Wet brushless ESC goes up in smoke

[Stretto]

"Robert Roland" wrote in message
news:ei9qt6p235hr6orqnrkt3j2mp37d83u6uu@4ax.com...

On Wed, 25 May 2011 05:58:30 -0600, "Stretto" <Stretto@nowhere.com>
wrote:

If the mosfets themselves burned up then it was most likely due to cross
conduction where the high and low side mosfets were both turned on at the
same time and no safety protection prevented it(you can current sense on
the
mosfets to make sure no cross conduction occurs).

Since the motor got very hot, it is more likely the problem was caused
by the commutation somehow getting stuck. The windings in the motor
are very low resistance, so it would have a very similar effect.

If the mosfets are hot and you throw some water on them they probably won't
like it.

Certainly. But I do not think that is what happened here. The MOSFETs
normally are just barely warm to the touch after a flight, and they
were not submerged in water. The water must have seeped slowly in
under the heatshrink.

It's difficult to say what the problem is when there is no real information
given.

I understand that, and I was not really seeking help with the specific
troubleshooting. I was curious to see if anyone could explain why
these controllers seem to fail so reliably in contact with water. This
is the third one I have seen in a short time.

DC motor controllers can come in a wide variety of designs with some
more prone to catastrophes than others.

Indeed. But for R/C hobbyists, cost is important, so the simplest
designs seem to be the most popular. In general, they all employ a
microcontroller (almost exclusively an Atmel) and a bank of MOSFETs
and a few passives to glue it together.

Can you determine if it is the mosfets themselves or the controller that is
the problem? Is it obvious what is broke or does it just not work?

I have now removed all MOSFETs that are bad, and shuffled the rest
around so that I have at least one on each rail on each phase. The
controller now runs properly on a low-power motor (hard disk spindle
motor), so only MOSFETs were broken. There is also a bipolar
transistor (gate drive for the high-side P-channels) that is seriously
scorched, but, amazingly, it seems to work.

You can also check the mosfets to see if they are working or not by
applying
a gate voltage(possibly not a good idea in circuit unless you are careful)
and checking their conductivity.

The voltage from the multimeter is actually enough to turn them on,
but since there are many of them in parallel, it is tricky to find the
bad ones by measuring. I first removed all that had a short from gate
to either D or S. After that, there were only a few left, so it got
much easier.

For "water tight" boards you can apply an epoxy. If the components are
getting very hot and not designed for rapid cooling there is not much you
can do as it is a mechanical problem with the stresses of rapid cooling on
the cases. If it is just the mosfets then possibly a heat sink will help if
one doesn't exist.

There is a small heat sink, but I think it's mostly cosmetic. It is
placed under the shrink hose, it does not contact all the MOSFETs, and
it does not get particularly hot. If overloaded (usually due to a too
large propeller), though, they get really hot.

Take a pic of the board and post it?

If you're interested, no problem. I have left the pictures at their
original resolution, so they're about 2MB each.

Here's the power side, with the seven surviving MOSFETs:

http://home.c2i.net/w-479147/temp/top.jpg

Here's the logic side:

http://home.c2i.net/w-479147/temp/bottom.jpg

Notice the 6-pin dual transistor in the upper right corner. It looks
completely mangled, but it still seems to work.

The two 7806s are not really part of the controller. They are used to
supply the radio gear from the motor battery, so that no dedicated
receiver battery is needed. It is called a "BEC", a Battery Eliminator
Circuit.

The SO-8 at the bottom, covered in glue, is the 5V regulator for the
CPU.

-------------------------------------------

What maybe happening is there is not enough gate drive to drive all the
mosfets. This can cause cross conduction due to the mosfets not turning off
and on fast enough(you get times when they are both on on the same side).

This would be a controller issue and not the problem with the mosfets. Since
you say it has happened several times that is most likely the problem if the
drive circuitry is sub-par.

I can't tell what exactly the circuit is doing but it looks like the uC is
directly driving the mosfets(considering all those resistors). Mosfets are
not as ideal as one wishes and the biggest problem with paralleling them is
the increased gate capacitance. Basically you can think of a gate of a
mosfet as having a capacitor across it and it takes time to charge the
capacitor. As it charges the mosfet transitions from open to closed or vice
versa. The transition acts a resistor varying from R_ds(on)(for your mosfets
it is 7.5mOhms) to some very large value. As current is flowing through the
mosfet the resistance causes significant thermal dissipation. The goal is to
have very quick transition so there is less time for the mosfet to heat up.
Fast transitions require low gate capacitance. When you parallel mosfets you
increase the effective gate capacitance(as seen by the driver) which slows
all the transitions down for each mosfet.

The design parallels 5 mosfets per leg. Each mosfet has 7.5mOhms so the
total resistance is 1.5mOhms but the gate charge is 105*5(*2) = 525nC. These
are best case. If the drive voltage is lower than 10V that 7.5mOhms will
increase which increases heat dissipation(but you said it wasn't getting too
hot to the touch so the drive voltage maybe ok all the incidences happen
when you are losing power).

You could possibly use 1 mosfet per leg instead of 5 by finding a better
mofset.

http://www.fairchildsemi.com/ds/FD%2FFDMS7650.pdf

While the wrong package it has about the same ratings or better(from what
I've checked).

http://www.vishay.com/docs/69063/si7137dp.pdf

For pch it's not as good but may work(if you use better nch's(lower R_ds(on)
then you can use worse pch if necessary).

The main thing you need to work out is if the controller really is failing
because it gets wet or if it's coincidence. There is only 1 way a mosfet is
going to fail in the way you are using it and it's overheating(I'm sure you
know there are other ways to ruin them such as ESD but these effects
shouldn't occur in SOC). For an h-bridge this will happen only due to cross
conduction issues(assuming it was properly designed for the rated load and
for a motor it generally is not meant run stalled).

Cross-conduction either occurs because the controller itself is
malfunctioning and not synchronizing the switching properly or because it is
not properly able to drive the mosfets.

Possible reasons: Stalled motor(draws more current), low power(increases
cross conduction and R_ds(on)), malfunctioning controller(sync issues =>
more cross conduction). Water(not sure but this is not part of the SOC).

Note that significant cross conduction should only occur when changing
polarity/reversing the motor. But if the controller or power is failing that
it will more likely occur. If you are drastically changing the polarity of
the motor(or the controller thinks you are) then it may be trying to reverse
the polarity very often which will increase power. So in that case it could
be an issue with the input to the controller(the controller should be able
to prevent this quite easily).

Hopefully I've given enough information that you might be able to figure out
what happened.

PS. That dual transistor you mentioned may be a clue to the cause. I see 2
others that look identical(I guess that’s a diode above each one). I don't
know what they are doing but possibly the transistor is going back causing
the mosfets to subsequently fail. You did say it's working but maybe not as
well as it should be.

BTW, normally these motor controllers are designed by using a driver with
high-side capabilities. p-ch mosfets generally have a much larger R_ds(on)
than similar n-ch and so it is better to use n-ch's instead of p-ch's for
the high side. This requires special driving circuitry though but generally
is better. (basically you have to generate a voltage larger than your supply
voltage to drive the high-side n-ch mosfets. There are dedicated IC's to do
this that cost a few dollars and some have significant driving capabilities
for paralleling(much more so than that uC).

 

[Winston]

Robert Roland wrote:

On Wed, 25 May 2011 07:03:23 -0700, Winston<Winston@BigBrother.net
wrote:

It might not be primarily a moisture issue.

It is possible, but the statistics suggest very strongly it is a water
issue. I and my buddies have been flying electrics off land for years,
and have never seen a controller fail in such a spectacular fashion.
In fact, they are remarkably reliable. Now, after less than ten flying
sessions off water, we have seen three controllers let the smoke out.
All of them, however, have been quite recently, so it looks like
repeated exposure might be a clue.

A bench test is in order then.

The following test will help you separate a 'water' caused fault
from a MOSFET 'circuit design fault'.

Can you set up your scope to read the gate voltage into each bank of
P-channel MOSFETs and bag up the controller as per normal?

RoRo: For flying models, however, weight is critical, so a lighter RoRo:
solution must be found. I have simply used a plastic bag with
RoRo: a zip tie around the wires.

Run your seaplane model until the battery is quite low and the
P-Channel MOSFETS are quite warm. Don't measure the heatsink
because as you say, it doesn't cool the high side pass elements.
Suggest tacking thermocouples to the Vcc leg of each P-MOSFET.

RoRo: There is a small heat sink, but I think it's mostly cosmetic.
RoRo: It is placed under the shrink hose, it does not contact all
RoRo: the MOSFETs, and it does not get particularly hot.

Keep an eye on the gate voltage into the paralleled P-MOSFETs
and the battery voltage.
The gate voltage should snap quickly almost to ground and
stay there during the duration of phase commutation. Then it
should snap back up very quickly and stay nearly at battery
voltage while that phase is turned off. Subtract gate
voltage from battery voltage to find out if your P-MOSFETSs
are likely saturated when they need to be and likely cut-
off when they need to be.

I think you will see that, at low battery voltage, your driver
chip stops saturating your hot P-MOSFETS and they 'go linear'.

While the P-MOSFETs roast in their cooking bag, you will probably
reach a point where one or two of them will shoot through into
their associated N-MOSFET partner, bringing the test to a
'smoking close'.

No water needed.

--Winston <-- Who *never* puts a bag around power electronics.

 

[Jamie]

Winston wrote:

Robert Roland wrote:

I am flying a R/C seaplane powered by two brushless DC motors. One of
the motor controllers went up in smoke, apparently from getting wet.

I have done some reading on how these controllers work, and as far as
I can tell, everything is really low-impedance stuff, so some water
should not be able to cause a catastrophic failure like this.

I had been flying the plane several time before, so it is entirely
possible the controller got wet on one or more of those occasions too.
Could it be that the first dip caused some corrosion that later caused
a low-impedance short?

I fly only off fresh water. There was some white powdery oxides on the
solder joints, so some amount of corrosion is definitely going on.

Can anyone explain to me the specific process how some wetness can
cause a full meltdown? Are the MOSFETs not water tight?


It might not be primarily a moisture issue.

http://www.onsemi.com/pub/Collateral/AND8199-D.PDF

Near the upper limits of a MOSFET's operating temperature,
insufficient gate drive can cause the device to fall out
of saturation and begin dissipating too much power, causing
it to fail.

Moisture *can* be a real enemy to electronics and is to
be avoided for sure. In this case however, it appears that
there is a design issue in the Electronic Speed Controller
which can kill it's pass element when operating under heavy
load, particularly at high device temperatures as the
batteries become depleted.

Advise you replace your MOSFET driver chip with one that
is capable of higher gate drive at lower battery 'state-
of-charge' to assure the pass element does not 'go linear'
on you again.

--Winston

Or, he's using a high side driver type and the pump cap is bad! Which

will do just about the same thing in the end.

Jamie

 

[Robert Roland]

On Wed, 25 May 2011 11:42:38 -0700, Tim Wescott <tim@seemywebsite.com>
wrote:

It may still be doing some good, by spreading the heat.

Sure, but it would definitely be an advantage if it was in actual
contact with all, or at least most, of the MOSFETs.

Normally modern
surface mount parts dump heat into the board copper -- are you sure the
heat sink wasn't contacting that?

Yes, the heat sink was just stuck on top of the MOSFETs with some sort
of sticky tape. After removal, I could see the impression of the
MOSFETs in the tape, but only a few of them had left an impression.

Here's a picture of the underside of the heat sink:

http://home.c2i.net/w-479147/temp/sink.jpg

As you can see, only 7 or 8 impressions are visible in the tape. In
fact, I think I can see a similar pattern when looking at the
surviving MOSFET placements (I have moved one or two of the remaining
MOSFETs to get all phases and both sides covered).

The board copper is definitely contributing to the cooling. I had to
switch to a thicker soldering tip, because the thin one kept getting
stuck as the copper sucked the heat right out of it. I just checked
the data sheet: The MOSFETs have *much* lower thermal resistance
between junction and foot than they have between junction and ambient.

Or it could just be cheap.

I specifically asked for decent quality when I bought them, and I paid
quite a bit more than I would have paid for the cheapest ones. Now
that I have taken them apart, I suspect it might be a ripoff. Just for
fun, I am going to order the cheapest ones I can find from China and
see how they compare.

Brand & model?

WayPoint 25A: http://www.elefun.no/p/prod.aspx?v=5787

The site is Norwegian, but the product description is in English.

If you can figure out what it was you should consider replacing it.

Parts are already ordered. Once repaired, this controller is going to
see some harsh water tests
--
RoRo

 

[Winston]

Jamie wrote:

(...)

Or, he's using a high side driver type and the pump cap is bad! Which
will do just about the same thing in the end.

RoRo stated that each phase has 3 paralleled P-MOSFETs on
each high side, so I don't think a pump cap is involved.

As battery voltage falls, Vgs falls with it, pushing the
high side switches toward linearity.

They can then become 'magic smoke emitters'.

--Winston

 

[Phil Allison]

"Robert Roland"

Although there is visible corrosion, it is not severe. After washing
off the oxides, there is no visible corrosion damage. It seems the
solder was the part that corroded the most.

** Solder corrodes very slowly in presence of water - until there is a DC
voltage available to cause electrolysis.

That's what I am thinking. A couple of layers of plastic varnish,

** Make sure the PCB is very clean a fee of all solder flux first.

Use a flux cleaning spray or denatured alcohol and warm air to dry.

and

then stick the controllers in a plastic bag while flying, and then
remove the plastic bag afterwards, so that condensation and minor
leaks can dry out.

** Either of those last two can result in a repeat event.



...... Phil

 

[Robert Roland]

On Wed, 25 May 2011 14:11:44 -0700, Winston <Winston@BigBrother.net>
wrote:

A bench test is in order then.

Yes, various bench tests will be performed once the controller has
been repaired. I am currently ordering parts.

Run your seaplane model until the battery is quite low and the

LiPo batteries are not run until "quite low". If their voltage gets
too low (about 3V per cell), they could die instantly. As a general
rule, I use only 80% of the battery's rated capacity. At this level,
each cell has well over 3.5V. The controller is rated for 3 or 4
cells, which essentially means a minimum of 9V. With an adjustable
bench power supply, I can push the limits and see what happens.

I think you will see that, at low battery voltage, your driver
chip stops saturating your hot P-MOSFETS and they 'go linear'.

I will test.
--
RoRo

 

On Wed, 25 May 2011 17:53:50 +0200, Robert Roland <fake@ddress.no>
wrote:

That's what I am thinking. A couple of layers of plastic varnish, and
then stick the controllers in a plastic bag while flying, and then
remove the plastic bag afterwards, so that condensation and minor
leaks can dry out. I'm planning on leaving the heatshrink off, so that
water does not get trapped under it.

Sounds like a plan. I recommend slowly heating the board to ~150 F or
so, soaking at temperature for 1/2 hour, then dipping it in varnish
until it cools (in the varnish) then letting it drip and dry.

Oil based varnish.

Heating drives off any moisture and cooling in varnish will allow any
contracting air pockets to suck in varnish, and the coating will be
thicker after it cools.

Switches, sockets etc.. will not work with this method.

For your idea of a bag, (opening it to allow water vapor to escape
when you're not using it). I recommend adding a bit of dry cloth,
sponge, or desiccant bag. In the event that some water does get in,
you don't want it rolling around in there, but give it someplace to
go.

I have a waterproofed electronic camera that I mount to the bow of my
kayak. SOP is to keep a #10 metal can with about an inch of super
dry desiccant in it, and pack the whole camera and case in desiccant
over night. Then I assemble it, when I'm ready to use it. The water
proofing is good (will stay submerged for hours without any ingress -
gasketed case), but just a slight amount of water vapor will condense
on the glass port, so pictures look out of focus.

One of these days I hope to find a camera light enough to waterproof
and attach to the kite that pulls the kayak. The "chewing gum - spy"
cameras work, but the photo quality is abysmal.

 

[Tim Wescott]

On Wed, 25 May 2011 23:57:12 +0200, Robert Roland wrote:

On Wed, 25 May 2011 11:42:38 -0700, Tim Wescott <tim@seemywebsite.com
wrote:

It may still be doing some good, by spreading the heat.

Sure, but it would definitely be an advantage if it was in actual
contact with all, or at least most, of the MOSFETs.

Normally modern
surface mount parts dump heat into the board copper -- are you sure the
heat sink wasn't contacting that?

Yes, the heat sink was just stuck on top of the MOSFETs with some sort
of sticky tape. After removal, I could see the impression of the MOSFETs
in the tape, but only a few of them had left an impression.

Here's a picture of the underside of the heat sink:

http://home.c2i.net/w-479147/temp/sink.jpg

As you can see, only 7 or 8 impressions are visible in the tape. In
fact, I think I can see a similar pattern when looking at the surviving
MOSFET placements (I have moved one or two of the remaining MOSFETs to
get all phases and both sides covered).

The board copper is definitely contributing to the cooling. I had to
switch to a thicker soldering tip, because the thin one kept getting
stuck as the copper sucked the heat right out of it. I just checked the
data sheet: The MOSFETs have *much* lower thermal resistance between
junction and foot than they have between junction and ambient.

Or it could just be cheap.

I specifically asked for decent quality when I bought them, and I paid
quite a bit more than I would have paid for the cheapest ones. Now that
I have taken them apart, I suspect it might be a ripoff. Just for fun, I
am going to order the cheapest ones I can find from China and see how
they compare.

Brand & model?

WayPoint 25A: http://www.elefun.no/p/prod.aspx?v=5787

The site is Norwegian, but the product description is in English.

If you can figure out what it was you should consider replacing it.

Parts are already ordered. Once repaired, this controller is going to
see some harsh water tests

Can't go wrong with Castle Creations.

--
http://www.wescottdesign.com

 

[Tim Wescott]

On Wed, 25 May 2011 23:57:12 +0200, Robert Roland wrote:

On Wed, 25 May 2011 11:42:38 -0700, Tim Wescott <tim@seemywebsite.com
wrote:

It may still be doing some good, by spreading the heat.

Sure, but it would definitely be an advantage if it was in actual
contact with all, or at least most, of the MOSFETs.

Normally modern
surface mount parts dump heat into the board copper -- are you sure the
heat sink wasn't contacting that?

Yes, the heat sink was just stuck on top of the MOSFETs with some sort
of sticky tape. After removal, I could see the impression of the MOSFETs
in the tape, but only a few of them had left an impression.

Here's a picture of the underside of the heat sink:

http://home.c2i.net/w-479147/temp/sink.jpg

As you can see, only 7 or 8 impressions are visible in the tape. In
fact, I think I can see a similar pattern when looking at the surviving
MOSFET placements (I have moved one or two of the remaining MOSFETs to
get all phases and both sides covered).

The board copper is definitely contributing to the cooling. I had to
switch to a thicker soldering tip, because the thin one kept getting
stuck as the copper sucked the heat right out of it. I just checked the
data sheet: The MOSFETs have *much* lower thermal resistance between
junction and foot than they have between junction and ambient.

Or it could just be cheap.

I specifically asked for decent quality when I bought them, and I paid
quite a bit more than I would have paid for the cheapest ones. Now that
I have taken them apart, I suspect it might be a ripoff. Just for fun, I
am going to order the cheapest ones I can find from China and see how
they compare.

Brand & model?

WayPoint 25A: http://www.elefun.no/p/prod.aspx?v=5787

The site is Norwegian, but the product description is in English.

If you can figure out what it was you should consider replacing it.

Parts are already ordered. Once repaired, this controller is going to
see some harsh water tests

I think the way to go would be to bring the heat sink down to the copper,
rather than the FET case. I've seen quite a few designs where the board
is thermally connected to a rather substantial case for thermal transfer,
but I'm not sure what modern practice is when the board is little and the
'case' is heat-shrink tubing.

Go to the local hobby shop and peer at a name-brand ESC and see what they
do, perhaps.

--
http://www.wescottdesign.com

 

[Robert Roland]

On Thu, 26 May 2011 09:48:47 -0500, Tim Wescott <tim@seemywebsite.com>
wrote:

I think the way to go would be to bring the heat sink down to the copper,
rather than the FET case.

If overheating was the actual problem, I'd agree. In normal operation,
however, the controller does not get much more than hand warm, so
cooling is probably sufficient. And once the commutation jams or the
MOSFETs shoot through, no amount of cooling is going to save the day.

I've seen quite a few designs where the board
is thermally connected to a rather substantial case for thermal transfer,
but I'm not sure what modern practice is when the board is little and the
'case' is heat-shrink tubing.

On designs that produce considerable amounts of heat, I'd expect
they'd use transistors that have a proper metal tang to bolt securely
to a heatsink?
--
RoRo

 

[Tim Wescott]

On 05/26/2011 11:18 AM, Robert Roland wrote:

On Thu, 26 May 2011 09:48:47 -0500, Tim Wescott<tim@seemywebsite.com
wrote:

I think the way to go would be to bring the heat sink down to the copper,
rather than the FET case.

If overheating was the actual problem, I'd agree. In normal operation,
however, the controller does not get much more than hand warm, so
cooling is probably sufficient. And once the commutation jams or the
MOSFETs shoot through, no amount of cooling is going to save the day.

Yes and yes. First, the ESC can't count on good airflow, and second you
don't want to burn up power in the ESC -- you want it going to the motor.

And if you have the Gabor sisters advising you on your circuit design,
you know that nothing's going to help you if you have serious
"shoosht-through".

I've seen quite a few designs where the board
is thermally connected to a rather substantial case for thermal transfer,
but I'm not sure what modern practice is when the board is little and the
'case' is heat-shrink tubing.

On designs that produce considerable amounts of heat, I'd expect
they'd use transistors that have a proper metal tang to bolt securely
to a heatsink?

The dividing line just keeps moving out, though -- more and more,
thermal management is being done by dumping heat into the PC board, then
(if necessary) removing heat from same.

If you waltz through the available selection of power parts, you'll find
_lots_ of surface mount chips that feature large tabs that are designed
to solder onto the board for heat dissipation. Some (I can never
remember the package name) are essentially TO-220 parts with the tab
trimmed and tinned, and the leads formed to contact the board.

TO-220 (D-PAK^2??) seems to be the current dividing line between
"reasonable to move to through-hole" and "let's keep this all surface
mount".

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[Winston]

Robert Roland wrote:

On Wed, 25 May 2011 14:11:44 -0700, Winston<Winston@BigBrother.net
wrote:

(...)

I think you will see that, at low battery voltage, your driver
chip stops saturating your hot P-MOSFETS and they 'go linear'.

I will test.

Excellent!
Please keep us posted.

--Winston

 

[Robert Roland]

On Wed, 25 May 2011 19:07:28 +0200, I wrote:

There is also a bipolar
transistor (gate drive for the high-side P-channels) that is seriously
scorched, but, amazingly, it seems to work.

I just poked around with the scope a bit. It does definitely NOT work.

So, one high-side MOSFET never turns on, which means that two of the
six phase combinations are lost. The remaining four are apparently
enough to get the hard disk motor going.

Have to wait for parts now, before I can do any more testing.
--
RoRo

 

[Winston]

Robert Roland wrote:

(...)

Have to wait for parts now, before I can do any more testing.

Can you 'reverse engineer' the connections from the Atmel chip
to your three bipolar transistors? It'd be reassuring to see
that all three Atmel 'high side' outputs are operating properly.

--Winston

 

[Robert Roland]

On Fri, 27 May 2011 13:25:30 -0700, Winston <Winston@BigBrother.net>
wrote:

Can you 'reverse engineer' the connections from the Atmel chip
to your three bipolar transistors?

I have had one half-hearted go at it, but got nothing sensible. I will
take another look.
--
RoRo

 

[Robert Roland]

On Fri, 27 May 2011 13:25:30 -0700, Winston <Winston@BigBrother.net>
wrote:

Can you 'reverse engineer' the connections from the Atmel chip
to your three bipolar transistors? It'd be reassuring to see
that all three Atmel 'high side' outputs are operating properly.

OK, I figured out the gate drive. The signal looks normal.
--
RoRo

 

[Winston]

Robert Roland wrote:

On Fri, 27 May 2011 13:25:30 -0700, Winston<Winston@BigBrother.net
wrote:

Can you 'reverse engineer' the connections from the Atmel chip
to your three bipolar transistors?

I have had one half-hearted go at it, but got nothing sensible. I will
take another look.

Here's the microcontroller spec. That should be helpful.
http://www.atmel.com/atmel/acrobat/doc2486.pdf
http://www.atmel.com/avr.

Looks like the PORT B pins are a popular source of
your PWM signals, particularly 13, 14 and 15:
http://www.atmel.com/dyn/resources/prod_documents/doc2542.pdf


--Winston

 

[Winston]

Robert Roland wrote:

On Fri, 27 May 2011 13:25:30 -0700, Winston<Winston@BigBrother.net
wrote:

Can you 'reverse engineer' the connections from the Atmel chip
to your three bipolar transistors? It'd be reassuring to see
that all three Atmel 'high side' outputs are operating properly.

OK, I figured out the gate drive. The signal looks normal.

Excellent news!

--Winston

 

[Jamie]

Robert Roland wrote:

On Thu, 26 May 2011 09:48:47 -0500, Tim Wescott <tim@seemywebsite.com
wrote:


I think the way to go would be to bring the heat sink down to the copper,
rather than the FET case.


If overheating was the actual problem, I'd agree. In normal operation,
however, the controller does not get much more than hand warm, so
cooling is probably sufficient. And once the commutation jams or the
MOSFETs shoot through, no amount of cooling is going to save the day.


I've seen quite a few designs where the board
is thermally connected to a rather substantial case for thermal transfer,
but I'm not sure what modern practice is when the board is little and the
'case' is heat-shrink tubing.


On designs that produce considerable amounts of heat, I'd expect
they'd use transistors that have a proper metal tang to bolt securely
to a heatsink?
Is it possible that your processor is acting strange when insufficient

voltage is present at intermitting moments? Maybe the drive circuit from
the uC does not have proper protection in case a long STh/Olap takes
place during a uC reset or reboot caused by unexpected dips in power?
I can only assume you're using a 3 pole full bridge with a 3 wire
motor? I kind of suspect some problems in phase firing if so. Maybe
you should be using fuse Links.

Jamie