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Using Arduino => General Electronics => Topic started by: nosredna000 on Jul 20, 2016, 07:29 pm

Title: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: nosredna000 on Jul 20, 2016, 07:29 pm
Hi,

I'm trying to PWM control a monster DC motor.

Motor (fan) is 30amp continous 12vdc and I'm seeing 100amp on start up.

I was attempting to use a http://www.nxp.com/documents/data_sheet/BUK953R5-60E.pdf (http://www.nxp.com/documents/data_sheet/BUK953R5-60E.pdf) mosfet

Vds 60V
Id 120A
Ptot 293W
Rds  .003723Ω


Diode was a 1N4004

Schematic less 10k resistor was   Didn't seem to want to work with the 10k.


http://bildr.org/blog/wp-content/uploads/2012/03/rfp30n06le-arduino-solenoid-400x218.png (http://bildr.org/blog/wp-content/uploads/2012/03/rfp30n06le-arduino-solenoid-400x218.png)

Code

Code: [Select]


int fan_power = 50;
int fan_commanded;
long time_at_this_power, last_changed_time;
long timer = 60000;

void setup() {
   pinMode(3, OUTPUT);
}

void loop() {
 
 
  fan_commanded = map(fan_power, 0, 100, 0, 255);
  analogWrite(3, fan_commanded);
 
  time_at_this_power = millis()- last_changed_time;
 
  if (time_at_this_power > timer){
   fan_power = fan_power + 25;
   last_changed_time = millis();
   }
   
   if (fan_power > 100){
    fan_power = 50;}
 
}






I'm a novice electrical guy.  I'm sure I'm missing something simple.  My guess is either PWM freq too high and diode not fast enough or Diode not up to snuff current or voltage wise?  This did work A-OK with a 10 amp motor.   Would someone please point me in the right direction?   Thanks :)


Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: CrossRoads on Jul 20, 2016, 07:32 pm
When the motor current is turned off, it will generate current of its own of the same amount.
1N4004 is only rated for 1A. Need a bigger diode to shunt that current back into the motor to let it dissipate.
http://www.digikey.com/product-search/en?keywords=1n4004 (http://www.digikey.com/product-search/en?keywords=1n4004)
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: MarkT on Jul 20, 2016, 10:12 pm
And 100A is too much for a single TO220 package, you'll fuse the bondwires, whatever the
datasheet says.

Think about something like this: http://uk.farnell.com/ixys-semiconductor/ixfn200n10p/mosfet-n-sot-227b/dp/1427322 (http://uk.farnell.com/ixys-semiconductor/ixfn200n10p/mosfet-n-sot-227b/dp/1427322)

Is in a rugged enough package.  Somewhat more on-resistance alas, but 700W rating and lots of
thermal mass make up for this.  Alternatively a pair of TO247 packaged MOSFETs in parallel is
reasonable - soldered onto thick bus-bars preferably.

And at these sorts of power levels a MOSFET driver is a definite requirement - don't imagine a logic level device
driven direct from an Arduino is adequate, get proper 12V powered driver like a MIC4422 to drive the MOSFET
gate.

Add a 15V zener to the gate-source directly on the terminals too to protect the gate from over and reverse
voltage.  Layout between MOSFET driver and MOSFET has to be low inductance and short - note this is a
4-terminal MOSFET, one source terminal is used for the load current, one for the gate driver.

Power the driver from a clean filtered 12V, not the main battery, as spikes on the driver supply that will push
right through to the MOSFET gate and pop it, zener or not.

And find at least a 100A peak-rated diode. That will definitely be mounted on the heatsink too, and
be nice and chunky.

Finally remember that difficulty of handling high currents goes up as the square of the current - 100A
is 100 times more tricky than 10A.  100A is well beyond any PCB trace, thick copper wiring
throughout, and ventilate everything, even the wiring can get hot quickly at 30 or 100A.
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: nosredna000 on Jul 21, 2016, 04:34 pm
Thank you guys so much for the help. 

last question  Would this be handled better with 2-3 of these same mosfets with a good driver in parallel?


I honestly didn't expect this fan to pull so much.   We where running another fan before this one but it only pulled 10-12 amps running and would spike maybe 20-30 at start up.   The fet was warm but well within specs with just a alligator clipped to the sink (for testing).



Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: MarkT on Jul 21, 2016, 09:10 pm
You shouldn't be expecting, you should be consulting datasheets and measuring!

If you drive multiple MOSFETs from the same driver chip, add a 10 ohm or so gate resistor
to each MOSFET to damp out any differential oscillation modes.

One thought: are you being careful to ramp up drive PWM on this motor - large motors
can have stall currents that are too large for the motor to handle, even briefly, so normally
have current-sensing driver units to protect against this.
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: raschemmel on Jul 22, 2016, 04:19 pm
The OP never actually mentioned whether he used a heatsink or not. There are some very substantial TO220 heatsinks available. At 30A, with an RDSon of 3.7 mohms, it should only have dissipated 3.3W (P= I2*R  = 302 * 0.0037 = 3.3W). Based on that, I don't think it was the power dissipation that blew the mosfet. I think it was the back EMF . Here's a (http://www.mouser.com/ds/2/427/sbl3030p-104148.pdf) 30A diode.
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: allanhurst on Jul 22, 2016, 05:54 pm
Hi..

on start up  a dc motor can easily take at least 10x it's running current - and it only takes a millisecond to blow the drive transistor!

get some (much) bigger devices - and follow the rest of the advice above.

regards.
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: raschemmel on Jul 22, 2016, 06:07 pm
Quote
I'm seeing 100amp on start up. 
It looks like that's your problem. If you has a "slow start" circuit (inrush current limiting inductor) , you probably would not have blown the mosfet.
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: allanhurst on Jul 22, 2016, 06:16 pm
Hi..

or a big ntc resistor - can you get them that big?

Don't fancy switching big inductors at that current - the spikes will be energetic and hard to control.

regards  Allan
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: raschemmel on Jul 23, 2016, 01:16 am
What is the response time of the motor ? (How long does it need to get up to speed)

50 mS ?

Do you know the winding resistance of the motor ?

If the inrush current is 100A with a voltage of 12V, the "shorted" resistance of the motor = 12/100A
(V = I*R =>< R = V/IINRUSH

12V/100A = 0.12 ohms

Assuming a current inrush inductor rated for 30A has a coil resistance of 0.1 ohms, then the inductor and the motor in series would have a resistance of 0.220 ohms.

If the motor startup time (response time) is 50 mS (0.050 S),

the slope of the current on startup with inrush inductor is:

t = L/R => L = t*R = 0.050 S * 0.22 ohms = 0.011 H (11 mH/50A)

I think such an inductor might be expensive.



Have you considered the possibility tgst your code is to blame?

Have you tried just ramping up the duty cycle one count at a time ?

I tested your code and the motor did a sort of slow start but I honestly don't know how gradually you need to ramp up such a large motor.

What is the motor RPM and how long does it take to ramp up to that rpm when given 100% duty cycle command ? ( I suppose that's academic since your mosfet is blown)
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: allanhurst on Jul 23, 2016, 08:09 am
And the energy stored in the inductor at switch-off is 1/2 x L x I^2  ->  13.75 joules!

That's big spike. Where will it go?

for comparison they reckon it takes 4mJ at a spark plug in an internal combustion engine to light the mixture.

regards

Allan.
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: raschemmel on Jul 23, 2016, 08:37 am
A dc motor turning off is hardly the same as a relay or other device because as you know, when you turn off the motor power the motor does NOT stop instantaneously. The decelerating motor would disipate the energy stored in the inductor because it has such a low resistance. I have not personally tried the proposed circuit but I don't see a problem with such a large rotating mass on turn off.
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: allanhurst on Jul 23, 2016, 12:30 pm
Hi..

  ... and a big flywheel shottky diode does the job of dealing with the off spike..... well OK. But I think you'd need some serious snubber components as well.

The OP wants to pwm control the speed... If directly from the arduino with analog.Write this is about a 1kHz
pwm driven by 0..255  giving 0..100% duty cycle - OK?

L di/dt = V.

so if I apply 12v across your 11mH inductor ( taking the motor as a short for now)
current will rise at 1091/amps second . In 1mS ( the pwm period) it will only get to 1.09 amps.

Hence the inductor is far too large by a factor of at least 30 if we want to deal with 30 amp max.
with 1kHz pwm.

Suggest 220uH/ >50 amp rating  - a bit more achievable. And ramp up/down the speed gently

regards

Allan.

still think big ntc thermistors might be easier - I know you can get 10 amp ones - siemens, phillips etc  make them.  Use several in parallel. I've used these to reduce inrush on big toroidal mains transformers.
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: raschemmel on Jul 23, 2016, 03:28 pm
It's 480 Hz or 960 Hz , depending on which pins you use but  this brings us to the question of what iis proper way to ramp up the pwm ? If you send analogWrite(pin, 1);  , what is the duty cycle ? (1/255)*100  or 0.392 %.
But why use analogWrite when you can use the PWM Library ? (@16 kHz)

I = V/R
  = 12V/0.22 ohms (0.12  + 0.1)

How do you get 1091A/ S ?
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: allanhurst on Jul 23, 2016, 03:49 pm
Hi Raschemmel...


    well ok - use 16kHz if you want, ( this reduces the inductor size by 16X ), but at these frequencies you're going to have worry about full switchmode power techniques - switching losses, even more fussy snubber efficiency/effectivity tradeoffs.. plus a VERY fussy  layout which you're unlikely to get right first time. And you can't breadboard crudely - a real layout would be required to have any chance.
And you'd probably be better off with a specialised swmode psu chip from LT, Texas, Maxim etc and programme that from the arduino.

It's not a lashup in an evening job. This is a proper product!

I'd go for my ntc thermistor idea -crude and cheap.

Allan

ps.. just a thought - don't some cars have variable speed big fans for engne cooling ? Could you get a second hand controller cheap from a scrapyard, and drive that somehow?



 
   
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: raschemmel on Jul 23, 2016, 04:17 pm
The motor the OP has is more like a starter motor so  a scrap surplus starter would be a good UUT Test Load for a prototype driver.
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: allanhurst on Jul 23, 2016, 05:17 pm
Sorry R.. missed your question.....



since L di/dt = V,   ( standard equation for an inductor.  For a capacitor  C dv/dt = I   ....  textbook stuff)

so di/dt = V/L =  12v/0.011H  = 1091 A/s  .

If you add resistance the current will approach the final dc current such that

I(t)  =  I ( final ) (1- e^-t/(L/R)     .  L/R  is the time constant , in this case a few tens of ms.

   

A starter motor can easily pull 500A at switch on.

Not easy to drive . still  recommend ntc thermistors and 1kHz pwm with schottky flywheel and suitable snubbers

regards

Allan

I'll send the resultsof simulations in my next post


Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: raschemmel on Jul 23, 2016, 05:25 pm
Please show your work.

1091A/S  ?

please show how you obtained this result.
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: allanhurst on Jul 23, 2016, 05:45 pm
Hi R..

you'lll note in my original sums I said that I presumed the motor was a short.  I also assumed the inductor had zero dc resistance.  The 1091 A/s then applies.

Even with added resistances , the inital slope will be this.

I enclose PSPICE circuit and resulting waveform, with included resistance. You'll note the risetime is far too slow to use a 1kHz PWM control.

regards.

Allan
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: raschemmel on Jul 23, 2016, 06:07 pm
I don't have PSpice so I found this (https://www.easycalculation.com/physics/electromagnetism/ac.php) online which only works if you enter the inductance and try increasing values for current until the result = 12.001 V and the value for current that obtains that result is the 1091A you cited.

In all honesty, math was never my strong suit. I learned basic electronics math including algebra but deriviatives is something I studied at DeVry but never mastered and frankly , don't remember.
I did find this, (http://www.allaboutcircuits.com/textbook/direct-current/chpt-15/inductors-and-calculus/) which helps, but frankly I am having a hard time grasping how the math can explain the typical ringing waveform you see across an inductor. I can generate that waveform and observe it on my scope,
(which I have done and taken photos of : see attached) but as yet , I don't understand how the math explains or describes the ringing seen in the photo.
(http://forum.arduino.cc/index.php?action=dlattach;topic=413804.0;attach=174825)

One of the things that I am trying to understand (again) is:
Quote
You can see according to this formula that the voltage is directly proportional to the derivative of the current. Since the derivative of a constant is equal to 0, if the current is a direct current (DC), the current across the inductor will be equal to 0. So if the current is a DC current, the current flowing through the capacitor will always be 0. This, again, is because the derivative of a constant is always equal to 0. A constant does not change. So if a user simply enters in a current such as 10A or 20A or 30A, the current will be 0, for all these values. This shows that no voltage can be across an inductor if it is connected to a DC power source. There is only voltage across an inducttor when it is connected to an AC source.
It seems to me the above statement does not take into account the case of turn on and turn off.


Since the OP's motor is a DC motor , Reactance does not apply, and Cos(t) does not apply.


In the attached schematic, how should the diode or diodes be connected ?
(http://forum.arduino.cc/index.php?action=dlattach;topic=413804.0;attach=174827)

@DLoyd,
Sorry about the price tag but this is an old photo taken before I removed the price tag. 8)

Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: allanhurst on Jul 23, 2016, 06:20 pm
Hi R

To get the ringing you see there must be some capacitance - do you have a circuit for the set-up for this waveform  ?

regards allan.
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: raschemmel on Jul 23, 2016, 06:26 pm
It was a diode test circuit where the diode is across a 100 uH inductor that is switched by a mosfet.
The point of the test was to compare the effects of different diodes in eliminating the back EMF.
This photo was supposed to represent the "CONTROL" where there is NO diode. The diodes being tested were schottky diodes. I had some caps across the diode originally but removed them. I honestly don't know if they were still in the circuit when this photo was taken.
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: allanhurst on Jul 23, 2016, 07:45 pm
Hi R..

     well you asked for it.  There are loads of textboks which will do better than me.

Another way of looking at the characteristics of inductors and capacitors is this.

the impedance of an inductor is  Z(L) = jwL               L is the inductance in henrys

and a capacitor                       Z(C)  =  -1/jwC        C is the capacitance in farads

w is the rate of change in radians/second . There are 2*pi radians in a circle, so w = 2*pi*Freq/Hz

j is the square root of -1. An imaginary number. But useful.  Treat it as a vector length 1 at right angles to the 'real' ( ie ordinary numbers) axis. Think of an x-y graph , but now  horizontal is real  and  vertical imaginary.  +j is above the horizontal , -j below.

Inductors are above the horizontal, capacitors below ( because of the - )

Let's think of a resonance - that where the overall impedance of an inductor and capacitor adds to nothing. ie he inductor length above = the capacitor length below.

So we can say that jwL = -1/jwC.  Multiply thru by jwC

so  j^2 * w^2 * L * C = -1.

But j^2 = -1     by definition.

so w^2 * L * C = 1

or w = 1 / ( sqrt (L*C)

but since w =  2* pi * freq/ Hz

the frequency of resonance / Hz is  1/(2*pi*sqrt(L*C)) 

been a long time.....

regards

Allan ..

I won't bore you any more if you don't want......


 










Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: raschemmel on Jul 23, 2016, 08:12 pm
Quote
the frequency of resonance / Hz is  1/(2*pi*sqrt(L*C))   
so basically, it's the resonance that causes the ringing and the amplitude is diminishing because the energy is being "depleted/dissipated " ?
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: allanhurst on Jul 23, 2016, 09:22 pm
Yes - the ringing you see is at the resonant freqency. The decay is because of resistance in the circuit -
whether fitted , or in the L-C components. Real components always have some resistance.

regards

Allan

ps I generally use s for seconds, as S is for siemens - ie reciprocal ohms. Confusing, eh?
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: allanhurst on Jul 23, 2016, 11:07 pm
R

 
another example - just for fun.

suppose I have a resistor of 10 ohms in series with
an inductor of impedance 10 ohms at a particular frequency.

I now apply a voltage of 1v at this frequency - what current flows?

1/(10+10)    amps?

 No, because the total effective impedance has to take into account of the fact that the inductive impedance is at right angles to the resistive.

Hence the answer is that the actual impedance is sqrt(10^2 + 10^2) or 10 * sqrt(2) - it's just finding
the resultant of 2 equal vectors at right angles.

Hence the actual current will be 1/10* sqrt(2)  amps and it will it will lead the applied voltage by

arctan( 10/10)  ie 45 degrees

regards Allan.

mess about with a signal generator and a 2-channel scope and you'll see it's true.


Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: raschemmel on Jul 24, 2016, 12:45 am
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/rlimp.html (http://hyperphysics.phy-astr.gsu.edu/hbase/electric/rlimp.html)

Yes. I said I was rusty. I didn't say I forgot everything I learned LOL.

A pure resistance has no phase shift while an inductive reactance causes the current to lag voltage by 90 degrees.

BTW, I the 11 mH was based on the assumptions of 50 ms for the motor rampup time and 0.22 for the sum of the inductor and motor series resistances. The inductor value would be different if the windup time was different or the series resistance was different. I'm inclined to believe the method used was correct even the assummed values and resultant inductance are not, mainly because I got the formula from a Phd electrical engineer.

FYI,
current LAGS voltage in an inductor and LEADS volyage in a capacitor (http://www.electrical4u.com/rl-series-circuit/) .
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: allanhurst on Jul 24, 2016, 01:11 am
Yeah sorry. got it backwards.

regards.

Allan
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: MarkT on Jul 24, 2016, 02:04 am
so if I apply 12v across your 11mH inductor ( taking the motor as a short for now)
current will rise at 1091/amps second . In 1mS ( the pwm period) it will only get to 1.09 amps.

Hence the inductor is far too large by a factor of at least 30 if we want to deal with 30 amp max.
with 1kHz pwm.

No, this is a complete non-sequitor.

What you should have said is that the inductor limits the rise of current to 30A to at least 30ms,
irrespective of the PWM frequency.

If a fast current control loop is wanted for motor control (ie for a servomotor) extra series inductance
is bad news, unless its upstream of a big capacitor.

Any way a 30A 11mH choke is probably a monster, perhaps as big as the motor itself!  That's enough to say
its not the solution to any reasonable problem!
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: allanhurst on Jul 24, 2016, 02:32 am
Hi MarkT...

      if you read through the thread you'll find it isn't my idea...  the quote is from a post where I pointed out it couldn't work

If you want to do  it an outline design is aooended.  But Iwouldn't do it like that- even 220uH at that current is a juicy device!   I've ommitted snubber stuff which would certainly be required..

My feeling is that big NTC thermistors could be used to limit inrush current.


take no notice of device types - it's just what PSPICE gives me.

regards

Allan
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: raschemmel on Jul 24, 2016, 10:12 am
For the record, I did say such an inductor would be expensive. So is anyone here saying it wouldn't work to limit the current or simply that it is not an elegant solution ? If you are saying it wouldn't work I think you should explain why. I have made such inductors ( for a completely different application) using very expensive   Nanocrystaline and Metglass cores that weigh at least 10 lbs and 2 1/2" wide sheet copper and Nomex paper or the plastic insulated sheet copper. The inductance was measured on a $5000 LCR meter.  I made the prototype which was tested and the 9 that were needed were made by a transformer manufacturer. As I stated, the inductance value of 11 mH was based on an arbitrary motor wind up time of 50 ms that I just pulled out of the air because the OP has not given us the response time of his motor. The 30A/ 11 mH inductor was not presented as  a solution for the OP's case but rather presented as an example of how to calculate the  value of inductance required to limit the inrush current. (presumably a working example but impractical due to cost and size). Once having done that it becomes clear that not only would it be as large as the motor, it would cost more.   I did not mean for it to be taken seriously as a solution.  That being said, is there any reason it would not perform the function for which it was designed ?

I don't know what a practical inrush current limiter would be fir the OP's case.  I have nit used the NTC resistors mentioned. ( I know NTC stands for Negative Temperature Coefficient)
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: allanhurst on Jul 24, 2016, 11:51 am
Hi  R   Look at

http://en.tdk.eu/blob/528070/download/4/pdf-inrushcurrentlimiting-an2.pdf

Several S464's in parallel could be a good bet

regards

Allan.
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: nosredna000 on Jul 25, 2016, 03:06 pm
Hi guys  sorry to be missing for so long.  I left this alone over the weekend.   

Anyway,  I tried two mosfets in parrallel with a 10k ohm resistor on the gate and a 10A01-T diode across the fan and it ran pretty much endlessly with no heat in the diode or mosfet

Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: raschemmel on Jul 25, 2016, 03:37 pm
Glad to hear that's working out for you.



@Allanhurst,
How would find one (http://en.tdk.eu/blob/528070/download/4/pdf-inrushcurrentlimiting-an2.pdf) that can handle 100A inrush current ?
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: allanhurst on Jul 30, 2016, 10:52 am
Hi R.. sorry to be so slow.

You're right - parallelling NTC thermistors is a bad idea.....

I'm sure if I  looked hard I could find a suitable device ..  I'll have a poke around and see what I can find

I'll be back.

regards.

Allan.
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: raschemmel on Jul 30, 2016, 11:44 am
Quote
You're right - parallelling NTC thermistors is a bad idea.....
 
I am ? About what ? (I don't recall commenting on paralleling NTCs, although considering the response time of these devices and the fact that probably no two devices have EXACTLY the SAME response time, the idea of paralleling them doesn't sound practical because the FIRST one to respond would get the bulk of the current and basically explode or self destruct from the current overload before the other NTCs could share the load. If you could measure the current on a 4-channel scope using TEK DC current probes (http://www.testequipmentdepot.com/tektronix/accessories/oscilloscope-probe/tek-a622-ac-dc-current-probe.htm?ref=gbase&gclid=CjwKEAjwoPG8BRCSi5uu6d6N5WcSJABHzD8FJEZ6BeSIdeD_cquuPAwRRs6Jl-PxjOqyALRbX0gFeBoC94zw_wcB) for EACH NTC and look at the waveforms of four current probes measuring four different NTCs, I'll bet they won't all respond at the same instant and the ones that respond first will conduct more current.
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: dlloyd on Jul 31, 2016, 04:20 pm
A late suggestion, but here it is anyways. I'm thinking an ICL (http://www.digikey.ca/en/ptm/a/ametherm/inrush-current-limiters-overview/tutorial) would have reduced the startup surge, available at up to 50A continuous rating.
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: raschemmel on Jul 31, 2016, 04:59 pm
Looks like " that's the ticket !"

Same question: " Is there any reason why they can't be paralled for more current limiting ?"

I recognize those as one of the components in our product at work. They are used to limit capacitor charging current in a volt/VAR power correctiion application.
Title: Re: Burned Mosfet and popped flyback diode.. help! Big DC motor PWM
Post by: dlloyd on Jul 31, 2016, 06:36 pm
Quote
Is there any reason why they can't be paralleled for more current limiting ?
Probably not due to their logarithmic response as they heat up and the resistance decreases. Precise matching would be near impossible. Also keeping both at the same temp would be near impossible. Therefore if paralleled, the one with slightly faster response would see overcurrent and fail, then the other. Just my opinion, haven't tried them.

They can handle a lot of power though, i.e. 50A @ 680VAC = 34kVA.