Protection in Solar Panels;Bypass diode

For protection of Solar panels bypass diodes are used details for protection of solar panel is as described below:-

Bypass diode is also known as free-wheeling diode and is connected in parallel to every solar cell or for group of solar cells. It is not advisable to use bypass diode for every solar cell as it is too costly so it is used for group of solar cells. Bypass diode is connected in parallel with opposite polarity to a solar cell. As we know that solar panel is constructed using individual solar cells and solar cells are made from layers of silicon semiconductor materials. One layer of silicon is treated with a substance to create an excess of electrons. This becomes the negative or N-type layer. The other layer is treated to create a deficiency of electrons, and becomes the positive or P-type layer similar to transistors and diodes. During normal operation of solar panels all solar cells are forward biased and bypass diode is reverse biased as bypass diode acts as open circuit. The damaging effects of hot-spot heating may be avoided through the use of a bypass diode. Bypass Diodes prevent the current(s) flowing from good, well-exposed to sunlight solar cells overheating and burning out weaker or partially shaded solar cells by providing a current path around the bad cell.

Bypass diodes are basically used for protection in case of shades/Dark. Normally one bypass diode is used for every 15 solar cells. Therefore for 60 cell module bypass diode required are 4 nos.

The voltage across the unshaded solar cells depends on the degree of shading. For example, if one cell is completely shaded, then the unshaded solar cells will be forward biased by their short circuit current and the voltage will be about 0.6V (this is taken as an example). If the cell is only partially shaded, the some of the current from the good cells can flow through the circuit, and the remainder is used to forward bias each solar cell junction, causing a lower forward bias voltage across each cell. The maximum power dissipation in the shaded cell is approximately equal to the generating capability of all cells in the group. 

Above figure is self explanatory showing how bypass diode provides protection from Hot Spot heating. 

Process by Which Hot-Spot heating is prohibited by Reverse bias Diodes

If a solar cell is reverse biased due to a mismatch in short-circuit current between several series connected cells, then the bypass diode conducts, thereby allowing the current from the good solar cells to flow in the external circuit rather than forward biasing each good cell. The maximum reverse bias across the poor cell is reduced to about a single diode drop, thus limiting the current and preventing hot-spot heating.