AN3432 datasheet(9/24 Pages) STMICROELECTRONICS

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AN3432

Bypass diode inside the junction box

Doc ID 019041 Rev 1

9/24

Figure 6.

Solar system diagram

Ideally, a bypass diode should have a forward voltage (VF) and a leakage current (IR) as low

as possible. However, these are conflicting objectives. Special care needs to be taken to

eliminate any risk of thermal runaway.

The junction box manufacturers use Schottky diode for its low forward voltage. The choice of

maximum reverse voltage is made versus the number and voltage of the solar cells in

series. Then the trade off “conduction voltage VF/reverse current IR” is selected according to

the total power losses ratings.

2.3

VRRM is the first rating criterion

The maximum repetitive reverse voltage (VRRM) of the bypass diode is directly linked with

the number of cells bridged by the bypass diode.

2.3.1

Maximum number of solar cells to bridge with bypass

The maximum number of cells to bridge is defined by the breakdown voltage (Vc). The

literature gives breakdown voltage (Vc) range for the poly-silicon cells from 12 V to 20 V. For

mono-silicon cells the breakdown voltage extends up to 30 V.

For an efficient operation, there are two conditions to fulfill:

●

Bypass diode has to conduct when one cell is shadowed.

●

The shadowed cell voltage Vs must stay under its breakdown voltage (Vc). It is defined

by the cell manufacturer and is the minimum value of the manufacturing distribution.

The maximum number of solar cells (n max) to bridge is calculated using both these

conditions:

Solar panel

Junction box

Inverter

AN3432 数据表(PDF) 9 Page - STMicroelectronics