Selection of Bypass Diodes

Selection of Bypass Diodes

The diode in the photovoltaic junction box is used as a bypass diode to prevent the hot spot effect and protect the components.

The selection of bypass diodes mainly needs to follow the following principles:

1. The withstand voltage capacity is twice the maximum reverse working voltage;

2. The current capacity is twice the maximum reverse working current;

3. The junction temperature should be higher than the actual junction temperature;

4. Small thermal resistance;

5. Small pressure drop.

The bypass diode is in the cut-off state when the component is usually working. At this time, there is a reverse current, the dark current, which is generally less than 0.2 microampere. Dark current reduces the current drawn by the component, albeit by a small amount.
From the ideal point of view, each photovoltaic cell should be connected to a bypass diode. Still, it is very uneconomical because of the impact of the cost of bypass diodes, dark current loss, and the existence of voltage drop under working conditions. In addition, the position of each cell of the photovoltaic module is relatively concentrated. Therefore, after connecting the corresponding diodes, it is necessary to provide sufficient heat dissipation conditions for these diodes.
Therefore, it is generally reasonable to use a bypass diode to protect multiple interconnected battery groups. This reduces the production cost of PV modules and adversely affects their performance. If the output power in a string of cells drops, the cell in the series, including those that usually work, will be isolated from the entire PV module system due to the bypass diode. As a result, the output power of the whole photovoltaic module will drop too much due to the failure of a particular cell.
In addition to the above issues, the connection between a bypass diode and its adjacent bypass diode must be considered carefully. In practice, these connections are subject to some stresses from mechanical loads and cyclical temperature changes. Therefore, during the long-term use of the photovoltaic module, the association mentioned above may fail due to fatigue, resulting in an abnormality of the photovoltaic module.
In addition, the effect of shading one cell is different from covering half of the two cells, so when shading is unavoidable, try to shade as many cells as possible, with as few shadows as possible for each cell.
In the construction of solar modules, individual cells are connected in series, so-called series connections, to achieve higher system voltages. Once one of the battery slices is blocked (for example, tree branch or antenna, etc.), the affected battery no longer works as a power source but becomes an energy consumer. The other unblocked batteries will continue to pass current through them, causing high Energy loss, "hot spots" will appear, and even battery damage.
To avoid this problem, bypass diodes are placed parallel on one or several batteries connected in series. The bypass current bypasses the blocked cell and is passed down through the diode.
When the cell is working, the bypass diode is usually cut off and has no effect on the circuit; if there is an abnormal cell in the cell group connected in parallel with the bypass diode, the entire line current will be determined by the cell with the minimum current. This is because the shielding area of ​​the battery determines the current size. If the reverse bias voltage is higher than the minimum voltage of the storm, the bypass diode is turned on. At this time, the abnormal working battery is short-circuited.

The harm of a hot spot is enormous, and the burning spot effect is straightforward when the module array power station is unmaintained. Therefore, avoiding or reducing the adverse impact of a hot spot on the module has become essential in module design.
It can be seen that the hot spot means that the module is heated or partially heated. As a result, the cells at the hot site are damaged, reducing the module's power output and even causing the module to be scrapped, seriously reducing the service life of the module and causing hidden dangers to the safety of power generation and other power plants.

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