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'[EE]: 12V motor and blown FETs'
2001\08\31@201700 by Jinx

face picon face
A friend of mine has asked me, in desperation, if the list
has any comments about his circuit.

http://home.clear.net.nz/pages/joecolquitt/12vmotor.html

The problem he's got is that this circuit tested OK on the bench,
and 2/3rds of the finished units seem to be alright in use, but
the others fry all 3 FETs short circuit, causing the motors to run
full tilt, causing a scramble to disconnect the battery. It even
happened twice this week, on separate occasions, with units
that were seemingly turned off, then spontaneously burst into
life a few minutes after being used. I've suggested a varistor
across the FETs, but that doesn't explain why the units start
on their own. Any clues ? (personally I'd have made the circuit
more fail-safe, but he's very cost-conscious. Perhaps to the
extent of false economy it seems)

TIA

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2001\08\31@212515 by Dave Dilatush

picon face
Jinx,

This is a hip-shot, but it might be worth investigating:

It might be that when the unit is turned off, the 555 keeps running
for a while off the charged stored in its 470uF supply capacitor.  In
principle it "shouldn't" be doing anything during this period, but my
guess is it could be continuing to run as a long-period astable
multivibrator off the leakage current in pins 2 and 6, putting out
brief pulses at a low rate (a couple per second, perhaps).

As this happens, the FETs are still seeing these pulses as drive
(albeit flakey drive); and they are still connected through the motor
to unswitched +12V.  As the 470uF supply capacitor slowly discharges,
the gate drive pulses to the FETs become weaker and weaker, until a
point is reached where the FETs no longer switch fully on, but instead
are only partially conducting.  When this occurs, these FETs are going
to be dissipating a LOT of power (hundreds or perhaps thousands of
times as much as during normal operation) and perhaps this is what's
frying them.  In other words, they might be burning to death instead
of being electrocuted.

It's also possible that during this wind-down period following turn
off, the 555 output at some point in time simply goes HIGH and stays
there; and from that point until the 555 supply voltage drops below
the Vth of the FETs, they will REALLY be dissipating power.

I haven't done any PWM motor control in years, and way back then only
with commutated SCRs, so I'm not enormously confident of the scenario
I've described; but it sounds borderline plausible and might be worth
checking out.

Having had a couple of beers already this evening, I'd probably better
refrain from proposing a fix.

Hope this helps...

Dave

Jinx wrote...

{Quote hidden}

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2001\08\31@213146 by Russell McMahon

picon face
> A friend of mine has asked me, in desperation, if the list
> has any comments about his circuit.
>
> http://home.clear.net.nz/pages/joecolquitt/12vmotor.html
>
> The problem he's got is that this circuit tested OK on the bench,
> and 2/3rds of the finished units seem to be alright in use, but
> the others fry all 3 FETs short circuit, causing the motors to run
> full tilt, causing a scramble to disconnect the battery. It even
> happened twice this week, on separate occasions, with units
> that were seemingly turned off, then spontaneously burst into
> life a few minutes after being used. I've suggested a varistor
> across the FETs, but that doesn't explain why the units start
> on their own. Any clues ? (personally I'd have made the circuit
> more fail-safe, but he's very cost-conscious. Perhaps to the
> extent of false economy it seems)

First - what may be happening.
Second - what to do.

The Taiwanese call this failure 'turbo mode" :-).

The present arrangement appears to be attempting to prevent the FETs being
over voltaged by turning them on if the drain voltage exceeds the 27v zener
ratings.
If this happens the gates can be pulled up hard as high as you want to pull
them until the FET turns on and clamps the spike. Which gets there first
depends on parameters which need to be examined in more detail.

The gates have no direct protection at the gate but only behind the 68 ohm
drive resistors. There is very significant capacitive coupling between gate
and drain during switching and transient oscillatory modes can occur which
generate large short term signals on the gate. Having protection right at
the gate (electrically AND physically) helps.

Zener gate protection is often OK but for negative going oscillatory
transients (which do happen in the real world and which can be quickly
fatal) a reverse biased schottky at the gate is even better. A zener can
still be used to limit the positive excursions. This adds cost and
complexity so if this is to be avoided then starting with zeners alone at
the gate would be worthwhile.
.

SO

I would try zeners slightly more than 12v at each FETS gate OR if he must
save money then tying all 3 FETS gates together and having a common zener to
ground from the gates. (I realise that this removes the spreading affect of
the 3 x 68r base drive resistors but as the FETs are being driven at well
above gate threshold voltage this loss should be more than made up for by
the gain in protection.

RShorting out the 3 x 68r and seeing how it affects operation and
reliability may be instructive.

If the 3 x gate to drain zeners are felt to be providing valuable
protection, consider placing a resistor in  series with each one so that
rapid positive dv/dt transients cannot pull  the gate above the FET's
maximum Vgs rating.

I question why these zeners are needed here.
IF the motor flywheel Schottky is operating correctly then the FET drains
should see minimal effects from the motors inductive reactance when the FETs
are off as all motor current will flywheel through the Schottky.

QUESTION - After blowup - is the motor Schottky still OK?
If this ever fails open the FET is indeed subject to motor current and
inductive kick.
If it fails short the FET is switching a hard DC supply.
Either case is liable to be fatal.




regards


       Russell McMahon

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2001\08\31@213759 by Brent Brown

picon face
Interesting circuit.

Looks like the BC327 could do with a B-E turn off resistor. When
the switch is open the base is floating. This could allow the
transistor to partially turn on or even oscillate, especially if noise is
present. This in turn means the circuit may try to activate the
MOSFETs, and as insufficient gate voltage is available they are
most likely operating in a linear mode outside of their safe operating
area. For example Vds = 6V, Ids = 30A, P=180W,- poof!

Maybe a 10k or 22k resistor? Might need to be higher to stop it
affecting the pot characteristics, but the lower the better. Make the
pot 1k and the capacitor 100nF to compensate.

Does the power switch need to be where it is? (Maybe it is part of
the pot). Better if it was independant of the timing circuit.

Looks like a 12V to 6V converter for a VW wiper motor.

To make it even safer, but add complexity, add a brownout circuit
that disables the MOSFETs when the power supply is below say 8
or 10V.

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/text: 025 334 069
eMail:  spam_OUTbrent.brownTakeThisOuTspamclear.net.nz

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2001\08\31@220410 by David VanHorn

flavicon
face
>Any clues ? (personally I'd have made the circuit
>more fail-safe, but he's very cost-conscious. Perhaps to the
>extent of false economy it seems)

I'd decrease the pulldown on the gates.
I'm also suspicious of tying the gates to the drains through those zeners.
When you turn off the fets, the drain voltage will rise, and likely turn
the diodes on.
At that point, I'm not sure what happens, could be gate breakdown, could be
a nasty period of linear operation.

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I would have a link to http://www.findu.com/cgi-bin/find.cgi?KC6ETE-9 here
in my signature line, but due to the inability of sysadmins at TELOCITY to
differentiate a signature line from the text of an email, I am forbidden to
have it.

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2001\08\31@221314 by Andrew E. Kalman

flavicon
face
Dave Dilatush wrote:

>  In other words, they might be burning to death instead
>of being electrocuted.


Ah, the PICList conjures up such beautiful imagery ... :-)
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2001\08\31@225118 by Jinx

face picon face
>
> Does the power switch need to be where it is? (Maybe it is
> part of the pot). Better if it was independant of the timing circuit.

It's part of the pot - his reasoning is that the unit can't be turned
off unless the speed control is down

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'[EE]: 12V motor and blown FETs'
2001\09\01@052057 by Roman Black
flavicon
face
Jinx wrote:
>
> A friend of mine has asked me, in desperation, if the list
> has any comments about his circuit.
>
> http://home.clear.net.nz/pages/joecolquitt/12vmotor.html
>
> The problem he's got is that this circuit tested OK on the bench,
> and 2/3rds of the finished units seem to be alright in use, but
> the others fry all 3 FETs short circuit,


I think the FET gate drive circuit needs
a lot more safety... The pull down resistor
is 470k (!!) which is way too high, and
the only mechanism to turn the fets off is
when 555 pin 3 goes low, which is a collector
of its output transistor. So unless the 555s
output transistor is reliably turned on, and
its collector pulled low, the fets can't be
reliably turned off.

I also question the 27v zener diodes to the
fets gates??? This allows commutation or
other spikes to turn on the fets!!

My suggestions; drive the fets from the 555
output, with as low ohm pull down resistor
as possible. 555 will output about 11.5v on
high, set it up for about 9v at the fets
gates. Lose the zeners that attach the fets
gates to the noisy motor!! You want to make
sure that unless the 555 output is pulled hard
high, the fets MUST be off. Your circuit is
almost the opposite of that. :o)

Use a small cap across the gate pull down resistor,
check on the CRO you just want to round the
corners of the squarewave going to the gates.

Your snubber is wimpy for a 30A motor, try 0.22uF
and a 10 ohm 5w resistor. Slightly smaller snubber
across the fets, D-S.

It's pretty vital to decouple the motor PWM as
close as possible to the motor and fets. This is
one of the main hassles with electric vehicles,
you need BIG caps to run a 30A motor and with low
impedances, so commonly used lots of smaller caps,
like 10x 1000uF 25v caps in parallel very close
to the motor and fets. And the caps may get hot...

There is a good web site,
http://www.4qd.co.uk/

They make some impressive high current DC motor
drivers, for electric vehilces etc, and they have
some great info on PWM controller design. If your
friend is going to build 30A controllers this is
a real good place to start. :o)
-Roman

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2001\09\01@054424 by Jinx

face picon face
> There is a good web site,
> http://www.4qd.co.uk/
>
> They make some impressive high current DC motor
> drivers, for electric vehilces etc, and they have
> some great info on PWM controller design. If your
> friend is going to build 30A controllers this is
> a real good place to start. :o)
> -Roman

I warned him not be too precious about his circuit when the
list made comments about it. I know little about what he's
doing apart from helping him source a few components. My
only experience with high power motors is a circuit I made
up to PWM a 1/2 HP winch motor. Lost a couple of FETs
to spikes before using a varistor, sure didn't have the flakey
effects he's had

Appreciate the help so far, he's certainly getting a critique !!

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2001\09\01@055051 by Jinx

face picon face
> There is a good web site,
> http://www.4qd.co.uk/
>

Aye Caramba !!!  Think he'd better pay me a vist and
read himself silly

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2001\09\01@062442 by Roman Black

flavicon
face
Jinx wrote:
>
> > There is a good web site,
> > http://www.4qd.co.uk/
> >
>
> Aye Caramba !!!  Think he'd better pay me a vist and
> read himself silly


Yep, you really have to respect a firm that put
excellent info and tutorials on PWM motor drivers
on their web page, explaining all the possible
problems and the bext way to get peak performance,
all for free. Then they present their products, which
gain instant respect based on the experience and
knowlege obviously present within their firm.

A skilled person could go to their site, pick up
all the info, then design their own unit based on
all the expert data. Then again, their prices are
pretty reasonable and with the advantage of being
built by the experts and ready to run... Love it.
Internet at its best. :o)
-Roman

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2001\09\01@150640 by Olin Lathrop

face picon face
> A friend of mine has asked me, in desperation, if the list
> has any comments about his circuit.
>
> http://home.clear.net.nz/pages/joecolquitt/12vmotor.html

Before commenting on the circuit I want to make it clear that I haven't
chased down any of the part numbers so I will have to assume that max
current ratings and the like have been checked.  I will also assume that the
555 produces a PWM signal that is nearly rail to rail at pin 3 and will
otherwise not look into that part of the circuit.  That being said, parts of
this circuit look rather confused to me.

1  -  My gut reaction is that the snubber circuit should be more aggresive,
but I haven't done calculations to verify that.

2  -  I don't like the shottkey diode accross the motor.  You do need a
diode there to short out the flyback current, but unfortunately it needs to
be both beafy (30A) and very fast recovery.  Perhaps your BYV143-40 (-50?)
can take the 30A, but what about its reverse recovery time?  Unless you've
already looked into this carefully, my top suspect is that the flyback diode
is too slow and the FETs are taking a serious beating every time they turn
on.  This will either kill the FETs directly, or fry the diode so that it no
longer clips the flyback pulses, which will kill the FETs by high voltage
spikes.

3  -  The diodes between the FET drains and gates don't belong there.  They
are probably contributing to the problem.  I'm guessing that you are trying
to use the FETs to clamp their own excessive drain voltages by turning them
on.  In my opinion the circuit has already failed if you get more than 28V
on the drains, which I assume is the max voltage rating of the FETs.  Trying
to turn the FETs on partially at this case seems pointless and even harmful.

4  -  I don't know what your PWM frequency is and what the apparent gate
capacitances are, but I bet the FETs are spending way too much time in the
transition region.  The whole gate drive circuit looks pretty flaky.  It can
easily take 1A or more for a short time to properly switch the gate of a
power FET.  The 555 certainly isn't up for that, and the extra resistors
will make the problem even worse.  The slow switching time is my second
suspect for killing the FETs.

So here is my recommendation:  Ditch everything from the 555 pin 3 on,
except the FETs and the motor and start over by doing some real
calculations.

You could either use commercial FET driver chips, or a double emitter
follower to provide the necessary current gain for the pin 3 signal.  The
double emitter follower consists of an NPN and a PNP transistor.  The two
bases are tied together, as are the two emitters.  The collector of the NPN
goes to the + supply, and the collector of the PNP to ground.  Pin 3 drives
the bases, and the emitters drive the FET gates.  You will loose about 700mV
of voltage swing on each end, but that should be fine here.  In return, you
get 100 times or so more gate drive current than the 555 can provide
directly.  This should switch the FETs much faster than they are being
switched now, keeping them out of the deadly linear region as much as
possible.  There should be a pulldown resistor on pin 3, and another one on
the FET gates.

Don't put anything between gate and drain.

Look into the snubber circuit.  It probably needs to be heftier than what
you showed.

Now for the flyback diode.  This part needs to be selected carefully.  You
have to assume that the motor looks like a pure inductor in the worst case.
Actually this is pretty close to reality when the motor is stalled.  If the
max forward current is 30A, then that is also the max flyback current.
Unless you can find (and pay for) a very fast recovery diode that can do
30A, you need to apply a little cleverness.  As I said before, this is my
prime suspect as to why the FETs are frying.  I would put a resistor in
series with the diode.  Since the max flyback current is 30A, a 500mOhm
resistor will develop 15V accross it worst case, which should still keep the
drain voltage below 30V (which I assume is the FET rating).  You still
should get a fast recovery diode, but at least it won't look like a dead
short when the FETs first turn on.  500mOhms is therefore the largest
tolerable diode resistor, but there are good reasons form making this lower
too.  Depending on the PWM frequency, this could dissipate significant
power.  This could not only be cumbersome to cool, but would also negate the
high efficiency of PWM drive in the first place.  Therefore, you could use a
lower resistor or multiple resistor-diode combinations in parallel.  I like
the latter better because it allows the use of lower current and therefore
faster recovery diodes and spreads the heat dissipation.  The series
resistors will guarantee that the diodes will share the current reasonably
evenly.  Maybe four sets, each using 10A diodes.  You'll have to work out
the resistor tradeoffs yourself, but I'm guessing around a few 100 mOhms
each.  These may end up being properly measured lengths of wire of know
thickness.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olinspamKILLspamembedinc.com, http://www.embedinc.com

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2001\09\01@175415 by Jinx

face picon face
> > A friend of mine has asked me, in desperation, if the list
> > has any comments about his circuit.
> >
> > http://home.clear.net.nz/pages/joecolquitt/12vmotor.html
>
> Before commenting on the circuit I want to make it clear that
> I haven't chased down any of the part numbers so I will have
> to assume that max

Thanks for your considered reply Olin, and to all others. I'll
pass all of these suggestions back to John. Should keep his
nose pressed firmly to the drawing board for most of next week

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2001\09\01@211607 by Reginald Neale

flavicon
face
>A friend of mine has asked me, in desperation, if the list
>has any comments about his circuit.
>
>http://home.clear.net.nz/pages/joecolquitt/12vmotor.html
>
>The problem he's got is that this circuit tested OK on the bench,
>and 2/3rds of the finished units seem to be alright in use, but
>the others fry all 3 FETs short circuit, causing the motors to run
>full tilt, causing a scramble to disconnect the battery. It even
>happened twice this week, on separate occasions, with units
>that were seemingly turned off, then spontaneously burst into
>life a few minutes after being used. I've suggested a varistor
>across the FETs, but that doesn't explain why the units start
>on their own. Any clues ? (personally I'd have made the circuit
>more fail-safe, but he's very cost-conscious. Perhaps to the
>extent of false economy it seems)


  Make the ON/OFF switch double pole. Use the other pole to
  directly ground the FET gate node. Also the pulldown on
  that point could stand to be lot lower in value.

  Reg Neale

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