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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
ok pics first.
This is called a ESC, this particular one is a "AfroESC" 30 amp. Named apparently by the Chinese without any thought of being politically correct.
Im going to try to identify this transistor.
Here this exact ESC blew out the tail fan while in flight, She went into a "Black Hawk Down" like flat-spin. I was able to lay it in flat, and save the machine but there was still some minor damage. Ok with that background heres the problem. I was in flight and it overheated, ive never had a 30 Amp ESC get hot enough to blow transistors. So, its obvioulsy heat, and there are no metal fins, in fact its heat shrunk with no airflow at all.
So, crazy thought here ... tell me so. if i use a single piece of brass, about 1 x1.5 inches silver epoxied onto the tops, with out short circuits, the heat should drop considerably right?
The flight above last about 4 continuous minute at 30 amps or so before it blew. and they do make a 45 amp version (thats the biggest) but i cant find the 45 amp one for sale.
Transistors are under the label, then under clear heat shrink.
Registered Member #2989
Joined: Sun Jul 11 2010, 12:01AM
Location: UK
Posts: 94
I would think the copper on the PCB and wires would act as heat sinks and it's cooled by a large fan, maybe removing the heat shrink alone would do the trick. or could it be a one off duff PCB.
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Mosfet is correct. I do not see why this thing should need a heatsink at 30A load.
Rated with 2mOhm, 32A = 2W power disitpation its totaly fine since the Die has max 23°K/W heat resistance to the outside world.
There is something different wrong... like overcurrent or the gatedriver for the highside Mosfet is too weak (bad bootstrap cap) or too low gatevoltage.. could be caused by too high input voltage to the regulator and overheating of the LM2940 regulator.. or something. So many causes besides the actual mosfets... which are fine and properly cooled by the PCB and wires imho.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Sulaiman wrote ...
This is your mosfet i think
That sure looks like it.
As for cooling through the vias and traces, its a very small board, with 6 transistors in close proximity, only 3 can conduct to the output wires. The rest are on the on the board. Maybe one or more of these are the ones that blew.
6 times 2 watts means 12 watts in a small space. it was much hotter than the others of the same flight. It is the yaw motor, and in a tri-copter that one does 15% more lifting than the others.
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
6 times 2 watts means 12 watts
Aha. So all your mosfets are conductin at the same time and all the time and with 32A.... dude, maybe thats why they blew. Did you notice that the motor maybe does not spin or something like that? Just anything that supports your well thought out argument?
That PCB is totally fine! There is so much more that can go wrong besides heat management.
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
why 0.7? the theoretical maximum factor is 0.5 because only one of 2 mosfets can conduct in a halfbridge. But then additionally you do actually drive a slightly inductive load, which implies that the average current is less than the peak current. Then generally a 30A switching device will not drive a 30A motor if the owner is in anyway sane.... this all makes the PCB totally fine. And remember that "hot" for humans is not "hot" for semiconductors. Your factor is practically much less than 0.5. And 5W are easily disipated in a forced air convection environment...
Please consider other failure causes. Like the stuff mentioned above and maybe more systematic reasons like a peak current that is generally too high or/and your flight stabilizer oscillates (which happens if you configure the PID-loop wrong) And of course there is allways the possibility that you simply got a bad component which does however not imply that the engineering was done wrong.
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
DerAlbi wrote ...
why 0.7? the theoretical maximum factor is 0.5 because only one of 2 mosfets can conduct in a halfbridge. But then additionally you do actually drive a slightly inductive load, which implies that the average current is less than the peak current. Then generally a 30A switching device will not drive a 30A motor if the owner is in anyway sane.... this all makes the PCB totally fine. And remember that "hot" for humans is not "hot" for semiconductors. Your factor is practically much less than 0.5. And 5W are easily disipated in a forced air convection environment...
Please consider other failure causes. Like the stuff mentioned above and maybe more systematic reasons like a peak current that is generally too high or/and your flight stabilizer oscillates (which happens if you configure the PID-loop wrong) And of course there is allways the possibility that you simply got a bad component which does however not imply that the engineering was done wrong. EDIT: In addition, the gatedriver will probably be weak, bearing in mind the (low) cost of such a device.
Your hypothesis does not take into account switching losses. The driver design is hard-switched, and there is going to be current flowing as the MOSFETs switch on and off.
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Three Phase Electronic Speed Control In-Flight Blow-Out.
