PV Module reverse behaviour

This tool presents the operation of a PV module when it is reverse-biased at negative voltages, as it can happen in the framework of an array or a module, when the cells are different, or if the irradiation is not homogeneous.

The tool show three typical situations:

"One single PV cell": we see that under bright irradiance, reverse bias of the cell rapidly involves high powers to be dissipated, as the current is already at least at the cell Photocurrent level. (See the reverse cell characteristics model).

The relevant coefficient brev, defined along with other PV module parameters in the "Components" section, can vary significantly from one cell to another and is highly temperature-dependent. But this behaviour is not critical in the qualitative evaluations of the PV array behaviour tools. If it is not precisely known, we can usually use the default value proposed by the programme.

"PV module without by-pass diode": the solid line represents the whole module characteristics, that is, all cells in series. With identical cells, the total dissipated power is equally distributed on every cell. The blue dotted line shows an arbitrary operating reverse current (identical current in all cells in series).

If one cell has a lower photocurrent—due to its quality or shading—or a better Brev (flatter curve), its voltage follows its own reverse curve, and at the imposed common current, it can generate much higher power than other cells, leading to a temperature rise (see the "Hot spot" phenomenon in the tool for one shaded cell).

"PV module with bypass diode": shows the resulting module characteristics when the module is protected with one or more bypass diodes mounted in reverse bias. In this case, the total module reverse voltage is limited to the forward voltage of the diode (approximately 0.7 V for one diode, 1.4 V for 2 diodes, etc.). The bypass diode carries the excess current, and excess power in the diode cannot develop because the voltage remains very low.