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

[Xtrem]

1. yes ,it help for faster switching,difference is because of different PPS.

2. I dont understand what exactly zener does, if zener was reverse connected
then it would have a reason to exist, (TURN-ON Voltage = Vbatt-2.2V)

3. 100k does not affect the ringing , i use a <10 Ohm in series with the Gate to
prevent ringing.

[Mr.Jaick]

thank you xtrem

1_ how does it help with faster switching? better rising and falling edge by bypassing a resistor?
as far as i know a resistor is frequency independent to some extent

2_ just can't figure it out
I have used zener to protect the gate
cathode on the source anode on the gate
but not sure about this fashion...

3- after thinkering a bit more i guess i found the answer
the parallel resistor is to ensure that the mosfet turns "OFF" if the input becomes open circuit or floats
basically without the 100k if drive transistors fail open then mosfet won't turn off and after a few seconds it will catch fire, maybe damage the battery or even the coil

[6666]

Xtrem would it be possible to see the circuit "and last the driver from Lorenz Pulse 5 (very good driver)"
thanks

[Xtrem]

thank you xtrem

1_ how does it help with faster switching? better rising and falling edge by bypassing a resistor?
as far as i know a resistor is frequency independent to some extent

2_ just can't figure it out
I have used zener to protect the gate
cathode on the source anode on the gate
but not sure about this fashion...

3- after thinkering a bit more i guess i found the answer
the parallel resistor is to ensure that the mosfet turns "OFF" if the input becomes open circuit or floats
basically without the 100k if drive transistors fail open then mosfet won't turn off and after a few seconds it will catch fire, maybe damage the battery or even the coil

1. maybe it react as a parallel R/C filter , im not sure , but i see it on the scope.

2. dont know .

3 yes you have right , its probably for protection.

[Xtrem]

Xtrem would it be possible to see the circuit "and last the driver from Lorenz Pulse 5 (very good driver)"
thanks

If you mean the test circuit that i have make

[6666]

If you mean the test circuit that i have make [ATTACH]54446[/ATTACH]

Yes , thank you.

[Ferric Toes]

I look at the turn off by inserting a 0.1ohm resistor in the ground lead from the coil and cable, just between the end of the cable and the circuit board. I use a thick film resistor, which has a tab looks like a TO220 size device, on the basis that it has minimal inductance. Scope is across the twin lead of the resistor. In the attached plot the scope is triggered by the falling edge of the drive pulse and we seen a delay of about 0.3uS before the Mosfet switchoff starts. The current pulse has a final peak of 1.5A, which develops the -150mV across the 0.1ohm resistor. The small turn on delay is of no importance in the light of the relatively slow build up of current due to the coil inductance.

In this case I am using the A.C. coupling as per 6 on pages 36 - 38 of the Texas paper, but without the zener. The diode gives just one diode's worth of negative swing to assist discharge of the gate/source capacitance. The Mosfet I am using is Infineon 17N80C3. Rgs is 100K and Cc is 0.47uF. Additionally there is a 10ohm resistor right where the gate leg is. For about 1uS the switchoff looks a mess, but from from 1.5uS onward it is clean without further ringing. Ignoring the initial noise the switchoff is fast enough for my present requirements. I am also using a series diode, HER208, to isolate the coil from the Coss of the Mosfet

It is hard to know what causes the ringing. Changing the scope probe from x1 to x10 and changing the scope sensitivity to match, gives a similar result but it is more symmetrical about the 0V line. Next thing to try is to disconnect the Rx and then replace the coil with an equivalent thick film resistor.

With my preamp setup using OPA1611 and OPA1612 with a total gain of 470x; a 10" 330uH fast coil and 1.2m of 75ohm coax, the earliest I can sample is 5.5uS. The limitation seems not to be in the Tx but in the receiver front end in terms of bandwidth and recovery time. The OPA1612 stage appear to need a faster device as there is no improvement even when the OPA1611 is replaced with a very fast AD8055.



Eric.

[green]

I look at the turn off by inserting a 0.1ohm resistor in the ground lead from the coil and cable, just between the end of the cable and the circuit board. I use a thick film resistor, which has a tab looks like a TO220 size device, on the basis that it has minimal inductance. Scope is across the twin lead of the resistor. In the attached plot the scope is triggered by the falling edge of the drive pulse and we seen a delay of about 0.3uS before the Mosfet switchoff starts. The current pulse has a final peak of 1.5A, which develops the -150mV across the 0.1ohm resistor. The small turn on delay is of no importance in the light of the relatively slow build up of current due to the coil inductance.

In this case I am using the A.C. coupling as per 6 on pages 36 - 38 of the Texas paper, but without the zener. The diode gives just one diode's worth of negative swing to assist discharge of the gate/source capacitance. The Mosfet I am using is Infineon 17N80C3. Rgs is 100K and Cc is 0.47uF. Additionally there is a 10ohm resistor right where the gate leg is. For about 1uS the switchoff looks a mess, but from from 1.5uS onward it is clean without further ringing. Ignoring the initial noise the switchoff is fast enough for my present requirements. I am also using a series diode, HER208, to isolate the coil from the Coss of the Mosfet

It is hard to know what causes the ringing. Changing the scope probe from x1 to x10 and changing the scope sensitivity to match, gives a similar result but it is more symmetrical about the 0V line. Next thing to try is to disconnect the Rx and then replace the coil with an equivalent thick film resistor.

With my preamp setup using OPA1611 and OPA1612 with a total gain of 470x; a 10" 330uH fast coil and 1.2m of 75ohm coax, the earliest I can sample is 5.5uS. The limitation seems not to be in the Tx but in the receiver front end in terms of bandwidth and recovery time. The OPA1612 stage appear to need a faster device as there is no improvement even when the OPA1611 is replaced with a very fast AD8055.

[ATTACH]54455[/ATTACH]

Eric.

Been wondering about gate drive. Need a gate resistor, what value? I have been driving the gate with op amp output in a control loop. I purchased some UCC27518 and UCC27519 drivers awhile back. Used them when I made bipolar Tx. Seemed to work ok. Replaced control loop with UCC27518 driver for my unipolar Tx. Some pictures with unipolar Tx. Wondering if they should look different. Don't have a .1R, tried a 1R and .01R. Oscillation at turn off. Same oscillation with scope lead connected to +batt(scope common lead). Thinking no oscillation?

Think my resistor is connected same location as yours.

[Ferric Toes]

I kept everything in place, including the cable, and substituted the coil with 6.6ohms of thick film resistor so that the pulse current is of the same order. The result is that most of that ringing hash is absent. Just one much smaller ring of a couple of cycles disappearing within 1.5uS. My coil is wound with 1/0.1 PTFE insulated wire with metallised fabric shield. Coil plus 1.2m Belden coax gives a total resistance of 6ohms. Resonant frequency with cable is about 1MHz.

Maybe my method of shielding causes high frequency ringing?

I'll do so more tests and plots tomorrow as the workshop is too cold to spend longer today. As far as I remember the series gate resistor is there to damp oscillation in the gate circuit. This really applies if there are long pcb tracks from the driver to the gate. 10ohms is adequate. My tracks are as short as I can make them, but I put the resistor there anyway.

Eric.

[Tinkerer]

I read somewhere, that a good "rule of thumb" for the Mosfet gate resistor is about 10 times the Mosfet's Source Drain Resistance. Have not really tried it yet as I was still trying different Mosfets.
Mosfet technology has changed and improved a lot in recent years. For my purpose the best, at present, is C3M75120J with 2 Ohm gate resistor and IX4340UE gate driver. Still have some switching noise to see to.
Full ground plane around the driver area has helped to make the switching noise "good enough" at the time, but maybe now is the time to make it better.

[Tinkerer]

Here is the Mosfet gate switch OFF.

Red trace is the Mosfet gate.
Blue trace is the preamp out.

Timing 1us

Sorry for the many pages. Can not find a way to send only the first page.

[Qiaozhi]

Sorry for the many pages. Can not find a way to send only the first page.

In the PDF reader, print the document to file and select page 1.

[Tinkerer]

In the PDF reader, print the document to file and select page 1.

Thank you for the help.
When I print a PDF page, it comes our very fuzzy, unreadable, when using Microsoft Edge.
I found I can use Print Screen and then crop the picture.

[Ferric Toes]

Still looking at the current waveform by scoping a 0.1ohm resistor in the ground return from the coil, except that the coil is replaced by a 6.6 ohm resistor (3x 2R2), with the coax still in circuit. All the hash has disappeared and we have a clean couple of cycles of decaying ringing. Next is for the resistor to be straight on the board i.e. no coax. Still a ring but all decayed before 1uS. I measured the coax with the resistor termination and it gives a reading of 0.475uH which together with a capacitance of about 80pF may account for the extended ringing in the first plot. Perhaps my wrap around shielding contributes some additional ringing frequency that explains the additional 'noise' on yesterday's plot.



Eric.

[bbsailor]

Still looking at the current waveform by scoping a 0.1ohm resistor in the ground return from the coil, except that the coil is replaced by a 6.6 ohm resistor (3x 2R2), with the coax still in circuit. All the hash has disappeared and we have a clean couple of cycles of decaying ringing. Next is for the resistor to be straight on the board i.e. no coax. Still a ring but all decayed before 1uS. I measured the coax with the resistor termination and it gives a reading of 0.475uH which together with a capacitance of about 80pF may account for the extended ringing in the first plot. Perhaps my wrap around shielding contributes some additional ringing frequency that explains the additional 'noise' on yesterday's plot.

[ATTACH]54465[/ATTACH] [ATTACH]54466[/ATTACH]

Eric.

Eric,

Based on my research, the energy in the oscillations which need to be damped before sampling can occur comes from any capacitance source as seen by the coil and comes from these sources.
1. Coil capacitance
2. MOSFET COSS
3. Coax cable capacitance due to cable type and length
4. Coil to shield capacitance
5. Circuit capacitance

I found that using Scotch 24 as a shield was not very detectable, had less surface area than a solid shield materiel and was pretty easy to lay around a homemade coil to keep the self resonance higher than other solid shield materiel.

I came to visualize that the oscillations that need to be damped comes from unwanted capacitance from all its sources.

I can see a new coil design where the coil wires continue up the plastic shaft connected to the coil where they directly connect to the first electronic active circuits either TX or RX to eliminate the coax capacitance and thus make for a potentially faster coil. This also eliminates the coax solder joints inside the coil housing that may become detectable small targets.

Thanks,

Joseph J. Rogowski