Teach you how to accurately analyze photovoltaic IV characteristics in 5 minutes

Because the power output of PV modules can vary greatly with temperature, it is important to measure their performance in their typical operating environment, which is often a sunny outdoor area such as a rooftop or undeveloped lot where it is difficult to power the measurement equipment or control the temperature.

Therefore, it is important that the measurement equipment used to characterize the module performance does not drift with temperature. In addition, the ideal IV measurement solution will be portable and consume very little power.

The single-supply rail operation and shutdown mode of the LTC2058, a 36V, low-noise, zero-drift op amp, enable battery-powered operation and maximize battery life. Its dual amplifiers enable simultaneous measurement of two channels (e.g., current and voltage). For applications that are subject to wide temperature variations, such as PV module measurements, the LTC2058's extremely low maximum input offset voltage temperature drift (0.025 µV/°C) maintains its accuracy despite large fluctuations in operating temperature. For example, in areas with very high sunlight, the ambient temperature can reach 45°C (113°F), which is equivalent to an additional 20°C over normal room temperature operating conditions. The maximum additional input offset drift of the LTC2058 under extreme conditions is only 0.5 µV.

The IV characteristic curve of a PV module is generated by applying a range of impedances to the PV module from short circuit to open circuit and measuring the current and voltage generated at each load. One method is to iterate through multiple settings of a high-rated power potentiometer or load box and take measurements at each point. This method has a drawback: brief periods of shading or illumination, such as birds, clouds, or bright reflectors passing overhead, can cause momentary drops or spikes in output power, introducing errors in the IV curve. A faster method is to open a shunt switch to a large capacitor, as the capacitor will effectively sweep its impedance from short circuit to open circuit during its few hundred milliseconds of charging time, minimizing the chance of transient effects affecting the IV curve.

In addition to the obvious advantages of this approach (i.e., speed, simplicity, and ease of measurement), very few high-power-rated components are required to implement transient capacitive sweeps. The components are exposed to high power for no more than a few hundred milliseconds. Therefore, with the proper choice of load capacitors and sense resistors, this accurate measurement circuit can be used to measure open-circuit voltage and short-circuit current for a wide range of modules, for example, in large-area PV module testers.

Figure 1 shows an implementation of an IV scan method for characterizing a PV module. C2 is the main capacitive load, and its size is a trade-off between measurement speed and accuracy: when a smaller capacitor C2 is selected, the scan speed is faster, which reduces the risk of error; when a larger capacitor C2 is selected, the scan speed is slower, while more accurate measurement sampling can be achieved.

Figure 1. PV sweep measurement using the LTC2058.

In the initial state, both SW1 and SW2 are shorted, so there is no voltage across C2. Both switches must be opened (SW2 first, then SW1) to start a measurement sweep of 150ms duration, ending with the full voltage of the module across C2. After the measurement, C2 is discharged in preparation for the next cycle by first closing SW2, where the series resistor R3 rated at 2W reduces the risk of sparking, and then closing SW1 to provide a true short circuit across C2 (RON = 0.3Ω) and pull the voltage across C2 to 0. For a full system implementation, these switches can be power MOSFETs driven by digital signals that control the timing and switching sequence.

The LTC2058’s robust 2.5MHz gain-bandwidth product is critical to accurately tracking the sweep rate of the PV current flowing through RSENSE. The largest current sense measurement error occurs during the sharpest transients in the sweep cycle. Although the input voltage across RSENSE has a low falling slew rate of 3.6 V/s (see Figure 2), the op amp’s group delay translates into real-time errors in the current sense output. Also, because RSENSE is fairly large, the closed-loop gain of the current sense circuit can be as small as 4V/V to produce a 2V full-scale output at a maximum short-circuit current (ISC) of 0.5A. This low gain is not a problem because the LTC2058 is unity-gain stable. Thus, the LTC2058’s high gain-bandwidth and low closed-loop gain requirement enable fast closed-loop response, minimizing errors caused by group delay.

Figure 2. The voltage across the sense resistor at a slew rate of approximately 3.6V/s.

A large capacitor, C2, together with a large RSENSE, determines the slew rate of the transient and, therefore, the error introduced by the fixed delay. The trade-off for using a larger C2 is that the time required for the IV measurement is slightly longer.

Diode D1 allows the output of the current sense channel to swing all the way to 0 V to measure the exact current in an open circuit condition at the end of the scan cycle. Diode D2 and 200Ω resistor R8 help protect IN+ of the current sense amplifier from damage due to electrical overstress.

For the voltage sense channel, R1 and R2 divide the full voltage of the module so that the output on VPV is within the 5V rail after a closed-loop gain stage of 5V/V. R1 and R2 are adjustable to divide any module open circuit voltage (VOC) as long as their current consumption is not too large relative to the module ISC. In this design, the current flowing through R1 and R2 produces an error of 19µA, or 0.0038% of ISC.

Figure 3. IV and power-v curves obtained using capacitive sweep and the LTC2058 circuit.

Figure 4. PV capacitive scanning circuit; module connections on the left, C2 on the right.

If the measurement equipment is placed close to the PV module, it will also be exposed to extreme temperatures in environments such as cold, bright sunlight or hot desert climates. However, it must maintain its accuracy to capture the changes in the performance of the PV module as temperature fluctuates. The maximum average input offset temperature drift of the LTC2058 is only 0.025µV/°C, which enables accurate measurement of solar panel performance over a wide temperature range. To learn more about the zero-drift operational amplifier LTC2058 , please click "Read the original article".

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