Inductor parasitic capacitance cancellation / reduction - Page 1
Forum section: electronics / projects

author: Manul
author_rank: Super Contributor
timestamp: September 20, 2020, 02:28:36 pm
content: In the picture attached you see a typical inductor model consisting of inductance, series resistance and some parallel capacitance Cp (parasitic). Up is the damping resistor Rdamp achieving critical damping for this inductor. The value of Rdamp needed to achieve critical damping depends on the value of prasitic capacitance Cp.If L,R and Cp are constant and inductor can not be modified in any way (secondary windings can not be added), is there any practical examples of adding negative capacitance for effective reduction of Cp? Like for example using negative impedance converter (NIC)? Complication: it involves fairly high voltages of hundreds of volts.Do you know any reference designs of such case?Thank you.

author: dmendesf
author_rank: Frequent Contributor
timestamp: September 20, 2020, 02:30:33 pm
content: What you really want to achieve? Higher ressonance frequency?

author: Manul
author_rank: Super Contributor
timestamp: September 20, 2020, 02:41:53 pm
content: Quote from: dmendesf on September 20, 2020, 02:30:33 pmFaster and controllable (through the value of Rdamp) magnetic flux collapse. It is easy to make it slower, but not faster, because it hits the boundary of underdamped condition which is unacceptable.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 20, 2020, 03:13:08 pm
content: Possible sure, but the cure is usually worse than the disease.For example, you might use an impedance inverter, but the op-amp involved has to 1. cover the full range of voltage, current and slew rate required, and 2. has to be fast enough (GBW high enough) that the assumptions used in the impedance inverter design hold true. Namely that the impedance is inverted properly, and not phase shifted into more resistance (or less i.e. negative resistance!). Likewise, noise gets a lot worse.The open loop / more traditional realization of the same thing, is to simply wire up the network by itself, then equalize its frequency response. More specifically, an EQ filter with zeroes strategically placed to cancel out the network's natural frequency response. Obviously, this requires enough GBW to boost the desired new cutoff point (which lies in the network's HF asymptote, so is probably heavily attenuated). And obviously, all that gain can't come for free: you're going to have a sucky SNR, simply because you're using all that gain.Quote from: Manul on September 20, 2020, 02:41:53 pmSo, pulsed something or other? Does it matter how it decays, rate aside? Are you measuring anything as well, or do you just want the external field to drop dead? Why not drive it with an inverse pulse to cancel out most/all the flux?Or if this is something like a pulse induction metal detector, then, the field doesn't really matter, but you need to subtract the excitation (and its resulting transient decay) from the measurement.Either way, you'll find the above analysis is spot on: maybe you drive an inverse pulse to cancel out the flux; but the exact rate at which the flux cancels, varies throughout the coil. That is, because the capacitance is distributed (and other higher order effects), you can't simply blip it and there you are; the flux doesn't reach all points simultaneously, and there will be peaks and dips as everything settles out.It's like stirring a cup of coffee, then trying to stop it spinning by stirring it backwards for a turn or two: the fluid crashes around the spoon, and sure you can cancel the bulk of overall rotation, but it's going to continue spinning in myriad vortices for quite some time, before finally settling out. (In this case, it definitely settles faster: viscosity is scale-dependent, and effectively the backwards stirring breaks the full-container vortex into a bunch of smaller vortices which dissipate much faster. This isn't true of linear electromagnetic systems though: those time constants are characteristic of the structure.)Tim

author: dmendesf
author_rank: Frequent Contributor
timestamp: September 20, 2020, 03:20:47 pm
content: Maybe you can use a series capacitor to cancel part of the L instead of the C but it's hard to say without the full circuit:https://www.electronicdesign.com/technologies/analog/article/21800911/resonant-circuit-generates-a-highfrequency-magnetic-field

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 20, 2020, 04:38:55 pm
content: Typical management request, fix the problem without changing the part causing the problem. The obvious answer is to change the inductor to something wound to meet your requirements. Look at how RF inductors are wound; Litz wire, pie windings, helical windings and spaced turns. Adding a NIC at high voltage or current, combined with high frequency, doesn't seem practical.

author: Manul
author_rank: Super Contributor
timestamp: September 20, 2020, 09:51:55 pm
content: Thanks, Tim, and others. In my spare time I'm working on pulse induction metal detector prototypes as a hobby. Mostly interested in more advanced techniques which allow gathering additional information about ground and target, not just detection itself.The deal is, I strongly suspect, that company x developed capacitance cancellation for their metal detectors, but I have little idea about specific implementation. I got interested in this idea, because it could be usefull together with my own existing techniques of gathering extra early stage information in the detection cycle.What came to my mind is the use of NIC, naturally. Yet, when I started practical design considerations, I quickly realised that it is a horribly messy thing to implement. Like I can put lots of time into this idea and get nothing in the end. So I'm in a dilema state to put effort here or not. Yet, knowing that someone did it succesfully is playing on my emotions (I'm sure a lot of people here knows the feeling).Thats the reason for my question about reference designs. I searched web a lot and there is not much about it.What matters about pulse decay? Well, noise IS a problem, I worked hard to get low noise and also external noise cancellation, so the addition of considerable noise would probably be unacceptable. I have controllable DC offset correction in the frontend, so DC bias is generally not a problem. Otherwise the cleanest possible decay is preferred.

author: David Hess
author_rank: Super Contributor
timestamp: September 21, 2020, 02:45:53 am
content: Quote from: Manul on September 20, 2020, 02:41:53 pmI am sure I have seen that done by modeling the parallel capacitance as part of the circuit which actively drives the current through the inductor.