3kV 2kA transient pulse switching problem - Page 1
Forum section: electronics / projects

author: matemathieu
author_rank: Contributor
timestamp: August 27, 2017, 10:23:23 am
content: Hi all,I am trying to put the best chance of success in implementation of a solution to a new system. I realized it is not easy to find a proper solution, but I don't want to miss an better option.The principle is simple but the implementation is to be defined. The system under test is provided with 3kV to an array of 4 parallel caps that are 30nF for a total of 120nF. The energy stored is hence 0.54J on each loading. The loading/deloading cycle is done every second and should be increased up to 3 times a second.Depending on the technology used to make the caps, the transient current is very high can peak up to a thousand of ampere! So how to I switch this massive quantity of electrons?After some initial research I found out reed relay would be a good option but there are all kind of issues with latching on and even melting the contacts. Then I wanted to evaluate what silicon based chips would give, and IGBTlike this oneseems very good at such high power super fast switching problem. But I wonder if it is a good choice and if there isn't a better option.Sincerely, any help is welcome.

author: mk_
author_rank: Regular Contributor
timestamp: August 27, 2017, 11:21:05 am
content: Quote from: matemathieu on August 27, 2017, 10:23:23 amI would use a flashtube with external trigger. No problems as long as your cables and connectors can handle the pulse-current.

author: daqq
author_rank: Super Contributor
timestamp: August 27, 2017, 02:25:40 pm
content: Take a look at Behlke - they do crazy high voltage - high current - high speed devices. Also, they are quite expensive.From a quick search, these fellows:http://www.behlke.com/separations/separation_c2.htmcome close. That said, there might be better ones.

author: oldway
author_rank: Super Contributor
timestamp: August 27, 2017, 02:27:46 pm
content: If you choose a semi-conductor (a thyristor for example) to discharge the capacitors, you will have to limit the peak current and the di/dt within the limit acceptable by the device you will use.

author: JohnG
author_rank: Frequent Contributor
timestamp: August 27, 2017, 03:45:50 pm
content: First off, I don't know of any relay that will not evaporate with 1000A. They are typically rated for 1-2A.More importantly, what does the system do? Are you testing the caps, or something else? Does it run indefinitely, or will be you be done after a few hundred or thousand pulses?Do you need a high di/dt, or is just a byproduct? You could consider a resonant charging system of some sort, i.e. charge it through an inductor and diode. It will ring up to about 2x the input voltage and the diode will prevent discharging. This will give you repeatable pulses with pretty good control over charging time.John

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 27, 2017, 08:38:36 pm
content: If you only need it done three times per second, use an IGBT or relay with a series resistor of R = 1.2 megohm. This will safely limit the current to just a few mA.If you have another specification regarding the time, duration or rate of these pulses, please do tell!Tim

author: matemathieu
author_rank: Contributor
timestamp: August 28, 2017, 07:14:33 pm
content: Thank you all for these awesome answers!Quote from: T3sl4co1l on August 27, 2017, 08:38:36 pmMy problem is that I need to carry this amount of transient current in my setup. I do not want to limit it.Quote from: JohnG on August 27, 2017, 03:45:50 pmYes I need to provide this amount of electrons to the setup. The system will run a thousand times then stop for a while then run again.Resonant charging system is a good idea, but I would need more details on that. If you may...Quote from: daqq on August 27, 2017, 02:25:40 pmThese specs look amazing! I have to look deeper but it might be the solution. Thanks!Quote from: mk_ on August 27, 2017, 11:21:05 amFlastubes might limit transient current going through, and still you have to trigger it, so the problem is still remaining.Let's keep it going, if anyone wants to step in

author: SeanB
author_rank: Super Contributor
timestamp: August 28, 2017, 07:37:16 pm
content: Flash tube is essentially a gas switch, it will keep a constant drop of around 60V when on, but can pass instantaneous currents in the kiloamp region. Want bigger current just use a bigger tube, or accept the small tube will have a shorter life. Trigger transformer for a flash tube is both common, reliable and simple to drive with just a RC circuit and a SCR to provide the sharp trigger pulse for it, along with a 300V or so power supply tap.

author: Marco
author_rank: Super Contributor
timestamp: August 28, 2017, 08:43:32 pm
content: Quote from: matemathieu on August 27, 2017, 10:23:23 amThe word commutation makes bugger all sense in this context, I think you just mean switching.With what kind of series resistance do you want to charge/discharge these capacitors?

author: oldway
author_rank: Super Contributor
timestamp: August 28, 2017, 08:52:14 pm
content: SCR able to switch up to 4.500V 43.000A peak......

author: Marco
author_rank: Super Contributor
timestamp: August 29, 2017, 12:11:42 pm
content: Thyristors won't ramp to 2kA very quickly though. Charging the capacitance will happen in the 100ns range at those currents. So switching time should be in the 10ns range really.

author: dmills
author_rank: Super Contributor
timestamp: August 29, 2017, 12:36:32 pm
content: Triggered spark gap (Possibly in Hydrogen), it will not quench particularly quickly but that does not sound like it matters, or a gas thyratron tube, very standard for this sort of thing in pulsed laser applications.Do respect the thyratron datasheets for things like warm up time and average power, but a mere few thousand amps at a few thousand volts of hold off is very much routine for those things.I would note that 2000A @ 3kV implies an impedance of about 1.5 ohms, which is going to make for interesting mechanical geometry to get the transmission line impedance that low.Regards, Dan.

author: oldway
author_rank: Super Contributor
timestamp: August 29, 2017, 12:39:18 pm
content: Indeed, di/dt of this scr is limited to 300A/µs....You must have an inductance to limit di/dt.Without knowing data of the circuit (ESR of capacitors, resistance of wiring, equivalente inductance of capacitors, inductance of the wiring), it is impossible to answer to the question because as described, the circuit has no resistance and no inductance.The peak current and the di/dt are in theorie infinites.....no device can switch infinite current with infinite di/dt....the problem has simply no solution at all if you don't add an inductance to limit di/dt and a low resistance to limit peak current.We need the data of this inductance and of this resistance.

author: Ian.M
author_rank: Super Contributor
timestamp: August 29, 2017, 02:33:40 pm
content: Another +1 for triggered spark gaps. However, why not do it cheap & dirty in air? Use a surface gap spark plug fitted to the cathode as the ignitor electrode in each and a CDI ignition module + coil to trigger them. The 'floating' gap feeding the capacitor will need its trigger coil capacitively coupled to stand-off the 3KV DC level. Put a 5KV gap across a coupling capacitor rated at 10KV so the coil discharge voltage doesn't break down the cap. The discharge gap has its cathode grounded so there's no problem there except protecting the CDI module from *massive* ground bounce.Build it with enough electrode surface area and the caps will puke their guts due to excessive dI/dt before the gap gets significantly damaged. Ear protection is essential - it will be *LOUD*!Done right a triggered spark gap can handle Megaamps:Reliable spark gap for capacitor bank switching - US DTIC 1963

author: oldway
author_rank: Super Contributor
timestamp: August 29, 2017, 07:16:03 pm
content: QuoteThis happened because you choosed too little contacts.Look for a big industrial contactor, it will work because the stored energy is not that big.

author: PartialDischarge
author_rank: Super Contributor
timestamp: August 29, 2017, 07:39:27 pm
content: Thyristor, thyristor, thyristor, unless the short circuit impedance is somewhat limited in which case go for igbts.The one suggested by oldway will do perfectly and you will not by any means exceed the stated 300V/us with those 4 caps.However you'll need to learn how to clamp them, trigger them... and then there is the snubber...

author: oldway
author_rank: Super Contributor
timestamp: August 30, 2017, 07:16:20 am
content: Quote from: MasterTech on August 29, 2017, 07:39:27 pmMy concern is about di/dt, not dv/dt....300A/µs...4µs to reach 1200A....you must have an inductance in the circuit to limit this di/dt

author: PartialDischarge
author_rank: Super Contributor
timestamp: August 30, 2017, 10:31:24 am
content: My bad, meant to write A/us. That kind of current can not be reached with those smallish caps. Do the math. Assuming a perfect short-circuit (a huge assumption) and perfect caps (a more than huge assumption), a 120nF 3000V cap will last 1us discharged at constant 360A. That already tells you something about what's going on here, now throw in all the component imperfections and no way.I've discharged low inductance kV capacitors with capacitances in the thousands of uF (compared to measly nF) with thyristors smaller that the one you linked. This was for internal arc flash testing (meaning very low short circuit impedance) and nope, A/us is not the limiting factor.

author: Marco
author_rank: Super Contributor
timestamp: August 30, 2017, 10:54:28 am
content: 100nf is doable with a wound metallized polyester capacitor with dimensions in the range of 10cm, if the foils are fully connected to the leads I don't see how it will have much inductance.

author: Zero999
author_rank: Super Contributor
timestamp: August 30, 2017, 11:13:24 am
content: Quote from: Ian.M on August 29, 2017, 02:33:40 pmAlso watch out for the intense visible and invisible (IR and UV) radiation which can cause severe eye damage, skin burns and cancer. The lower frequency end of the emission spectrum (microwave and below) can also get you in trouble with the law.

author: Ian.M
author_rank: Super Contributor
timestamp: August 30, 2017, 11:49:27 am
content: Quote from: Hero999 on August 30, 2017, 11:13:24 amAlso avoid breathing the traces of condensed metal, metal oxide and nitride vapours ablated from the electrodes, or the ozone or nitrogen oxides the arc produces, which can cause lung damage. DO NOT make the electrodes from alloys containing or coated with Lead, Cadmium, Beryllium, or other metals known for theiir toxicity.Hopefully that's Californicated it sufficiently for the muppets.

author: matemathieu
author_rank: Contributor
timestamp: September 01, 2017, 09:54:59 am
content: Quote from: Marco on August 28, 2017, 08:43:32 pmThank you, I corrected the wording.Quote from: dmills on August 29, 2017, 12:36:32 pmI was completely ignorant of this solution, thanks a lot!I've also learned something surprising today from Wikipedia : The term "thyristor" was derived from a combination of "thyratron" and "transistor"and it makes complete sense!Quote from: oldway on August 29, 2017, 12:39:18 pmYes I should have specified a circuit inductance / impedance. It is basically connecting wires to the different part.A 2 ohm resistor is used close to the switching part. I can't tell much more at the moment.Quote from: Ian.M on August 30, 2017, 11:49:27 amI'll be carefulIt also might generate soft x-ray too.Again, thanks to all the great comments I am not putting here.A very interesting discussion, thank you all.