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How to choose the diode of the PV junction box?

Bypass diodes are commonly used in photovoltaic junction boxes, with a parallel bypass diode diode method to reduce the impact of hot spots. First look at the formation of hot spots principle: the blocked cell is no longer power, itself is equivalent to a power consumption; at both ends of the S-1 chip in the direction of bias, such as no bypass diode protection, then Component current will flow through the heat. The shadows formed by the forward I-V characteristic of the component and the reverse I-V characteristic of the blocked cell are the maximum power consumption of the cell. In the solar cell (string) at both ends of the parallel bypass diode, bypass diode to work, will be blocked a string of battery chip bypass, component current flows from the bypass diode to ensure that the components work properly and protect the block The battery pack will not be damaged.

In addition, since the bypass diodes are connected in parallel to the ends of a string of cells, the normally lower diodes are in the reverse state, and the reverse voltage drop depends on the reverse voltage drop of about 0.5 NV (the number of strings of N ), By the reverse current characteristics of the diode, the diode reverse bias when there is leakage current, the current is very small, generally in the micro-level. The effect of reverse bias voltage and temperature on reverse current. Temperature and reverse bias voltage drop on the IR; the temperature rise makes the IR doubly increased, the same, the reverse voltage drop can lead to an increase in diode leakage current.

So the ideal state is a piece of the battery plus a bypass diode, but in practical applications, no manufacturers will do so, only to meet the requirements of the use of components in the case, consider each bypass diode bypass The number of solar cells. Since then, the performance requirements for bypass diodes are particularly important.

As most of the diodes installed in the junction box, the box by the limited cooling space and junction box structure and material constraints, the thermal performance of the diode must be good, hot spot occurs, the component current flows through the bypass diode, There is a current flow will have heat generated at the same time, because the junction box inside the diode heat also made a request on the junction box: Junction box to have a good heat and good heat dissipation characteristics. For the thermal performance of diodes to consider, for the choice of diodes, the main parameters to follow the following points:

1, the thermal resistance coefficient of the smaller the better;

2, the smaller the better the positive pressure drop;

3, the positive resistance to the greater the better;

4, the smaller the better the reverse current;

5, the temperature characteristic curve is better;

6, ESD performance is better (refer to IEC61000-4-2 electrostatic discharge immunity test standard);

7, in addition, the installation of bypass diodes in the junction box, limited to the junction box space and the environment, from the junction box to consider, the main consideration how to heat out: 1, diode select pin welding installation, pin large 2, the junction box filling irrigation gel, enhance the heat dissipation performance, this time, thermal conductivity, sealant can play a role in the heat of the diode.