Accurate Measurement - Tools:
What is NPLC and How it works in Solar Cell/ PV testing?
Solar cell I-V characterization
A solar cell or photovoltaic (PV) is a device that absorbs photons from a light source and then releases electrons, causing current to flow when the PV is connected to a load to generate electricity. PV researchers and manufacturers are committed to achieving maximum efficiency at the lowest cost. Therefore, the analysis of electrical properties of photovoltaic cells and photovoltaic materials is an important part of the research, development and manufacturing process. Current-voltage characteristic analysis of solar cells results in important parameters for PV performance, including maximum current (Imax) and maximum voltage (Vmax), open voltage (Voc), short-circuit current (Isc), and efficiency (η).
Power measurement equipment (Source-Measure-Unit, SMU) used in solar cell I-V characterization.
Why uses SMU for I/V testing of solar cells?
Many semiconductor and electronic device tests involve outputting voltage and measuring current as quickly as possible. The overall test time is a function of charging time, measuring time, discharge time, setting and processing test time. Traditional power supplies can only output voltage or current, not input. However, the four quadrant SMU instruments not only output and input voltages and currents, but also measure voltage, current, or resistance. The four quadrants of the SMU instruments operate in automatic input mode, allowing the rapid absorption of all charges from the equipment under test (DUT) and wiring, effectively accelerating discharge times. In addition, if this output and measurement capability are tightly integrated into a single instrument, independent digital meters (DMMs) and power supplies are no longer required. This will improve testing time, simplify the overall testing system design, and improve availability. Therefore, in the I/V test of solar cells, the four-quadrant SMU (Source-Measure-Unit) is the most common and recommended.
The largest source of noise in low current measurement: Interference from AC power supplies.
In low-current testing, the effect of noise on the reading values needs to be considered. The general noise sources are: 1) electrostatic coupling; 2) Vibration/ Deformation 3) Drift current 4) AC power interference. Interference from AC power supplies is the most serious interference.
How does AC power produce interfering noise? Simply speaking, the power supply system of electrical measuring instruments is mostly powered by AC voltage from the the internal full-wave or half-wave rectifier circuit, to generate “DC voltage”, which is supplied to related amplifying circuits, AD circuits, etc. . In fact, “DC voltage” still has “ripple voltage”. This ripple voltage is a kind of tiny “unstable”power source, and the reading of the measuring circuit will be affected by it. This means that we have “jumping” in the reading of the meter, which forms noise interference.
Figure. Full-wave rectifier and half-wave rectifier circuit. Both can rectifier and smooth the input AC voltage. The output is close to the “DC voltage”, but still retains the “ripple” characteristic. This ripple creates a “noise” of the reading value on the meter.
Therefore, whether it is SMU or DMM, the noise effects by this internal power ripple must be considered in their design. In addition to its own refinement in the AC-DC conversion circuit, in low-current measurements, there is a need to separate or eliminate this ripple noise from the true signal. The SMU or DMM has built-in NPLC and average filters in its measurement functions to eliminate noise interference. Here’s a quick description of both features.
How SMU eliminates measurement noise: Introduction to the built-in NPLC/filter/moving-average functions.
NPLC filter function
NPLC is the cycle multiple of the sampling power supply, N represents how many times it is, and PLC is Power Line Cycle. As mentioned above, the interference of AC power is very serious. To reduce the interference of AC power, a common method is to take the measurement period as an integer multiple of the AC cycle as much as possible. In this way, various interferences can cancel each other out in one cycle.
The above figure is a schematic diagram of NPLC filtering and sampling in the Keithley source meter manual. NPLC refers to the number of power line cycles, which represents the duration of signal sampling (or integration time). Using integer multiples of NPLC can produce the most accurate measurement, but it will be limited by the power line cycle frequency (1 PLC = 60 readings/s at 60Hz, or 50 readings/s at 50Hz).
Tektronix’s website mentions the definition and importance of NPLC for electrical measurement equipment. Regardless of the measurement resolution and accuracy of DC voltage, DC current and resistance measuring meters, they will be degraded due to AC power noise. Using integer multiple sampling of the NPLC power line cycle can reduce AC power noise. The use of NPLC with N=1 or greater will increase the integration time of data acquisition and eliminate noise. Thereby it will improve the measurement resolution and the accuracy. NPLC=100 can bring the highest accuracy, but the entire test time will be 100 times of NPLC=1.
Let’s take a look at how Keysight (formerly Agilent) describes the built-in NPLC function and noise filtering. The setting of N is equivalent to determine the sampling period and the sampling integration time. N=1 means that the sum of the sampling time of the signal (or integration time) is equivalent to the total time of a power cycle, so the noise can be “averaged” and the output reading value is zero.
The abscissa of the figure above is the time axis, and the ordinate is the voltage value. The “DCV level of the signal of interest” is assumed to be 115.5mV, and the blue sine wave is “power line noise”. When NPLC=1, the entire sampling period is just the “power line noise period”, so the average noise value collected is 0. And this period of time is called the integration time.
Let’s look at the situation when NPLC=0.1. Assuming NPLC=0.1, the time period of each measurement is integration time = 1ms. Data sampling only obtains a small part of the power line noise cycle, so the measurement result will contain noise. This is the result we see in the above figure: “Signal of Interest” is 115.5mV but the measured result is 116mV or 114mV.
Readers may feel very unfamiliar with the “integration time” of an electric meter, and it is difficult to understand. You can understand NPLC in another way: the setting of NPLC determines the number of “automatic sampling and averaging” of the SMU. Assume that a reading value of SMU sampling time is fixed = 1ms. The AC power frequency is 50Hz, which means that one power line cycle is 0.02sec. When setting NPLC=1, the source meter will automatically collect (0.02sec/1ms)=20 times, and then average the 20 data to output the reading. The 20 samples are evenly distributed in a complete cycle of the power line cycle, so the average is 0, which eliminates the noise. When the single sampling time is much faster than 1ms and is close to infinitesimal, it is the concept of “integration time” mentioned above.
There is a dilemma between measurement speed and performance. For example, the alternating current used is 50 Hz and a cycle is 0.02 seconds. Therefore, if NPLC=1, the measurement sampling period is 0.02 seconds, if NPLC=10, the measurement sampling period is 0.2 seconds, and if NPLC=50, the measurement sampling period is 1 second. Only by improving the NPLC can the noise be suppressed and the AC common-mode rejection ratio can be improved. Of course, sometimes the measurement speed must be pursued, and the sampling period is less than 0.02 seconds, which will inevitably sacrifice the test performance of the electric meter. That is to say, NPLC=100 can bring the highest accuracy, but the entire test time will be 100 times that of NPLC=1.
Average filter function
The function of the averaging filter is similar to the concept of NPLC=1. By averaging after multiple sampling, it can effectively eliminate power noise or other noise.
The above figure takes Keithley’s meter as an example, which has a built-in Filter function. It provides two filter types, moving average and repeated average. It can set the number of filter counts and filter window. If the moving average filter is set to filter count=10, its first reading value will display the average of the 1st to 10th sampled values. The second reading shows that the sampled values from 2 to 11 will be averaged. And so on. If the filter count is set to 10 for the repeated filter, the first reading will be the average of the 1st to 10th samples, and then the reading will be output. The second reading will display the average value of the 11th to 20th samples.
The “Filter” function of the source meter is a built-in concept of “average” to effectively eliminate noise. By sampling and averaging multiple times, noise can be eliminated to improve the accuracy and resolution of the measurement. Similarly, the more average counts, the more accurate the reading results, but the longer time it takes. Not suitable for dynamically changing samples (non-equilibrium) or unstable samples.
How to start the function of NPLC?
Because each source meter SMU has built-in NPLC or averaging filter functions. The user can use the front panel of the instrument or program to control the NPLC or filter functions.
The picture above shows the NPLC function of Keysight’s SMU, which explains its NPLC parameters and performance. Readers can use the buttons on the front panel to set the functions of the NPLC in accordance with the instructions in the manual.
The picture above is the description of how to set the mains frequency and NPLC in the Keithley SMU2400 manual. Readers who want to program control can refer to the 2400 operation manual, which has a very detailed instruction. It is recommended that readers who want to program control need to have a basic understanding of the functions of the SMU and the concept of the instrument communication with the computer. So that they are more able to get started. It is suitable for readers who have basic instrument control experience.
How the IVS-KA6000 controls the NPLC functionality of the SMU?
How to control the NPLC function of the source meter by IVS-KA6000?
We mentioned earlier that the source meter SMU is the best I/V test tool for the measurement of solar cells. As the most powerful software for solar cell I/V measurement, IVS-KA6000 can not only control a variety of SMUs, but also collect and analyze current and voltage data based on the set measurement parameters. After the acquisition is complete, the software can directly analyze the relevant parameters of the solar cell to be tested, including short-circuit current, open-circuit voltage, fill factor, maximum power, conversion efficiency, etc. Therefore, the setting of SMU’s NPLC is the basic function module of IVS-KA6000.
Follow the graphic steps below then to control many functions of SMU, including NPLC settings.
１. Confirm the communication link between IVS-KA6000 and SMU. Just check Source-Meter in the lower left part of the main screen, and a green light will indicate a successful connection with the SMU. The SMU can be controlled through IVS-KA6000.
２. Click the “Instr. Setup” at the top to enter various instrument control pages.
３. Enter the setting screen, the first tab (red) “Main” is the SMU communication setting page. The user only needs to select the SMU by the drop-down menu, and the communication settings in the blue box on the right will be automatically set. There is no need for users to read the SMU manual to set the relevant values.
４. Select “Config” in the yellow box below to enter further settings of SMU. After entering, you can see the settings of NPLC (“Number Power Line Cycles”). After selecting through the drop-down menu, press the “OK” button to complete the setting.
５. On this function setting page, in addition to the NPLC setting, the settings of “Current Compliance” and “2-wire/4-wire method” inside the SMU can also be simply set on this page.
６. Back to the main screen, you can start the I/V test of the solar cell.
The most suitable meters in the solar cell I/V measurement is the four-quadrant source meter SMU. In the electrical test process, the biggest noise source usually comes from the periodic noise of the power line. The periodic noise of the power line source comes from the periodic ripple after the AC power is converted into the DC power, and then the interference noise is generated. Therefore, the source meter SMU has built-in NPLC and filtering averaging function to eliminate periodic ripple noise. Generally, the NPLC or averaging filter function can be set and controlled through the SMU panel control function, or it can be programmed and controlled through communication commands. The easiest way is to simply control the NPLC filter averaging function through the most powerful I/V test software IVS-KA6000.
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