How to choose diodes for solar panel junction box

How to choose diodes for solar panel junction box

Introduction

Diodes are semiconducting devices that are capable of allowing current in one direction while blocking it in the reverse direction. This capability of diodes is extensively used in solar panels as well as solar junction boxes. Depending on how they are connected to the PV solar system, they can differently affect the system. When connected in series with a string of solar PV panels, they prevent back flow of current down the module string (called blocking diodes) and when they are connected in parallel with the modules allow passing of current through a shaded area of a pv module (called bypass diodes). The following figure shows both types of diodes.

Diodes in Solar panel junction box

Bypass diodes are not usually mounted directly on pv solar panels; rather, they are screwed or soldered in the solar junction box placed at the back of the solar panel. There are different configurations of diodes in junction boxes, but most of the time they contain three diodes in series. The 

following figures show their connections in typical junction boxes:

Proper choice of diodes in solar PV panels:

Type of diodes

Currently, there are two types of diodes available in the market to be used as bypass diodes in PV panels and arrays:

• PN-junction silicon diode
• Schottky barrier diode.

Both types are available in the market with a wide variety and a wide range of current ratings. The forward voltage drop in Schottky barrier diode is about 0.4 volts as compared to the PN silicone diodes with 0.7 volt drop.

Since each diode has its own voltage drop and a leakage current characteristics, the best choice for a bypass diode is the one with the lowest forward voltage (VF) and lowest leakage current (IR).

Number of diodes

It should be mentioned that for an efficient operation, there are two conditions that ought to be fulfilled

• Bypass diode should act and conduct current when one cell is shadowed.
• The voltage Vs of the shadowed cell should always stay under its breakdown voltage (Vc). Vs is defined by the cell manufacturer and is the minimum value of the manufacturing distribution.

In order to calculate the maximum number of solar cells (nmax) to bridge these conditions should be met:

Vbypass = Vs – (n – 1)*VOC

With Vs < VC and VOC = 0.5 V

VF = VC – (nmax – 1)*0.5

So

Nmax < 1 + (VC – VF)/ 0.5

As an example, let us find nmax for a commonly used setting by module manufacturers. For poly silicon PV cells with VC = 12 Volt and a bypass diode with VF = 0.5, the above equations can be used to fine nmax = 24, meaning that a maximum of 24 solar pv cells can be bridged by a bypassing diode.

Maximum number of PV cells to be bridged by bypassing diodes

http://www.wxminyue.com/how-to-choose-diodes-for-solar-junction-box/