System for determining the current-voltage characteristics of a photovoltaic array

• US 4,129,823 A
• Filed: 11/03/1977
• Issued: 12/12/1978
• Est. Priority Date: 11/03/1977
• Status: Expired due to Term

• Current Assignee: Sensory Technologies LLC (DSI Video Systems LLC)
• Original Assignee: Sensory Technologies LLC (DSI Video Systems LLC)
• Inventors: Rosinski, Louis S. Jr., Belli, Johann B., van der Pool, Kees
• Primary Examiner(s): Tokar, M.
• Application Number:
  US05/848,081
• Time in Patent Office: 404 Days
• Field of Search: 324/20 R, 324/22, 324/24, 324/25, 324/158 R, 324/158 D, 315/149, 356/229
• US Class Current: 324/761.01
• CPC Class Codes: G01R 31/308 using non-ionising electromagnetic radiation, e.g. optical radiation investigating or analysing materials by the use of optical means G01N21/00; image analysis G06T7/00
• CPC Class Codes: H02S 50/10 Testing of PV devices, e.g. of PV modules or single PV cells testing of semiconductor devices during manufacturing H01L22/00
• CPC Class Codes: Y02E 10/50 Photovoltaic [PV] energy
• CPC Class Codes: Y10S 136/29 Testing, calibrating, treating, e.g. aging

Abstract

A nonlinear load circuit, consisting of a transistor and a resistor in series, is provided for the photovoltaic array under test. Base bias for the transistor is supplied via the source-to-drain path of a field effect transistor (FET). A ramp signal is fed to the gate of the FET. As a result of the nonlinear relationship between the gate voltage and source-to-drain current of the FET, equal ramp steps result in a nonlinear effective load for the array under test. This produces a very gradual change in load impedance for each ramp step in the regions of high current output from the array ("current mode"), and relatively greater changes in load impedance for each ramp step at output levels of lower current and higher voltage from the array ("voltage mode").

Advantageously, the array is illuminated by a pulsed flash lamp. A photosensor detects the light level incident on the array, and comparison circuitry provides a "sample" pulse each time that the incident light level is at a selected value. This "sample" pulse gates a pair of sample and hold circuits that respectively sample the array output current and voltage under the load condition determined by the present ramp step. Consecutive like operations facilitate measurement and plotting of the complete current-voltage curve.

Claims

1. A system for determining the current-voltage characteristics of a photovoltaic array, comprising:

a flash lamp mounted to illuminate said photovoltaic array at a pulsed rate,

a separate photosensor also mounted so as to be illuminated by said pulsed flash lamp,

comparator means, connected to said photosensor, for providing a gate pulse each time that the illumination level striking said photosensor reaches a preselected level,

nonlinear load means for providing successively different load impedance values across said photovoltaic array, and

sampling means for sampling the current and voltage from said photovoltaic array at each of said successive load values, said sampling means being enabled by said gate pulse.

2. A system according to claim 1 wherein said load means comprises:

a ramp generator for producing a ramp signal,

a load element and a transistor connected together as a controllable load for said photovoltaic array, and

control circuit means for conditioning the conductivity of said transistor in response to said ramp signal so that for relatively heavily conducting conditions of said transistor, said conductivity and hence the net impedance of said controllable load changes very slightly with each ramp step, thereby permitting the successive sampling of the photovoltaic array output at slightly different high current levels, and that for relatively lesser conductivity conditions of said transistor the net impedance of said controllable load changes more greatly for each ramp step, thereby permitting a larger current change for each ramp step.

3. A system according to claim 2 wherein said load element is a resistor connected in series with said transistor across said photovoltaic cell, and wherein said control circuit means comprises:

a field effect transistor connected to the base of said transistor so as to control the conductivity thereof,

said ramp voltage being used to control the gate voltage of said field effect transistor, so that the nonlinear relationship between the gate voltage and the source-to-drain current of said field effect transistor results in the desired nonlinear control function.

4. A system according to claim 1 wherein said nonlinear load means comprises:

a. a first transistor and a low resistance resistor connected in series across the output of said photovoltaic array,

a field effect transistor (FET) connected to provide base bias to said first transistor via the source-to-drain path of said FET, and

a ramp generator connected to provide a ramp signal to the gate of said FET, whereby equal steps of said ramp signal result in unequal changes in the conductivity of said first transistor, and hence of the effective load across said array, as a result of the nonlinear relationship between gate voltage and source-to-drain current of a field effect transistor.

5. A system according to claim 4 wherein said sampling means comprises:

a first sample and hold circuit connected to sample the voltage across the output of said photovoltaic array,

a second sample and hold circuit connected to sample the voltage developed across said low resistance resistor, said developed voltage being indicative of the output current from said photovoltaic array, said gate pulse enabling both of said sample and hold circuits.

6. A system for determining the current-voltage characteristics of a photovoltaic array, comprising:

a ramp generator producing a ramp signal,

a first transistor connected in a load circuit across the output of said photovoltaic array, the conductivity of said first transistor thereby establishing the net load impedance for said array,

a field effect transistor connected to provide base bias to said first transistor via the source-to-drain path of said field effect transistor, said ramp signal being provided to the gate of said field effect transistor, so that the net effective load impedance for said photovoltaic array changes in relatively small steps at near full conductivity of said first transistor and in relatively larger steps for lesser conductivity of said first transistor, this nonlinear change in net effective load impedance resulting from the nonlinear relationship between gate voltage and source-to-drain current in a field effect transistor.

7. A system according to claim 6 wherein said ramp generator produces a ramp signal having steps of substantially equal incremental value, together with an operational amplifier having its output connected to the gate of said field effect transistor, the output of said ramp generator being connected to the non-inverting input of said operational amplifier, and a Zener diode connected between the output of said operational amplifier and the inverting input thereof so that the gate of said field effect transistor is biased by the Zener diode feedback to a level at which pinch-off of the field effect transistor will begin within the first few steps of said ramp signal.

8. A system according to claim 6 wherein said photovoltaic array is illuminated by a pulsed flash lamp, together with:

a photosensor situated adjacent to said photovoltaic array so as to sense the light level incident on said array,

illumination intensity selection means for providing a reference signal corresponding to a desired light level at which said current-voltage characteristics are to be determined,

comparator means for comparing the output signal from said photosensor with said reference signal and for providing a "sample" pulse when these compared signals are equal, said "sample" pulse thereby occurring at each pulse of said flash lamp when the light level incident on said array is at the desired level, and

sample and hold circuitry, gated on by said "sample" pulse, for sampling and holding the current and voltage from said photovoltaic array.

9. A system according to claim 8 together with an X-Y plotter connected to said sample and hold circuitry so as to plot the values of current and voltage held thereby, said plotter thus producing a curve of the current-voltage characteristics of said photovoltaic array.