Page 94 - Applied Photovoltaics

Figure 5.13. Effect on total output of a ‘bad’ cell with a bypass diode. When the
combination is short-circuited, the total power dissipated in the bad cell and
bypass diode is approximately equal to the output capability of one good cell.

In practice, however, one diode per cell is generally too expensive and diodes are
usually placed across groups of cells, as shown in Fig. 5.14. The maximum power
dissipation in the shaded cell is then approximately equal to the generating capability
of all cells in the group.

If one cell is shaded, current is
shunted around all members of the
cell’s group.
This group is reverse-biased to a
voltage equal to the ‘knee-voltage’
of the diode.
Therefore, the shaded cell is
reverse-biased to the generating
capability of the other cells in the
group plus this knee voltage.
Therefore, the maximum power
dissipated in the shaded cell
approximately equals the
generating capability of all cells in
the group.

Figure 5.14. Bypass diodes used across groups of cells in a module.

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