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Old 07-17-2010, 02:15 PM   #21 (permalink)
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Electro, good to hear you had a good trip and good news about the surviving mosfet.

Sounds like you have a good method for checking the isolation of your power supplies. You can also check by measuring the voltage difference between your 2 0v from your different power supplies when they are not connected together, you are likely to see they are floating around. Note: your multimeter might bring them together, i know its annoying

Sounds like your soldering method is not ideal but seems like the best you can do unless you look at using a different mosfet that has a tab with a hole so you can screw it on rather than solder it. At the end of the day if the mosfet works the soldering doesn't matter

Regarding your switching frequency of 16 8 and 4 they are all in the audible frequency range so you are likely to hear it when your circuit is working which you could think of as a feature . If it gets too annoying you can look at increasing it to 30kHz or above.

Looks like you dont need that gate resistor, you should just take it out. the gate itself is isolated anyway so current isn't going to flow. If you change to something other than a mosfet then you would look at using a gate resistor again.

When i finished up with university, there was a whole lot i had no idea about. those first few months developing electronic products was a steep learning curve.

So looking at this oscillation, it is HUGE!. Due to the time base on your image which is 200ns id assume this is mostly ESR related. I am guessing you have several huge electrolytic caps leave them as they are because they will hold up the power during a pulse of your 16kHz but add some more caps physically close to the mosfet rails these caps dont need to be a really high value, the ESR is the important thing, they could be 100 uF maybe, i would have to do the sums on your oscillation frequency and current draw but if it was me i would just put 100uF across the rails really close to the mosfet and see what happens. A contributing factor to this oscillation may be due to inductance/resistance between the cap and the mosfets.
PS you are interested here in the driver caps not the controller caps. Hopefully you have enough capacitance on your controller card and nice stable voltage rails on the controller card otherwise your drive signal would be almost unusable. If your controller card doesnt have stable voltage rails seperate the voltage rails of the controller card and driver card and put a filter into the controller card (inductor and capacitor on the voltage rail)
As you have indicated your voltage swing will just get much worse if you drive higher than 14A so best to get this fixed before increasing current otherwise your might blow things up. This might explain why you blow transistors with your starter motor and not with your smaller motor.

I shall look forward to seeing what you find

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Old 07-17-2010, 03:59 PM   #22 (permalink)
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Quote:
Originally Posted by apowers View Post
maybe it is the bendix.
I read the Wikipedia article on Bendix and I'm still not sure what exactly you are referring to. Bendix apparently refers to the way the sprocket is attached to the starter, specifically with a helical cut. I do not believe that is what you meant.

I will assume jackbauer got it right, and no I bypassed my starter solenoid.
I was hoping to use it initially as a safety cutoff. The problem is the solenoid and starter share a common ground, so it is not possible to turn the solenoid off separately. (without doing something fancy or mutilating the starter)

saand:
Yeah the meter would bring them together.. I suppose one could try to measure impedance across the ground connections. If they are connected it will be a short. If they float, then you measure voltage between the two and you should see the difference induced by the meter, 3V in my case.

Experimentation agrees with theory. I get open circuit measurement on ohmmeter and -1.8V on voltmeter, dropping to -1.7V very very slowly.
So they are isolated, and my ground wire between them is mandatory.

Thanks for confirming my gate resistor removal; I was sure everyone would tell me to but it back in since it seems almost like dogma at this point lol

Yep, the hands-on at university is so lacking. That's why I'm doing this project! Well that, and fun


Yep those oscillations are quite impressive, and they make me quite sad.
I see we have a problem with terminology which should be addressed at this juncture.
I consider having 3 main components:
1. The microcontroller board with outputs logic level PWM.
2. The mosfet driver board, one chip - takes 5v PWM input and drives 12V pwm output.
3. The controller, which consists of buss bars, capacitors, flyback diodes and mosfets.

I have those metallized polypropylene capacitors, only 4.7uF but with really low ESR and high ripple current capability.
Digi-Key - PF2475-ND (Manufacturer - ECW-F2475JB)

Two of them are on the controller between BM+ and B- along with the large electrolytics. Saand you suggest to add more capacitance here with low esr; do these caps not count? Or do you think I just need more of them?

The third is on the driver board since the wall wart powering it can't handle those kind of current spikes.

My microcontroller (you say controller card) has enough capacitance, but perhaps I should add some inductance as you suggest. There is a slight sag and oscillation, but not enough to be detrimental I think.



Just now I have begun to suspect the connection between the capacitor board and the buss bars. I epoxied them together with some two-part epoxy that claimed to be a solder replacement. (I knew soldering was going to be a pain)
The problem is that the solder was ineffective on the components; it was a weak physical bond, making an intermittent electrical connection. A fun story for another time.
The cap board is still connected to the buss bars with that epoxy.
The resistance across the epoxy is negligible, within the tolerance of my meter (it is a Fluke, but more electrician rather than electronics version)
As I said before, precharging takes several seconds so the capacitance is connected in a DC sense.
I suspect now that the AC connection is not as good; that the epoxy has a high ESR. This would keep the good capacitors from fulfilling their duty.

My only course of action is to replace this epoxy. I'll give it a try and cross my fingers!


I appreciate all of your help everyone. I hope I can give back to the community some day
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Old 07-17-2010, 05:21 PM   #23 (permalink)
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I looked up your starter and the selenoid is mounted on the starter. That is what engages the bendix. It draws alot of amps. Mosfet might still be too small for starter motor.
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Old 07-17-2010, 10:02 PM   #24 (permalink)
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I looked up those polyproplene caps their ESR is about 1 ohm each at the frequencies your interested in (your ringing oscillations are up around 10 MHz). So the ESR would be a problem when drawing high current, you can do some calcs to work out if you are likely to have an issue.
The standard way of dealing with this sort circuit which switches high currents very quickly is to put down multiple types of capacitors,
- some with high capacitance which likely have high ESR so that it keeps up the rails on low frequency transients
- some with mid range capacitance with his average ESR for higher frequencies
- Some very low capacitances that will have low ESR for very high frequencies (above 1 MHz)
If you see a digital circuit design there is always a 100nF capacitor physically right beside every digital chip (if the circuit has been designed properly). This is so that any digital switching takes current from the very close capacitor rather than the power supply and therefore stops ringing in the rails.
You are essentially running a really really big digital switch so you need some more of these very low ESR caps physically close to your mosfet
the wikipedia page might explain it a bit better than i have
Decoupling capacitor - Wikipedia, the free encyclopedia
Also if you ever see a product designer have issues on their rails due to ringing they wont even think about it and just put some more caps on the rails, the caps are cheap and easy and they wont hurt your circuit

you might also want to consider is your oscillation real at these sort of frequencies if your oscilliscope ground is not connected it might cause you to see this. Although from my understanding of your circuit in this instance it seems real.

Your epoxy sounds very interesting, i have never come across this sort of compound before, any plastic/ polymer based electrical connection i would be hestitant about, i think your right on the money thinking it might provide higher resistance at higher frequencies.
Can you put solder on then epoxy.
You mention you have a connection between your capacitor board and your bus bar. I assume your mosfets aren't on the capacitor board in which case you will want to be very careful about making this connection very low inductance and very low resistance. If you have a spare one of those polyproplene caps you should just physically hold it across the rails, one leg on the mosfet connection and one leg on the return of the motor. This might just get rid of or reduce the oscillation due to the physical closeness.

I also appreciate everyones help on this forum, i wouldn't be trying half of the crazy mods to my car without people here. I am currently trying to modify the parameters in my ECU its proving to be very difficult but people on here are helping a lot.
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Old 07-18-2010, 03:21 AM   #25 (permalink)
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so how did you ever find the ESR of these caps at 10MHz??
I can't find that information for either kind of cap that I have.

Some other cap datasheets have a curve of esr vs. frequency but I've found nothing of the sort for these particular caps.


I had assumed that for the most part, my 1" by 1/4" by 6" buss bars would have very little inductance. Apparently this is not so, there is a 1V difference in the amplitude of the rails oscillation along the length of the BM+ bar.

However the real inductance is the 1oz copper clad of the capacitor board.
I tinned it heavily in an effort to increase its thickness, and the traces are all as wide as possible, 0.8" at the thinnest, >1" everywhere else.

The ripple at the far end of the cap board has an amplitude of only 0.4V when it is 5 volts at the mosfet. Thus adding more capacitance here would be of little use; the inductance of the traces would dominate. It needs to go elsewhere.
The connection between buss bar and cap board is solid; very little difference in ripple amplitude across the fresh solder joint.

I tried attaching a 100uF electrolytic I had lying around across the buss bars near the mosfet and it did wonders for reducing the ripple. The spare poly cap I tried was not as effective, likely due to its high ESR as you mentioned.
(spare = I ripped it off the driver board hehe)
I'm sad, I thought those had a lower ESR

What type of cap would you suggest? I am leaning towards ceramic.
My oscillations occur at about 1MHz and 15MHz.
Looking at HF caps, their resonance point is very small, such that no one cap could counter both the 1MHz and 15MHz noise. Since the 15MHz has a higher amplitude I would want to beat it first.

This cap Digi-Key - 495-1024-1-ND (Manufacturer - B37986G1222J054)
has an impedance of 50mOhm in the MHz range, but apparently the leads have to be the correct length to get the impedance right for the resonant frequency to move. Seems reasonable but how would you design with this in mind? Apparently adding 2cm of lead shifts the resonant point from 100MHz to 10MHz.


Hmm yes, I knew you can get fake signals at high freqs but it didn't even cross my mind here. Since it has different magnitudes at various locations and since I can change it by adding capacitance I agree that it is real.

I want to return that epoxy but I lost the receipt. I should note that the bond was quite strong between the FR-4 and buss bars; I took off some copper clad when I pried them apart.




apowers: the solenoid (and thus the bendix) is disabled. However it would draw way less current than the starter itself. I know the starter draws 370A stalled. Once I had 2 mosfets (total 380A rated) and they still fried in <2s, and were cold afterward. No sign of overheating, which is an indication for overcurrent AFAIK.
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Old 07-18-2010, 03:31 AM   #26 (permalink)
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I forgot to mention, I found a way to decrease the oscillations.. Obvious really but it has a downside.

I add gate resistance! :P

I threw a 100ohm pot in to see the effect of changing gate resistance on supply ripple.
Turning the mosfet on more slowly allows time for the inductance to be overcome.
I can reduce the amplitude of ripple from ~30V to ~5V.
The frequency of oscillation is nearly unchanged.

At higher currents I imagine this would fry the mosfet as it has to survive high currents with a large Rds during the longer transistion time. I remember playing with heat dissipation formulae for switching power supplies, I might have to revisit them.

Still, with 30 times the current draw, the starter should cause 30 times the ripple, or 900V (still 150V with slow turn-on). Seems excessive, but maybe *that* is what is killing my mosfet...
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Old 07-18-2010, 12:16 PM   #27 (permalink)
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A bit of searching found the manufacturer data, digikey didn't have the best link for information. A link to the link is below, you have an ECWF cap according to digikey so click on characteristic data in this link and you will find it
Radial lead type capacitor specification list

I have not researched caps for this sort of application before so i cant make a informed recommendation for you but looks like you are on a good track for making a better decision than i can with what you have found so far. I will say though that the cap you have listed from digikey looks to be a particularly low value, 2.2 nF will likely not do much as it cant supply much energy. Basically the cap is place close to the switcher so the switching device will take current directly from the capacitor rather than through line inductance from your power supply. So the capacitor needs to supply the full current for say 50 ns. 140 A for 50ns is a fair amount of energy, its 0.000084 joules.
you can calculate the capacitor by seeing how much energy it can supply with this formula
E = 1/2 * U^2 * C

Where
E = Energy in capacitor in joules
U = Voltage in capacitor in volts
C = Capacitor capacitance in farads

So to match this energy if your voltage is to only drop by 2 volt you will need
42 uF

If a single cap will not do the job use multiple caps, nothing wrong with extra capacitance (except for inrush currents when you turn your car on)

Regarding lead length and resonant frequency, this just goes to show you how critical even very small inductance plays in your circuit

When doing this sort of thing i usually dont look at resonant frequency because you usually avoid resonance, i typically go for the smallest ESR and put it as close as possible to the switching device within 5 millimeters if possible (5 mm is just picked out of the air)

hahaha yea i knew adding a bit of gate resistance and capacitance would reduce your ringing but i didn't suggest it because you might just give up completely. Also as you have written if you are driving 140A you want a really quick turn on time due to heat and ideally you should fix the oscillation another way or you might blow up your mosfet again

It is a very good chance that the oscillations were killing your mosfets and seems to be confirmed because your current fet has survived with lower loads.

either way looks like your making progess on what is a relatively hard thing to design
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Old 07-24-2010, 06:21 PM   #28 (permalink)
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Thanks once again saand.

Now I understand what you mean by doing calculations.

As for doing actual work, the project is on hold for one month while I sort out my personal life (i.e. further education) and spend more time with my family - sailing Georgian Bay to Lake Huron for a week.

I'm still thinking about it, and reading online when I can.

I was reading this appnote:
http://www.national.com/an/AN/AN-558.pdf

At the bottom of Page 12 it reads:
Quote:
There
is also the possibility that the drain-source voltage will exceed
its maximum rated voltage due to ringing in the drain
circuit. A protective RC snubber circuit or zener diode may
be added to limit drain voltage to a safe level.
So here's the question - do I actually *have* to add capacitance to hold up my power rails, or can I just protect the mosfet by putting a 90V zener across Drain-Source?

I know what the correct solution is, but I specced out capacitors based on my values and the calculations you showed, and we're talking $100 of capacitors; there are already $50 on the board. I'm starting to feel silly here... There must be a compromise!


My last option is to start from scratch on the capacitor board. Clearly the one I have is too inductive, even with its hugely wide traces; they are just too long - and too far away from the mosfets!
I think that is why P+S's controller has the capacitors along the length of the controller. I thought with a short controller (come on its only four inches of HUGE buss bar) that I could get away without doing that.

Thoughts/opinions?
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Old 07-25-2010, 12:50 PM   #29 (permalink)
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I like the idea of the zener but you may have to look at getting a large zener if you think you will have significant currents flowing.
I guess give it a go and see what happens, zeners are cheaper than giant caps. PS you might look at using say a 30v zener which might clip your ringing and make it very small.

If you are looking at redoing the capacitor board. I would recommend making up a capacitor and mosfet board so the caps are right beside the fets. that way you will have a better overall design with less inductance in critical locations and you will probably not need so much capacitance as you will likely have less ringing.
But if you do decide to go down that route make sure you put in provision to have a range of capacitor values with different ESR's, so that you can be sure you will catch the fast ringing.

A bit of time off will hopefully give you some time to think about the best way forward and good to see you are not neglecting your family. Some of these projects get very interesting and its always hard to tear me away from a good project.
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Old 09-12-2010, 01:38 PM   #30 (permalink)
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Problem solved yay

Hey guess what? My controller no longer blowing mosfets!!!

Problem source:
Switching ~380 amps is no small task.
Doing it quickly is substantially more difficult.
Even tiny inductances will cause voltage spikes
V = L * di/dt - Vdd

To get a 100V spike (as I must have, to fry the old mosfets)
L = (100 + 12) / (di/dt)
di = 380 amps
dt = 50ns
<I know that's not proper calculus but it gives us an approximate linear slope that in fact should be the best case! i.e. lowest slope>
L = 112 * 50n / 380 = 15nH
One inch of buss bar (or so I have read) at best case is about 10nH. No wonder it fried the mosfets! The voltage spiked over V_(BR)DSS and took it out.

The only thing we can change is dt, the time to switch. I have increased it to 200ns. The theoretical inductance needed to fry my mosfet is now 60nH.


So my starter motor is very happy, spinning away for me, singing me a song. Or so it feels.. This success has been a very long time coming.

I want to thank everyone that helped out in this thread, especially saand for staying with me all the way and giving a lot of sage advice.
Thank you!!!!

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