Fried Digital Side Car

[jee7s]

Did I read that schematic right? A 10 A breaker? Don't the FRC rules say to attach the DSC to use a 20 A breaker?

[cgmv123]

Originally Posted by jee7s

Did I read that schematic right? A 10 A breaker? Don't the FRC rules say to attach the DSC to use a 20 A breaker?

You read it right. I'm not sure why FIRST differs. That's a question for EricVanWyk.

[jee7s]

Well. the datasheet for the FET says it's good to 33A. But, that doesn't mean there isn't a lot of heat building up anyway.

Also, is that schematic for Rev 7 or Rev 8 of the DSC? Q1 has decidedly different packages on those two revs.

[EricVanWyk]

That schematic image is from Rev 7 or earlier. I think CB1 was removed before we went to actual production.

The updated FET part number is FDS8817NZ. There is also an added 10k gate resistor.

That part can blow in a few situations:
1) The DSC ground was used to return something else's current. This can happen if a wire shorts to a ground pin on the DSC. This is one of several reasons we do chassis isolation tests during inspection.

2) An ESD event hit the gate of the FET, and subsequently a large but nominally acceptable current was passed through the part (which is now probably a bad diode).

3) An ESD event hit, and then the DSC was plugged in with reversed polarity.

4) Aliens.

5) A short across the power inputs just down stream of this protection. This is rare: The switchers have very well behaved protection against short circuits at their loads, and the Big Flashy Light port has a breaker.

You can swap the part out with a decently hot iron and try again, but I would check to find where the bonus current is coming from. I'd turn it on attached to a current limited bench supply the first time after the repair.

Originally Posted by jee7s

Did I read that schematic right? A 10 A breaker? Don't the FRC rules say to attach the DSC to use a 20 A breaker?

I'd be happier with a 10 Amp breaker, but 20 is enough like 10 to be just fine. In the end, it wasn't worth the supply chain changes. There are vanishingly few cases where the difference between 10A and 20A breakers would matter to a team - At this point the first line defenses are already down and the board would need to be repaired or replaced.

[Brian Selle]

Thanks for the info. I'm thinking the issue may have been a PWM ground that momentarily touched a +12v spike input. Would that do it?

[EricVanWyk]

Originally Posted by btslaser

Thanks for the info. I'm thinking the issue may have been a PWM ground that momentarily touched a +12v spike input. Would that do it?

Definitely.

[Crazyboy]

Originally Posted by EricVanWyk

Definitely.

I can back that up. I've actually done it! We had wires come loose and the DSC shorted. We discovered this 5 minutes before our quarter finals match, and I changed it in 3. One of those "can't believe that happened" moments.

[Tungrus]

pay close attention to the picture, the culprit is mutated Woodstock, the bird from Charlie Brown cartoo n (white smiley face next to FET)!