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bravo88
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Short Circuit Protection for Triac

11/18/2011 6:02 AM

Triac in AC mains circuit, under load short circuit condition, will be damaged with certainty, even with fuse or MCB protection. Fuse and MCB are not fast enough to cut off short circuit current.

Does any one have idea on how to protect such circuit?

Thank you.

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electricalexpert65
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#1

Re: Short Circuit Protection for Triac

11/18/2011 7:33 AM

Haven't you heard of semiconductor fuses?
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bravo88
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#3
In reply to #1

Re: Short Circuit Protection for Triac

11/18/2011 6:57 PM

But this is not resetable.

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Winfield Hill
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#2

Re: Short Circuit Protection for Triac

11/18/2011 12:05 PM

You need to use a switch that can be turned off. So, use an IGBT instead. These are easily protected by monitoring the voltage across the device when it's on. It should be under a few volts, but if it's over say 3 to 5 volts for more than 5 to 10us, then simply turn off the IGBT gate drive (no sense resistor is needed). This scheme is widely used and protects the IGBT from failure, even up to fault currents of hundreds of amps! For example, the Fairchild FGA60N60 pictured at right, $6 ea qty 10 at DigiKey (link).

There are high-voltage gate drivers that incorporate the shutoff circuitry, and give you back a convenient logic-level FAULT signal.

IGBTs are taking have taken over from triacs in industrial power control.

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bravo88
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#4
In reply to #2

Re: Short Circuit Protection for Triac

11/18/2011 6:58 PM

GA, thanks !

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bravo88
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#5
In reply to #2

Re: Short Circuit Protection for Triac

11/26/2011 7:03 PM

Found out an disadvantage with using IGBT. It causes a significant power loss constantly due to voltage drop by it , example at 30A load, with 1.5 V drop, it will have a constant loss of 45 Watt , a hugh heat amount to deal with.

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Winfield Hill
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#6
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Re: Short Circuit Protection for Triac

11/26/2011 10:08 PM

Yes, that's clearly shown in the datasheets, and I mentioned as well, that not until well over a 2V drop has occurred should one infer an over-current fault. But remember these are high-voltage switches, and a 1.5V drop out of say 450V is only 0.3%. Also, high-voltage MOSFETs are usually worse, at least those in the same size category and price range. A 1.5V drop at 30A would require RDS(on) <= 50mΩ, and only the largest 600V MOSFETs meet or exceed that spec. This is especially true at high junction temps, where RDS(on) rises by 2x or more.

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