Diode for my custom Solar Panels

[toxsick]

Hello,

I am building some custom Solar Panels using 3x6inch cells rated at 3.5A 0.5v (1.8watt)

I need help selecting 2x diodes for my panels.
I can easily get the normal rectifier diodes 6A that is rated with 0.95 F voltage drop 1000v
better diodes have drops of .55 is it worth it?? only .4v
I am thinking of using a Schottky type diode because they have low forward drop.
SR504 SB540 5A 40v diode is it ok to use. (0.55v F drop)

1. I will be using 4 strings of 9 cells for a total of 36 cells giving out 18v @ 3.5A (60watts)
I will be putting 4 of these 60watt panels in series for a output voltage of 48v nominal.

I don't understand how the panels being in series would effect the diode.
40v, will be fine because it is only handle 1 panel? or the panels having a high voltage maybe about 60-80v open circuit voltage.
does this example have truth? say 3 panels with open voltage of 60v 4th panel would have 60volt going thou it? so i would require a larger diode?

2. my other panel will be 18 cells @ 7A 4.5v to push my MPPT a little higher. this will be used for my current 2x 120Watt Sharp panels. each rated at 7A 18v (20-21 open v.)
it will be a 120(7A18V)+120(7A18V)+32watt(7A4.5V)

if someone can kindly tell me how to install the diode, i'd also applicate that (I currently think on the Positive side, with the diode's stripe facing the away from the Panel. (towards the connector)

with a Professional Solar Panel have a maximum rated system voltage of 1000v, the diode is what limited them?

Thankyou
Shaun

 

[RCinFLA]

If you are using a commercial controller you will not need the blocking diodes as there is usually a disconnecting MOSFET switch within the controller.

The purpose of a blocking diode is avoid a backwash of leakage current through panel from battery when there is little or no generation from panel. There is a second application where bypass diodes are placed on a panels or every 3 to 6 cells to bypass them when they are shaded. This only works if you have high enough voltage stack and an MPPT controller where there is enough overhead voltage to afford the drop out of a panel that is shaded. You actually don't need an MPPT controller but having an excessive voltage stack would be wasteful for a simple ON-OFF PWM controller.

Cells act like an illumination based current source that is clamped by inherent diode of cell. No load voltage, Voc, is where all the illumination current goes down the inherent diode of the cell. Normal MPP voltage is just below where the inherent cell diode starts to conduct significant current. Typically about 3% of illumination current is allowed to leak down the inherent diode so you get the highest possible voltage on the cell. At 25 deg C this is about 0.53 volts D.C. but, as a normal silicon diode, it has a negative temperature coefficient and the MPP voltage goes down at higher temperature. In the sun, in summertime, depending on ambient temp, the cells may get to 60 deg C and the MPP voltage will drop to about 0.42 vdc per cell.

The reverse leakage on a PV cell in the dark depends on quality of cell and its temperature. Good quality monocrystaline cell has very low reverse leakage (assuming you stay well below the stackup sum of the inherent diode forward bias voltage).

A schottky diode has a low reverse breakdown voltage, as you said 40 volts PIV. If the battery side to cell side exceeds this voltage the schottky diode will zener out.

The larger the diode area, the lower the forward current density and therefore the lower the diode forward voltage drop. If you use a silicon diode with higher current rating it will generally have less forward drop. Keep in mind that the forward voltage drop times the current through the diode will be the heat dissipation in the diode. A larger diode area (higher current rating) will have more junction capacitance but this is of little importance in this application.

 

[toxsick]

Thanks for the reply.

you mention bypass diode..
I think thats what I want to setup incase of total panel shading the mppt will just lower the output state.

as it turns out i got a few 6A1 100v diodes with a .9v drop

I just put these on the + output in series?