Battery Bank Isolation

[SolarEngineer]

I have a situation where I have multiple battery banks feeding a single load, and I need to avoid having one bank dump power into the other banks. I've been told to diode isolate the banks, but I'm having trouble finding a good way to do this. International Rectifier has schottky modules that would work, but the have non-isolated bases. I'd appreciate any advice.


John

 

[ScottyUK]

Semikron, IR, IXYS, Toshiba, others all have isolated base diode modules capable of handling very high current and voltage. Why Schottky? Conventional silicon has better voltage blocking characteristics - Schottky is limited to less than 100V. What voltage and current are you working with? We could be talking about anything from a PCB mounted signal diode up to a device capable of controlling power in the MW range.


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Sometimes I only open my mouth to swap feet...

 

[itsmoked]

Schottky because Si has a horrible Vf.

You really need to tell us more. If is a low voltage bank you would want to avoid diodes and what they do to charging systems,(depending on your topology).

How many Amps.
What kind of load?
Charging system?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[BobbyNewmark]

Can you not find suitable mica insulators for the base?

This is probably of no use to SolarEngineer, but several companies make chips that switch MOSFETs so that they act like rectifiers. It eliminates the diode drop so the efficiency can be very high. Anybody seen preassembled modules based on these chips?

 

[ScottyUK]

"Schottky because Si has a horrible Vf."

Agreed 100%, but on, say, a 100V array the 0.4V difference is really of academic interest whereas on a 6V array it is hugely significant. There's just not enough information in the OP.


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Sometimes I only open my mouth to swap feet...

 

[SolarEngineer]

Thanks for the responses.

The battery banks are 3 48V (44-60V), 500Ah banks feeding a single inverter. Each bank consists of 2 strings of batteries, and each bank will be charged seperately. I'm not thrilled by the concept, but sales sold it, and a customer wants it. They promised delivery in the next few weeks, so I basically need to look for parts Newark or Allied have in stock.

The maximum continuous current I expect is 40A. I'm concerned about the forward voltage drop not for system performance, but because of the amount of heat I'll need to dissapate. This is going into the desert and I really want to avoid a fan.

I was looking at International Rectifier diodes in half-pak and TO-244AB packages. But I'm not sure how to connect to the non-isolated base and still maintain isolation from the heatsink. I know it's not rocket science, but I just haven't seen it before.

I'll see what I can find from Semikron,IR, IXYS, and Toshiba. Thanks for that tip.

 

[itsmoked]

Thanks for the details.

Look at Sil Pads they may have isolated thermal conductive, electrical insulators specifically for your footprints(any footprints for that matter).

Try to stay below 50% of any of the device's ratings for trouble free use.

http://www.bergquistcompany.com/tm_sil_pad_detail.cfm?oid=104254&tab=spec&prod_type=silpad

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ScottyUK]

Beware of Semikron's ratings: they look really good compared to most of their competitors of similar size and price, but they used to play some cute specmanship games which required serious heatsinking to reach the quoted ratings. The products themselves are good, just be careful you don't find you need a liquid cooled heatsink to reach the datasheet spec!

Have a look at

http://www.irf.com/package/pkmstdrec.html

for typical isolated base modules.

There is a big range of diodes in those package styles:

http://www.irf.com/product-info/diodes/

One thought is that the silicon is likely to be the weakest part of the system in the event of a fault - if you want to avoid expensive semiconductor fuses, ensure the diodes have plenty margin to give a conventional fuse a chance of clearing the fault before the silicon melts to protect the fuse .

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Sometimes I only open my mouth to swap feet...