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IGBT ratings/magnetic projectiles

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Jack A

Wed Feb 06 2013, 08:18AM

Registered Member #2975 Joined: Wed Jul 07 2010, 12:19AM
Location:
Posts: 28

@DerAlbi and Yandersen
I'm not quite sure what you're accusing me of DerAlbi, but I'm heartbroken that you feel that I'm 'milking' a project out of you... I am not doing this for money, I am not even doing this for the approval of my peers, I am; in fact, doing it purely out of personal interest in the subject, and longing to gain experience. You said that I'm trying to make you do the project for me so that I might badge it as my own, I cannot help but disagree; I designed the entirety of the system and the greater project myself using resources I found online and the experience gained after two successful coilgun models (however these two factors were unable to make me instantly capable of complex control circuitry), however I only realised recently that my switching plan was not going to work, and that is why I came back to this thread seeking your advice (but not physical assistance obviously). It is true (as I read the entirety of the datasheet before posting it) that there is an example circuit for the MOSFET driver, thank you for pointing it out, but due to my limited experience, I didn't know if this was the right way to run it, I still seek help on how to use this with the pwm output on my arduino. You also made some rather upsetting comments about my 'losers attitude', I also disagree with this; I am completely committed to this project, and have done challenging projects in the past, none of which turned out perfectly by themselves. You said that I couldn't succeed with the driver chip because I have no experience/knowledge, this is probably true, which is why I am seeking advice, which will allow me to get experience and knowledge, and then succeed; please don't tell me you were capable of complex control circuitry the first time you picked up the components... If I am being 'disrespectful', I am sorry; If you find the way in which I form my questions disrespectful, I am sorry; but I do not see the way in which I have run my project to be in any way deserving of such harsh comments.

Anyway, here is the plan;
I will be using only approximately 30J per stage, which will allow me to use the IGBT's I have bought (

) without exceeding the specifications.
The charging circuit is done, it currently only goes to 310V, but this is mostly just a problem with the input voltage, which is easily fixable later on, if it is even necessary.
The first coil is done, I have 'sparked' it, and the final velocity of my testing projectile (20g) is promising.
The other parts of the setup relevant to my project are also ready to go.
Using the circuit found in the datasheet, which I assume is good to go, the gate will be quickly driven back to zero, after the single pwm maximum brings the gate high, right?
Once the circuit is working for the first stage, I will finish the other two stages, these will be easy as the requirements for them will be even less that for the first stage.
I will then mill a 20g projectile.

I hope you all understand that the only reason I am asking you all for help is because I lack the experience and knowledge in control systems, which is partly why I am building it using these elements, as it will allow me to do all the control systems myself in future.

Thank you all for your help.

DerAlbi

Wed Feb 06 2013, 03:37PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

:-)
I am not someone who forgot the difficulties of the learning process after something is learned. I know its hard to read a Datasheet, and you have to read many to learn how and what they tell you. But this is the problem: asking for the info will not give you the experience. Its ok if you are unsure after reading to ask some specific stuff or what terms like "as short as possible" in the datasheet means - this is what caused nightmares for me^^

For all the switching stuff: lets make something where you can actually see how stuff works - this seems to be one of you biggest problems: you dont keep the bigger picture in mind.

It may require some sheets of paper but:
1) Draw a very very abstract schematic including ALL the parts of your design. (Arduino, driver, IGBT, coil, capacitor, dampener..)
2) Now think about what the different blocks actually are.
2.1) you know your arduino gives you a signal (0-5V)
2.2) follow the signal path through your block diagram and replace all the necessary blocks by a real schematic of the block - taken from datasheet or intuition. Stuff like bypasscaps for the driver can be omitted by now.
3) You just developed a complete schematic with all the interesting components for switching!
3.1) Now draw the expected voltage waveform like _/\_ at the signal lines with a voltage description (e.g. peakvoltage) and risetimes to the edges. The datashets will help you there - there are even diagramms that show you the switching waveform

4) You now have a complete impression of your signal path! So the basics and the system should become more understandable.
5) Now the tricky part:
Take your schematic and an other pen color and draw some parasitics in the signal path! You dont know which one matters? Thats ok -> just follow some rules: 5.1) a low voltage signal that transports information via voltage will have a capacitive dominated degeneration (like the gate voltage rises clowly due to RC-lowpass effect)
5.2) Paths with high current will suffer from resistance (The ESR of the cap.. or coil)
5.3) path with high switching frequencies will suffer from inductive parasitics. The higer the current the smaller is the inductance that needs to be taken into account. (e.g. the wire from your collector to the coil will have an inductance that produce a voltagespike when switched of and this can destroy your IGBT)
Just let your imagination guide you - more parasitics hurt much less than ignored parasitics in the end^^

5.4) Add some values to the parasitics

You cant estimate them? Right. No one can^^ You can google for some coarse values.. that would be ok. Im completely unsure: just write down what caused the parasitiv effect (wire length for inductances, Resistors or the cap it self for parasitic caps...)

5.4) You now have a feeling how your schematic change due to the physical dimensions of your circuit and what the dominating effects are and how you could influence them.

6) Put the schematics back into the individual blocks from 1)

Why? Because you ave the understanding whats inside the blocks, you know the input and output on every block - there is no reason why you should work with the whole schematic again

7) Simulations time!!
Use LtSpice to simulate your circuit and oberserve what the effects of the parasitics do to the signal

7.1) e.g. the IGBT can be aproximated by paralleling 4 Mosfets.. doest matter here. Interesting is the Collector voltage spiking and stuff like this.
Yes you cant simulate you driver circuit completely due to lack of models - doenst matter. Relie on the datasheet - they wont tell you something wrong as long as you are close to the example application - which in most cases will be excact your application, because these ICs are so specialized for a certain task that you cant use them differently. But you know the signal rise time from the datasheets and this value is a good parameter for a pulse voltage source rise time in the simulation...

Some tips:
DONT stop doing things because you are not sure. If you dont make mistakes we have nothing to correct - and you have nothing to learn from. Its just paper now - later "experiencegain" is more expensive

DONT stop reading the datasheet because you are not sure if its interesting or you are unsure if you use the chip in the end. Working with a datasheet of a mosfetdriver and getting used to it will help you very quickly to determine the usability of an other DriverIC, because the datasheets all are some kind of equal for the same application.

You know what? Please make pictures of your signal-schematic development

I want to see some wrong estimated parasitics!

But i DONT want to (not)see any forgotten stuff

After all this you watch at your drawing, watch at your signal waveform you expect, and then you ask yourself: WHY THE HELL did i write something about the PWM-Pin^^

Yes, its work. Sry^^

Jack A

Wed Feb 06 2013, 08:36PM

Registered Member #2975 Joined: Wed Jul 07 2010, 12:19AM
Location:
Posts: 28

Thanks for the advice!

DerAlbi

Thu Feb 14 2013, 02:12AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Some Progress?

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