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.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present applications benefits from U.S. application 61/050268 filed on May 5, 2008 by the same inventor. 
     
    
     FIELD AND BACKGROUND 
       [0002]    1. Field 
         [0003]    The present invention relates to energy conversion and specifically to circuitry which combines multiple voltage inputs from serially connected direct current sources into a combined output. 
         [0004]    2. Description of Related Art 
         [0005]    Sunlight includes a spectrum of electromagnetic radiation emitted by the Sun onto the surface of the Earth. On the Earth, sunlight is filtered through the atmosphere, and the solar irradiance (Watts/meter square/nanometer W/m 2 /nm) is obvious as daylight when the Sun is above the horizon. The Earth receives a total solar irradiance determined by its cross section (π·R E   2 , R E =radius of the earth), but as the Earth rotates the solar energy is distributed across the entire surface area (4·π·R E   2 ). The solar constant is the amount of incoming solar electromagnetic irradiance per unit area, measured on the outer surface of Earth&#39;s atmosphere in a plane perpendicular to the solar rays. The solar constant is measured by satellite to be roughly 1366 watts per square meter (W/m 2 ) or 1.366 W/m 2 /nm. Hence the average incoming solar irradiance, taking into account the angle at which the rays strike and that at any one moment half the planet does not receive any solar irradiance, is one-fourth the solar constant (approximately 0.342 W/m 2 /nm). At any given moment, the amount of solar irradiance received at a location on the Earth&#39;s surface depends on the state of the atmosphere and the location&#39;s latitude. 
         [0006]    The performance of a photovoltaic cell depends on the state of the atmosphere, the latitude and the orientation of the photovoltaic cell towards the Sun and on the electrical characteristics of the photovoltaic cell. 
         [0007]      FIG. 1  shows schematically a graph of a solar irradiance  100  versus wavelength. Irradiance  100  is distributed around a peak wavelength at about  550  nanometers.  FIG. 1  also shows schematically an absorption spectrum  102  of a typical solar photovoltaic (PV) cell with a given band-gap which allows only a portion of the solar irradiance to be converted into electrical power. The finite characteristic of the band-gap of the photovoltaic cell causes a substantial part of the sun&#39;s energy to remain unutilized. In order to improve photovoltaic efficiency, multiple junction cells have been designed which include multiple pn junctions. Solar irradiance not absorbed, because its energy is less than the band gap is transmitted to the next junction(s) with a smaller band gap and the transmitted radiation is preferentially absorbed and converted into electrical energy. 
         [0008]      FIG. 2  shows the graph of solar irradiance  100  versus wavelength and three absorption spectra  202 ,  204  and  206  respectively of three photovoltaic junctions used in a single multi junction cell designed to absorb different parts of the solar spectrum. The first photovoltaic junction having the largest band gap has an absorption spectrum  206 , the second photovoltaic junction has an absorption spectrum  204 , and the third photovoltaic junction which has the smallest band gap has an absorption spectrum  202 . Combining the three pn junctions of photovoltaic junctions into a single multi junction  30  cell increases the efficiency, theoretically to about 60% and practically today to above 40%. 
         [0009]      FIG. 3  illustrates multiple multi junction cells  30  connected in series. Each multi-junction cell  30  has three serially connected photovoltaic junctions  300 ,  302 , and  304  which operate with three absorption spectra  206 ,  204  and  202  respectively. Multiple multi junction cells  30  connected in series form a multi-spectral photovoltaic panel  3000  with output terminals  310  and  308 . 
         [0010]      FIG. 4  illustrates characteristic current-voltage curves of a single photovoltaic junction cell at different illumination levels. Curve  400  shows the maximum power point (MPP) for low light levels, curve  402  show the maximum power point MPP for higher light levels, and curve  404  shows the maximum power point MPP yet higher light levels assuming a constant temperature of the cell. As can be seen, at the different light levels the maximum power point is achieved at nearly identical voltages, but at different currents depending on the incident solar irradiance. 
         [0011]    Reference is now made to conventional art in  FIGS. 5   a  and  5   b  which shows a typical photovoltaic installation  50  operating in dark or partially shaded conditions and bright mode respectively. Bypass diodes  500   a - 500   c  are connected in parallel across photovoltaic panels  502   a - 502   c  respectively for instance according to IEC61730-2 solar safety standards (sec. 10.18). Photovoltaic panels  502   a - 502   c  are connected in series to form a serial string of photovoltaic panels. Referring to  FIG. 5   a,  bypass diode  500   a  provides a path  510  around photovoltaic panel  502   a  during dark or partially shaded conditions. Current path  510  allows current to flow through bypass diode  500   a  in the forward mode, preventing common thermal failures in photovoltaic panel  502   a  like cell breakdown or hot spots. During forward mode, bypass diode  500   a  preferably has low forward resistance to reduce the wasted power.  FIG. 5   b  refers to normal operation or bright mode, forward current  512  will flow through photovoltaic panels  502   a - 502   c  while bypass diodes  500   a - 500   c  will operate in the reverse blocking mode. In reverse blocking mode, it is important that bypass diodes  500   a - 500   c  have the lowest high temperature reverse leakage current (I R ) to achieve the highest power generation efficiency for each photovoltaic panel  502   a - 502   c.    
       BRIEF SUMMARY 
       [0012]    According to the present invention there is provided a circuit including multiple direct current (DC) voltage inputs which including one or more shared terminals. A primary transformer winding includes a high voltage end and a low voltage end. The primary transformer winding has a tap or taps operatively connected to the shared terminals through a first switch. A secondary transformer winding includes a high voltage end and a low voltage end. The secondary transformer winding is electromagnetically coupled to the primary transformer winding. The secondary transformer winding has one or more taps operatively connected to the shared terminal(s) through a second switch. A direct current voltage output terminal connects the high voltage ends of the primary and secondary transformer windings. A low voltage direct current output terminal operatively connecting said low voltage ends of said primary and secondary transformer windings. 
         [0013]    Diodes are typically connected in parallel with the first and second switches or the diodes are integrated with a transistor in a single package. The switches may be metal oxide semiconductor field effect transistor (MOSFET), junction field effect transistor (JFET), insulated gate field effect transistor (IGFET), n-channel field effect transistor, p-channel field effect transistor, silicon controlled rectifier (SCR) and/or bipolar junction transistor (BJT). A third switch optionally connects the low voltage end of the primary transformer winding to a common terminal; and a fourth switch optionally connects the low voltage end of the secondary transformer winding to the common terminal. Diodes are typically connected in parallel with the third switch and the fourth switch. Bypass diodes are operatively connected across the DC voltage inputs. Photovoltaic cells are optionally connected to the DC voltage inputs. The photovoltaic cells may be optimized for maximal optical absorption of different respective portions of the electromagnetic spectrum. The direct current voltage output terminal may be connected to a DC to DC converter. 
         [0014]    According to the present invention there is provided a circuit including multiple direct current (DC) voltage inputs; multiple transformers including primary windings and secondary windings; multiple first switches connected respectively in series with the primary windings into a multiple of switched primary windings; and multiple second switches connected respectively in series with the secondary windings into multiple switched secondary windings. The switched secondary windings are parallel connected respectively with the switched primary windings by the DC voltage inputs. The switched secondary windings are adapted for connecting to a combined direct current power output combining the DC voltage inputs. The first and second switches are: metal oxide semiconductor field effect transistor (MOSFET), junction field effect transistor (JFET), insulated gate field effect transistor (IGFET), n-channel field effect transistor, p-channel field effect transistor, silicon controlled rectifier (SCR) and/or bipolar junction transistor (BJT). 
         [0015]    According to the present invention there is provided a circuit for combining direct current (DC) power including multiple direct current (DC) voltage inputs; multiple tapped coils including respectively primary ends, secondary ends and taps. The taps are adapted for connecting individually to the DC voltage inputs. The first switches connect respectively in series with the tapped coils at the primary ends of the coils. The second switches connect respectively in series with the coils at the secondary ends of the coils. The taps serially connect respectively the first and second switches. A combined direct current power output is adapted for connecting between the tap of highest voltage and a reference to both the inputs and the output. 
         [0016]    According to the present invention there is provided 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. A direct current voltage output is connected across one of the DC voltage inputs and a reference to both the inputs and the output. 
         [0017]    According to the present invention there is provided a method for combining direct current (DC) power. Multiple direct current (DC) voltage inputs are connected to respective inductive elements. Multiple switches are connected respectively with the inductive elements. The switches are switched periodically. 
         [0018]    A direct current voltage output is combined by connecting across one of the DC voltage inputs and a reference common to both the DC voltage inputs and the direct current voltage output. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
           [0020]      FIG. 1  is a graph illustrating typical spectra of solar irradiance and solar absorption of a single photovoltaic junction, according to conventional art. 
           [0021]      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. 
           [0022]      FIG. 3  illustrates serially connected multi junction cells, according to conventional art. 
           [0023]      FIG. 4  illustrates a current-voltage (TV) characteristic curve (arbitrary units) of a photovoltaic cell at three different illumination levels, according to conventional art. 
           [0024]      FIGS. 5   a  and  5   b  illustrates a typical photovoltaic installation operating in during dark or partially shaded conditions and bright mode respectively, according to conventional art. 
           [0025]      FIG. 6  illustrates a block diagram of photovoltaic installation with a power combiner according to an embodiment of the present invention. 
           [0026]      FIG. 7  illustrates a power combiner circuit, according to an embodiment of the present invention. 
           [0027]      FIG. 8  illustrates a power combiner circuit, according to another embodiment of the present invention. 
           [0028]      FIG. 9  illustrates a photovoltaic system including multiple power combiners, according to an exemplary embodiment of the present invention. 
           [0029]      FIG. 10  illustrates a flow diagram of a method, according to an embodiment of the present invention. 
       
    
    
       [0030]    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 
       [0031]    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. 
         [0032]    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. 
         [0033]    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&#39;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. 
         [0034]    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. 
         [0035]    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. 
         [0036]    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&#39;s convenience. 
         [0037]    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  606   a - 606   c  connected in series. Photovoltaic cells  606   a - 606   c  are preferably multi-junction photovoltaic cells, photovoltaic cells or other direct current sources. An anode and cathode of a bypass diode D 1  connects across in parallel with photovoltaic cell  606   c  at node F and node A respectively. An anode and cathode of a bypass diode D 2  connects across in parallel with photovoltaic cell  606   b  at node A and node B respectively. An anode and cathode of a bypass diode D 3  connects across in parallel with photovoltaic cell  606   a  at node B and node C respectively. Voltages V 1 , V 2  and V 3  are the voltage outputs of photovoltaic cells  606   c,    606   b  and  606   a  respectively. Voltages V 1 , V 2  and V 3  are applied to three voltage  15  inputs of power combiner  604  as between nodes C &amp; B, B &amp; A and nodes A &amp; F respectively. Power combiner  604  has a single output voltage V out . 
         [0038]    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 1 , V 2  and V 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 2  and V 3 . Similarly, node A is on a “shared input terminal” of V 1  and V 2 . One end of inductor L 1  connects to node C, the other end of inductor L 1  connects to one end of inductor L 3  to form node W. The other end of inductor L 3  connects to one end of inductor L 5  to form node X. The other end on inductor L 5  connects to the drain of MOSFET G 1  and the source of G 1  connects to node F (ground). One end of inductor L 2  connects to node C, the other end of inductor L 2  connects to one end of inductor L 4  to form node D. The other end of inductor L 4  connects to one end of inductor L 6  to form node E. The other end on inductor L 5  connects to the drain of MOSFET G 2  and the source of MOSFET G 2  connects node F (ground). The drain of MOSFET G 5  is connected to node W, the source of MOSFET G 5  connects to the source of MOSFET G 6 . The drain of MOSFET G 6  connects to node D. The drain of MOSFET G 4  is connected to node X, the source of MOSFET G 4  connects to the source of MOSFET G 3 . The drain of MOSFET G 3  connects to node E. The output voltage V 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 5 , L 6 , L 3 , L 4 ,L 1  and L 2 . The winding polarity of L 5 , L 3  and L 1  is preferably opposite of the winding polarity of L 6 , L 4  and L 2 . The two inductors within each of the inductor pairs L 5 -L 6 , L 3 -L 4  and L 1 -L 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. L 1  and L 2 , L 3  and L 4  and L 5  and L 6 ), to adjust the typical relative MPP voltage of each of the input voltages. Each of the three voltages V 1 , V 2  and V 3  are applied across each of inductors L 5 , L 3  and L 1  respectively with for instance a 50% duty cycle when switches G 1 , G 4  and G 5  are closed and switches G 2 , G 3  and G 6  are opened. Each of the three voltages V 1 , V 2  and V 3  are applied across each of the inductors L 6 , L 4  and L 2  respectively with typically a 50% duty cycle when switches G 1 , G 4  and G 5  are opened and switches G 2 , G 3  and G 6  are closed, thus completing a full switching cycle. The output voltage (V out ) of power combiner  604  is the sum of the input voltages V 1 , V 2  and V 3 . The input voltages V 1 , V 2  and V 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 5 , L 6 ), (L 3 , L 4 ) and (L 1 , L 2 ) respectively; a result of applying control pulses to switches G 1 -G 6  for instance with a 50% duty cycle. Switches G 1 -G 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. 
         [0039]    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 1 , V 2  and V 3  are input to power combiner  604  between nodes A &amp; F, B &amp; A and nodes C &amp; B respectively. One end of inductor L 1  connects to node C, the other end of inductor  30  L 1  connects to the drain of MOSFET G 1  the source of G 1  connects to node B. One end of inductor L 3  connects to node B, the other end of inductor L 3  connects to the drain of MOSFET G 3 , the source of G 3  connects to node A. One end of inductor L 5  connects to node A, the other end of inductor L 5  connects to the drain of MOSFET G 5 , the source of G 5  connects to node F (ground). One end of inductor L 2  connects to node C, the other end of inductor L 2  connects to the drain of MOSFET G 2 , the source of G 2  connects to node B. One end of inductor L 4  connects to node B, the other end of inductor L 4  connects to the drain of MOSFET G 4 , the source of G 4  connects to node A. One end of inductor L 6  connects to node A, the other end of inductor L 6  connects to the drain of MOSFET G 6 , the source of G 6  connects to node F (ground). The output voltage V 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 5 , L 6 , L 3 , L 4 , L 1  and L 2 . The winding polarity of L 5 , L 3  and L 1  is preferably opposite of the winding polarity of L 6 , L 4  and L 2  respectively. The two inductors within each of the inductor pairs (L 5  and L 6 ), (L 3  and L 4 ) and (L 1  and L 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. 
         [0040]    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  904   a  - 904   c  which are for instance multi-junction photovoltaic cells which have three voltage  20  outputs V 1 , V 2  and V 3  with three bypass diodes connected across each voltage output of photovoltaic cells  904   a - 904   c.  Connected to each photovoltaic cells  904   a - 904   c  is a three voltage input power combiner  604 . Power combiner  604  has a single voltage output (V out ) which is applied across the input of DC to DC converters  92   a - 92   c.  The outputs of DC to DC converters  92   a - 92   c  are connected in series to form the input to DC to AC converter  900  and the output of multiple series strings  902 . 
         [0041]    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. 
         [0042]    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”. 
         [0043]    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.