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Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Thanks! To answer your question, the coil is an inductor, and inductors store energy. When you switch off the current through inductor, the stored energy generates a voltage that tries to keep the current flowing. This generated voltage can reach thousands of volts and destroy the transistor used to switch the coil. The solution is to give the energy somewhere else to go. This is done by placing a diode from transistor collector to coil + (or anti-parallel with the coil). The inductor voltage drives current through the diode and coil until the stored inductor energy is all dissipated as heat.
However, current remaining in the coil after the projectile reaches the half-way point slows the projectile down, so the current decay has to happen very fast to avoid this "suck-back" effect. Placing multiple diodes in series allows the inductor voltage to rise higher (due to stack up of diode threshold voltage) before the diodes conduct to shunt the inductor energy. In my case, using 4 diodes reduces the commutation time from 0.74ms with one diode to 0.2ms with 4.
A much better method is to recover the stored inductor energy rather than wasting it as heat. This can be done with a half-bridge or with the two-coil design Yan has. Neither works for my battery-powered gun though, so I go with the series doiodes (as much as this upsets Yan).
About the IGBTs, although the batteries are only 50V, the aformentioned voltage spike reaches about 600V before the diodes conduct, so I need IGBTs for their high voltage ability.
I'm surprised to hear about 600V spike for the shunted coil - how did you determined that without an oscilloscope and how the hell is it possible at all?!
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
DerAlbi wrote ...
..and what about >>active clamping<< at the maximum voltage of a _Mosfet_..?
You're right, I hadn't really though of that (and I already had the IGBTs laying around). Future designers should probably look into this, since the MOSFETs will have lower power loss, and being smaller they will also allow for more stages to be fit into the gun, both effects will improve efficiency. But since my design is done and the construction is 85% complete, I think I'll just move along if that's ok with you
Yandersen wrote ...
I'm surprised to hear about 600V spike for the shunted coil - how did you determined that without an oscilloscope and how the hell is it possible at all?!
LTSpice simulation. Also, inductor equation is E=L*di/dt. My coil is 99uH, and current goes from 300 to zero amps. Left by themselves, my IGBTs turn off in 8us. Maths: E=99u*300/8u=3.7kV possible if you don't slow it! This is also how boost converters work.
P.S. about the scope, I've kind of decided to get a USB scope so I can get a true measurement of efficiency by the time this thing is done. But that won't be for a couple months. If anyone has a recommendation on a good brand or model, please let me know.
Registered Member #9349
Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102
Thanks for the answers.
I tried to google "active clamping" but I don't get how it works, and how this could be applied to a mosfet powered coilgun. I find datasheets about that but I don't understand the principle behind that. How would it looks like on a simple circuit with a battery, a coil and a mosfet ?
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Active clamping is: put a TVS diode between Gate and Drain. If the drain voltage rises too much, the Gate voltage rises too (due to the TVS-Diode) This enables the Mosfet to "switch on" and conduct as much current as is needed to hold the voltage below a certain level. In this way any parasitic inductance can be "discarged" safely.
@Saz: I really dont understand where your 600V come from your descruiption. If your coil is "reverse protected" by a diode any drain voltage about VCC would be supressed immediately :-/ The only way there can be a voltagespike is if there is a huge Drain/Collector-Inductance or/and an Inductance in seriens to the diode. (e.g. long diode leads) Of course these are always there - however usually they are verry small so they dont store much engergy - so a avalance rated Mosfet would easily do the job. However i understand, if you have the parts, its better to use them. Dont worry about efficiency that much... i am fine as long it shoooots! - i wouldnt love a 50% eff. if the projectile just falls out of the barrel^^
I would really love to see your Spice simulation with your estimated parasitic inductors.
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Sure thing. The diode gives the current somewhere to go but the diode's threshold voltage still allows a voltage above Vcc to build at the transistor collector, especially when there are many diodes in series.
My simulation doesn't account for parasitics, since I have no good way to estimate them. Also, the projectile flying through the coil will alter the coil waveform significantly (increasing inductance and reducing current). This simulation was used to give me an idea of about how many diodes i could put in series before the collector voltage rises too high, and the max commutation current my diodes have to pass. The plots are for a 5ms forward pulse.
We have now beat the record for longest coilgun thread with slowest progress. I'm going to get back to work now!
Registered Member #9349
Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102
DerAlbi wrote ...
Active clamping is: put a TVS diode between Gate and Drain. If the drain voltage rises too much, the Gate voltage rises too (due to the TVS-Diode) This enables the Mosfet to "switch on" and conduct as much current as is needed to hold the voltage below a certain level. In this way any parasitic inductance can be "discarged" safely.
Active clamping is similar to put an anti parallel diode across the coil ? I don't understand how this method could hasten the current drop in the coil, because, if you hold low the mosfet gate after firing the coil, a voltage spike will build up, thus the TVS diode make the voltage rise again on the mosfet gate, and it starts to conduct angain ? it's weird. Is it a way to limit the voltage spike on mosfet drain by slowly turning off the coil current ? I thought that slowly turning off the current in the coil is exactly what you do not want to have in a coilgun, because, as the projectile get to the center of the coil, you want to de-energize it quickly to prevent sucking-back.
I have found that when looking for "active clamping"
On page 213, it shows how this is coming together, and multiple variants of active clamping,. This schematic is made for IGBT, but will it works on mosfet without any modification ?
So the way I understand it : the upper (active clamping) TVS voltage rating should be a little below the mosfet Drain to source max voltage, and the second TVS (between gate and source) voltage rating should be little lower than the mosfet gate to source max voltage. Is that right ?
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Thats right, but active clamping is in no way a method to fasten anything (if used that way it probably overheasts the junction due to high powerdisipation outside the SOA)... its only a protection mechanism for high voltage _spikes_ from parasitic inductances.
Saz: i see where your idea of a high voltage spike comes from This is however a simulation and like any simulation its parameters must be right Your diodes use the standard model that really sucks This is nothing nearby anything a real diode would do - i think the junction resistance in this model is far to high Just right click and select a MBRS360 or so.. this is a real diode (not made for the current) however you get a much closer result to reality as this crap suposes. this also will slow down the current decay... sorry to say that.
Next thing: any switching transients _can_ compromise this setup.. i recommend to put a 50k-500k resistor in parallel to the diodes (just for simulation) Spice doesnt like transient voltages with diodes in series... the diodes parasitic L and C can oscilate in ps-time steps -.- that drives one crazy
i would recommend to put 20nH + some ESR (20mOhm) in series with the diodes and in series to the collector. This would simulate the parasitic effects
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
The diodes in my simulation are 1N914, not ideal diodes. With 4 ideal diodes, commutation takes 1.91ms which is actually much longer than the 0.2ms with the 1N914.
Just out of curiosity, where did you get your estimates from? I've tried this simulation with parasitics before and found that the results varied wildly with small changes in my parasitics, so without a good way to guess at them, I just left them out and called it a good guess, which is really all it needs to be. None the less, I tried re-simulating with your parasitics. Here's what I got (both cases 5ms pulse and 1N914 diodes):
So not really all that different. I'm not too surprised since L and R of the parasitics are small compared to the coil's. What do you think? Picture attached in case I got what you said wrong.
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