AN3401

Application overview

Doc ID 018842 Rev 1

3/18

1

Application overview

The photovoltaic effect allows each PV cell to generate current once irradiated. Therefore,

the PV cell can be represented as a current generator, whose voltage and current generated

depend on the cell technology, cell size, and irradiation level.

Typically, the voltage provided by a single PV cell is too low for most of the applications; so

far, their connection in series is preferred. For this reason, PV panels are assembled by

connecting in series a proper number of PV cells.

In optimal conditions the PV cells of a PV panel are equally irradiated and generate the

same current, assuming negligible the spread among each PV cell.

Due to topological constraints, even if only one PV cell on the panel is partially shaded, the

whole series operates at the lowest current level forced by the shaded PV cell.

Therefore, shaded cells behave like a load and the current generated from the fully

irradiated cells can cause them to overheat (Hot spot) or in some cases, also lead to

permanent damage.

In order to prevent these events, the series of cells of a PV panel are arranged in strings and

a bypass device is connected in parallel to each string, as shown in Figure 1.

Figure 1.

Bypass diodes internal connection

The following is a brief list of the main requirements of the bypass devices:

●

To prevent the hot spot issue, bypass devices are connected in parallel to the cell string

●

During normal operation (no shadows) the reverse leakage current must be very low

●

When the cells are shaded the voltage drop must be very low.