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Old 07-09-2010, 04:07 PM   #11 (permalink)
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...and subsequent failure

Disconnecting the TVS had no effect on gate waveform; my capacitance theory was wrong. I replaced it immediately.

I remembered I used to measure gate voltage *at the driver* rather than four inches away at the mosfet (which I know is a lot of inductance for these pulses)
That explains why I remember a much faster gate drive.

I shortened the wires to about 2 inches each, the lowest I can go in prototype form. Still very sluggish driving.
I put one scope probe on either side of my 10ohm gate resistor. 100ns on the driver side, 600+ns on the transistor side.

I shorted the gate resistor to see how the circuit would act without one. The motor changed pitch slightly and the gate drive looked fairly nice.
The ringing effect was still evident but nowhere near as prominant.

I reduced the PWM to 0%.
As I disconnected my scope leads from the resistor, the mosfet died.

NOT AGAIN!

Yup it's dead all right.
-It's cold even though it had been running for a few minutes before it died.
-The transistor was completely off when it died, 0% duty cycle.
-ESD? I would expect the TVS to protect from esd as well, but the coincidence of me touching the probes and the mosfet dying is compelling.

I'm sure stumped now. Thoughts?

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Old 07-09-2010, 10:51 PM   #12 (permalink)
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Quote:
Originally Posted by electrowizard View Post

I reduced the PWM to 0%.
As I disconnected my scope leads from the resistor, the mosfet died.

NOT AGAIN!

I'm sure stumped now. Thoughts?
Hello ElectroWizard,

I suspect that you are not done blowing parts just yet.

I think that a gate pull down resistor is in order.
Put a 10k resistor between the gate and emitter.
The very slight (perhaps 10Mohm) load of the probe
may have been enough to pull the gate low or keep
it out of the linear range a bit more. I don't
think that you are blowing the gate insulation,
I think that you are unintetionaly operating the IGBT in it's linear
region and they have VERY low tolerance to that!

I didn't notice the value of the TVS, but it should
probably limit to 15vdc as is commonly done with
15v 1W zener diodes. The TVS is a perfectly good
replacement for that if the voltage limit is similar.

When you do begin to parallel the IGBT's you will
want to match them for V(SAT) and may want to
set up a jig for doing that.

I agree with you that this stuff is a blast, and blasting a
few parts is just part of that fun!

Cheers

Mark Weisheimer
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Old 07-09-2010, 11:23 PM   #13 (permalink)
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Thanks for the reply Mark;

I've read enough about IGBTs to understand what you're talking about, however my controller is using MOSFETs (after three weeks of reading and deciding)

The TVS is a unidirectional 15.2V unit so it should be quite similar to a 15V zener. My max Vgs is 16V so I should be safe there.

I know a lot of people use the 10k pulldown resistors.
Maybe my driver connection came loose and my mosfet went into triode... I don't believe they are as sensitive to that as you say IGBTs are. I suppose I will include it also just to be safe.

Six mosfets down, four to go! Good thing I bought lots of spares :P



Unrelated question: mosfet gate resistors (not pulldown)- absolutely necessary? I have looked high and low; it seems they are chosen empirically rather than actually calculated. My circuit seems to want a very low gate resistance. Maybe I should have chosen a more powerful gate driver? I don't really understand the tradeoffs here.
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Old 07-09-2010, 11:39 PM   #14 (permalink)
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Ok,
a lot of info here to work with,

As i understand it (please correct me if wrong) your light load tests are not blowing up mosfets? and the FETs are not getting hot in light load tests.
If this is the case it would be good to see the current waveform when the system is powering the full load. See if you can triggure on the hall effect and capture the peak current while the mostfet is still alive.
As a side note i have not often had success with home brew hall effect sensors so if yours is home made before you can trust the levels too much would be worth checking with a known current effectively calibrate it

Generally the schematic looks to have no issue, the design is solid, has adequate heat sinking (assuming the drain is soldered solidly to the terminal in that picture) the fly back diode is present assuming it hasn't blown up.

The gate resistor is generally a good idea but bare in mind the gate is electrically isolated from the drive and source so almost no current flows through the gate anyway so the gate resistor as you have found doesn't do very much. It will save your driving circuit if the mostfet starts conducting through the gate though.

As for the semiconductor temperature, your train of thought is a good one although not too relevant in your case considering what you have mentioned so far. If your case temperature was getting warm at all you would start to consider how well the semiconductor is bonded thermaly to the case, from the datasheet you can do calcs on the temperature rise of the semiconductor based on the thermal bonding of the case to your heat sink and the case temperature. But looks like you dont have to worry about this one until you start driving your motor under significant loads

If you system works (without blowing the FET) when driving the 1A motor and the precharge resistor the problem is likely to be the instantanious current limit of the FET 190A might be being violated your motor start current might be higher than 190A. If you are getting above 190A even in pullsed currents you should start worrying about this value. This datasheet value is related to the heat that the FET can dissipate through its case.

Can you tell me what your switching frequency is, you probably dont want to be above 100k Hz and you could likely be down as low as 1kHz. Although to avoid hearing your switching of the high currents you might want to be above the human audible frequency limit i think it is 26kHz

The mosfet direction looks good according to the datasheet

The oscillation is an interesting one, this will be developed by inductance and capacitance in your circuit which is inherent in parts and wires you likely havn't considered. Strange that it still happens under a resistive load did you try with a low resistance?, you might want to check your supply rails also do not have these oscillations. Also at these frequencies 12MHz you need to start considering ground loops so is your oscilloscope connected to a power outlet in which case you are likely to be connecting the 0V to your mains earth. Without knowing your countries mains arrangement and your oscilloscope construction i cant tell you for sure. If you are just running of a battery for your 12v this may not make a difference but if you have a mains connected power supply generating your 12v or a battery charger connected to the 12v this might affect your measurement.
A way to get rid of these oscillations (if not generated by your measurement methods) is to place a snubbing circuit (resistor and capacitor) wherever the oscillation is occuring however this is a last option. the best method to get rid of this is to find the source and fix it that way. So check with a low resistance load, try putting a capacitance on your gate drive, make sure the capacitance value is suited to the frequency of your 12MHZ as it will slow down your turn on time for switching if it is too large. Try things like moving the wires around while its switching and see if the frequency of ringing changes, the wiring inductance may be affecting it.
If you are correct in your idea that Vds ringing is affecting Vgs this could only happen if the source (s) is not coupled to 0v effectively. Check that the source of the mosfet has a low impedance connection to the 0v of the driving circuit

Its unfortunate the mosfet died again, you must be getting good at replacing them quickly now . ESD is a good idea however the affect of ESD is very rarely observed in this way. I have walked up to a board accidentaly zapped it ( I felt the Zap) and the board works fine. ESD usually compromises the component silicon which then degrades over time. It is worth keeping an eye on ESD but i dont think its the cause of the dead FET.
It is interesting that when you removed the oscilloscope that the FET died, did it happen to occure right when you removed the oscilloscope 0v connection rather than the probe. As i mentioned above the 0v on the oscilloscope is connected to the mains earth effectively grounding your circuit. If you have FETs to spare and a bit of time might be worth repeating this, see if it is consistant, if it is this points directly to your problem well at least 1 problem. Check the connection of the 0v of the driving circuit, if the 0v on the drive is not coupled to the source of the mosfet then the drive could float higher than +20v and kill your FET and as soon as you try to measure the Vgs you would get what appears like OK levels but as soon as you remove the oscilloscope the FET could die.
You also mention you checked the temperature of the FET after it died, i recommend you put a temperature probe on the FET case so you can check the temperature when it is in a working state. alternatively put your finger on the case when it is working see if it gets warm under the full load condition. If you check the temp after it has died the temperature could dissipate within seconds or less considering you have such a large heat sink.

Well sorry i can not tell you the exact problem you have, its always hard to diagnose an issue without seeing the circuit and playing with it yourself

so good luck, look forward to seeing what else you find, just think how happy you will be when you finally work out the problem and you will have learnt a lot in the process
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Old 07-09-2010, 11:54 PM   #15 (permalink)
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saand you rock my friend!

Yes I know how hard it is for me to diagnose hovering over the circuit, it is even harder when just being described on a forum. I'm doing my best though!

I think your idea of a poor ground connection floating Vgs too high could be bang-on. My soldering iron is not capable of heating anything attached to the buss bars, so the driver ground to the source lead was only clipped on and may have slipped loose. The oscilloscope ground lead is grounded through the mains, which shares a common ground with my 12V gate driver. And the oscilloscope ground lead was more securely fastened than the gate driver.
The battery was not being charged, so no ground loop there.
However, I just remembered.. my TVS is permanently soldered between source and gate. Shouldn't that prevent the gate from breaking down regardless of the actual potential of the source lead? Hmm..

I decided on a last minute trip to the cottage for several days with family, so I will disappear for about 5 days. I'll try and digest the problem while I'm away and come back at it full-force.

And yes, my propane-torch soldering skills have improved dramatically. I can replace a mosfet in about 45 mins now... the copper bars take almost 10 mins apiece to heat up and longer to cool down. I know that kind of exceeds the recommended soldering profile... I wonder how much damage I am doing to them. Hmm.

Last edited by electrowizard; 07-10-2010 at 12:04 AM..
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Old 07-10-2010, 02:25 AM   #16 (permalink)
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Glad to help out,

your definately doing well in describing what you see and your methods of testing, I often have clients give me the worst description of what is going wrong becomes like a puzzle just to work out what they are doing.

Regarding your comment on the soldering, you could well be compromising the device if you are trying to heat up the solder on the tab for longer than probably 30 seconds. Not sure what this would do to the device but this is the reason these type of devices are connected to heat sinks using a tab with a hole so you can screw them down.
A tip for soldering next time, when you are solding on the tab which will be the hardest to connect heat up the copper bars until the solder starts to flow, you can see it when the solder gets shiny. when this happens then place the part down the solder should take to the tab, and you can take your soldering iron straight off, it will then cool down fairly fast an no issue.
when doing the legs should be able to use the same method for the legs that connect straight to the copper bar.
If your heating up the tab directly with the soldering iron for more than say 15 seconds your likely causing damage.

enjoy your trip, maybe inspiration will strike
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Old 07-10-2010, 04:34 PM   #17 (permalink)
Mark Weisheimer
 
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Quote:
Originally Posted by electrowizard View Post
Thanks for the reply Mark;

I've read enough about IGBTs to understand what you're talking about, however my controller is using MOSFETs (after three weeks of reading and deciding)

The TVS is a unidirectional 15.2V unit so it should be quite similar to a 15V zener. My max Vgs is 16V so I should be safe there.

I know a lot of people use the 10k pulldown resistors.
Maybe my driver connection came loose and my mosfet went into triode... I don't believe they are as sensitive to that as you say IGBTs are. I suppose I will include it also just to be safe.

Six mosfets down, four to go! Good thing I bought lots of spares :P



Unrelated question: mosfet gate resistors (not pulldown)- absolutely necessary? I have looked high and low; it seems they are chosen empirically rather than actually calculated. My circuit seems to want a very low gate resistance. Maybe I should have chosen a more powerful gate driver? I don't really understand the tradeoffs here.
EW,

I had to laugh. I was just so sure you were using IGBT's!

I pulled up the datasheet. They show using gate drive resistors in diagram 21.
The resistors are usually chosen for "turn on time"
(dv/dt, or delta voltage / delta time)

It can be calculated as an RC time constant using C(ISS), drive voltage,
and gate drive resistance value. You are shooting for a fast turn on
without creating severe ringing.
That device is made to use "logic level drive" so you should be able to drive
it with 5vdc.

I would still use gate pull down resistors, and 10K is fine.
Your TVS value is fine for voltage protection too.

I wonder if the power supplies are as isolated as you are led to believe.
I have seen supposed isolated supplies that had resistors or caps
tied to ground or a supply line and were NOT isolated.
Do you have any surplus switching supplies lying around,
such as are commonly used for laptops or LCD TV's and etc?
I have found them to be good sources for test supplies
and I am very careful when using transformer based wall warts.

I was trying to draw a relationship to the device blowing when
you pulled your probe, and did not even think of the common ground
issue. I tend to use a battery powered portable scope and
ignore common/grounding issues as a result.

Good Luck!

Mark
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Old 07-17-2010, 03:22 AM   #18 (permalink)
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Back from the cottage

It was a great trip; I'm sure I gained another 5 pounds over the few days I was there. Went for a 19km bike ride to town though, was impressed with myself.

I came home and guess what? You know the mosfet that died before I left?
Apparently it is not dead

I immediately checked the others and yes they are all dead for sure.


I realized it was not broken with my ohmmeter; I proved it still worked by powering the circuit up.
Thinking back to the grounding issues raised by saand, I disconnected the oscilloscope ground leads from B-. Immediately the motor shot to full speed as if the mosfet died again, but reconnecting ground fixed the problem.

I tried to check whether or not my power supplies were isolated from line ground or not. (driver and microcontroller supplies) I had assumed they were not isolated. Testing continuity with my ohmmeter between the power supply output and the 110 line contacts indicates no connection on either supply. This means they are *both* isolated. This causes me to suspect that my testing method is not valid. Does anyone have a better way or is this correct?

To correct the issue in the mean time, I connected my microcontroller ground to my driver ground, such that they have the same reference. The problem is apparently resolved.


saand:
Thanks for the encouragement
As I said I am not using an iron but rather a propane torch to heat the bars.
I heat away from the devices, and once the solder is hot enough, I stick the SMT devices down and add some fresh solder around the tab while pressing down on the case to ensure a good contact. Cooling down takes at least ten minutes, even with using scrap copper bars to absorb heat and dump it into cooling water.

I am switching at 16kHz though I am free to choose 4 or 8 easily.

The gate oscillations again.. I removed the 10ohm gate resistor. My drive waveform is now at 100ns with only 3% overshoot and minimal ringing. That gate resistor is a menace. However you were right, the supply rails also have this oscillation. See below for more on this.

Thanks for the ESD information, very good to know. 5 years of engineering school and they never taught us that. . .

All components are cool under load at 50% duty cycle for about an hour. Except the motor :P


Weisheimer:
No problem, it happens to the best of us hehe
As you said the gate resistor can be calculated but it is more empirically chosen to reduce ringing right? So if in my application I have no ringing with no gate resistor and a reasonable turn-on time then I should not include it. Do you agree?

What is your take on my supply isolation; how can I test this properly?




Ringing revisited:
My battery voltage (BM+ to B-) rings an extraordinary amount on mosfet turn-on.
Click image for larger version

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Blue = Vdd; Yellow = Gate

I would expect this to occur due to a lack of controller capacitors.
I have verified that I have about 27mF of capacitance due to a 1.1 second time constant with my 42 ohm precharge resistor. I know that does not indicate a low esr or good ripple capability but it does prove the caps are connected.
I also have two of those red metallized polypropylene capacitors (where P+S's much larger controller uses 3 and some other electrolytics) so I would expect my voltage swing to be minimal on a 14A load.

With a nominal 12V battery voltage the buss bar voltage swings from 28V down to 1V. The period of oscillation is close to the risetime of the driver signal.
Remember the mosfet is logic-level and is thus turned on completely, thus unable to oscillate when Vgs varies in the image.

So I am puzzled by the Vdd oscillations. My controller capacitors should prevent it from happening. These caps can even turn the motor ~1/2 turn from a standstill after the battery was disconnected. Are my caps to blame - bad ESR - or is there something I'm not thinking about?

Also, I still do not know why I am blowing transistors with the starter motor. The window motor works excellently. Curious curious..
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Old 07-17-2010, 10:06 AM   #19 (permalink)
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Quote:
Originally Posted by electrowizard View Post
Also, I still do not know why I am blowing transistors with the starter motor. The window motor works excellently. Curious curious..
maybe it is the bendix. I have a starter that draws alot of amps when it engages. I disabled the contacts for that and it made a difference.
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Old 07-17-2010, 11:23 AM   #20 (permalink)
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Are you using the starter solenoid? I didnt. I just wired the motor to run all the time by bypassing the solenoid.

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