<< Click to Display Table of Contents >> Navigation:  Physical models used > PV Module - Standard one-diode-model > PV module reverse behaviour

This tool presents the operating of a PV module when it is polarised towards 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 the other parameters of the PV module in the "Components" part, can vary strongly from one cell to the other, and is highly dependent on temperature. 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), then its voltage will follow its own reverse curve, and at the imposed common current it can produce a much higher power than for the other cells, therefore giving a rise of temperature (see the "Hot spot" phenomenon in the tool for one shaded cell).

"PV module with by-pass diode": shows the resulting module characteristics when the module is protected with one or more by-pass diode, mounted in reverse bias. In this case the whole module reverse voltage is limited to the "forward" voltage of the diode (about 0.7V for one diode, 1.4V for 2 diodes, etc). The excess current is drawn by the diode, and cannot give rise to excess powers in the diode since the voltage stays very low.