Totem-pole gate driver VS Active pull down VS Fast Fet turn off

[Ferric Toes]

Target Taus have been calculated in threads on this forum. However, due to not knowing the exact Allow conductivity calculated values are approximate. We have also empirically determine Tau values for common targets such as coins.
Check threads by Green where he post Log/Lin plots of decay to determine Tau.
Here is one on gold nuggets:
https://www.geotech1.com/forums/atta...2&d=1547586775

Like Green, I prefer to measure actual targets as many are alloys have a considerably reduced conductivity compared to the parent metals. I made a special PI for this on which I can vary the Tx pulse up to 1mS in width. Here is the decay curve plotted for a US silver quarter. Note the Tx pulse is 800uS wide which is approaching 5 Tau for this target. I put the first time cursor on a convenient point of the curve and measure with the amplitude cursor. This gives a value of 832.8mV. I then move the other cursors to a point on the curve where the amplitude value is 37% of the first figure. The delta value on the time line is 165uS, which is the Tau.


Eric.

[green]

Received the .1ohm resistors. Made a tester. Was thinking some of the oscillation was caused by noise pickup using a regular scope probe with ground lead. Cut a BNC cable and soldered across .1ohm resistor to reduce pickup. Thought it would reduce oscillation, NOT. Including some test scope pictures. Top left picture, .1ohm and 1ohm resistor are in series with coil to +12V. How can I get over 1A oscillation across the .1ohm resistor and no oscillation across the 1ohm resistor? Any thoughts why or what I might try? I have a thin film .1ohm and some different Mosfets to try. 1ohm resistor is a 1/4W metal film.

[green]

Received the .1ohm resistors. Made a tester. Was thinking some of the oscillation was caused by noise pickup using a regular scope probe with ground lead. Cut a BNC cable and soldered across .1ohm resistor to reduce pickup. Thought it would reduce oscillation, NOT. Including some test scope pictures. Top left picture, .1ohm and 1ohm resistor are in series with coil to +12V. How can I get over 1A oscillation across the .1ohm resistor and no oscillation across the 1ohm resistor? Any thoughts why or what I might try? I have a thin film .1ohm and some different Mosfets to try. 1ohm resistor is a 1/4W metal film.

I see a had scope connected to wrong end of MUR460 when measuring C.

[green]

Don't know if I should. Added a 100 ohm resistor in series with cut off BNC cable to reduce oscillation. Recorded decay with Rd to +12V and Rd across coil. Recorded C at drain not coil(error reply #122).

[green]

Don't know if I should. Added a 100 ohm resistor in series with cut off BNC cable to reduce oscillation. Recorded decay with Rd to +12V and Rd across coil. Recorded C at drain not coil(error reply #122).

Tried a IRF840 with the 100 ohm resistor in series with he BNC cable.
Couple questions. Can the BNC cable be excited in resonance or was the oscillation without the 100 ohm resistor real current oscillation? Have more Mosfets, what parameters should I record?

[waltr]

Tried a IRF840 with the 100 ohm resistor in series with he BNC cable.
Couple questions. Can the BNC cable be excited in resonance or was the oscillation without the 100 ohm resistor real current oscillation? Have more Mosfets, what parameters should I record?

Yes, Coax cable can oscillate. They are distributed inductance and capacitance and will be 'resonant' at some frequencies.
100 Ohm resistor in series with the coax can help by lowering the 'Q' of the coax LC.

In Ham radio we do use 1/4 or 1/2 wavelength of coax as resonant elements.

[green]

Received the .1ohm resistors. Made a tester. Was thinking some of the oscillation was caused by noise pickup using a regular scope probe with ground lead. Cut a BNC cable and soldered across .1ohm resistor to reduce pickup. Thought it would reduce oscillation, NOT. Including some test scope pictures. Top left picture, .1ohm and 1ohm resistor are in series with coil to +12V. How can I get over 1A oscillation across the .1ohm resistor and no oscillation across the 1ohm resistor? Any thoughts why or what I might try? I have a thin film .1ohm and some different Mosfets to try. 1ohm resistor is a 1/4W metal film.

reply#122

Still trying to measure oscillation. More recordings. CH1(normal scope probe) CH2(cut BNC cable soldered across .1 ohm resistor).
Four left pictures compare CH1 connected to scope ground or point A(oscillation is noise pickup?). Any suggestions on how to determine if oscillation on CH2 is actual current oscillation? Think it isn't but don't know to prove it.
Third row, labeled CH1 wrong. Should be point E(555 out), scope trigger for all recordings. Forth row, coil connected to 1 ohm resistor in series with .1 ohm resistor. Ch1 connected to scope common or point B(noise pickup on both?)

If oscillation CH2 is caused by BNC cable resonance, any suggestions on how to solve it or what is causing it?

[Ferric Toes]

reply#122

Still trying to measure oscillation. More recordings. CH1(normal scope probe) CH2(cut BNC cable soldered across .1 ohm resistor).
Four left pictures compare CH1 connected to scope ground or point A(oscillation is noise pickup?). Any suggestions on how to determine if oscillation on CH2 is actual current oscillation? Think it isn't but don't know to prove it.
Third row, labeled CH1 wrong. Should be point E(555 out), scope trigger for all recordings. Forth row, coil connected to 1 ohm resistor in series with .1 ohm resistor. Ch1 connected to scope common or point B(noise pickup on both?)

If oscillation CH2 is caused by BNC cable resonance, any suggestions on how to solve it or what is causing it?

It is difficult to find out what is going on at switchoff. I found out that there is a lot of variation between different Mosfets. A lot of ringing disappears if you avoid avalanche. If you are in avalanche, put some series resistance in the coil circuit to reduce peak current. It is worth substituting a thick film resistor for the coil and seeing if it still rings. I tried a 5W resistor that is not metal film and it has some inductance because I was getting about 20V of flyback voltage. Thick film gives none. I still get one overshoot spike on the scope across the 0.1ohm resistor. Can't explain that as yet. By the way, I only ground one of my four scope probes as all the BNC sockets are connected together in the scope. This avoids a ground circuit ring. I'll post some plots by Monday.

Eric.

[Mr.Jaick]

reply#122

Still trying to measure oscillation. More recordings. CH1(normal scope probe) CH2(cut BNC cable soldered across .1 ohm resistor).
Four left pictures compare CH1 connected to scope ground or point A(oscillation is noise pickup?). Any suggestions on how to determine if oscillation on CH2 is actual current oscillation? Think it isn't but don't know to prove it.
Third row, labeled CH1 wrong. Should be point E(555 out), scope trigger for all recordings. Forth row, coil connected to 1 ohm resistor in series with .1 ohm resistor. Ch1 connected to scope common or point B(noise pickup on both?)

If oscillation CH2 is caused by BNC cable resonance, any suggestions on how to solve it or what is causing it?

using bare caox i.e transmission line is a tricky business
those ringings are reflections caused by a low impedance source which is the power rail in your case and high impedance load = 1M scope input
caox has quite high capacitance, for low frequencies it has high impedance and it's ok just remember you are putting a capacitor in the circuit by using coax(27pF / feet)
but when you get higher in frequency like your rise time it causes all sort of reflections, oscillations and crap due to your scope 1M input impedance and low source impedance
terminate the coax with a 50ohm termination(if coax is 50ohm) on the scope input to get rid of reflections i.e widening the bandwidth
you have to use series resistor with your coax to lower the loading effect if the source impedance is higher
for example:
if you're using 50ohm coax
use 450 ohm resistor in series with the core so you get 500 ohm overall impedance and also your signal will get divided by 10:1 at the scope so watch out for that
you can also use a 950 ohm resistor to get 1k input impedance so lower loading of your signal but 20:1 ratio is bad for your low level measurement
also try to get shortest tip length to the board to minimize tip inductance
my solution for the last one is to solder a coax connector to the board and then connect your coax to that
but it's not that important for this application
hope it helps a bit

[Qiaozhi]

using bare caox i.e transmission line is a tricky business
those ringings are reflections caused by a low impedance source which is the power rail in your case and high impedance load = 1M scope input

It is quite likely that the unexplained ringing is indeed due to transmission line issues. This is a big problem in high-speed digital design, or anywhere where signal edges change very quickly, because this emphasizes the behaviour of passive circuit elements. A very good book to read is: High-Speed Digital Design (A Handbook of Black Magic) by Howard Johnson and Martin Graham.

[Mr.Jaick]

yes indeed
I have to make a T-shirt --> "know your instrument"
they can sometimes easily fool you, either it's the humble DMM, an oscilloscope or even a simple power supply!

here is a very good representation from Doug Ford about scope probes, it's a joy to read.
THE SECRET WORLD OF PROBES OCt09.pdf
warning: read this with Australian accent

[green]

Thanks for the replies. Don't have an oscillator. Thinking a steep response might work. Used UCC27518 output(connected to CH1 and CH2 only). 50 ohm resistor soldered to coax center wire.

[green]

Some recordings, IRF840. Coax with 50 ohm resistor picks up less noise than x10 probe. Middle left picture, coil connects to +12V. Shouldn't be any current thru .1 ohm resistor(some signal CH2, CH1 not good). Like Eric, I get less oscillation if Mosfet doesn't avalanche. Rd to +12V not coil. Not posting how to measure oscillation, posting the problems I'm having.
UCC27518 driver

[Tinkerer]

Gate resistor

Something about Mosfet gate resistors

[green]

Some tests with different gate resistor values turning Tx on and off. Guessing some of my problem is the circuit is hard wired. Different gate resistors effected oscillation across .1 ohm resistor at Tx on. Not much effect at Tx off.
Coil connected to A, avalanches. Should different gate resistors effect oscillation at Tx off. Would a good circuit board layout look a lot different?

I use a coil with different Tx and Rx coils. Noise level at integrator out seems to be the same whether Tx is on or disabled. What negative effect does the oscillation at Tx on or off have? Should I be seeing integrator out noise increase if Tx is enabled?