Optimizing Target Responses

[bbsailor]

Thoughts to consider when trying to reduce coil capacitance.

Sources of coil- seen capacitance.

1. The coil windings, turn to turn capacitance governed by the dielectric constant of the wire insulation and the insulation thickness and coil to shield capacitance.
2. MOSFET and other diode capacitance as seen by the coil.
3. COAX cable capacitance based on the length between the coil and the control box that is about 100pf.

We can reduce number 3 by being creative and building a remote circuit located at the end of the aluminum shaft, near the coil that puts the necessary circuits in the available space to reduce or eliminate the coil- seen coax capacitance.

The less capacitance that the coil sees, the higher value the damping resistor can be and the steeper the coil discharge sloop will be to better stimulate smaller targets. However, smaller targets may be parts of the coil that retain eddy currents such as coil wire, shield, or other metal parts that were not previously detected.

These thoughts are just to get the creative minds of this forum to push forward.

Thanks

Joseph J. Rogowski

[alex---1967]

Thanks. I'll think about it.
663

[Mr.Jaick]

What is SRF?
I am not very good at English abbreviations.

How does a fast diode minimize this effect? By what means?

SRF = Self-Resonant Frequency
is the frequency of undamped coil(aka ringing), which is determined by inductance and over all parasitic capacitance.

A fast reverse recovery diode with low junction capacitance in series with the drain of you power MOSFET will reduce your overall solid state parasitic capacitance seen from coil.
how?
your diode capacitance will be in series with Coss of your MOSFET thus two caps in series, and I suppose you know what happens to the overall value?
but the story has not finished yet
since capacitors hold charge and there are two of them in series one smaller(diode) and the other larger(MOSFET) they're gonna trap part of the flyback charge and distribute between themselves after turn off, and the major chunk of this charge will be on the MOSFET's Coss obviously because larger capacitor eats more.
that charge or voltage whatever you're comfortable with will remain there and part of it slowly discharges due to Leakages(Drain-Gate, Drain-Source, P-N, PCB) all the OFF time when you're busy with signal processing, now what that remaining charge does is it will put these P-N junctions in Hard reverse bias which will decrease their capacitance by orders of magnitude (good news).
for example your typical MUR460 will have around 100pF for 1V of reverse bias but for 200V it'll be lower than 10pF which is great.
circuit

blue trace is Drain(where you trap the charge)
Red is RX


as you can see the blue trace basically starts at 626V and discharges down to 273V in 400us
you may say why doesn't it go all the way to the 900V flyback then decay form there?
the answer to that is my MOSFET can take 900V flyback but not my 600V rated diode hence starting form there, the blue trace actually does go to 900V for a moment but diode breaks down fast and continues from 600V
if the MOSFET was 600V as well then the flyback wouldn't go that far.

[Qiaozhi]

What is SRF?
I am not very good at English abbreviations.

How does a fast diode minimize this effect? By what means?

SRF = Self-resonant frequency

https://www.geotech1.com/forums/showthread.php?23660-What-is-the-purpose-of-the-series-diode

Also, Appendix D of The Voodoo Project gives a more detailed analysis.

https://www.geotech1.com/forums/showthread.php?26504-The-Voodoo-Project-Published-October-2020

[alex---1967]

What is SRF?
I am not very good at English abbreviations.

How does a fast diode minimize this effect? By what means?

[green]

I have a question - when measuring the natural frequency of a coil, is only a coil with a cable used?
Does the transistor capacity not count? This is right?
It seems to be logical, yes.
But then, when determining the coil time constant tau_RX (in a real metal detector), it seems to me that the capacitance of the switching transistor should also be taken into account.
That is, it is necessary to use in the calculations not the capacity of the coil, but the total capacity of the coil + cable + transistor ...
Am I right or wrong?

Fet capacitance can have a large effect on SRF. Adding a fast diode between fet and coil minimizes the effect, small effect.

[alex---1967]

Let's look at a realistic example...

Measured coil parameters:
L = 300uH
Rs = 5 ohms (total series R)
SRF = 500kHz (inc. cable etc)

I have a question - when measuring the natural frequency of a coil, is only a coil with a cable used?
Does the transistor capacity not count? This is right?
It seems to be logical, yes.
But then, when determining the coil time constant tau_RX (in a real metal detector), it seems to me that the capacitance of the switching transistor should also be taken into account.
That is, it is necessary to use in the calculations not the capacity of the coil, but the total capacity of the coil + cable + transistor ...
Am I right or wrong?

[Carl-NC]

Waveform drawings are in Visio, graphs are in Excel.

[detecto]

Hello everyone.
Car, question for you.
What software do you use to make graphs that you posted?
Thanks!

[Carl-NC]

No, I was missing a square-root:



I've corrected post #24, good catch!

[alex---1967]

Yes, better late than never!

Part 5:

The require damping resistor for critical damping is

[latex]
R_D = 0.5 \times \frac{L}{C_L} = 471\O

I think this is a mistake.


The result should be reduced by 1000 times.


This is how this formula should probably be written:





Rd = 0.001 * 0.5 * L / Cl






Example from calculations on the first page:






Rd = 0.001 * 0.5 * 300uH / 338pF = 0.001 * 0.5 * 0.0000300 / 0.000000000338 = 444 Ohm

[Koala]

Tried viewing the forum on a Windows 10 machine with Edge What I have. If I right click on image and open image in a new tab it seems to work for post #24. I'm computer stupid, so probably not understanding what is meant by broken. Just trying to learn something.

correct

it doesn't display but will open in a new tab fine

[6666]

Looks like its been fixed.

[green]

Tried viewing the forum on a Windows 10 machine with Edge What I have. If I right click on image and open image in a new tab it seems to work for post #24. I'm computer stupid, so probably not understanding what is meant by broken. Just trying to learn something.

[Qiaozhi]

Chrome in both Android and Windows 10 show the broken image icon.

The same in MS browser in W10.

I looked at the HTML source and it seems the image URLs are nor url-encoded, that might be the cause.

If I copy the image address and paste it on the address bar then the images show, but still not on the web page even after reloading.

Tried viewing the forum on a Windows 10 machine with Edge, Samsung Android phone with Chrome, and Samsung Android tablet with Chrome ... LaTeX images broken in all cases.

However, everything is ok on a Linux Mint laptop with Firefox.

Also just tested on Windows 10 with Firefox, and no broken LaTeX images.

Update: Tried Linux Mint with Chromium and FlashPeak Slimjet browsers ... LaTeX links broken.
So it appears that all browsers tested fail to display the LaTeX images, except Firefox.