Who makes high-current diodes?

williaty

I've got a DC power system fed by an AC>DC power supply to charge the batteries when the grid is up. When the grid goes down, the power supply is supposed to just turn off, but testing reveals that it turns into a current sink instead. I'd like to put a blocking diode in the wire between the power supply and the main DC bus to prevent the power supply from draining the batteries when the grid fails. However, the diode must pass the full charging current, which is 50A. So I need a diode rated at an Imax of 50A minimum with a minimum Vmax of 35V. Do such beasts exist?

NorthGuy

Re: Who makes high-current diodes?

Sure. For example, this one.

niel

Re: Who makes high-current diodes?

do know 3 things,
1> diodes are polarized so putting them in in the wrong direction will cause it not to work as you wish it to.
2> diodes that large need a large metal heatsink or the diode will not be handling that 50a for very long.
3> diodes introduce a voltage drop often in the area of .5v to .75v and that represents a loss of power and also can cause the battery to not reach its proper charging potential.

an alternative may be to operate a relay attached directly to the output of the pvs that would automatically switch the outpower wires of the supply in and out of the circuit. it would need very heavy contacts to handle 50a if mechanical, but there are solid state relays too.

inetdog

Re: Who makes high-current diodes?

niel wrote: »

3> diodes introduce a voltage drop often in the area of .5v to .75v and that represents a loss of power and also can cause the battery to not reach its proper charging potential.
... but there are solid state relays too.

A properly designed FET circuit can actually have a close to zero voltage drop when turned on. That will both improve the output voltage and allow you to use a smaller heat sink. The downside is more complexity and more to fail.

Cariboocoot

Re: Who makes high-current diodes?

Another option: replace power supply with proper battery charger that doesn't do this.

Sounds like what you've got there has some internal problems since the output of a DC is through diodes (rectifiers) and should not leak current back. Possibly a big old filter cap on the output is causing the drain.

mike95490

Re: Who makes high-current diodes?

niel wrote: »

......
an alternative may be to operate a relay attached directly to the output of the pvs that would automatically switch the outpower wires of the supply in and out of the circuit. it would need very heavy contacts to handle 50a if mechanical, but there are solid state relays too.

How about a AC powered Relay/Contactor or even a SSR for the dc powersupply output switching ? Grid on = relay powered and contact made to the batteries.

If you insist on a diode, you need a single diode rated for the full power, or you need to make a complex array of lesser diodes with swamping resisters (more loss) on them to force proper current sharing. A SCHOTTKY diode is suggested as being less loss then conventional diodes.
Here's a nice 200V 120A one: http://www.digikey.com/product-detail/en/APT100S20BG/APT100S20BG-ND/1494212
For $8, and a substantial, isolated heatsink (the case is live on this package) this part should work .
http://www.microsemi.com/document-portal/doc_download/6582-apt100s20b-g-c-pdf
At 50a, the drop would be about .55V, giving a loss of 30watts, which will need a beefy heatsink & fan.

niel

Re: Who makes high-current diodes?

all further good ideas. the fet circuitry is beyond most in designing so i kept it as simpler solutions that most can do. that is somewhat the basic idea behind many ssrs anyway and you can buy them outright without having to design them.

RCinFLA

Re: Who makes high-current diodes?

No solution is without its issues. A mechanical relay is probably the best way to stay out of trouble. A relay with 50 amp capability will likely draw 2 watts if DC coil and more if AC coil.

SSR must be DC type. 99% of SSR's are AC triac based which will not work for DC. You also need to have DC SSR work with higher voltage on either side of switch which most DC SSR's don't do. You can place two of the DC SSR's in series, reverse polarity to each other to ensure it takes higher DC voltage on either end of switch. Turn both on and off together.

http://www.futurlec.com/Relays/SSRDC50V80Apr.shtml

williaty

Re: Who makes high-current diodes?

A relay would quickly exceed it's lifetime cycle count as this has a 26-second period. I've ordered the high-current relay listed earlier. I'll see if that fixes it.