What is a bypass diode?

Can somebody explain me the function of bypass diodes and why they are helpful in a solar module? Thanks

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This is a great question! Thanks for asking.

Intro 1) When part of a PV module is shaded, the shaded cells will not be able to produce as much current as the unshaded cells.

Intro 2) Since all cells are connected in series, the same amount of current must flow through every cell.

Result: The unshaded cells will force the shaded cells to pass more current that their new short circuit current. The only way the shaded cells can operate at a current higher than their short circuit current is to operate in a region of negative volt age that is to cause a net voltage loss to the system.

The shaded cells will dissipate power as heat and cause “hot spots”. And the shaded cells with drag down the overall IV curve of the group of cells. The effect of this shading is also dependent on how the module is shaded.

Function of a bypass diode: One way to minimize the effect shading has on a single module in a series string, is to use bypass diodes in the junction box. Bypass diodes allow current to pass around shaded cells and thereby reduce the voltage losses through the module. When a module becomes shaded its bypass diode becomes “forward biased” and begins to conduct current through itself. All the current greater than the shaded cell’s new short circuit current is “bypassed” through the diode, thus reducing drastically the amount of local heating at the shaded area. 

Hope it clears up. Here are some resources:

bypass diode

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8 years 3 months ago
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Levent Bas
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