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Aug 24, 2019, 12:53 PM
anti-wind activist
thrashaero's Avatar
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Discussion

Trying to replace ESC burnt mosfet chip


OK so this ESC fried a mosfet.
It is a Turnigy Sentry 40A. I took off the heatshrink and revealed which mosfet it was, as you can see in the picture it is the bottom left.
I hope to replace it. But I'm not sure on where to get the specific mosfet.
I know it is a 7832Z by International Rectifier as are all of the mosfets on the board, but there are 2 distinct sets.
As long as it says 7832Z is all I should care about, the other numbers are before it are not important?

Also there was a thin layer, floppy grey pad that was on top of the mosfets and had the heatsink plate on top, i guess as a thermal conductor instead of paste.
Since the mosfet burnt a hole in it, where do I get this material for a new one?
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Aug 24, 2019, 02:05 PM
Registered User
The "P"-line and the "U"-line are date code and lot and are irrelevant.
The two sets are the high and low side drivers for the three phases of the motor, 2 transistors each.

But maybe the molten silicon powered the microcontroller back, so it's fried, too.
And the FETs are rated 20A each @25C, i'd say getting two of them to power 40Amps load is hard on them.

The gap filler material should be available where you buy the FETs from.
Aug 24, 2019, 02:15 PM
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thrashaero's Avatar
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Should I replace them all or I'll be fine with just replacing the burnt one?
Aug 24, 2019, 02:25 PM
Registered User
I would consider getting a new ESC. There is a very good chance that something else is damaged so replacing one FET may not fix it. If you try to replace all of the Mosfets, you would almost certainly be better off buying a new one.
Aug 24, 2019, 03:12 PM
anti-wind activist
thrashaero's Avatar
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Quote:
Originally Posted by Volt_Ampere
I would consider getting a new ESC. There is a very good chance that something else is damaged so replacing one FET may not fix it. If you try to replace all of the Mosfets, you would almost certainly be better off buying a new one.

Yeah the one fet might not be the only problem.
But even if I replace all of them I'm only out $3.16, much better than $30 for another esc.

I'm pretty sure the fet just got hot from a hot day and the heatsink wasn't upright towards open air (also under heatshrink and couldn't tell what side it was on, it was mounted towards the body of the aircraft)

Since the 'heatsink' was only a metal shim and was covered by heatshrink I'm planning on just leaving the heatshrink off and using an actual heatsink with fins on it to dissipate better.
Aug 25, 2019, 07:35 AM
Dave the Rave
dmccormick001's Avatar
Quote:
Originally Posted by thrashaero
Yeah the one fet might not be the only problem.
But even if I replace all of them I'm only out $3.16, much better than $30 for another esc.
You might want to check your math on this...

I think the only way you can get a MOSFET with those specs for a quarter each is if you buy them in lots of 2000-3000. Individually you'll pay more like a buck each. And as the other posters are telling you, it is very possible, maybe even likely, that the microprocessor on the other side is damaged, too. And you can't replace that because it's programmed by the manufacturer.

Take the advice of the "seasoned" flyers here and don't risk your whole model trying to save a couple of bucks. Get a new one. A bigger one. The outside temperature on a hot day didn't cause that, too much current did. Your motor pulled too much current through it, you need a bigger ESC.
Aug 25, 2019, 09:59 AM
anti-wind activist
thrashaero's Avatar
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Quote:
Originally Posted by dmccormick001
You might want to check your math on this...

I think the only way you can get a MOSFET with those specs for a quarter each is if you buy them in lots of 2000-3000. Individually you'll pay more like a buck each.
Nope:
10 for $2.63


Quote:
And as the other posters are telling you, it is very possible, maybe even likely, that the microprocessor on the other side is damaged, too. And you can't replace that because it's programmed by the manufacturer.
Maybe. I would think the burnt fet would act as a sort of fuse. Once it's done the esc just doesn't function. Also they're on separate pcb's. I'll have to see if it works when i get a new fet soldered in. If that doesn't work I'll replace all the fets. If that doesn't work, oh well, I tried at least.


Quote:
Take the advice of the "seasoned" flyers here and don't risk your whole model trying to save a couple of bucks. Get a new one. A bigger one. The outside temperature on a hot day didn't cause that, too much current did. Your motor pulled too much current through it, you need a bigger ESC.
I'm not too worried about that since I already risked my model when it was flying before the fet smoked. As I tried to take off again it just got weaker and weaker to the point of the fet sizzling and wouldn't take off. If the replacement fet doesn't work, then it still won't take off.
Also maybe you missed the part where I mentioned the heat sink wasn't mounted upright as well as covered by heatshrink and I'll be using no heatshrink and an actual heat sink with fins instead of a flat plate to have the same problem being less likely to happen.
I've had this esc since around 2008 and have only had a problem on a day when it was particularly toasty, probably the last straw to make it capitulate.
Aug 25, 2019, 03:43 PM
Registered User
Quote:
Originally Posted by thrashaero
I would think the burnt fet would act as a sort of fuse.
It's a piece of silicon that normally has 3 wires to it: one power, one motor, one control.
Power and Motor are usually switched by control, control is isolated to both, it works by applying voltage not flowing current.

When the FET fails, the silicon melts and you have a drop of metal that may or may not short any of the three leads.
So you may get a Power/Motor short, which will burn some more (because the other FET controlling the other battery pole to this motor pole will switch, which will result in a battery short over the ESC). This most likely is what made yours explode...

You may get a complete isolate and the motor just stops or vibrates because one phase is at least half open.

And you may get a Control/anything short, which will give battery voltage to the pin that normally is nonprotected and connected to the low power microcontroller pin. In this case the micro will definitely die, but the structures are so small they just die, not pop.
So you won't see but the defect, but it may have the same problems as it is basically the same tech, just on a very small scale, like 20mA max instead of 20A.

And beware if the micro output is shorted to high, which will activate the FET without control, resulting in instant redestruction.
If it's stuck low the motor will run weird if at all and the missing phase may destruct more of the ESC.
And even if it works it still may have some dents from the first crash and not perform to the datasheet, which may cause trouble further down.

Personally, i would replace the FETs when i'm ordering anyways, and then try that thing under controlled environment, like with a mini motor on a short circuit proof lab supply or fused battery.
I'd invest like 10$ max and learn something.
And if that thing misbehaves the littlest bit it's in the trash...
Aug 25, 2019, 05:46 PM
Registered User
Quote:
Originally Posted by thrashaero
Ebay and Aliexpress sellers are known for selling fake chips, mislabeled chips, rejects, etc... Granted, not likely to make a lot of money selling 10 MOSFETs for less than $3 with free shipping, but I'm not sure I would trust any drone flying with MOSFETs that might be questionable

It's true that you won't waste much money trying (but desoldering and resoldering that takes time and can result tin damage if you are not experienced (it looks like you are, not suggesting you are not, just a consideration).

But I would give that ESC a good, long test before trusting it with a flying drone...
Aug 25, 2019, 08:26 PM
anti-wind activist
thrashaero's Avatar
Thread OP
Quote:
Originally Posted by learningarduino
It's a piece of silicon that normally has 3 wires to it: one power, one motor, one control.
Power and Motor are usually switched by control, control is isolated to both, it works by applying voltage not flowing current.

When the FET fails, the silicon melts and you have a drop of metal that may or may not short any of the three leads.
So you may get a Power/Motor short, which will burn some more (because the other FET controlling the other battery pole to this motor pole will switch, which will result in a battery short over the ESC). This most likely is what made yours explode...

You may get a complete isolate and the motor just stops or vibrates because one phase is at least half open.

And you may get a Control/anything short, which will give battery voltage to the pin that normally is nonprotected and connected to the low power microcontroller pin. In this case the micro will definitely die, but the structures are so small they just die, not pop.
So you won't see but the defect, but it may have the same problems as it is basically the same tech, just on a very small scale, like 20mA max instead of 20A.

And beware if the micro output is shorted to high, which will activate the FET without control, resulting in instant redestruction.
If it's stuck low the motor will run weird if at all and the missing phase may destruct more of the ESC.
And even if it works it still may have some dents from the first crash and not perform to the datasheet, which may cause trouble further down.

Personally, i would replace the FETs when i'm ordering anyways, and then try that thing under controlled environment, like with a mini motor on a short circuit proof lab supply or fused battery.
I'd invest like 10$ max and learn something.
And if that thing misbehaves the littlest bit it's in the trash...
Thanks for the helpful information.
It was going bad the previous day as my onboard voltage indicator (super bright led that changes colors) was going low rather early. Then the day it failed, I was able to take off fine but then got low voltage pretty quickly to the point I had to make a quick autorotation. I thought the battery was going bad so I pulled it off and checked it's voltage and of course it was fine.
I then put the battery back on but the heli was then acting really weak, barely able to hold a hover to the point it had to sit back on the ground and just got slower and slower to stopping. I gave it throttle again and then heard the sizzle and saw the smoke. The motor was also pretty damn hot too despite not really used much for that flight.

I'm aware that sensitive electronics like ic chips can flake out without any visual indication. I had a lesson on ESD prevention which showed under microscope the effects otherwise unseen of esd in small amounts can make things crap out. Something I truly hate about electronics !! As I've lost a nice PC motherboard to the same sort of deal...
Aug 25, 2019, 08:37 PM
If it flies, I can crash it.
rocketsled666's Avatar
Part of my job includes failure analysis of electronics. I've seen plenty of failures for "Electrical Over Stress". I don't think I'm taking much risk in saying there is absolutely more damage than just the blown FET. The microcontroller is probably damaged. Discrete components like diodes, resistors, and capacitors can be damaged and show no visible evidence. And traces on the PCBA could also be damaged, some of which are on internal layers you cannot see.

The FET isn't a fuse. By the time it finally died, a lot more had to have been killed in the process...
Last edited by rocketsled666; Aug 25, 2019 at 08:46 PM.
Sep 10, 2019, 11:00 PM
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thrashaero's Avatar
Thread OP
Alright well I got the new fets in.
I soldered a new one in the burnt position first of course. But then I connected battery and burnt that one too.
Thinking I maybe used too much heat while soldering, I removed that one and put another in again and burnt that one too right as I connected the battery.
So I figured screw it I need to replace them all anyway, so I did that. Took about 2 hours.

I didn't have much confidence it would work since I thought i probably overheat the chips with an iron I don't know the temp of, but I powered it up and it works fine. Used programming card with no problems. Motor spins fine. Yay so far.

Of course I have yet to actually mount it and have it really put to work and see if it's going to handle it's previous task but so far seems to be promising.


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