125kHz, high voltage, low esr capacitor for near-(B-)field RFID sender - Page 1
Forum section: electronics / general

author: fgrandel
author_rank: Contributor
timestamp: October 20, 2023, 10:42:30 am
content: Hi!I'm trying to build an amplifier for an OOK modulated near-(B-)field signal @ 125kHz generated via tank circuit but have difficulties to find an appropriate capacitor at a reasonable size/price. Can anyone recommend an adequate part? (Posted in this sub-forum due to similarity to typical power applications.)The design requirements are as follows:a) tank circuit oscillating at 125kb) min. current and resistance in the tank circuit for highest Q (i.e. minimal ohmic loss / heat dissipation, minimal signal distortion).c) For max. B field at min. current, the inductor should have max. inductance. Currently the circuit operates at ~1.5 Amps through ~800uH. 2 Amps would be better.d) a and c imply a capacitance of ~2nF with a max. voltage of ~1-1.5kV depending on max current.e) I'm not discharging the capacitor while the baseband is off (i.e. the tank does not oscillate). I use zero-crossing detection to disconnect the inductor from the capacitor at the precise moment when all energy has been dumped in the capacitor. This provably gives me very fast start/stop times plus very little energy dissipation and distortion due to baseband modulation.f) Not surprisingly I want to make the whole thing as small as possible plus keep BOM small and cost low.Now I've found that the critical component in this circuit is the capacitor, more specifically ESR at the target frequency. Static losses while baseband is off should not be too high but are not that important as the limiting factor for energy loss currently turns out to be timing and parasitic capacitance of the switch:- Ceramic caps are small at the required voltage but even the lowest ESR parts turned out to have much too high resistance at 125kHz. I tried to put lots of them in parallel but that is bulky and increases the risk of failure.- I found some specialized RF film capacitors with much lower resistance at the target frequency. But they are expensive, bulky and I still need to place several of them in parallel.- I looked at capacitors used in inductive heating but these are even bulkier and more expensive.Any ideas?

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 20, 2023, 01:22:33 pm
content: That doesn't sound right, typical MLCCs should work fine.For example, C3225C0G3A103J250AC (10nF, 1kV, 1210 size) has typical ESR of <4mohm around 125kH, giving a Q in the tens of thousands.Is this not enough?

author: TimFox
author_rank: Super Contributor
timestamp: October 20, 2023, 02:20:44 pm
content: Ceramic capacitors: stating only "ceramic" does not tell anything about ESR.The MLCC capacitor cited above by mtwieg has a "C0G" ceramic dielectric (also called NP0), which is very good for ESR and Q.Other common ceramics, such as X7R and Z5U are much worse for ESR, and have non-linear problems, as well.("MLCC" refers to the construction, multi-layer ceramic capacitor, that can use any of the common dielectric materials.)

author: nctnico
author_rank: Super Contributor
timestamp: October 20, 2023, 03:01:33 pm
content: What kind of distance does this system need to work at? Sounds like either you want to achieve read-out distances approaching 1 meter or your receiver circuit is seriously under performing. Either way, foil capacitors work well for this application.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 20, 2023, 03:43:50 pm
content: Is this a custom application, or standard 125kHz backscatter RFID stuff? Perhaps with extended range or something, hence the pressure for high Q?Terminating the transmission very suddenly, sounds like all manner of potential problems with ringdown, harmonic generation/emissions, wideband sensitivity --> noise degradation and interference/susceptibility, etc.The more canonical approach would likely be, set it up as a bridge, so that signal -- subtle changes in load (real) or reactive current -- can be extracted from the bulk DC current flow, or from slower-varying changes due to proximity (people moving around, or the transponder itself not being stationary, or the reader for that matter), preferably with some suitable line coding to keep the modulation bandwidth down, facilitating such separation. (I forget how much redundancy or coding is in the usual kind; I know it's pulse coded, and probably with lead-in pulses at the front and CRC or EC at the end, but beyond that I don't know.)Maybe it's not a problem at all, or there's enough signal processing going on to deal with all that (the noise and stuff), but suffice it to say there's good reason why it's typically done differently; a linear, continuous-time method with mixing and filtering to extract exactly the sidebands desired, maximizes dynamic range.Tim

author: nctnico
author_rank: Super Contributor
timestamp: October 20, 2023, 03:57:22 pm
content: Demodulation from measuring the current flow is not going to yield good results. In the end the tags will AC modulate the voltage across the tank circuit leading to a signal with a much better signal to noise ratio. The 125kHz carrier is far enough apart that you can suppress it enough with some simple analog filters and then do the rest of the demodulation in the digital domain.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 20, 2023, 04:09:07 pm
content: Normal RFID should not need that much power.C0G / NP0 capacitors can have pretty low loss and would be probably my favorite. Not all of them are the same loss wise, but the target high voltage already limits the choice.If nothing else helps air or vaccum capacitors would be lowest ESR, but bulky / expensive.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 20, 2023, 05:35:29 pm
content: Quote from: nctnico on October 20, 2023, 03:57:22 pmImpedance is impedance... current or voltage, doesn't matter. More specifically, take either/or depending on series or parallel equivalent tank.Quote from: Kleinstein on October 20, 2023, 04:09:07 pmI've seen plenty of signal just holding a card up to a loop and driving it with 110-140kHz or thereabouts with a function generator; it seems to be a pretty noncritical sort of thing. At least -- again, assuming average LF RFID thingys. Any higher Q then (beyond approx. 0) is simply to give better range, or maybe it's a power transfer application too.Tim

author: nctnico
author_rank: Super Contributor
timestamp: October 20, 2023, 05:43:33 pm
content: Quote from: T3sl4co1l on October 20, 2023, 05:35:29 pmYou'd say that but have you actually tried to build a 125kHz RFID reader or tried to do communication over wireless energy transfer (both technologies are pretty similar BTW)? I have and from my experience it is far easier to use an AM style detector on the tank circuit's voltage rather than reading the current. Keep in mind that for measuring the current you'll need some filtering + amplification next to a circuit which is throwing out a large magnetic field which induces the carrier frequency into every trace near it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 20, 2023, 06:16:23 pm
content: Sounds like you were using a CC drive then, which will read changes in impedance as voltage amplitude.I've done all this plenty of times yes, the impedance stuff that is, not RFID specifically. Mainly induction heating, so, doing it at scale, albeit not very precisely.Tim

author: nctnico
author_rank: Super Contributor
timestamp: October 20, 2023, 07:33:22 pm
content: 125kHz RFID readers are very simple devices. Typically the tank circuit driven from a half-bridge. The communication from a tag (or wireless power receiver) is done with a very small modulation depth. You can have several amps through the tank circuit while the current change due to the communication signal from the tag/receiver is in the uA or mA range. You can easely deal with 4 orders of magnitude of difference between the drive current and the current changes due to modulation.

author: f4eru
author_rank: Super Contributor
timestamp: October 21, 2023, 07:40:51 am
content: Do you have infos on the max allowed field for a 125kHz RFID reader, from regulatory, and people safety point of view ?Also, do you take any measures to not kill tags that are too close to the reader ?

author: nctnico
author_rank: Super Contributor
timestamp: October 21, 2023, 09:48:48 am
content: There are regulatory limits to 125kHz RFID systems where it comes to the field strength. In the NL the limit is 62dBuA at 10 meters (last time I checked).

author: fgrandel
author_rank: Contributor
timestamp: October 21, 2023, 12:27:24 pm
content: Very interesting discussion, thanks everyone for taking the time to answer. And above all thanks to the pointers to specific parts. Very much appreciated.To answer a few of your questions:- As some of you have guessed, it's a custom active tag application requiring additional range (~3m). We modulate the field on the sender side and demodulate it on the receiver side, the other way round as is common with impedance-modulating passive tags.- Yes, we do have a bridge topology on the sender side and that works nicely.- We've tried different modulation approaches and control-system topologies. The one we've now yields by far the best results in practice with our specific setup and requirements (trade-off heat emission/energy loss, SNR, PER, RSSI stability, etc.). From a theoretical viewpoint, "true" OOK is not as bad as it seems btw. I think it's just not very common in magnetic field modulation for practical reasons. For "true" OOK (in the theoretical sense), you need a phase-locked closed control loop with zero start/stop time (i.e. zero energy loss in the tank during off times). That's impossible with the typical RFID open loop bridge topologies that burn all energy in the tank for each baseband bit. They are terribly inefficient and noisy at the level of power and sensitivity that we need.- Yes we are aware of legislation and systematically ensure that we stay within regulatory limits relevant to our application area. And as you've guessed, that's a challenge.

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 21, 2023, 01:24:01 pm
content: I'm still curious why the capacitor presents a challenge. IMO that should be one of the most trivial components in this system, compared to the coil, bridge, demodulation, etc. Maybe you were hoping to find a suitable cap in a 0805 package?

author: fgrandel
author_rank: Contributor
timestamp: October 22, 2023, 12:02:28 pm
content: @mtwieg:> I'm still curious why the capacitor presents a challenge.It probably never did (had I known). The simple and honest answer is: I'm not an electronics but a software/maths guy, the rest is autodidactics, a lot of reading and experiment. You just closed a rather embarassing knowledge gap of mine. :-) Thanks for that.The topology I'm building is not my invention btw, it's coming from magnetic underwater communications - usually researched by the military for submarines as farfield doesn't work in that environment and nearfield is harder to eavesdrop on. The second field of application is in communication with medical devices through the skin where similar technology is being used due to its high resistance to interference and NLOS performance. Wireless energy transfer not so much - they'd rather keep the field energized all the time and communicate over impedance modulation as you know. But these are all not my applications. I'm interested in RSSI-based real-time localization. Google for "AmfiTrack" if you want to get the hang of it. I'm not related to them in any way, but they have nice videos demonstrating the active principle behind my project, too, which inspired me.And as we're at it: two more critical components I haven't found an ideal solution for, yet...1) Any recommendation for a high voltage (~1.5kV), very low source-to-drain capacitance and very low on-resistance MOSFET (or similar)? My circuit works great but now that the price/volume problem with the capacitor seems to be solved, this is the next limiting factor that could still be improved on the sender side (although optional, as the whole thing works fine already).The MOSFET is in the tank circuit and all current flows through it in "on" state. It is switched off at zero current to keep the energy trapped in the capacitor. At that moment it obviously takes over all the voltage which will rapidly charge it's parasitic capacitance through the inductance which represents lost energy inevitably transferred back to the coil. Not much as compared to overall energy in the circuit but enough to be relevant and measurable. That energy will have to be burnt through a freewheeling diode switched in parallel with the blocking MOSFET - otherwise it would result in high frequency oscillation at high amplitudes across the parasitic MOSFET capacitance (which dominates the tank circuit in "off" state). With precision timing this all works astonishingly well. But I'm sure there's an order of magnitude still hidden in practical improvements at the detail level. Of course there's a design trade-off between low on-resistance and capacitance as those are usually inversely related. But maybe you're aware of some category of MOSFETs that would optimize that trade-off?2) Any recommendations for a low noise, high sensitivity 3-channel voltage or current amplifier as a front-end between 3-D coils and ADC on the receiver side? The signal across the receiving coils is in the order of tens of µV (rms) but may ride on much higher transient voltages induced by interference of other magnetic fields (including the earth's magnetic field which is much stronger than the field I measure and manifests itself as soon as the receiver is being moved around). Maybe you even know of sample designs including amplifier, preprocessing, ADC and MCU from other areas of application that I'm unaware of? :-) I currently use an off-the-shelf RFID receiver IC which works reasonably well but has certain limitations when it comes to resolution, speed and precision for mobile tags (especially under rotation). It would probably be an advantage if I could keep signal preprocessing and RSSI measurement from the 3D receiver coils separate from demodulation. Plus preprocessing would allow me to feed the resulting filtered isotropic signal directly into a simple one-channel off-the-shelf Manchester decoder (or start decoding the signal myself in a DSP or the MCU directly).Any ideas?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 22, 2023, 02:18:04 pm
content: Rds(on)*Coss (or Ciss or etc.) is a figure of merit that varies from generation to generation (so, look for newer parts), and proportional to the stats of the given material in use. So, among materials in present use for power switching, Si > SiC > GaN. Lower FoM being better, of course.But again, it sounds like you're trying to solve a problem that no one else in the world has. Maybe there's some merit to that, that it's a new and unique problem; but it's more likely that you're approaching it from the wrong angle, and standard solutions exist, or if not solutions like existing products, then at least standard analyses that could be used. Suggesting, perhaps, further gaps -- in signals analysis, or network theory, for example. (Which should hardly be an embarrassment: most graduate EEs barely skate by signals theory as it is, and network theory isn't even part of undergrad, at least in most places, and I mean actual network theory, beyond the basics of AC steady state analysis.)And no, of course not really "no one else", there's pulse type metal detectors for example which sound very similar. But why not just whole-ass pulse the coil, why use capacitors at all? (Other than switch capacitances, which will be easier to dampen against coil and stray inductances, than an extended network will.) Otherwise, you're just tuning for a narrower frequency band, at the antenna itself (the antenna is part of the signal filter chain), and starting and stopping any resonant current in the antenna, faster than its own time constant (~Q/Fo) would decay, simply isn't gaining you much of any signal energy (which only manifests over the same time constant!). More importantly, you're setting yourself up for all manner of nonlinearity (MOSFET Coss varies extraordinarily widely with Vds), making dynamic range a huge challenge. And maybe all this can be compensated out, or trimmed, or clever solutions found; but do you really want to spend so much time on that, only to find you've missed your targets on SNR or field strength or etc. and need to redo the whole damn thing -- and the whole thing will be keenly sensitive to device parameters so you have to go through the whole tuning process again?A key insight, by way of anecdote:Also, not to project someone else on you; not that you know anything about them other than I'm writing right here and now, obviously, but just to be clear, the guy was a bit of a jerk.Anyway, bossman from my first job. Induction heating power supply design. He had an idea. Concern was, their existing designs were all so stupidly slow. And, they generally were: most used dominant-pole compensation with a time constant of fractional seconds; the audio-frequency ones, you could hear, feel even, as they ramped up. There's not really any reason to choose that, other than it's slow enough not to care about any particular load, and being that induction heating applications are usually pulsed over the seconds time scales, and the startup time is a constant adder, it's not really a big deal for anything like process consistency in production. But anyway, his idea was, not just to speed them up a little bit, but to create anew, revolutionary designwithsub-cycle controlthat could meter power into the tank on a 1/4-cycle sample rate basis! It would be a digital control and FPGA and ARM soft core (??) and all that and it would be amazing and blow the socks off everyone else and...Well, you can probably see a couple of odd points standing out from that already, and yes, they were odd, and have odd explanations (ranging from "we got the license might as well use it" to "we must succeed where others have failed"); but those aren't important here... Anyway, the relevant part here was the obsession with 1/4-cycle control.His thought process was, if voltage and current are swapping around every 1/4 cycle, ...can't wedo somethingabout that?Alas, no amount of argument would convince him that 1/4 is an utterly arbitrary measure, or that no, we simplycan't, it's a physical impossibility; granted, I knew hardly anything about network theory at the time, so it's not like I could offer any sort of comprehensive proof; but also, now that I do know more about it, it's not like a comprehensive proof would've been at all convincing anyway (and, to be fair, I still don't know how I would go about constructing such a proof from first principles). That is: network theory is on a high enough level that, one cannot understand a proof written within it, without also understanding much of it; and any simplification or reduction thereof, must be taken for granted, and at that point it's just a confidence game like anything else.Bossman had a degree in EE at least, maybe not at a practicing level for some years (decades?), but enough to be dangerous anyway; but also a degree in business, and I think the latter was the more primary interest. And, what is business but a confidence game? You roll the dice on new technology and you get what you get, right? Well, not to get political here, but you can see where that kind of mindset comes from, and what effect it has on, well, all of us, to some extent or another...In the end, I convinced him that, no, we're not going to do 1/4 cycle control, we don't have thetime and resources to develop that, but you can have 1-cycle control (which was done at a sample rate of 1 cycle with a pipeline delay of 1, so the actual maximum control bandwidth was 2 cycles, but shhh..).Anyway, these days, I would present, at least the outline of a proof, something like this: for a given Q factor, and given excess inverter capacity (typically the inverter VAs are, say, double the required power, to allow some mismatch / adjustable range, without having to change the tuning settings -- transformer or inductor taps, resonant capacitors), the difference necessarilymustresonate in the tank by itself. That is, say we have 100kVA in the tank, a Q of 10, and inverter capacity of 20kVA. We're delivering 10kW and have 10 to spare*, so at most we could change the resonant tank energy by 20kVA per cycle, or five cycles to fully start/stop it.*Which, if it's reactive, could then be 17.32kVAR, they sum vectorially; but if we're talking increasing or decreasing tank energy in as much of a stepwise manner as possible, we'll want that to be mostly real power, since we're delivering power to, or drawing power from, the resonant tank, exchanging with the DC link (which then also needs at least enough capacitance to absorb the tank energy at once, which will usually be fine). So we might constrain the reactive power output, and adjust timings to optimize for real power to dominate those 20kVA.Which you'll notice is a more basic sort-of-time-domain and energy-based argument, which is also more convincing. It gets harder when the network is more complex; for a voltage-fed series resonant, or current-fed parallel resonant network, the above is straightforward, but it's less obvious if it's a voltage-fed inductor-matched parallel resonant circuit for example, which is actually what we were tasked with at that job.And, control can be faster when Q is low, or indeed when Q <= (inverter capacity factor), it can be direct drive entirely (who needs capacitors!), but those are also less interesting as there aren't as many applications where that is true, and, well, it's fast to control regardless, so it doesn't really matter if you have a "1/4 cycle update rate" or anything.Anyway -- that was quite a long story, but I wanted to make it clear I know where you're coming from (at least, to an extent), and provide some background about what I'm suggesting and why.So, to say that I've thought about these things before, things like hard shutoffs, or fractional-cycle control; and I will likely think about them again from time to time. As have others; and indeed it seems at least one person had made it a small obsession -- always stuck sitting in one place, the destination in sight but the path otherwise unknown, lacking the knowledge to be able to circumscribe the canyon separating them from their destination. It's not that these are strictly impossible problems (you can scale or bridge a canyon!), but their solutions generally aren't practical; even if you resolve all the transients, and keep noise down, signal quality up, what do you have to gain, a dB or two of SNR? Such extensive effort needs justification; on what theoretical grounds can you prove such SNR improvement is possible, right? Stuff like that.Also to be clear, I'm probably coming on too strong with things here, maybe a gentle nudge would work better than "you're wrong and this is why", well not quite that strong, but that's probably still within the realm of what one could read it as...I'm not coming at this from a position of -- well, okay, to be even clearer, ego is inseperable, that's the human condition unfortunately, but Iat least like to think thatI'm more interested in the science of this stuff, than to merely be right. I can't say it's not pleasing to discover things that are right, or point out things when they are wrong. But science is cool and everyone should know some, and I at least like to think that's the stronger priority.And yeah, knowledge gaps happen, learning is the process of being wrong, less and less often; and humility is hard, especially as you learn more things and get (over?)confident in some areas, but that doesn't always translate to confidence in others. (You can read into this, whatever amount of projection you like, lol.) My hope is, not to point at gaps and laugh, but to highlight what misconceptions might've formed around them, and to show pieces that fit better. The real challenge is, the pieces are extended, with lots of wiggly tendrils that connect to myriad other topics; it simply takes years to develop that network of knowledge, no length of forum post can do that all at once.CheersTim

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 22, 2023, 02:18:17 pm
content: Quote from: fgrandel on October 22, 2023, 12:02:28 pmFET options at such high voltages are very limited. But I don't see how a high voltage FET is necessary or beneficial to you. If your circuit is just a simple half bridge driving a series-resonant, load, then yes turning off the bridge at zero current means there will be max voltage across the capacitor. If the FETs were ideal switches, that charge would stay on the cap until the bridge is enabled again. But that won't work with real FETs. One of FETs will immediately have its body diode forward biased (since the cap voltage is much greater than your DC bus voltage), meaning the bridge will continue to conduct current. This will happen regardless of the FETs' Vdds rating. Ultimately what happens is the LC circuit continues to ring down until most of its energy has been moved back to the DC bus.I'm guessing there's some other feature of this circuit which prevents this from happening... but what is wrong with above scenario, where the remnant energy in the load is rectified back to the DC bus? The efficiency of this process should be similar to the efficiency of driving energy into the load (you can even drive the bridge to eliminate the conduction losses of the FETs' body diodes).QuoteFrom your description so far, it doesn't sound like you need anything terribly specialized. Most critical aspect probably will likely be the circuitry coupling the receiver coil(s) to the ADC driver. Using resonant receiver coils will provide some build-in bandpass filtering, and can boost the signal voltage/impedance to make coupling it to the ADC easier. But this would come at the cost of bandwidth, of course. Again, without knowing more about your modulation/demodulation approach it's hard to give specific advice.The ADC itself is usually the last thing you select, after the bandwidth and SNR of the signal is estimated.