Active bypass circuit for a solar generator

Abstract

The invention relates to a bridging and connecting circuit (1) with a first terminal (11) for connecting to a positive pole of a solar generator and a second terminal (22) for connecting to a negative pole of the solar generator, with a series circuit arranged between the first terminal (11) and the second terminal (22) comprising a first diode (2), a capacitor (3) and a first semiconductor switch (S1), wherein the first diode (2) and the first semiconductor switch (S1) have a common orientation in a forward direction towards the second terminal (22) and a second semiconductor switch (S2) which is arranged in parallel to the part of the series circuit consisting of the first diode (2) and capacitor (3), wherein the second semiconductor switch (S2) has an orientation opposite to that of the first diode (2).The circuit includes a control circuit (4) which is designed and configured to switch at least the second semiconductor switch (S2) on when a voltage applied to the capacitor (3) exceeds a first threshold value, and to switch it off when the voltage falls below a second threshold value which is lower than the first threshold value.

Description

[0001] The invention relates to a bridging and connecting circuit, in particular a bridging and connecting circuit for a solar module, and to a solar module with such a circuit. The invention further relates to a photovoltaic system with an inverter and a string of solar modules, at least one of which has such a bridging and connecting circuit.

[0002] Solar modules consist of individual solar cells, or sub-modules, connected in series. Under normal operating conditions, such as homogeneous sunlight, these cells deliver individual voltages that sum to form the total voltage of the solar module. On a larger scale, a so-called string consists of several solar modules connected in series, with a single total voltage across the entire string. Current flows in a predetermined direction. If one or more cells are partially shaded, or if the sub-modules have different characteristics, the voltage across the shaded sub-modules reverses under load. In the worst-case scenario, the entire total voltage of the unshaded cells drops in reverse bias across the shaded solar module or sub-module. This can exceed the permissible reverse voltage of the affected solar module or sub-module and lead to irreversible damage.The harmful reversal of the preferred voltage and current direction can have a variety of causes, with partial shading being just one example.

[0003] For this reason, it is known in the prior art to connect a bypass diode in parallel to each solar cell or sub-module or each solar module in a string.

[0004] Bypass diodes are typically housed in a junction box connected to a solar module. In the event of a voltage reversal, for example due to shading, a significant power loss occurs. This power loss can be absorbed by the diode in the short term. However, during prolonged shading, it becomes difficult to dissipate the resulting heat energy from the diode and the junction box. A further problem is that the highest loads, i.e., the highest solar currents, usually occur when both ambient and module temperatures are high. This problem is exacerbated with larger solar modules, as their short-circuit or rated current increases proportionally to their surface area.

[0005] Dissipating heat from bypass diodes in the junction box of solar modules presents a challenge for passive components. Therefore, a high operating temperature for the diodes is often accepted, which also results in particular challenges and disadvantages for the assembly technology; cooling is especially problematic, and consequently, expensive and technically complex manufacturing processes and materials must be used.

[0006] Active bypass diodes can be used, which incorporate a suitable semiconductor switch to bypass sub-modules of a solar module string. This has the disadvantage that, when the bypass is activated, the solar module cannot contribute to the power supply of a safety circuit that controls the semiconductor switches. Therefore, another power source must be used, incurring additional costs. To prevent a bypass circuit from remaining in a static, powerless state, the associated control circuit is typically powered by the solar module connected to the input of the bypass circuit.

[0007] Such a circuit can advantageously be used in a photovoltaic system for switching individual solar modules on and off. For example, DE 10 2006 026 661 A1 discloses a solar module for a PV system that reduces the power loss of the solar module in the event of shading or a defect. For this purpose, a semiconductor switching element is connected in parallel to the solar module, and control electronics are provided that block the semiconductor switching element when the solar module is generating current and otherwise switch it to a low-impedance state to bypass the solar module.

[0008] The invention is therefore based on the objective of providing a circuit for a solar module that takes over the function of a bypass diode, while providing low power loss, being simple in design, and ensuring long-term reliability.

[0009] The idea of ​​protection by means of a bypass diode is not limited to the field of solar modules, but is also known in other areas with similar technical requirements, meaning that the inventive solution is also applicable to such technical problems.

[0010] The object of the invention is achieved by a bridging and connecting circuit with the features of independent claim 1. Dependent claims 8 and 10 relate to a solar module with such a bridging and connecting circuit, and a photovoltaic system with a string of such solar modules, respectively. Dependent claims 2 to 7 relate to preferred embodiments of the bridging and connecting circuit, and dependent claim 9 relates to a preferred embodiment of such a solar module. Advantageous further developments and improvements are possible by means of the measures specified in the dependent claims.

[0011] A bridging and connecting circuit according to the invention has a first connection for connecting to a positive pole of a solar generator and a second connection for connecting to a negative pole of the solar generator and comprises: - a series circuit arranged between the first and second terminals comprising a first diode, a capacitor and a first semiconductor switch, wherein an antiparallel body diode of the first semiconductor switch, or a diode connected antiparallel to the first semiconductor switch, and the first diode have a common orientation in the forward direction of the first semiconductor switch towards the second terminal, - a second semiconductor switch arranged in parallel to the partial series circuit of first diode and capacitor, wherein an antiparallel body diode of the second semiconductor switch, or a diode connected antiparallel to the second semiconductor switch, has an orientation opposite to that of the first diode, and - a control circuit designed and configured to switch at least the second semiconductor switch on when a voltage applied to the capacitor exceeds a first threshold, and to switch it off when the voltage falls below a second threshold that is lower than the first threshold.

[0012] The proposed circuit behaves between its two terminals like a diode with very low forward losses, exhibiting very low power dissipation compared to passive diodes, but a longer reverse recovery time.

[0013] The central idea of ​​the bridging and connecting circuit according to the invention is to temporarily interrupt the current flow by means of the second semiconductor switch, which is connected in series to the first semiconductor switch, wherein the first semiconductor switch represents the actual active switching element in this circuit, in order to generate a voltage drop from which the control circuit can draw power.

[0014] This enables the semiconductor switch's control circuit to be self-powered, with the overall circuit requiring only two terminals, similar to a diode, and its behavior not being influenced by additional external control signals. The invention takes advantage of the fact that in a system with current-limited sources, such as solar generators, it is not a significant disadvantage if the circuit transitions from a conducting to a blocked state with a delay when the current direction is reversed.

[0015] The semiconductor switches, preferably implemented as transistors, can, when appropriately controlled, block current flow or conduct current with significantly lower losses than a passive diode, such as a PN diode or Schottky diode. The second semiconductor switch, connected in anti-series to the first, allows the current flow to be interrupted in any direction.

[0016] The circuit according to the invention does not require any further control signals. Rather, the only criterion used for the switching state of the semiconductor switches is the level of the supply voltage of the control circuit, i.e., the voltage applied to the capacitor.

[0017] When the capacitor used as an energy storage device is discharged, current can flow from the first terminal through the first diode, the capacitor, and the body diode of the first semiconductor switch to the second terminal. This charges the capacitor, and the voltage across it increases. If the voltage across the capacitor exceeds a certain threshold, the control circuit closes the second semiconductor switch, thus enabling conduction. This allows direct current flow from the first terminal through the second semiconductor switch and the first semiconductor switch to the second terminal.

[0018] Due to the power consumption of the control circuit, the voltage across the capacitor gradually decreases. When this voltage falls below a second threshold, which is lower than the first, the control circuit opens the second semiconductor switch, i.e., it switches to the off state. This restarts the charging cycle of the capacitor until the voltage across the capacitor again exceeds the first threshold.

[0019] In this way, a constant supply to the control circuit via the capacitor is ensured in the forward direction of the circuit.

[0020] If the current direction reverses and an unwanted reverse current flows through the circuit, this current can initially flow from the second terminal, through the first semiconductor switch and the second semiconductor switch, back to the first terminal, especially if the supply voltage of the drive circuit is higher than the lower, second threshold value (i.e., the smaller of the two voltage thresholds). In this case, the storage capacitor also discharges. As soon as the voltage across the capacitor falls below the lower threshold value, the drive circuit opens the second semiconductor switch, which then switches to its off state. However, in this case, the overall circuit does not return to the capacitor's charging cycle because the first semiconductor switch is now reverse-biased with respect to current flow.As long as the polarity of the voltage between the second and first terminals does not reverse, the capacitor will not be recharged and the entire circuit will remain in this blocked state. The self-power supply of the control circuit is therefore designed so that a supply voltage can only build up when the current flows in the desired direction. This means that the entire circuit switches to the desired conducting or blocked state without any further sensors or control mechanisms.

[0021] A particularly advantageous embodiment of the bypass circuit is one in which the control circuit controls the first semiconductor switch together with the second semiconductor switch. In this way, the first semiconductor switch is also switched depending on the voltage thresholds. If the voltage across the capacitor exceeds the first, upper threshold, both the first and second semiconductor switches are then switched on, allowing a direct path from the first terminal to the second terminal. This minimizes power loss through the body diode of the first semiconductor switch. When the current direction reverses and the voltage across the capacitor, i.e., the supply voltage of the control circuit, falls below the lower threshold, the control circuit switches off, and the first semiconductor switch switches to the off state, thus actively interrupting the current flow.Both semiconductor switches are controlled by a common control circuit, which switches both semiconductor switches on as soon as a sufficient supply voltage is available. If the supply voltage falls below a minimum value, both semiconductor switches are switched off together. This design can be particularly advantageous in current-limited systems, such as solar cells.

[0022] In one embodiment of the bridging circuit, the first semiconductor switch has a further control circuit which is designed and configured to block the first semiconductor switch in response to a current flowing from the second terminal via the first semiconductor switch to the first terminal, i.e., to block the current flow, and to close the first semiconductor switch in response to a current flowing from the first terminal via the first semiconductor switch to the second terminal, i.e., to allow the current path to pass through.The advantage of this variant of the bridging and connecting circuit according to the invention, through the provision of a further control circuit configured to switch only the first semiconductor switch, lies in the immediate blocking of the first semiconductor switch when the current direction changes from forward direction (i.e., from the first to the second terminal) to reverse direction (i.e., from the second terminal to the first terminal). This variant of the invention is preferable in applications where a slight time delay in blocking the entire circuit is undesirable.

[0023] Advantageously, the first semiconductor switch is directly connected to the second terminal. Equally advantageously, the first diode is directly connected to the first terminal. In particular, the anode of the first diode is directly connected to the first terminal. This allows a solar module connected directly to both terminals to provide a supply voltage or a charging voltage for the capacitor even when the solar module is de-energized within a photovoltaic system. The overall circuit according to the invention thus directly forms a bypass path for a connected solar module.

[0024] In an advantageous embodiment, the capacitor is arranged between the first diode and the first semiconductor switch. This optimizes the charging cycle of the capacitor. In particular, it prevents any residual capacitance of the capacitor from negatively affecting the reverse recovery time of the circuit when the desired current direction is reversed.

[0025] The semiconductor switches can be implemented as IGBTs (Insulated-Gate Bipolar Transistors), HEMTs (High-Electron Mobility Transistors), or FETs (Field-Effect Transistors). It is particularly advantageous for the first and / or second semiconductor switch to be implemented as a MOSFET (Metal-Oxide Semiconductor Field-Effect Transistor). A semiconductor switch implemented as a MOSFET is characterized by a lower on-resistance compared to an IGBT. This minimizes the power dissipation across the first switch when it is closed. Consequently, the necessary cooling of the semiconductor switches can be implemented more simply and cost-effectively, or even eliminated entirely. The body diode inherent in the MOSFET can briefly handle the entire solar generator current and, at low solar generator currents, can be used continuously as a bypass diode.A lower blocking voltage of a MOSFET compared to an IGBT can be tolerated if a sufficient number of solar modules in a series connection incorporate a bridging and interconnection circuit according to the invention. In this way, the voltage to be blocked is distributed among the number of MOSFETs present in the series connection of solar modules.

[0026] Another aspect of the invention relates to a solar module to which a bridging circuit according to the invention is connected in parallel. By connecting the solar module in parallel with the bridging and connecting circuit according to the invention, the latter functionally acts as a kind of bypass diode for the solar module. In this application, the current requirement of the drive circuit is negligibly small compared to the load current. As a result, in the forward direction, the charging cycle time during which the capacitor is recharged is very short compared to the time during which the first and second semiconductor switches are switched on. This results in a very low average voltage drop, and the power dissipation and thermal load are very low compared to a passive diode.Furthermore, in this case, the second semiconductor switch can preferably be equipped with a lower reverse voltage withstand capability, thereby reducing both its contribution to overall losses and its contribution to overall costs. The bypass circuit according to the invention can be inherently connected to the solar module to implement a bypass path, or it can be part of an external protection circuit configured to lock, unlock, or discharge the solar module in response to an external communication signal.

[0027] In one embodiment, the solar module comprises a plurality of sub-modules, each sub-module having a bypass circuit according to the invention connected in parallel. Because each associated sub-module of the solar module has its own bypass circuit, the maximum voltage load on all or at least some of the semiconductor switches can be limited to the open-circuit voltage of a sub-module. This allows the use of significantly less expensive semiconductor switches than if the entire solar module had to be de-energized by a bypass circuit. Semiconductor switches with lower internal resistance can also be selected, thus reducing electrical losses during normal operation and, in particular, improving the thermal load on the components used.

[0028] Another aspect of the invention relates to a photovoltaic system with a string of the described solar modules and an inverter. A photovoltaic (PV) system according to the invention comprises an inverter with at least one series connection of solar modules, a so-called string of solar modules, connected to the inverter. At least one of the solar modules has a bridging and connection circuit according to the invention. Advantageously, several solar modules of the string have a bridging and connection circuit according to the invention. The number of these circuits can be selected such that any voltage occurring within the string of solar modules is always kept below a maximum permissible voltage. In one embodiment, all solar modules of the string have the circuit according to the invention.In a further embodiment, at least one partial module of at least one solar module of the string of solar modules has the circuit according to the invention.

[0029] The invention is illustrated below with the aid of figures, of which Fig. 1 shows an embodiment of the bridging and connecting circuit according to the invention in an operating mode, Fig. 2 shows an embodiment of the bridging and connecting circuit according to the invention in a different operating mode and Fig. Figure 3 shows another embodiment of the circuit according to the invention.

[0030] The Fig. Figure 1 shows the structure of a first embodiment of a bridging and connecting circuit according to the invention. For simplicity, this will be referred to below as the overall circuit 1. The overall circuit 1 has a first terminal 11 and a second terminal 22. If the overall circuit 1 is used as an active bypass circuit for a solar module, the first terminal 11 is provided for connection to a positive terminal of a solar module and the second terminal 22 is provided for connection to a negative terminal of a solar module. The first terminal 11 is connected to the second terminal 22 via a series circuit consisting of a first diode 2, a capacitor 3, and a first semiconductor switch S1. The first semiconductor switch S1 is controlled via a control circuit 4. The control circuit 4 is supplied via an energy storage device, which in this case is formed by the capacitor 3.Furthermore, the overall circuit 1 includes a second semiconductor switch S2. Both semiconductor switches S1 and S2 are shown in the figures with a diode connected in parallel. These diodes are to be understood as part of the respective semiconductor switch S1 or S2 and can be formed by the body diode of the respective semiconductor switch S1 or S2. The second semiconductor switch S2 is connected in parallel to a portion of the series circuit consisting of the first diode 2 and capacitor 3 and is also driven by the control circuit 4. The first diode 2 is oriented with respect to its forward bias such that it has a forward bias from the first terminal 11 to the second terminal 22 and a reverse bias from the second terminal 22 to the first terminal 11.The first semiconductor switch S1 is arranged such that it shares a forward bias orientation with the first diode 2, pointing towards the second terminal 22; that is, its body diode also has the same bias orientation as the first diode 2. The second semiconductor switch S2 is oriented oppositely to the first diode 2 with respect to its forward bias, or rather the forward bias orientation of its body diode. The semiconductor switches S1 and S2 are preferably implemented as transistors, in particular as MOSFETs.

[0031] The control circuit 4 is designed and configured to switch at least the second semiconductor switch S2, and preferably also the semiconductor switch S1, on when a voltage applied to the capacitor 3 exceeds a first threshold value, and to switch it off when the voltage falls below a second threshold value, which is lower than the first. The control circuit 4 can preferably be implemented as a simple cut-off trigger. This automatically keeps the supply voltage within the desired range.

[0032] Depending on the switching states of the two semiconductor switches S1 and S2, two different current paths are thus opened between the first terminal 11 and the second terminal 22.

[0033] When the capacitor 3, used as an energy storage device, is discharged, a current can flow from the first terminal 11 via the first diode 2, the capacitor 3, and the body diode of the first semiconductor switch S1 to the second terminal 22. This charges the capacitor 3 and increases the voltage across it. This current path is described in Fig. 1 is indicated by the dashed line. If the voltage applied to capacitor 3 exceeds a first threshold value, the control circuit 4 closes the second semiconductor switch S2, thus making it conducting. This allows a direct current flow from the first terminal 11 via the second semiconductor switch S2 and the first semiconductor switch S1 to the second terminal. This current path is shown by the dashed line in Fig. 2 shown, which in relation to the embodiment of the overall circuit 1 with the in Fig. The embodiment shown in Figure 1 is identical. In this case, the current can flow through the body diode of the first semiconductor switch S1. Preferably, both semiconductor switches S1 and S2 are controlled together, so that the first semiconductor switch S1 is also switched to conduct, thus preventing losses across the body diode of semiconductor switch S1.

[0034] In this switching state, the voltage across capacitor 3 gradually decreases again due to the self-consumption of the control circuit 4. When this voltage falls below a second threshold value, which is lower than the first, the second semiconductor switch S2, and, if controlled together, also the first semiconductor switch S1, are opened again by the control circuit 4, i.e., switched back to the off state. This again activates the Fig. 1. The charging cycle of capacitor 3 shown is started until the voltage applied to capacitor 3 again exceeds the first threshold value.

[0035] If the current direction reverses and an unwanted reverse current flows through the entire circuit 1, this current can initially flow from the second terminal 22 via the first semiconductor switch S1 and the second semiconductor switch S2 to the first terminal 11. This also discharges the capacitor 3. As soon as the voltage across capacitor 3 falls below the lower threshold, the control circuit 4 switches semiconductor switches S2 and S1 off. There is therefore a slight reverse bias time of the entire circuit 1 until the lower threshold is crossed and the first semiconductor switch S1 is off. In this case, however, the entire circuit 1 does not return to the charging cycle of capacitor 3, since the first semiconductor switch S1 is now reverse-biased with respect to current flow.As long as the polarity of the voltage between the second terminal 22 and the first terminal 11 does not reverse, the capacitor 3 is not recharged and the entire circuit 1 remains in this blocked state. The entire circuit 1 thus switches to the desired conducting or blocked state without any further sensors or control.

[0036] In Fig. Figure 3 shows another embodiment of the overall circuit 1. This differs from the ones shown in Fig. 1 and Fig.The only difference between the two embodiments shown is that the first semiconductor switch S1 has a further, separate control circuit 5, which is designed and configured to switch the first semiconductor switch S1 off in response to a current flowing from the second terminal 22 via the first semiconductor switch S1 to the first terminal 11, i.e., to block the current path, and to close it in response to a current flowing from the first terminal 11 via the first semiconductor switch S1 to the second terminal 22, i.e., to enable conduction and allow the current path to flow in the forward direction. Thus, the first semiconductor switch S1 can be immediately blocked when the current direction changes from the forward direction, i.e., from the first terminal 11 to the second terminal 22, to the reverse direction, i.e., from the second terminal 22 to the first terminal 11.This variant of the invention is preferable in applications where a slight time delay in the blocking of the entire circuit 1 is undesirable. The control circuit 5 can, for example, be formed by a comparator that detects the drain-source voltage of the first semiconductor switch S1 or the voltage between the first terminal 11 and the second terminal 22. Reference symbol list 1 Bridging and connecting circuit 2 diode 3 Capacitor 4 Control circuit 5 Control circuit 11 connection 22 S1 semiconductor switches S2 semiconductor switch

Claims

[1] Bridging and connecting circuit (1) with a first connection (11) for connecting to a positive pole of a solar generator and a second connection (22) for connecting to a negative pole of the solar generator, comprising: - a series circuit arranged between the first terminal (11) and the second terminal (22) comprising a first diode (2), a capacitor (3) and a first semiconductor switch (S1), wherein an antiparallel body diode of the first semiconductor switch (S1), or a diode connected antiparallel to the first semiconductor switch (S1), and the first diode (2) have a common orientation in a forward direction of the first semiconductor switch (S1) towards the second terminal (22), - a second semiconductor switch (S2) arranged in parallel to the part of the series circuit consisting of the first diode (2) and capacitor (3), wherein an antiparallel body diode of the second semiconductor switch (S2), or a diode connected antiparallel to the second semiconductor switch (S2), has an orientation opposite to that of the first diode (2), and - a control circuit (4) which is designed and configured to switch at least the second semiconductor switch (S2) on when a voltage applied to the capacitor (3) exceeds a first threshold value, and to switch it off when the voltage falls below a second threshold value which is lower than the first threshold value. [2] Bridging and connecting circuit (1) according to claim 1, wherein the control circuit (4) controls the first semiconductor switch (S1) together with the second semiconductor switch (S2). [3] Bridging and connecting circuit (1) according to claim 1, wherein the first semiconductor switch (S1) has a further control circuit (5) which is provided and configured to switch the first semiconductor switch (S1) to blocking in response to a current flowing from the second terminal (22) via the first semiconductor switch (S1) to the first terminal (11) and to conducting in response to a current flowing from the first terminal (11) via the first semiconductor switch (S1) to the second terminal (22). [4] Bridging and connecting circuit (1) according to one of the preceding claims, wherein the first semiconductor switch (S1) is directly connected to the second terminal (22). [5] Bridging and connecting circuit (1) according to one of the preceding claims, wherein the first diode (2) is directly connected to the first terminal (11). [6] Bridging and connecting circuit (1) according to one of the preceding claims, wherein the capacitor (3) is arranged between the first diode (2) and the first semiconductor switch (S1). [7] Bridging and connecting circuit (1) according to one of the preceding claims, wherein the first semiconductor switch (S1) and / or the second semiconductor switch (S2) are designed as a MOSFET. [8] Solar module to which a bridging and connecting circuit (1) according to one of the preceding claims is connected in parallel. [9] Solar module, wherein the solar module has a plurality of sub-modules, wherein a bridging and connecting circuit (1) according to any one of claims 1 to 7 is connected in parallel to each sub-module. [10] Photovoltaic system comprising a string of solar modules according to claim 8 or 9, and an inverter.

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