Abstract:
A circuit and method that provides an inexpensive and easily implemented rapidly-changing load test circuit for photovoltaic cells, which can be under pulsed light conditions. Embodiments of the present invention permit the testing of photovoltaic cells by causing a sweeping voltage across the photovoltaic cell in a very short time period.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of the filing of U.S. Provisional Patent Application Ser. No. 61/029,789, entitled “Rapid Photovoltaic Cell Tester”, filed on Feb. 19, 2008, and the specification thereof is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention (Technical Field) 
     Embodiments of the present invention relate to a circuit for providing a rapidly sweeping load, which can be used in numerous applications, particularly for use in testing a photovoltaic cell. 
     2. Description of Related Art 
     Testing of PV cells under pulsed conditions requires the voltage of the cell to be swept over a range of greater than the open circuit voltage to a negative bias or vice versa. Although numerous manners and circuits are known for testing photovoltaic cells, particularly in an attempt to maintain consistent quality controls for the manufacturer thereof, known systems cannot meet the high speed timing requirements to sweep the load in less than about three milliseconds. Accordingly, known systems do not have the ability provide a sufficiently rapidly changing test load so that changing load tests can be easily and inexpensively incorporated into large-scale photovoltaic cell manufacturing facilities. There is thus a present need for a method and apparatus which provides an inexpensive and easily implemented rapidly-changing load test for photovoltaic cells. Embodiments of the present invention are able to sweep a load within a time of about 10 milliseconds to about 10 microseconds. This allows for testing of PV cells in much faster time frames than currently available. 
     BRIEF SUMMARY OF THE INVENTION 
     An embodiment of the present invention relates to a circuit for providing a rapid sweeping load which includes first and second power supplies; a first capacitor coupled in a parallel configuration to the first power supply; a second capacitor coupled in a parallel configuration to the second power supply; a first current limiting component connecting the first power supply to the first capacitor; a second current limiting component connecting the second power supply to the second capacitor; third capacitor comprising a capacitance substantially smaller than each of the first and the second capacitors; and first and second switches respectively coupling the first and second capacitors to the third capacitor. 
     In one embodiment, at least one of the switches is a solid state switch and more preferably, both of the switches are solid state switches. In one embodiment at least one of the first or second capacitors has a capacitance of less than about 10 millifarads and/or less than about 5 millifarads. Optionally, the third capacitor can have a capacitance which is greater than that of the first or second capacitors. In addition, the circuit can also have a third current limiting component and the third current limiting component can include a current limiting resistor and/or an inductor. In one embodiment, the first and second capacitors can be replaced with first and second inductors. The third current limiting component can be electrically connected between the load capacitor and the first and second charged capacitors. 
     An embodiment of the present invention also relates to a method for testing a photovoltaic cell which includes connecting the photovoltaic cell to a testing circuit; connecting a load capacitor to a first charged capacitor; disconnecting the load capacitor from the first capacitor; connecting the load capacitor to a second charged capacitor; applying a light source to the photovoltaic cell; and obtaining one or more measurements. The method can also include disconnecting the load capacitor from the second charged capacitor. Applying a light source can include applying a pulsed light source. The one or more measurements obtained can include a voltage measurement and/or a current measurement. In the method disconnecting the load capacitor from the first capacitor and connecting the load capacitor to a second charged capacitor preferably causes a sweeping voltage across the photovoltaic cell. 
     In one embodiment, the one or more measurements can be obtained during the time in which the sweeping voltage across the photovoltaic cell occurs. The measurements are preferably obtained during a time period of from about 10 microseconds to about 100 milliseconds and/or about 10 microseconds to about 10 milliseconds. 
     Embodiments of the present invention include but are not limited to methods where a photovoltaic cell connects to a photovoltaic cell in a parallel configuration with a load capacitor. Connecting with a load capacitor to a first charged capacitor includes but is not limited to connecting a load capacitor to a first charged capacitor which has substantially larger capacitance than the load capacitor. And connecting a load capacitor to a first charged capacitor includes but is not limited to connecting a load capacitor to a second charged capacitor which has larger capacitance than the load capacitor. 
     Objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The accompanying drawings, which are incorporated into and form a part of the specification, illustrate one or more embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating one or more preferred embodiments of the invention and are not to be construed as limiting the invention. In the drawings: 
         FIG. 1  is a drawing illustrating a circuit according to an embodiment of the present invention; 
         FIG. 2  is a drawing illustrating a circuit according to an embodiment of the present invention; 
         FIG. 3  is a computer-generated image illustrating a graph of a theoretical measurement of a simulated circuit according to an embodiment of the present invention; 
         FIG. 4  is a drawing illustrating a preferred embodiment of a sweeping load circuit according to an embodiment of the present invention; and 
         FIG. 5  is a computer-generated print out of results obtained by a circuit according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention are directed to circuits and methods for rapidly testing photovoltaic cells at various loads. 
     An embodiment of the present invention relates to a high speed load. Although those skilled in the art will readily recognize numerous uses for such a load, the invention of the present invention is particularly useful for testing photovoltaic (PV) cells and loads under pulsed conditions. The load consists of 2 or more high speed switches and capacitors arranged to allow a voltage sweep across the cell in time frames ranging from about 10&#39;s of microseconds to about 10&#39;s of milliseconds. The voltage sweep can range from millivolts to several kilovolts and photovoltaic currents from about a few milliamps to about several kiloamps. In addition, the load can sweep between 2 or more load points, positive and/or negative, either high to low or low to high or any combination thereof. The same circuit configuration can be used for a load which can consist of more than 2 load points up to any number desired, simply by providing additional branches to the circuit. 
     A single sweep circuit according to an embodiment of the present invention is illustrated in  FIG. 1 . As illustrated therein, circuit  10  preferably comprises photovoltaic (“PV”) cell  12 , or another device which desires to make use of the rapid sweeping load of the present invention, connected to load capacitor  14 . Load capacitor  14  is preferably removably connected to capacitor  16  (C 1 ) and/or capacitor  18  (C 2 ) via switch  20  (S 1 ) and switch  22  (S 2 ). S 1  and S 2  are most preferably solid state switching devices, but can optionally be non-solid state switching devices. Elements  24  and  26  are preferably provided to limit current from power supplies  28  and  30  respectively. Elements  24  and  26  can comprise any component or combination thereof known to provide desirable results for limiting a flow of current, but most preferably comprise resistors and/or inductors. Element  32  is preferably provided so that the rise and/or fall time of the voltage across load capacitor  14  can be provided at a rate which is desirable to measure. For example, element  32  most preferably comprises a resistor or inductor of sufficient impedance such that the charging and/or discharging of load capacitor  14  occurs at a predetermined rate. As illustrated in  FIG. 2 , element  32  can optionally be provided on only one leg of the driving circuit. 
     Operation of the circuit of an embodiment of the present invention is preferably substantially as follows:
         1) Connections are made and measurements are continuously or intermittently made of the voltage across and the current from PV cell  12  by a measurement device as shown in  FIG. 1 .   2) Capacitors C 1  and C 2 , each of which most preferably has a capacitance of substantially more than that of CL, are charged to preset level  1  and preset level  2  respectively.   3) Switch S 1  is fired and the voltage on CL charges to preset level  1 , which is seen as the load for PV cell  12 .   4) as PV cell  12  is pulsed with an external light source shown in  FIG. 1  and CL holds the voltage at preset level  1 .   5) Switch S 1  is then preferably opened and S 2  is closed, thus causing the CL to have a voltage which is swept from level  1  to level  2  in a time determined by CL and element  32 .       

     Of course multiple other branches can be added to the circuit of the present invention, such that the voltage across CL is caused to go through any number of changes to any number of predetermined values. 
     By using the switch geometry according to the teachings of the present invention, a load can be swept very quickly and accurately thereby allowing for a very consistent load voltage across a device under test, which can include a photovoltaic cell, and can even provide a positive or negative bias to reduce the effects of the connection and series resistance. Optionally, protection diodes can be placed in series with the photovoltaic cell capacitors  14  of  FIGS. 1 and 2  or capacitor  44  of  FIG. 4  and the photovoltaic cell to protect against reverse biasing if necessary. In one embodiment, empirical equations are preferably used to determine the values of initial charge voltage on capacitors  14 ,  16 ,  46 , and  48  and to give the appropriate voltage sweep across the load. By doing this, the user only need supply the start and stop voltage and the charge voltage can be calculated from those values. In the embodiment of  FIG. 4 , resistors  62  and  63  can optionally comprise the same value or can comprise different values which allow different discharge slopes to be provided. In this embodiment, circuit  40  preferably comprises photovoltaic cell  42  connected in parallel to load capacitor  44 . Circuit  40  also preferably comprises first and second power supplies  58  and  60  which are respectively connected to charge capacitors  46  and  48  through resistors  54  and  56 . Switches  50  and  52  which are most preferably solid state switches are preferably used to discharge capacitors  46  and  48  through resistors  63  and  62  and into capacitor  44 , which is therefore seen as a changing load by photo cell  42  or another device connected in its place. If the present invention is used to test a photovoltaic cell, preferably one or more flashes of a predetermined light source is flashed onto the cell during the course of time of the voltage sweep created by the present invention. 
     INDUSTRIAL APPLICABILITY 
     The invention is further illustrated by the following non-limiting example. 
     Example 1 
     A circuit testing program was used to simulate an embodiment of the present invention.  FIG. 3  is a computer-generated graph depicting theoretical results obtained from the circuit simulation program.  FIG. 4  illustrates one embodiment of the present invention for use in testing a photovoltaic cell under a sweeping load conditions. In this embodiment, capacitor  44  comprises a value of 5 millifarads, capacitors  46  and  48  comprise values of 2.4 millifarads each, resistors  54  and  56  comprise values of about 1,000 ohms each, resistor  63  comprises a value of about 4 ohms, and resistor  62  comprises a value of about 1 ohm. 
     A circuit was constructed according to an embodiment of the present invention and measurements were obtained by a measurement device as shown in  FIG. 4  which generated the graph of  FIG. 5 . In this embodiment, first switch  50  was closed and the voltage was charged to a preset value of −200V. The switch was then turned off and the second switch  52  was turned on, which swept the voltage back to −150 volts. In this case the charge voltage on power supply  58  was negative and the voltage on power supply  60  was positive. This swept the voltage from −200 volts to −150 volts. The sweep was almost linear. Capacitor  44  across load  42 , which in this case was a photovoltaic cell, acted to sink the current generated from the solar cell under test when it was illuminated with a flash. 
     The preceding example can be repeated with similar success by substituting the generically or specifically described components and/or operating conditions of this invention for those used in the preceding examples. 
     Although the invention has been described in detail with particular reference to these preferred embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents. The entire disclosures of all references, applications, patents, and publications cited above are hereby incorporated by reference.