Nanosecond magnetic flux change - Page 1
Forum section: electronics / general

author: ricko_uk
author_rank: Super Contributor
timestamp: October 20, 2019, 06:50:23 pm
content: Hi,I am trying to create a nanosecond-rise spike across an inductor (as shown in the picture below - from a commercial product). All I care about is the generated magnetic field and that:- it has at least one spike in the 1 to 20 nanosecond rise (or fall) time- in a coil of inductance anywhere between 100uH and 100mH (I can pick the value but it has to be in that range)- there is no ringing (but a nice clean waveform like in the picture)Because of the large inductance I think the only way is to connect the coil between the positive rail and ground through a N-mosfet like when you drive a solenoid, but without the classic freewheeling diode. And then rely on the fast voltage rise when the mosfet switches off and the magnetic field then collapses. Mosfet voltage rating clearly chosen appropriately.Two questions:1) Does anybody have any other circuit suggestion? Any circuit works for me, all I care is to have an extremely fast magnetic field change in a 100uH to 100mH coil.2) As you can see in the picture, in that product they manage to have a nice smooth decay into the 6.2V. With my solution that I described above I get a lot of ringing even with short large traces, close components, SMD components etc (i.e. trying to minimise the parasitic inductance). Any suggestions?Many thanksRico

author: amspire
author_rank: Super Contributor
timestamp: October 20, 2019, 11:51:43 pm
content: Your picture is missing, but is the goal to make a rapidly changing flux or the voltage spike?If the goal is to use the magnetic flux spike somehow, we need to know what physically you need.If you just look at the basic physics, if you have a 100uH inductor and you want to change the current by 1A in 1ns, then you need a voltage of 100uH/1ns = 100,000V.If you use a 100mH inductor, you need 100 MV.If you are talking about uA of current and more reasonable voltages, then the level of flux is not going to be very high. Do you need an inductor solution at all? There is plenty of discussion in this forum of avalanche transistor pulse generators with sub-nanosecond rise times, and you can vary the width of the pulse by varying the length of a coax cable stub.

author: dexters_lab
author_rank: Unknown
timestamp: October 21, 2019, 09:38:28 am
content: Quote from: ricko_uk on October 20, 2019, 06:50:23 pmalthough i don't think this is right what you want it might be of some use...look into TMS (trans-cranial magnetic stimulation) medical equipment as this deals with generating hi peak magnetic fields from an inductor using high voltage capacitors, many of the documents do mostly concentrate on the clinical side but they often also include a lot of theory about the electrical aspects which may apply to your setupA long while ago i looked at a Magstim 200 machine, a lot of it was documented here somewhere, search on the forum for 'magstim' and you should find it

author: ejeffrey
author_rank: Super Contributor
timestamp: October 21, 2019, 04:13:45 pm
content: It is weird to say you only care about the magnetic field created and also have a spec for the inductance and also have that spec be variable over 3 orders of magnitude.It is also weird to not have any amplitude spec. For instance, a CMOS logic gate followed by a 5 kOhm resistor driving a 100 uH coil will give a rise time of approximately 20 ns but the peak current will only be a fraction of a milliamp -- I assume not what you want but technically meets your listed requirements.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 21, 2019, 06:07:49 pm
content: What are you really doing? This is just dripping withX-Y problem. If you let us know about the underlying problem, we likely can give much better answers.Tim

author: ricko_uk
author_rank: Super Contributor
timestamp: November 07, 2019, 04:54:40 pm
content: Thank you all for your replies. I will address each one separately now. Apologies it took me this long to get back but had to take time off then it took a while to get hold of the system I used for previous experiments and which I used as reference.What I'm trying to do build is a system exactly like a PEMF/TMS system whose most important feature is the magnetic flux spiking in 10ns or less to a value (during the spike) of up to 1,000 gauss and a minimum of 500 gauss. After the spike the coil to have no energy stored. The frequencies of the spikes up to 15Hz. The 1,000 Gauss spike in less than 10ns is the crucial parameter of the entire system.The only other constraint is the coil dimensions. The ID to be at least 20mm (OD not important) and coil length ideally at least 30mm.The options I can think of are either a capacitor damping its charge into the coil through an SCR or a coil connected to ground through a mosfet/BJT which is suddenly turned off.How/where can I find full calculations so that I can:- set as input parameters the desired magnetic flux density along the centre axis 1) inside coil or 2) at a distance in front of it- get as output the voltages and coil inductance so I can have a custom coil madePlease let me know if I missed any other information that might be useful to any of you and/or that avoid the X-Y problem.As a reference from previous experiments we have a coil from a off the shelf system with- with iron core inserted: 95mH (measured @ 10KHz - we don't have equipment to measure at higher frequencies)- with iron core removed: 40mH (in my initial post I put 100uH because on a different smaller system that was the smallest value but 40mH would do if that relaxes the parameters)- 25R DCRIf of interest, attached are the original scope trace across the power coil used to generate the magnetic field (not sure why it did not attach the first time) and also the schematic of the power stage now that I reverse engineered the PCB traces.

author: ricko_uk
author_rank: Super Contributor
timestamp: November 07, 2019, 05:14:50 pm
content: Thank you Amspire,please see the general update/reply I just posted. Yes we need the inductor because the magnetic flux is what we need (it is a PEMF application).The 1,000 Gauss spike in less than 10ns is the crucial parameter of the entire system. The voltages are not a constraint, within reason.As I just mentioned in that general reply we have that inductor as reference because we used it before for PEMF applications. So it sounds like we need to reduce the inductance by quite a lot in order to have resonable voltages (in the range of up to the mains voltage - or perhaps even doubled through a diode doltage doubler).How/where can I find full calculations so that I can:- set as input parameters the desired magnetic flux density along the centre axis 1) inside coil or 2) at a distance in front of it- get as output the voltages and coil inductance so I can have a custom coil madeThank you again for your help!!

author: ricko_uk
author_rank: Super Contributor
timestamp: November 07, 2019, 05:22:53 pm
content: Thank you Ejeffrey,please see update above I just posted. Please let me know if I left anything out.

author: ricko_uk
author_rank: Super Contributor
timestamp: November 07, 2019, 05:28:23 pm
content: Thank you T3sl4co1l,I posted the specs and details of what I am trying to build (effectively a PEMF/TMS machine). Please let me know if I left out any important information.Any infos or pointers about schematics and also calculations are much appreciated.Many thanksRiccardo

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 07, 2019, 05:42:49 pm
content: Ah, that makes more sense, it's a typo then. I see as a typical example,https://www.frontiersin.org/files/Articles/460932/fnins-13-00703-HTML/image_m/fnins-13-00703-g001.jpgshows ~100us pulses, still pretty good power but not at all impossible.The circuit shown, with the inductors specified, will take 1-2 milliseconds to charge up the inductor, and a few tens of ms to discharge. The flux density waveform will rise slowly, flatten off, then fall slowly (B proportional to inductor current). I think I can quite confidently say you won't get any biologically noticeable EMF around such a coil.Tim

author: Marco
author_rank: Super Contributor
timestamp: November 07, 2019, 10:02:57 pm
content: Just don't pulse it that fast but say you did ... same difference.A quick search for a non magnetic core inductor on digikey with ~100 MHz self resonance frequency suggests to me you're looking more at 5 uH than 50 mH. Calculator says it needs about 30A ... that's surprisingly low, well it surprised me because I forgot how small Gauss is relative to Tesla.Still takes around 30 kV to get it up there in 5 ns.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: November 07, 2019, 11:11:30 pm
content: So is it really 10 ns? that's 10 * 10^-9 seconds.

author: ricko_uk
author_rank: Super Contributor
timestamp: November 13, 2019, 12:41:42 am
content: Just a question...What would happen if you have a coil of lets say 100uH and after being energised (through a mosfet/IGBT to ground BUT wihtout the freewheeling diode) I suddenly disconnect it from ground? In other words to create an inductive kick situation (but without having the freewheeling diode)? The voltage would raise until the magnetic field collapses right? Wouldn't that magnetic field collapsing mean that the magnetic field changes "instantaneously"?If that is correct, what dictates the speed of the magnetic field collapse? And how can it be practically tested / measured?Ahbushnell, yes the target is 10ns.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: November 13, 2019, 02:55:21 am
content: Lets do some math. Assume a field of 1000 gauss (.1 tesla) and assume the field is only in the volume. Of course there will be more but this will give us a ball park. the coil is 30 mm long and the ID is 20 mm. The energy in the field is:Vol*B^2/uo=.075 joules. If we put the field in 10 ns the peak power is.075/10 ns=7.5 MW. But it's a short time.Now how do we make the field. If we assume the field is only in the center line of the coil and we use ampere's law, integral H * dl=N*I N is the turns and I the current. H is B/uo.For one turn the current is 2.387 kA.the energy in an inductor is 1/2 * L * I^2. So we know the energy and the current for a one turn coil which gives an inductance of 26.3 nH.We can calculate the voltage we need to apply to the one turn coil is Lcoil * I/10 ns which is 6.3 kV.You could change the number of turns. Say we do 10 turns. The current goes to 238 amps and the voltage goes to 63 kV.This an order of magnitude calculations. It's actually optimistic.

author: duak
author_rank: Super Contributor
timestamp: November 13, 2019, 03:39:00 am
content: A collapsing field is simply a magnetic field where the current is decreasing quickly. ie., the magnetic field strength is a function of the coil current so when the MOSFET opens, the current should fall. How fast it falls depends on the resistance it has to discharge into t = f(L/R). A clamp diode across the coil is almost a dead short so the current fall time will be long. Ideally, for the shortest time, you'll want to discharge into an open circuit but this also means the highest voltage.In the real world it's hard to find a switch that will not break down due to the voltage generated by the coil as it tries to dump the stored energy. Mechanical switches break down and arc at a low voltage when opening. As long as the MOSFET is within its Safe Operating Area and its drain to source voltage is kept within its breakdown voltage rating it can work quite well. There might be other devices that have fast opening times with high standoff voltages but I can't think of any right now.In light of what ahbushnell has pointed out, there is a lot of current and voltage to be dealt with in a short period of time. It might be possible but it's not easy or cheap. Speed costs money - how fast can you afford to go?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 13, 2019, 11:42:48 am
content: Mind, words are crude. There is no distinction between "building" and "collapsing" a magnetic field, it is always a linear, continuous and symmetrical system.In an ideal inductor, the current is proportional to flux, and flux is simply the time integral of voltage.If voltage spends some time negative, then flux is moved negative. How much depends on how much voltage was applied, and for what time. If voltage goes positive, flux moves positive. How much, same thing.If you switch voltage into an inductor, with a transistor, you've delivered some flux, at a rate determined by the supply voltage. If the transistor switches off and there is no explicit load for the inductor's energy to flow into, then it will simply flow into circuit capacitances and anything the inductor couples too (including EM radiation). Eventually that energy will be dissipated as heat, and everything settles down again.So:1. There is nothing special about "collapse", it is merely the word chosen to relate to switching off a charged inductor. "Flyback" or "freewheeling" are equivalent.2. The rate of change is controlled by the equivalent circuit, so,3. We can calculate behavior based on the RLC equivalent circuit of the system.A note, the RLC behavior doesn't end with the inductor having some (low frequency equivalent) inductance. Real inductors are made from wire, which has finite length; EM waves travel along that wire, and it is the sum action of waves over time that gives us the low frequency equivalent inductance. At very high frequencies, we can't ignore the action of individual waves, and we may find we run into problems if we neglect this.Namely, if the wire length of the inductor is longer than a fraction of the waveform of interest (i.e., an electrical length of a few ns in this case), we need to consider wave action.So, we should use an inductor made with, at most, a few meters of wire, otherwise we'll end up with more complicated behavior (such as, an external field that is uneven and which doesn't couple to all the electrical energy we thought we were using). In a 1-inch-scale inductor, that limits the inductance to a maximum of a few uH, which is fine. (The minimum is around 20nH, for a one-turn hoop.)Now that we have an approximate range for L, we can consider C and R. R should be small enough that we can consider the LC circuit to ring down on its own, and we can correct that slightly as needed.C mostly comes from the transistor (but also from the wire length, because again, wave effects). A typical power transistor might have 100pF output capacitance, so a 1uH coil will resonate at 16MHz, or a 1/4-wave time (which is from peak flux to the first zero crossing) of 15.7ns.So you will have a hard time doing that with transistors. A smaller inductance can use lower voltages, but the transistors are "thicker" (more capacitance).Pulsed power systems are better modeled as RLC networks or wave systems. You should consider a spark-discharge system: a small capacitor charged to a high voltage, switched with a spark gap.Tl;dr get an ESD gun. Not even kidding, serious. That seems to be what you are describing, in a very roundabout (or unaware?) manner. I still have no freaking clue where 10ns comes from or what your goal is, and I am quite confident it won't have any bearing on neurological processes (which respond in the 100s us at best, AFAIK). But that's the closest thing to what you seem to be after. You won't be getting amazing flux densities out of it, not without calling upSandiaat least. On the upside, it shouldn't be too destructive to play with (ESD is annoying to touch, but rarely fatal), so whatever it is you are doing, you probably won't kill yourself in the process.Tim

author: ricko_uk
author_rank: Super Contributor
timestamp: November 13, 2019, 04:36:29 pm
content: Thank you all for the prompt and very detailed answers!!

author: Marco
author_rank: Super Contributor
timestamp: November 13, 2019, 06:36:01 pm
content: Quote from: ahbushnell on November 13, 2019, 02:55:21 amInductance goes n^2, also smaller radius coils improve the math a bit if all you're interested is B ... of course the field strength falls off even faster away from the coil (or core).

author: duak
author_rank: Super Contributor
timestamp: November 14, 2019, 04:30:45 am
content: There might be another way to generate high current narrow pulses - Look for "magnetic pulse compression". I remember seeing various papers from CERN and from SLAC. I don't know if this technique can get down to ns rise and fall times but it can certainly generate pulses on the order of dozens of ns wide.The attached diagram shows a circuit that contains a cascade of inductors and capacitors. The inductors have ferromagnetic cores and, as such, can support a certain current before the incremental inductance drops because the core saturates. An inductor's current is limited by the inductance until the core saturates after which the inductance drops like a rock. The current then spikes upwards easily 1/3 as wide and three times as high. A pulse generated on the input is transferred from stage to stage narrowing and increasing in amplitude as it proceeds - see the dashed inductor currents Il1 to Il3.I accidently encountered this phenomonon when helping find out why an assembly would fail during ESD testing. It turned out some ferromagnetic cores added to reduce EMI susceptibility were saturating and then dumping an enhanced and sharpened pulse into the assembly causing it to fail.

author: ricko_uk
author_rank: Super Contributor
timestamp: November 15, 2019, 01:01:39 am
content: Thank you the additional infos and answers!!