The amount of electricity produced by solar cells is not only related to its conversion efficiency, but also related to solar irradiance and the area of the solar cell. In order to make the output power of different solar cells comparable, the solar cells must be tested under the same standard conditions. The internationally accepted standard test conditions include:
(1) Solar irradiance: 1000W/m2.
(2) Solar spectrum: AM1.5.
(3) Test temperature: (25±2)°C.
Standard test conditions place high requirements on solar simulators and ambient temperature. In addition, the electrical performance test of solar cells is also closely related to the measurement circuit. Let’s start the discussion from the solar cell measurement circuit.
The test principle diagram of the solar cell sorting tester includes voltmeter, ammeter and variable load, as shown in Figure 1.
The size of monolithic solar cells is developing in the direction of large area and high current, but the voltage of solar cells is only about 0.6V. For example, the current of 156mmx156mm silicon solar cells is about 8A, which requires good contact when testing the solar cells. , Because even 0.01Ω in series will produce a voltage drop of 0.01 x8=0.08(V), which is absolutely not allowed in the solar cell test. Therefore, Kelvin electrodes must be used to measure large-area solar cells, which is The so-called four-wire system is shown in Figure 2.
The solar cell measurement circuit includes electronic load, sample-and-hold and compensation power supply. Here we will focus on the electronic load.
Electronic load is a kind of load that uses electronic components to absorb electrical energy and consume it. Electronic loads are widely used in electric power, electronic power product testing, chemical power testing and physical power testing. Various loads are required in electrical laboratories, and traditional loads are generally replaced by slip wire rheostats. The slip wire rheostat has a low resolution of resistance change, and the resistance value will change due to poor contact or heat. Some experiments require a stable load with higher resolution. Most of the electronic loads have constant current characteristics, and some have resistance characteristics. The so-called constant current characteristic means that the current flowing through the electronic load is determined by the electronic load itself and does not change with the power supply voltage. The electronic load with resistance characteristics is equivalent to a resistance, and the current flowing through it is equal to the power supply voltage divided by its equivalent resistance. .
The electronic components of the electronic load are generally power semiconductor devices such as power MOSFETs and insulated gate bipolar transistors (IGBTs). Since sub-power semiconductor devices are used instead of resistors and the like as the carrier of electric energy consumption, it makes the adjustment and control of the load easy to realize, and can achieve high adjustment accuracy and stability. At the same time, through flexible and diverse adjustment and control methods, not only can the actual load situation be simulated, but also some special load waveform curves can be simulated to test the dynamic and transient characteristics of the power supply equipment. This is not possible with load forms such as resistors.
Electronic loads are divided into two types: DC electronic loads and AC electronic loads. DC electronic loads can have constant current, constant resistance, constant voltage, dynamic loads, and short-circuit loads. AC electronic loads can simulate constant current, constant resistance, different crest factors, different power factors, and short-circuit loads.
The working mode of the electronic load includes the following 4 kinds.
(1) Constant current mode (CC mode). In the constant current working mode, the load current flowing into the electronic load remains constant according to the set current value, regardless of the size of the input voltage, that is, the load current remains unchanged at the set value, as shown in Figure 3.
The core of the circuit is essentially a current sampling negative feedback control loop, where the transistor or MOS tube is used as the current control device and also as the load of the measured power supply.
(2) Constant resistance mode (CR mode). In the constant resistance working mode, the load current flowing into the electronic load depends on the size of the set load resistance and input voltage. At this time, the load current is proportional to the input voltage, and the ratio is the set load resistance, that is, the load The resistance keeps the set value unchanged, as shown in Figure 4.
(3) Constant voltage mode (CV mode). In the constant voltage working mode, the load current flowing into the electronic load depends on the set load voltage. At this time, the load current will increase until the load voltage is equal to the set value, that is, the load voltage remains unchanged at the set value. , As shown in Figure 5.
(4) Constant power mode (CP mode). In the constant work mode, the load current flowing into the electronic load depends on the set power. At this time, the product of the load current and the input voltage is equal to the load power setting value, that is, the load power remains at the set value. change.