direction in comparison to regular diodes. Accordingly, if the voltage at this Zener diode rises above a breakdown voltage, the diode will sud- denly conduct and thus avoid further rising of the voltage. With respect to the voltage level of the second predetermined value being higher than the breakdown voltage of regular Zener diodes, two or more Zener diodes might be provided in a row. However, an avalanche diode can also be used instead of the proposed Zener diode. The avalanche diode has the advantage of currently being available for higher voltage ranges.

With respect to one embodiment the limiter comprises a Zener diode or avalanche diode and a transistor, in particular a MOSFET or IGBT, whereby the Zener diode or avalanche diode is connected to the gate of the transistor. Additionally, the drain of the transistor is connected to a terminal comprising the positive voltage of the inverter input voltage, which is also connected to the other connection of the Zener diode or avalanche diode. Accordingly, when the voltage of the inverter reaches the second predetermined value, i.e. the breakdown voltage of the Zener diode or avalanche diode plus the gate threshold voltage of the transistor, the Zener diode or the avalanche diode will conduct a current and accordingly the transistor will become conductive and the main current will flow through the transistor. As a result this limiter will act in a manner of a "power- Zener diode" or "power-avalanche diode".

According to a further embodiment, the DC control unit is adapted to receive a DC input voltage in the range of 0- 1000V. According to a further embodiment the DC control unit is adapted to control and/or limit the voltage at the control output to a value in the range of 0-800V.

In a preferred embodiment the DC control unit further comprises a step up con- verter, also known as boost converter for rising the voltage at the control output.

The step down converter is active, if there is a high voltage at the DC control unit input. The step down converter will control this high voltage at the input down to a first predetermined voltage at the control unit output. However, if the voltage at the control unit input is below the first predetermined voltage at the control output or below a third predetermined voltage, the step up converter is used to rise this low input voltage. In this case, the step down converter is inactive.

Advantageously, either the step down converter is active or the step up converter is active. According to one solution the first predetermined output voltage is above the third predetermined output voltage. Accordingly, the step down con- verter is active for input voltages above that first predetermined voltage and the step up converter is only active for input voltage below the third predetermined output voltage. As a result, none of the step down converter and step up con- verter is active when the input voltage is between the first predetermined output voltage and the third predetermined output voltage. In particular, this solution provides a hysteresis like behaviour for a nominal operating state

Such a step up converter might as well be provided within an inverter to which the DC control unit is connected. However, according to a further embodiment it is advantageous to have a step up converter as part of the DC control unit, whereby the step down converter and the step up converter use the same inductor. This has the advantage, that only one inductor is needed for these two converters. As described above, the step down converter and the step up converter are not active at the same time.

In a preferred embodiment an inverter is proposed for transforming a DC voltage into an AC voltage, comprising a control unit according to the present invention. If a control unit can ensure, that the DC input voltage of an inverter does not exceed a first predetermined voltage, the inverter can be dimensioned accordingly. I.e. the semi-conductor components, in particular the power semi-conductor components can have no or very few tolerance with respect to the maximum input voltage. Accordingly, to avoid any failure when planning a solar system the inverter should also be provided with a control unit according to the invention to provide an exact and reliable limit. This can be achieved by integrating the control unit into an inverter.

For an optimised operation, one embodiment proposes a solar system comprising at least one solar generator for generating a DC voltage, a control unit being connected to the solar generator for controlling and/or limiting the voltage of the solar generator, and an inverter being connected to an output of the DC control unit for transforming the DC voltage into an AC voltage.

According to one embodiment of the invention a method for controlling a DC voltage of a solar generator which is to be fed into an inverter is provided for transforming the DC voltage into an AC voltage, by means of a DC control unit whereby the DC control unit receives an input voltage from the solar generator and controls an output voltage to be input into the inverter such, that the output voltage remains at a first predetermined value, if the input voltage is equal or above this first predetermined value. In normal operating condition a solar gen- erator is directly connected to the input of an inverter by means of a switch, in particular a mechanically operated switch. In this normal working condition the inverter is feeding electrical energy into the grid. If for any reason the inverter stops feeding electrical energy into the grid the DC voltage at the input of the inverter will rise. When this voltage rises above a second predetermined value a passive limiter will limit the input voltage at the inverter. I.e. a Zener diode or the like will become conductive and will cause a current to flow from the positive voltage terminal to the negative voltage terminal and/or the ground. If this happens, the switch will still be closed and bypass a control circuit, in particular a step down converter. In the next step, the switch will open and the control circuit will start to operate. This control circuit will control the DC voltage of the solar generator, to remain a first predetermined value, which is slightly smaller than the second predetermined value given by the passive limiter i.e. the Zener diode or the like. If there is some energy provided by the solar generator the control unit and thus the solar system might remain in this situation for a while.

However, in another situation, due to a lack of light the solar generator might only provide a small or no DC voltage. In this case, the step down converter or the like will basically conduct this small DC voltage to the input of the inverter. Once more energy is available from the solar generator the inverter will start to operate and thus to feed electric energy into the grid. In this case the inverter will provide a sufficient load to bring the system into a condition of the MPP or close to it. As a result, the voltage at the solar generator output will also drop below the maximum allowable voltage of the inverter input. To avoid losses in the step down converter, the mechanically operated switch or the like will then be closed again and the solar system is back in normal operation condition, as described above.

The embodiments of the invention will now be described with respect to the accompanying figures, wherein similar elements may be designated with the same reference numerals.

Fig. 1 shows a characteristic current-voltage diagram of a solar generator.

Fig. 2 shows a circuitry diagram of a DC control unit according to one embodiment of the present invention connected to an inverter. Fig. 3 shows a passive limiter according to a further aspect of the present invention.

Fig. 4 shows a circuitry diagram of a DC control unit according to a further embodiment of the present invention.

Fig. 5 shows a circuitry diagram of a DC control unit according to a still further embodiment of the present invention.

The characteristic line according to figure 1 shows the voltage of a solar generator Vsg at the x-coordinate and the current of the solar generator at the y- coordinate. Accordingly, the current falls with rising voltage. The diagram also shows an area between a minimum voltage Vmin and the maximum voltage Vmax. Below the voltage Vmin an inverter connected to the solar generator cannot be operated sufficiently. The voltage Vmax is the maximum voltage to operate an inverter connected to the solar generator. If the input voltage rises above Vmax a risk of damaging the solar generator occurs. The optimum and/or normal working point is indicated by MPP. If the current, starting from the maximum power point (MPP) drops, it will soon cause the voltage of the solar generator to exceed the maximum voltage Vmax. This happens, if the inverter stops working. On the other hand, whenever the voltage of the solar generator has exceeded the value of Vmax, it can be lowered by increasing the current, whereby the working point can be brought in the area of the MPP.

According to the present invention as illustrated in figure 2 an inventive DC control unit 1 is connected to an inverter 2. The control unit 1 comprises an input having a positive and a negative terminal 4 and 6 respectively (+Vsg and -Vsg). This input 4, 6 is to be connected to the DC output of a solar generator. The control unit 1 is connected with an output having a positive and a negative terminal 8 and 10 respectively to the DC input of the inverter 2. The inverter 2 is adapted to convert the DC voltage at the DC input 8, 10 into an AC voltage at the inverter output 12. This is just illustrated diagrammatically.

When connected to an operating solar generator the solar generator voltage will be provided at the input capacitor 14. This capacitor 14 will work as an energy storage and might provide some smoothing of the voltage. In starting operation, when the switch S2 is open as illustrated in figure 2 and the inverter 2 is not in working condition yet. The control unit 1 will provide a sufficient voltage at the output 8, 10 at the capacitor 16. The control is performed by the switch S1 , the coil L1 and the diode D1 and the additional diode 2, which work as a step down converter. Accordingly, assuming that the voltage of the solar generator is above the maximum allowable voltage of the inverter input 8, 10 the switch S1 will frequently close and open to generate a current through the coil L1 . This current will flow through the diode D1 when the switch S1 is open. The switch S1 will therefore be controlled depending on the resulting voltage at the capacitor 16. Accord- ingly, there will be provided a voltage at the capacitor 16, which is limited to a first predetermined maximum voltage but which is also sufficient to let the inverter 2 switch to working conditions.

If the inverter switches to working conditions and in particular if a MPP is reached, the voltage of the solar generator at the capacitor 14 will drop, as de- scribed with respect to figure 1 . If according to this effect the input voltage falls below the first predetermined voltage the mechanically operated switch S2 is closed and it will bypass the step down converter. Accordingly the voltage of the solar generator is provided at the capacitor 14 and the capacitor 16 in the same way.

If the inverter 2 will switch back into non-working condition while the switch S2 is closed, the current will drop and the voltage of the solar generator will rise as explained with respect to figure 1. If the voltage at the solar generator at the capacitor 14 and accordingly at the capacitor 16 will rise above a second predetermined value the Zener diode ZD1 , which is connected in parallel to the capaci- tor 16 will start to conduct a current, because the voltage will rise above the breakdown voltage. I.e. the Zener diode ZD1 will basically short the output of the solar generator. However, if the voltage at the Zener diode ZD1 will drop again below the breakdown voltage the current will stop again and thus the voltage at the Zener diode ZD1 as well as at the capacitor 16 will not drop down to zero.

The described effect of limiting the voltage by means of the Zener diode ZD1 is immediately effective, because the Zener diode ZD1 as being a semi-conductor component comprises no considerable response time. At least with respect to the time needed for the voltage of the solar generator to rise, a response time of the Zener diode ZD1 is neglectable. Therefore, the Zener diode ZD1 provides a quick and reliable protection of the inverter against an overvoltage. However, it is unwanted to have too much current flow over the Zener diode ZD1 . And accordingly the mechanically operated switch S2 will be opened once the Zener diode ZD1 limits the voltage at the output 8, 10 at the capacitor 16. I.e. the operation of the Zener diode ZD1 allows the mechanically operated switch S2 to be somewhat slow.

Once the switch S2 is opened and there is enough solar energy i.e. enough light available, the voltage of the solar generator at the input 4, 6 of the DC control unit and also at the capacitor 14 will rise. The step down converter will then again start operating and will provide a voltage at the output 8, 10 and thus at the capacitor 16, as described above. However, if for any reason the voltage of the solar generator at the capacitor 14 will fall below the maximum voltage allowed at the inverter input, the switch S1 will permanently be closed and the low voltage of the solar generator will thus be fed through the coil L1 to the capacitor 16. However, the mechanically operated switch S2 may be kept open because having such a low voltage of the solar generator might not cause the inverter to start operating again. Instead, such a start of operating is expected, when the solar generator provides more energy and thus usually in the first place the voltage at the capacitor 14 will rise, before there is enough energy for the inverter to start working again. As described above the step down converter is then starting to open and close the switch S1 and accordingly will take care for the appropriate voltage at the output 8, 10 and thus the output capacitor 16. The step down converter is adapted to respond within a short time, because the switch S1 is a semi-conductor switch such as an IGBT or a MOSFET.

According to the present invention illustrated in figure 2 the response time of the mechanically operated switch S2 might reach a value of up to 20ms. During this time the passive limiter provided by the Zener diode ZD1 has to absorb the power generated by the solar generator. The Zener diode ZD1 must be dimensioned accordingly.

According to the circuitry of a further embodiment of the control unit shown in fig. 4 a step up converter 30 is provided, that uses the same inductor L1 of the step down converter. If the voltage at the capacitor 14 is below a first predetermined or third predetermined value, at least one of the switches S1 and S2 will be closed and the input voltage will therefore appear at the diode D1 . This can be seen as an input voltage of the step up converter 30. The step up converter 30 basically comprises the inductor L1 , the diode D4 and the switch S3. To rise the voltage from the input of the step up converter 30 to its output at the capacitor 16 the step up converter 30 operates in a generally known way. I.e. the switch S3 closes to generate a current in the inductor L1 . Opening the switch S3 will force the current through the inductor L1 to pass through the diode D4 and charge the capacitor 16, resulting in a rising voltage at the capacitor 16. Accordingly the voltage at the capacitor 16 can be controlled by means of opening and closing the switch S3. In particular, the voltage at the capacitor 16 can be controlled dependent on a pulse pattern such as a pulse-width ratio.

It is to be noted, that the Zener diode ZD1 is also in this embodiment provided for preventing a short-time overvoltage. However, depending on the dimension of the current control unit this diode might not be necessary. The switch S3 is provided as a semi-conductor switch such as a transistor. The diode D3 is provided as an auxiliary diode D3 for the semi-conductor switch S3, in particular to prevent negative voltages.

As mentioned above, the inductor L1 is used by the step down converter as well as by the step up converter. However, comparing the circuitry according to fig. 4 to the circuitry according to fig. 2 shows that the switch S2 now only bridges the diode D2 and the semi-conductor switch S1 , but not the inductor L1. This is important for the function of the step up converter, but does hardly influence the step down converter.

The step down converter and the step up converter interact as follows: For high voltages at the input capacitor 14 the switch S3 is left open and the step up converter 30 is inactive. The step down converter will then function as described above with reference to fig. 2. If the voltage at the input capacitor 14 is low, such that rising of the voltage is desired, at least one of the switches S1 and S2 is constantly closed and accordingly the step down converter is inactive. Accordingly, depending on the input voltage at the capacitor 14 either one of the step down converter and the step up converter is active, or none of it, and the active one of both then uses the inductor L1 . As a result, there is no conflict of the step down converter and the step up converter and only one inductor L1 is needed.

The circuitry of the control unit of a still further embodiment shown in fig. 5 is quite similar to that shown in fig. 4. However, the difference is that the negative potential is controlled according to this embodiment, whereas the positive potential is controlled with respect to the embodiments shown in fig. 2 and fig. 4. This can be advantageous in view of voltage potentials with respect to ground.

It is to be noted that figs. 2, 4 and 5 comprise some identical reference numerals. However, the corresponding elements might not be identical, but fulfill similar purposes. The identical reference numerals shall not indicate the particular element chosen nor indicate the particular seize of the corresponding element. A person skilled in the art will - depending on the implementation - choose the kind and size of the necessary elements.

A further possibility of dimensioning the passive limiter is illustrated in figure 3. This passive limiter 20' is also to be connected to a positive and negative terminal 8' and 10' respectively of an output of the control unit. I.e. the passive limiter 20' can replace the Zener diode ZD1 illustrated in figure 2. If the voltage at the positive and negative terminals 8', 10' is small and in particular below a second predetermined value, the Zener diode ZD and accordingly the transistor D1 will block the flow of current. If the voltage exceeds the sum of breakdown voltage of the Zener diode ZD and the gate voltage of the transistor T1 , the Zener diode ZD will allow the current to flow and so will the transistor T1 accordingly. The main current will then flow over the resistor R2 and the transistor T1 . These two components will therefore absorb the main part of the power generated by the solar generator. The passive limiter shown in figure 3 has to be dimensioned accordingly.

To conclude, the present invention provides a solution to operate an inverter always connected via a DC control unit to a solar generator. The input voltage of the inverter remains below a predetermined maximum voltage. Even small and/or short overvoltages are avoided.

Claims

1 . DC control unit (1 ) for coupling an output of a solar generator to a DC- load such as an input of an inverter (2), comprising: a control input (4,6) for receiving an output signal of a solar generator, a control output (8,10) for providing an output signal for feeding into the inverter (2), and a control circuit for limiting the control output voltage to a first predetermined value comprising a step down converter (S1 , D1 , L1 ) for controlling the voltage at the control output (8,10) to a value equal or smaller than the first pre- determined value.

2. DC control unit (1 ) according to claim 1 , whereby the step down converter (S1 , D1 , L1 ) is adapted to control the voltage at the control output (8, 10) by means of a pulse-width modulation.

3. DC control unit (1 ) according to claim 2, comprising a switch (S2), in particular a mechanically operated switch (S2) for bypassing the step down converter (S1 , D1 , L1 ) or part of it.

4. DC control unit (1 ) according to any one of the preceding claims, comprising a passive limiter (ZD1 ) for limiting the voltage at the control output (8,10) to a second predetermined value.

5. DC control unit (1) according to claim 4, whereby the limiter (ZD1 ) comprises a Zener diode (ZD1 ) or an avalanche diode.

6. DC control unit (1 ) according to claim 4 or 5, whereby the limiter comprises a Zener diode (ZD) or an avalanche diode and a transistor (T1 ), in par- ticular a MOSFET or IGBT, whereby the Zener diode (ZD) is connected to the

Gate of the transistor (T1 ).

7. DC control unit (1 ) according to any of the preceding claims, being adapted to receive a DC input voltage in the range of 1 -1000 V.

8. DC control unit (1 ) according to any of the preceding claims, being adapted to control and/or limit the voltage at the control output (8,10) to a value in the range of 0-800 V.

9. DC control unit (1 ) according to any of the preceding claims, further comprising a step up converter (S3, D4, L1 ) for rising the voltage at the control output.

10. DC control unit according to claim 9, whereby the step down converter and the step up converter use the same inductor (L1 ).

1 1 . Inverter (2) for transforming a DC voltage into an AC voltage, comprising a DC control unit (1 ) according to any one of the preceding claims.

12. Solar system comprising: at least one solar generator for generating a DC voltage and a DC control unit (1 ) according to any of claims 1 to 10 being connected to the solar generator for controlling and/or limiting the output voltage of the solar generator.

13. Solar system according to claim 12, further comprising an inverter (2) being connected to an output of the DC control unit (1 ) for transforming the DC voltage into an AC voltage.

14. Method for controlling a DC voltage of a solar generator to be fed into

A DC-load such as an inverter (2) for transforming the DC voltage into an AC voltage, by means of a DC control unit (1 ) whereby the DC control unit (1 ) receives an input voltage from the solar generator and controls an output voltage to be input into the inverter (2) such, that the output voltage remains at a first predetermined value, if the input voltage is equal or above this first predetermined value, whereby the output voltage is controlled by means of a step down converter (S1 , D1 , L1).

15. Method according to claim 14 , whereby a switch (S2) is closed such, that the solar generator is directly connected to the inverter (2), if the input voltage is below the first predetermined voltage.

16. Method according to claim 14 or 15, whereby the output voltage is limited by means of a limiter (ZD1 ), if the input voltage rises above the first predetermined voltage or a second predetermined voltage.

17. Method according to claim 14 or 15, whereby a/the switch (S2) is opened, if the input voltage rises above the first predetermined voltage.

18. Method according to any of claims 14 to 17, whereby the step down converter controls the output voltage by means of a pulse-width modulation.

19. Method according to any of claims 14 to 18, whereby the output voltage is controlled by means of a step up converter, if the input voltage is below the first predetermined voltage or below a third predetermined voltage.