The application of photovoltaic junction boxes of ultra-high current modules has potential safety hazards

Some time ago, an article mentioned that the high-current junction box has fully met the needs of ultra-high power components, and showed the relevant data of the 25A junction box. In fact, according to the author's understanding, the 25A junction box cannot meet the needs of the use of ultra-high current double-sided modules. This article will give you a comprehensive analysis on the rated current standard of the junction box, the inventory of high-current junction box requirements, and reliability.

1. Selection criteria for rated current of photovoltaic junction boxes

According to the bypass diode test regulations in IEC61215, the rated current of the junction box needs to meet the Isc (single-sided module) greater than 1.25 times. The theoretical maximum current of the junction box is equal to IscX1.25X1.3. Therefore, using the short-circuit current of 18.36A for the G12 double-sided module, the maximum theoretical current of the junction box is equal to 29.84A, and a junction box with a rated current of 30A needs to be selected. The double-sided module of M10 is calculated according to the short-circuit current, the maximum theoretical current is 22.5A, and a junction box with a rated current of 25A needs to be selected. Through the above calculation, it can be clearly seen that the safety margin left by the 30A junction box is only 0.5%, which means that the subsequent improvement of the module current through other means (such as battery efficiency improvement, new component efficiency improvement technology) is restricted, and a slight fluctuation is reliable. Sex is risky. If the current conventional 10% safety margin design is considered, it is even necessary to develop a junction box above 33A. For M10 components, the safety margin of the junction box is 10%, which reserves sufficient space for the subsequent application of new technologies, which is a scientific consideration that takes into account reliability and long-term development.

2. Inventory of photovoltaic high current junction box requirements

From the electrical performance parameters of typical high-current modules of photovoltaic module factories and the selection of rated current of junction boxes, it can be seen that the M10 module with a current of 13.9A adopts the mature single-core solution in the industry and has sufficient safety margin, and uses diodes of higher specifications. The reliability of the high-current junction box is guaranteed. The 18.4A G12 ultra-high current module adopts a new dual-core solution (not verified by the industry), and the safety margin of the double-sided module is too small.

3. Reliability analysis of high current photovoltaic junction box

According to the survey, most of the M10 module junction boxes are the mature axial process and single-core design in the industry. The axial diode junction box has developed early, and is currently relatively mature and has a low failure rate. Now most of them adopt the trench chip structure, which has small leakage current and strong antistatic ability, and its reliability has been fully verified. As for the design of 30A junction box dual-core, the industry has not used it before. If the performance of the two chips is quite different when the chip is packaged, there will be the risk of uneven current shunting during operation, which will cause the diode to heat up and burn out.

The analysis of the shunt characteristics of parallel diodes is shown in the figure below. The forward current curve analysis of 150mil Schottky diodes commonly used in the industry is used. If the forward voltage drop of the two diodes is VF=50mv, the difference between the currents through the two diodes is 10A. Assuming a working current of 30A, the current of one of the diodes will reach 20A, which is close to the upper limit of the allowable current of this diode (22A, when the junction temperature exceeds 200°C) The risk of failure is significantly increased.

Since the junction temperature performance is very important for the selection of the junction box, the Isc of the M10 bifacial module is much lower than the current of the 18.4A bifacial module, and the junction box with the same configuration has lower heat, so the junction temperature test can be easily passed under normal conditions. The data are shown in the table below. For a more comprehensive evaluation of this junction box, the junction temperature performance was tested under tighter conditions as follows:

First, further increase the theoretical maximum current from 22.5A to 26A, and then perform the same test with other conditions. Record the diode case temperature one hour later. The results are as follows:

It can be seen that even if the test current exceeds the rated current of the M10 junction box, the diode is still safe and reliable. In order to simulate the influence of outdoor comprehensive aging factors (high temperature, high humidity) on the junction box, PCT48h (high-voltage accelerated experimental aging) + junction temperature test (26A) was performed on the M10 component junction box, and the junction temperature was observed. The results are as follows:

The experimental results show that even if the outdoor junction box is superimposed with ultra-high current in a high temperature and high humidity environment, the M10 module junction box still meets the needs of use.

To sum up, the evaluation of high-current junction box not only needs to consider the factors of junction box material, but also need to comprehensively consider the safety margin of the junction box design current, the maturity of the junction box packaging structure and the performance in harsh environments. The M10 photovoltaic module junction box is safe and reliable in the above aspects through rigorous design, while the G12 ultra-high current double-sided module junction box obviously does not meet the needs of use.