Abstract
A circuit for combining direct current (DC) power including multiple direct current (DC) voltage inputs; multiple inductive elements. The inductive elements are adapted for operatively connecting respectively to the DC voltage inputs. Multiple switches connect respectively with the inductive elements. A controller is configured to switch the switches periodically at a frequency sufficiently high so that direct currents flowing through the inductive elements are substantially zero. A direct current voltage output is connected across one of the DC voltage inputs and a common reference to both the inputs and the output.
Claims
1-17. (canceled)
18. A method comprising: connecting a first set of inductive elements, via a first set of switches, to a first input configured to receive a first voltage, and connecting a second set of inductive elements, via a second set of switches, to a second input configured to receive a second voltage; operating the first set of switches and the second set of switches based on a respective duty cycle, wherein each respective duty cycle is based on a winding ratio between the respective set of inductive elements; and outputting, based on the operation of the first set of switches and the second set of switches, an output voltage.
19. The method of claim 18, wherein the operating comprises operating the first set of switches based on a first respective duty cycle substantially equal to fifty percent.
20. The method of claim 19, wherein the operating comprises operating the second set of switches based on a second respective duty cycle substantially equal to fifty percent.
21. The method of claim 18, further comprising generating at least a portion of the first voltage or at least a portion of the second voltage using at least one photovoltaic cell.
22. The method of claim 18, further comprising opening and closing the first set of switches and the second set of switches at a frequency to combine the first voltage and the second voltage while reducing direct current to flow through the first set of inductive elements or the second set of inductive elements.
23. The method of claim 18, further comprising combining the first voltage with the second voltage.
24. The method of claim 18, further comprising connecting a third set of inductive elements, via a third set of switches, to a third input configured to receive a third voltage, wherein the output voltage is a combination of the first voltage, the second voltage, and the third voltage.
25. An apparatus comprising: a first set of inductive elements; a second set of inductive elements; a first set of switches; a second set of switches; a first input; a second input; and an output, wherein: the first set of inductive elements is connected via the first set of switches to the first input, the first input is configured to receive a first voltage, the second set of inductive elements is connected via the second set of switches to the second input, the second input is configured to receive a second voltage; the first set of switches and the second set of switches are configured to be operated based on a respective duty cycle, wherein each respective duty cycle is based on a winding ratio between the respective set of inductive elements; and the output is configured to output, based on the operation of the first set of switches and the second set of switches, an output voltage.
26. The apparatus of claim 25, wherein the first set of switches is configured to operate based on a first respective duty cycle substantially equal to fifty percent.
27. The apparatus of claim 26, wherein the second set of switches is configured to operate based on a second respective duty cycle substantially equal to fifty percent.
28. The apparatus of claim 25, wherein at least one of the first input or the second input is connected to at least one photovoltaic cell.
29. The apparatus of claim 25, wherein the first set of switches and the second set of switches are configured to open and close at a frequency to combine the first voltage and the second voltage while reducing direct current to flow through the first set of inductive elements or the second set of inductive elements.
30. The apparatus of claim 25, wherein the output voltage is a combination of the first voltage and the second voltage.
31. The apparatus of claim 25, further comprising: a third set of inductive elements; a third set of switches; and a third input, wherein: the third set of switches is connected to the third input, the third input is configured to receive a third voltage, and the output voltage is a combination of the first voltage, the second voltage, and the third voltage.
32. A system comprising: a voltage source configured to generate a first voltage and a second voltage; a first set of inductive elements; a second set of inductive elements; a first set of switches; a second set of switches; a first input; a second input; and an output, wherein: the first set of inductive elements is connected via the first set of switches to the first input, the first input is connected to the voltage source and configured to receive the first voltage, the second set of inductive elements is connected via the second set of switches to the second input, the second input is connected to the voltage source and configured to receive the second voltage; the first set of switches and the second set of switches are configured to operate based on a respective duty cycle, wherein each respective duty cycle is based on a winding ratio between the respective set of inductive elements; and the output is configured to output, based on the operation of the first set of switches and the second set of switches, an output voltage.
33. The system of claim 32, wherein the first set of switches is configured to operate based on a first respective duty cycle substantially equal to fifty percent.
34. The system of claim 33, wherein the second set of switches is configured to operate based on a second respective duty cycle substantially equal to fifty percent.
35. The system of claim 32, wherein the first set of switches and the second set of switches are configured to open and close at a frequency to combine the first voltage and the second voltage while reducing direct current to flow through the first set of inductive elements or the second set of inductive elements.
36. The system of claim 32, wherein the output voltage is a combination of the first voltage and the second voltage.
37. The system of claim 32, further comprising: a third set of inductive elements; a third set of switches; and a third input, wherein: the third set of switches is connected to the third input, the voltage source is configured to generate a third voltage, the third input is connected to the voltage source and configured to receive the third voltage, and the output voltage is a combination of the first voltage, the second voltage, and the third voltage.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:
[0018] FIG. 1 is a graph illustrating typical spectra of solar irradiance and solar absorption of a single photovoltaic junction, according to conventional art.
[0019] FIG. 2 is a graph illustrating three different absorption spectra of three stacked photovoltaic junctions of a multi-junction photovoltaic cell, according to conventional art.
[0020] FIG. 3 illustrates serially connected multi-junction cells, according to conventional art.
[0021] FIG. 4 illustrates a current-voltage (TV) characteristic curve (arbitrary units) of a photovoltaic cell at three different illumination levels, according to conventional art.
[0022] FIGS. 5a and 5b illustrates a typical photovoltaic installation operating in during dark or partially shaded conditions and bright mode respectively, according to conventional art.
[0023] FIG. 6 illustrates a block diagram of photovoltaic installation with a power combiner according to an embodiment of the present invention.
[0024] FIG. 7 illustrates a power combiner circuit, according to an embodiment of the present invention.
[0025] FIG. 8 illustrates a power combiner circuit, according to another embodiment of the present invention.
[0026] FIG. 9 illustrates a photovoltaic system including multiple power combiners, according to an exemplary embodiment of the present invention.
[0027] FIG. 10 illustrates a flow diagram of a method, according to an embodiment of the present invention.
[0028] The foregoing and/or other aspects will become apparent from the following detailed description when considered in conjunction with the accompanying drawing figures.
DETAILED DESCRIPTION
[0029] Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures.
[0030] By way of introduction, different embodiments of the present invention are directed toward compensating for current variations in multiple junctions cells or in serially connected photovoltaic cells and/or panels such as during partial shading while maximizing power gain, by avoiding the loss of power from one or more photovoltaic cells and/or panels shorted by the cells and/or panels respective bypass diode.
[0031] Reference is now made back to FIG. 3, which illustrates conventionally multiple multi-junction cells 30 connected in series, each with multiple serially connected photovoltaic junctions 300, 302, and 304. It is well known that the spectrum of solar irradiance on the Earth's surface is not a constant but varies according to many variables such as season, geographic location, time of day, altitude, atmospheric conditions and pollution. Hence, it becomes apparent that photovoltaic junctions 300, 302, and 304 sensitive to different spectrum bands may be absorbing a different amount of light depending on season, geographic location, time of day, altitude, atmospheric conditions and pollution. Since photovoltaic junctions 300, 302, and 304 are connected in series, the same current flows through all of the junctions. Thus, the best power point of serially connected photovoltaic junctions 300, 302, and 304 maximizes the overall power from photovoltaic junctions 300, 302, and 304, while each junction is typically producing a less than optimal amount of electrical power. On the other hand, a parallel connection of photovoltaic junctions and/or multi-junction cells, while allowing a better maximum power control for all photovoltaic junctions or multi-junction cells suffers among other possible power losses from an increase of ohmic power loss of the system since ohmic power loss is proportional to the square of the current. Furthermore, a parallel electrical connection of stacked pn junctions in a multi-junction cell is not particularly practical since multi-junction cells are typically stacked in a single production process and since the MPP voltage of each of these stacked pn junctions is different; the bandgap voltage for each pn junction is different.
[0032] The present invention in different embodiments may be applied to multiple photovoltaic cells and/or multi-junction photovoltaic cells connected in various series and parallel configurations with power converters/combiners to form a photovoltaic panel. Multiple series and parallel configurations of the photovoltaic panel and substrings within a panel with multiple power converters/combiners are used to form a photovoltaic installation. The present invention in further embodiments may be applied to other direct current power sources including batteries, fuel cells and direct current generators.
[0033] Embodiments of the present invention may be implemented by one skilled in the electronics arts using different inductive circuit elements such as transformers, auto-transformers, tapped coils, and/or multiple coils connected in serial and/or in parallel and these devices may be connected equivalently to construct the different embodiments of the present invention.
[0034] The terms “common”, “common terminal, “common reference” are used herein interchangeably referring to a reference common to both inputs and the output in the context of embodiments of the present invention. Typically, “common terminal” is ground, but the whole circuit may also be ungrounded. References to common terminal as ground are only illustrative and made for the reader's convenience.
[0035] Reference is now made to FIG. 6 which illustrates a block diagram of photovoltaic installation 600 with a power combiner 604 according to an embodiment of the present invention. A photovoltaic panel 60 has three photovoltaic cells 606a-606c connected in series. Photovoltaic cells 606a-606c are preferably multi-junction photovoltaic cells, photovoltaic cells or other direct current sources. An anode and cathode of a bypass diode D.sub.1 connects across in parallel with photovoltaic cell 606c at node F and node A respectively. An anode and cathode of a bypass diode D.sub.2 connects across in parallel with photovoltaic cell 606b at node A and node B respectively. An anode and cathode of a bypass diode D.sub.3 connects across in parallel with photovoltaic cell 606a at node B and node C respectively. Voltages V.sub.1, V.sub.2 and V.sub.3 are the voltage outputs of photovoltaic cells 606c, 606b and 606a respectively. Voltages V.sub.1, V.sub.2 and V.sub.3 are applied to three voltage inputs of power combiner 604 as between nodes C & B, B & A and nodes A & F respectively. Power combiner 604 has a single output voltage V.sub.out.
[0036] Reference is now made to FIG. 7 which illustrates, according to an embodiment of the present invention, circuit details of DC power combiner 604. Three voltages V.sub.1, V.sub.2 and V.sub.3 are input to power combiner 604 between nodes A and F, nodes B and A and nodes C and B respectively. Node B is on a “shared input terminal” of V.sub.2 and V.sub.3. Similarly, node A is on a “shared input terminal” of V.sub.1 and V.sub.2. One end of inductor L.sub.1 connects to node C, the other end of inductor L.sub.1 connects to one end of inductor L.sub.3 to form node W. The other end of inductor L.sub.3 connects to one end of inductor L.sub.5 to form node X. The other end on inductor L.sub.5 connects to the drain of MOSFET G.sub.1 and the source of G.sub.1 connects to node F (ground). One end of inductor L.sub.2 connects to node C, the other end of inductor L.sub.2 connects to one end of inductor L.sub.4 to form node D. The other end of inductor L.sub.4 connects to one end of inductor L.sub.6 to form node E. The other end on inductor L.sub.5 connects to the drain of MOSFET G.sub.2 and the source of MOSFET G.sub.2 connects node F (ground). The drain of MOSFET G.sub.5 is connected to node W, the source of MOSFET G.sub.5 connects to the source of MOSFET G.sub.6. The drain of MOSFET G.sub.6 connects to node D. The drain of MOSFET G.sub.4 is connected to node X, the source of MOSFET G.sub.4 connects to the source of MOSFET G.sub.3. The drain of MOSFET G.sub.3 connects to node E. The output voltage V.sub.out of power combiner 604 is derived between nodes C and F (ground). A transformer core 601 is used to electromagnetically couple all inductors L.sub.5, L.sub.6, L.sub.3, L.sub.4, L.sub.1 and L.sub.2. The winding polarity of L.sub.5, L.sub.3 and L.sub.1 is preferably opposite of the winding polarity of L.sub.6, L.sub.4 and L.sub.2. The two inductors within each of the inductor pairs L.sub.5-L.sub.6, L.sub.3-L.sub.4 and L.sub.1-L.sub.2 typically have the same number of winding turns, although there can be a different number of turns to each of the inductor pairs (eg. L1 and L2, L3 and L4 and L5 and L6), to adjust the typical relative MPP voltage of each of the input voltages. Each of the three voltages V.sub.1, V.sub.2 and V.sub.3 are applied across each of inductors L.sub.5, L.sub.3 and L.sub.1 respectively with for instance a 50% duty cycle when switches G1, G4 and G5 are closed and switches G2, G3 and G6 are opened. Each of the three voltages V.sub.1, V.sub.2 and V.sub.3 are applied across each of the inductors L.sub.6, L.sub.4 and L.sub.2 respectively with typically a 50% duty cycle when switches G1, G4 and G5 are opened and switches G2, G3 and G6 are closed, thus completing a full switching cycle. The output voltage (V.sub.out) of power combiner 604 is the sum of the input voltages V.sub.1, V.sub.2 and V.sub.3. The input voltages V.sub.1, V.sub.2 and V.sub.3 of power combiner 604 are forced by power combiner 604 to have the same ratio as the winding ratio of their inductor pair (L.sub.5, L.sub.6), (L.sub.3, L.sub.4) and (L.sub.1, L.sub.2) respectively; a result of applying control pulses to switches G.sub.1-G.sub.6 for instance with a 50% duty cycle. Switches G.sub.1-G.sub.6 are optionally metal oxide semiconductor field-effect transistors (MOSFET). Alternatively the switches can, in different embodiments of the invention, be a silicon controlled rectifier (SCR), insulated gate bipolar junction transistor (IGBT), bipolar junction transistor (BJT), field effect transistor (FET), junction field effect transistor (JFET), switching diode, mechanically operated single pole double pole switch (SPDT), SPDT electrical relay, SPDT reed relay, SPDT solid state relay, insulated gate field effect transistor (IGFET), DIAC, and TRIAC.
[0037] Reference is now made to FIG. 8 which illustrates, according to another embodiment of the present invention, an alternative circuit of DC power combiner 604. Three voltages V.sub.1, V.sub.2 and V.sub.3 are input to power combiner 604 between nodes A & F, B & A and nodes C & B respectively. One end of inductor L.sub.1 connects to node C, the other end of inductor L.sub.1 connects to the drain of MOSFET G.sub.1 the source of G.sub.1 connects to node B. One end of inductor L.sub.3 connects to node B, the other end of inductor L.sub.3 connects to the drain of MOSFET G.sub.3, the source of G.sub.3 connects to node A. One end of inductor L.sub.5 connects to node A, the other end of inductor L.sub.5 connects to the drain of MOSFET G.sub.5, the source of G.sub.5 connects to node F (ground). One end of inductor L.sub.2 connects to node C, the other end of inductor L.sub.2 connects to the drain of MOSFET G.sub.2, the source of G.sub.2 connects to node B. One end of inductor L.sub.4 connects to node B, the other end of inductor L.sub.4 connects to the drain of MOSFET G.sub.4, the source of G.sub.4 connects to node A. One end of inductor L.sub.6 connects to node A, the other end of inductor L.sub.6 connects to the drain of MOSFET G.sub.6, the source of G.sub.6 connects to node F (ground). The output voltage V.sub.out of power combiner 604 is derived between nodes C and F (ground). A transformer core 601 is used to electromagnetically couple all inductors L.sub.5, L.sub.6, L.sub.3, L.sub.4, L.sub.1 and L.sub.2. The winding polarity of L.sub.5, L.sub.3 and L.sub.1 is preferably opposite of the winding polarity of L.sub.6, L.sub.4 and L.sub.2 respectively. The two inductors within each of the inductor pairs (L.sub.5 and L.sub.6), (L.sub.3 and L.sub.4) and (L.sub.1 and L.sub.2) preferably have the same number of winding turns, although there can be a different number of turns to each of the inductor pairs, so as to adjust the typical relative MPP voltage of each of the input voltages.
[0038] Reference is now made to FIG. 9 which illustrates a photovoltaic system 90 including multiple power combiners 604, according to an exemplary embodiment of the present invention. Photovoltaic system 90 has multiple series strings 902 connected in parallel to the input of DC to AC converter 900. Series strings 902 have photovoltaic cells 904a-904c which are for instance multi-junction photovoltaic cells which have three voltage outputs V.sub.1, V.sub.2 and V.sub.3 with three bypass diodes connected across each voltage output of photovoltaic cells 904a-904c. Connected to each photovoltaic cells 904a-904c is a three voltage input power combiner 604. Power combiner 604 has a single voltage output (V.sub.out) which is applied across the input of DC to DC converters 92a-92c. The outputs of DC to DC converters 92a-92c are connected in series to form the input to DC to AC converter 900 and the output of multiple series strings 902.
[0039] Reference is now made to FIG. 10 which illustrates a method 10 according to an embodiment of the present invention. In step 11, DC voltage inputs are connected to inductive elements. In step 13, the inductive elements are switched at a high frequency dependent on the inductance values so that the inductive elements do not tend to “short” the input DC voltages. In step 15, a single output combines the DC inputs by connecting across typically the highest input voltage and a reference or ground common to both the DC inputs and the single output.
[0040] The definite articles “a”, “an” is used herein, such as “a multi-junction photovoltaic cell”, “a power combiner” or “a coil” have the meaning of “one or more multi-junction photovoltaic cells”, “one or more power combiners” or “one or more coils”.
[0041] Although selected embodiments of the present invention have been shown and described, it is to be understood the present invention is not limited to the described embodiments. Instead, it is to be appreciated that changes may be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and the equivalents thereof.
