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Preventing Reverse flow on Low Voltage/Large Currents
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Preventing Reverse flow on Low Voltage/Large Currents

by james_lan on Wed Oct 06, 2010 10:27 am

I'm powering something that will end up needing about 5-40 amps at 2+V, but needs to not have a reverse current more than 25microAmps, and less than (-)3V.

I'm wondering what sort of protection can handle it. All of the diodes I've looked at have been rated for much lower current. I'm considering wiring them in parallel. While I think that will work in an ideal case but I think that potentially causes a problem, in that if one of the diodes were to fail, tossing the increased current on the others, in a cascade.

Ideally, I'd like to control this with PWM, but noting the comments on a 25Amp motor, combined with a few comments elsewhere about a high negative voltage if power is turned off in general. So I'm considering this as a possibility, though not sure if this might create a reverse voltage. Having two circuits, at the same voltage one of which being a high current one say 18 Amps, and a smaller one of an Amp or so, which is always on, and drops it below the threshold of actually powering the device, using a mosfet for the large source.

(If you are wondering, it's a laser bar diode, and yes, it is that crazy. I've taken (or will*) all the safety precautions I've found about in research. The reverse current one is one I've never had to deal with before though in that sort of power levels. *Not all of the safety equipment has arrived, so I haven't tried to power it up yet.)

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Re: Preventing Reverse flow on Low Voltage/Large Currents

by cstratton on Wed Oct 06, 2010 11:19 am

If these things exist as a product, there's got to be an accepted way to drive them. Talk to the manufacturer? Google for a reference design?

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Re: Preventing Reverse flow on Low Voltage/Large Currents

by zener on Wed Oct 06, 2010 3:24 pm

There are plenty of diodes that big. You don't say how much Vf you can handle. You can also do "synchronous rectification" with a fet but you would need a way to turn the fet on and off of course.

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Re: Preventing Reverse flow on Low Voltage/Large Currents

by uoip on Wed Oct 06, 2010 4:14 pm

Have you considered something like this:

http://search.digikey.com/scripts/DkSea ... I45-16A-ND


Can handle up to 45A forward, allows no more than 20 uA through backwards, even when reverse biased to 1600V.

Don't try to parallel diodes to increase the current handling capacity. If the Vf's of the diodes don't match exactly absolutely perfectly, then one diode (the one with the low Vf) will end up carrying more than its share of current. That will cause it to fail, probably as an open circuit, transferring current to the others, causing the next one to fail, etc.
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Please be positive and constructive with your questions and comments.