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

[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?

[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]

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?

[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]

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]

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.

[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.

[KingJL]

The MUR460 also benefits from holding upwards of 100V reverse potential on the drain/cathode junction reducing the capacitance to around 15pF.

[ATTACH]54627[/ATTACH]

Eric, as your previous thread on the subject had shown, not all diodes are suitable for producing these effects. In my own experiments, I have successfully used the through hole mur460 and BYV28-200 (as used in the SD2000). For SMD's I have successfully used the MURS320, MURS340, and MURS360. I keep a stock of about 10 each BYV28-200's, MURS320, and MURS340 on hand. Simulation shows that almost any diode will work, but in the real world they don't... I have tried general purpose diodes with very disappointing outcomes. I once tried a 1004 because simulation showed that it would work... IT DIDN'T!! Eric, I guess all you can do is keep putting valid information forward with supporting real world measurements/results and hope that some use it to their benefit.

[Ferric Toes]

From a combination of calculation and simulation it appears that the diode eliminates about 50% of the mosfet capacitance, but how much improvement you see in practice depends on what you're trying to achieve. Either way, the diode doesn't make things worse.

Considering the so-called "sweet spot" sounds like it may assist in finding the point of maximum benefit.

The MUR460 also benefits from holding upwards of 100V reverse potential on the drain/cathode junction reducing the capacitance to around 15pF.

[Qiaozhi]

Intrigued by your calculation for "Coss effective" I looked at some datasheets.
Looking at the Coss of IRF740B or IRF740AS series and comparing with the diode MUR460 Junction capacitance, it becomes obvious, that the diode may not always be of much benefit.
Reading a lot of datasheets and comparing the many available parts in detail has become imperative.

From a combination of calculation and simulation it appears that the diode eliminates about 50% of the mosfet capacitance, but how much improvement you see in practice depends on what you're trying to achieve. Either way, the diode doesn't make things worse.

Considering the so-called "sweet spot" sounds like it may assist in finding the point of maximum benefit.

[Mr.Jaick]

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

found it

[Mr.Jaick]

There is in use a very nice auto snubber circuit, that hold the flyback lower than
200V.

that's correct
but what i'm looking into is:
they didn't use any method to flat top coil current at some point
I think Bruce said "bugger that" to all high conductive targets

[Monolith]

That's an interesting observation, although it's difficult to determine the exact values from looking at the graph in the datasheet.

With reference to Appendix D of "The Voodoo Project", where I investigated the use of a series diode in a PI TX circuit, there is an equation for Coss effective. In the case of the IRF740, Coss effective was calculated as 211pF at a voltage of 320V. This appears to more or less match the graph that you posted.

Coss effective is defined as a fixed capacitance that would give the same charging time as the output capacitance of a mosfet while VDS is rising from zero to 80% VDS with VGS = 0V.

Intrigued by your calculation for "Coss effective" I looked at some datasheets.
Looking at the Coss of IRF740B or IRF740AS series and comparing with the diode MUR460 Junction capacitance, it becomes obvious, that the diode may not always be of much benefit.
Reading a lot of datasheets and comparing the many available parts in detail has become imperative.

[waltr]

I never know how you guys determine a specific target's Tau
is it empirical or calculated?

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

[Qiaozhi]

Attached is the graph showing the Drain to Source capacitance of the Mosfet IRF740.
At about 200V is what I call the sweet spot, where the Coss starts increasing fast, as the voltage diminishes.
At 0 Volt the Coss is highest.

That's an interesting observation, although it's difficult to determine the exact values from looking at the graph in the datasheet.

With reference to Appendix D of "The Voodoo Project", where I investigated the use of a series diode in a PI TX circuit, there is an equation for Coss effective. In the case of the IRF740, Coss effective was calculated as 211pF at a voltage of 320V. This appears to more or less match the graph that you posted.

Coss effective is defined as a fixed capacitance that would give the same charging time as the output capacitance of a mosfet while VDS is rising from zero to 80% VDS with VGS = 0V.