Sizing (Volts & Amps) for Schottky Bypass diodes

[mopat]

Does anyone know how to specify the voltage and amperage rating for replacement bypass diodes?

Panels are 230W, Vmp=29.6V, Imp=7.78amps, Voc=36.8V, Isc=8.34A
They are 60 cell poly-crystalline and there are 3 diodes.

Thanks for your help.

 

[BiduleOhm]

You need to chose diodes with a Vrrm voltage higher than your string Voc so for example if you plan to put 4 panels in series you need a 150 V diode but I'd add some margin on top to be safe so I'd chose a 180 or 200 V diode minimum.

You also need a diode who has a If higher than the Isc current of your panel (9 A). I'd chose a diode with lots of margin, like 3 or 4 times, so a 30 to 40 A diode minimum.

And ideally you want a diode with the lowest Vf possible as it'll limit the losses when it conducts (althought it's not as important as for a blocking diode who always conducts in normal conditions, bypass diodes should only conduct rarely)

So a search result with the values from my example would be this

Unless you chose a really high spec diode (more expensive...) it'll need some heatsinking, but fortunately not a lot thanks to the high margin on the If and low Vf

Also, don't buy on Ebay, Aliexpress, Amazon, ... they are fake ones 99 % of the time, always buy on a reputable distributor (Mouser, Farnell, Digikey, ...).

 

[snoobler]

Are they marked on the diode?

Personally, I would snip them out - even out of a good one and get the same kind.

 

[apctjb]

You need to chose diodes with a Vrrm voltage higher than your string Voc so for example if you plan to put 4 panels in series you need a 150 V diode but I'd add some margin on top to be safe so I'd chose a 180 or 200 V diode minimum.

These are bypass diodes we are talking about right? so the voltage across the diode is the the voltage across a string of cells in the module if the module is in a reverse bias condition (shaded). This voltage should not be higher than the Voc of a single module. 50V for bypass diodes is fairly typical.

 

[BiduleOhm]

Yea you're right, it's for blocking diodes you need the full string Voc, sorry for that

 

[efficientPV]

Typically bypass diodes are placed about every 10V open circuit. In schottky diodes There is usually a penalty for going higher voltage in the form of increasing V forward. Keep it 45V or under. Only the data sheet tells.

 

[mopat]

thanks for all the input

in this solar farm 10 of these panels are in series with a Voc of 368 volts.

if I could summarize the diodes could be replaced with...

Voltage > 40 v and Current > 10 amps

So these 78-VS-10TQ040-M3 from Mouser 60 pieces for $58.08 are good/recommended?

 

[svetz]

Late to the party, but if replacing diodes you might be interested in What ever happened to the Maxim Chip?

 

[efficientPV]

Workable, but probably not the best choice. 15SQ045 is a 15A, dirt cheap and has almost half the forward voltage at 10A. Slightly different package. In this same TO-220 package you can get a dual diode which many panels use rated at 20A or more.

 

[mopat]

Workable, but probably not the best choice. 15SQ045 is a 15A, dirt cheap and has almost half the forward voltage at 10A. Slightly different package. In this same TO-220 package you can get a dual diode which many panels use rated at 20A or more.

Thank you; by golly I think efficientPV's got it!