So, I'm getting my first ArduCopter dropped off a little later this week and I've ordered a separate switching BEC to prevent any brownouts coming from the ESC's BEC circuit. I'm a little paranoid, plus I like the efficacy of switching hardware more then linear. :)
The main problem with MOSFET's, however, is that when they fail they normally fail in the on or off position. This means that if the BEC blows the copter will crash, but there's a 50% chance of it also killing a chunk of the electronics. I can't just add a fuse because a sudden increase in voltage is unlikely to have a sudden increase in current, so the fuse won't blow.
The thought occurred to me that I can build a little circuit with a fuse and a TVS diode, so that if the voltage exceeds a certain point the excess voltage will be dumped out of the circuit. Ideally, this should trigger a BEC shutdown, but adding a fuse would likely be a good idea in the event the BEC doesn't shut down fast enough to prevent the diode (or wires) from burning up.
Does this sound like something that would be worthwhile to add, or since the copter is going to crash anyway, should I just not worry about it? :)
Thanks.
Replies
My concern with fusing the main output is that potentially a non-essential item such as a servo could stall, blow the fuse and take out your vehicle. If you do want to use a fuse, it may be preferable to separate out power to non-essential items and fuse them separately.
>The thought occurred to me that I can build a little circuit with a fuse
Google "SCR crowbar"
That'll take care of ya.
In my experience, well-designed Buck converters are incredibly reliable. Failure is generally associated with exceeding the ratings, either on the input or the output. If you arrange your protection so that ratings can never be exceeded, then short of implementing power supply redundancy you should have a reliable system.
I do not know whether output current limiting is implemented on all commercial BECs, so I guess it would be risky to presume it. When implemented, current limiting is usually reliable, as failure of the shunt is invariably open, resulting in no output. The resistive element is often in series with the lower FET or diode, or indeed the intrinsic resistance of the lower FET itself. If the lower FET fails short then the output is clamped to ground anyway.
Of course you are quite right - TVS devices are only specified for transient conditions. But they can take a massive hit and survive. I am always a little nervous about 5c fuses blowing on a transient current surge and causing total vehicle loss.
I have destroyed countless Buck converters, and the upper FET has universally failed open. Of course this may have been preceeded by failing short, but I've never had an oscilloscope attached at the right time to determine the exact sequence of events.
I think a TVS has merit. The fuse may not be necessary, as the converter should go into current limit.
Cheers,
Andrew.