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

[waltr]

Been thinking detection distance should decrease. Tried adding resistance in spice, signal lower but looks like can sample sooner. Need to try with real circuit.

Yes, the sooner sample will improve the SNR since the signal is larger.
One could also then have a longer sampling time which will also improve SNR. This is very helpful for higher conductive targets but not so much for small lower conductive targets.
MY HH PI pulse rate is 1600Hz with smallish caps in the integrator (100nF). The standard PI integrator design, R's & C's verse sample rate, is another none linear function. Best way to analyze is a spreadsheet doing regressive calculations per time step.

As I said, when I built the HH it was not great. Adding an R in series with the coil improved sensitivity. Did additional experiments with different values of series R to increase coil current and distance did not improve significantly. I documented this in my HH2 build thread.
My HH works quit well and have been surprised at how it detects very small nails and lead.

[green]

Played with real circuit adding 8ohms series resistance. Signal less but not as much as I was thinking. Noise level doesn't seem to change(avalanche, snubbed or adding resistance to limit peak current). Maybe because I have separate Tx and Rx coils.

[dbanner]

Walltr is absolutely correct.
I've observed, which doesn't come without much effort, a strange behavior. Now I need more analysis power, other wise I cannot make heads or tails.

[dbanner]

Remember, we are essentially flashing a target. The feedback of which we try to make sense of.
Notwithstanding
The prf, after a certain frequency doesn't improve the response.
That's when I realized......

[dbanner]

I apologise, I shall retreat.

[Ferric Toes]

Having gone through a considerable number of different Mosfets, I am now revising which ones are good for a rapid switch off and those that are not. This is a result of looking at plots of Tx current waveform, high voltage flyback, and gate drive. One thing it is best to avoid is avalanching as this generates a lot of uncontrolled ringing noise. There are many variables which influence the result and the coil is obviously one of them. My coil is 10inches diameter, 330uH and 5.4ohms, with 1.2m of 75ohm Vandamme coax giving a total of 5.8ohms. In series, there is the 0.1ohm current sensing resistor, plus a series 4.7ohm metal oxide resistor to improve Tau and reduce coil current to 0.9A. This was necessary to bring the high voltage pulse below avalanche using a 12V supply to the Tx. Resistive damping to the coil is quite critical and was adjusted to give the fastest response without coil ringing.

As to the Mosfet, I found that those with a Vds of >500V were necessary. A commonly used one is the IRF740, but this avalanched and rang in my test circuit. Types with very low On resistances did not work well, but the STP9NK50 with a typical 0.75ohms Rds(on) worked fine with very low ringing and 500V Vdss. The ringing we are talking about and measured on many Mosfets had a period of 80nS which equates to 12MHz, so it's not coil ringing. Another candidate is the STP10NK60. All of these Mosfets are in the same ballpark as regards voltage and current ratings.

One puzzling feature on the blue trace for coil current, is the positive going spike which crosses the zero line to indicate a reverse current and then returning to -0.8A before decaying more slowly to zero which is the damped coil response. Some Mosfets have a much bigger excursion into the reverse current which results in higher amplitudes of the 12MHz ring. Green commented on this a while back and I thought is was an artifact of the back emf pulse. However the two do not coincide in time, except that the voltage starts to rise rapidly as the coil current approaches zero. Maybe this blip in the coil current is caused by one or more parasitic capacitances in the Mosfet. The true coil current decay is the curve that ends between 1.5uS and 2.0uS and ideally continue smoothly to join the 0.9A level.

The top green trace shows the back emf peaking just below 500V and then decaying to around 350V, then with a slow decay towards 200V when it is shorted to ground by the turning on of the next pulse. this voltage is stored on the Coss of the Mosfet and blocked by the series diode, which is now the MUR460 rather than the HER208 used in previous plots. Changing to the MUR460 seemed to make a small difference, but will look into that later.

The two best Mosfets are P9NK50 and P10NK60. The latter having more margin before avalanche sets in. Both Mosfets have the blip in the current trace ending close to zero.

The P9NK50 then had the supply voltage increased to 13.5V with the result that the flyback exceeded the avalanche voltage. Now the ringing appears. . Using the 600V device you can increase the voltage to 15V before ringing starts.

A device I recently advocated, 17N80, because of its very low Rds(on) and Vds of 800V didn't come out so well and there was a longer delay time before switching started. Notice the much larger amplitude of the spike in the blue trace and that we are still at least 300V away from avalanche.

Lastly, how does the good ol' IRF740 fare? Avalanche at 450V and ringing with a big negative spike.

Remember that all these plots are with extra resistance in the coil circuit and only 0.9A maximum coil current. Lower coil resistance or higher supply voltage will likely cause avalanche to occur, sometimes to the point that the Mosfet will be turned on a second time for a brief period.

Where we have delays of >10uS these effects will be long gone and may not matter, but the original question was for fast switching a device and these plots show show what is happening at early times. One can only proceed further when we have a preamp attached that will operate in its linear region much quicker than we have at the moment.

Eric.

[Mr.Jaick]

why can't we lower the Pulse width to avoid avalanche instead of higher resistance or lower drive voltage?

[Ferric Toes]

why can't we lower the Pulse width to avoid avalanche instead of higher resistance or lower drive voltage?

Attached is a plot of the whole transmitter waveform that I have. Aside from the initial current growth period of 134uS, the current is constant for nearly 200uS. If the pulse width were shortened, nothing would change until you reach the point where the current starts to fall at a width of 134uS.

The green trace is the back emf, or flyback as it is sometimes called, and you can see the slow decline between pulses, presumably due to the small reverse leakage current in the series diode and Mosfet. I have tried a bleed resistor from the Mosfet drain/diode junction which takes the voltage to zero between pulses, but this slowed the switchoff slightly. The wide, 350uS, Tx pulse is there to give good excitation to larger good conductors e.g silver and gold coins, as well as small objects.

In practice, I would use a considerably higher repetition rate and stick with the low pulse current as a means to avoid avalanche.

I tried the complementary emitter follower gate drive which was slightly faster than the single transistor drive, but this causes the avalanche situation to occur sooner.

Eric.

[Monolith]

Attached is a plot of the whole transmitter waveform that I have. Aside from the initial current growth period of 134uS, the current is constant for nearly 200uS. If the pulse width were shortened, nothing would change until you reach the point where the current starts to fall at a width of 134uS. [ATTACH]54614[/ATTACH]

The green trace is the back emf, or flyback as it is sometimes called, and you can see the slow decline between pulses, presumably due to the small reverse leakage current in the series diode and Mosfet. I have tried a bleed resistor from the Mosfet drain/diode junction which takes the voltage to zero between pulses, but this slowed the switchoff slightly. The wide, 350uS, Tx pulse is there to give good excitation to larger good conductors e.g silver and gold coins, as well as small objects.

In practice, I would use a considerably higher repetition rate and stick with the low pulse current as a means to avoid avalanche.

I tried the complementary emitter follower gate drive which was slightly faster than the single transistor drive, but this causes the avalanche situation to occur sooner.

Eric.

The Mosfet Coss capacitance has a sweet spot at a certain voltage level, where the capacitance is lowest. I used to choose the bleed resistor such, that this voltage level was reached at the end of the OFF time.

[waltr]

Very good summary Eric.
Thanks.

[6666]

Hi Eric great work thanks.
Did you happen to try an IRF840 in your experiment, was it better or worse than the 740 ?

Changing to the MUR460 seemed to make a small difference, but will look into that later.

Thats interesting to.

[Ferric Toes]

Hi Eric great work thanks.
Did you happen to try an IRF840 in your experiment, was it better or worse than the 740 ?

The IRF840 is one Mosfet I don't have. The 500V Vds must help. I'll swap back the HER208 to see what the difference is.

Eric.

[Mr.Jaick]

thank you Eric for the answer
sorry if i ask too many noob questions, i'm just an electronics student yet

The green trace is the back emf, or flyback as it is sometimes called, and you can see the slow decline between pulses, presumably due to the small reverse leakage current in the series diode and Mosfet. I have tried a bleed resistor from the Mosfet drain/diode junction which takes the voltage to zero between pulses, but this slowed the switchoff slightly.

1. does this bleeding off help with linearity of the Pre-Amp? because more stable virtual ground?

The wide, 350uS, Tx pulse is there to give good excitation to larger good conductors e.g silver and gold coins, as well as small objects.

I'm a bit confused here
2. is it the flyback that excites the targets OR the pre magnetic field from the coil ON moment? presumably both in different ways

In practice, I would use a considerably higher repetition rate and stick with the low pulse current as a means to avoid avalanche.

I was thinking about this discussion about limiting the coil current by means of a series resistor to decrease the Tau so we can use lower pulse width and therefore lower power and also saturate the coil
3. why saturating the coil is important?
we want high flyback voltage but lower than avalanche
flyback depends on inductance, peak coil current and how fast it shuts down
4. so what's wrong with just using enough pulse width to reach desired flyback voltage? without using any limiter
for example the SD2000 which doesn't use any limiter and its TX is far below any Tau valeu with that 0.4ohm coil

I tried the complementary emitter follower gate drive which was slightly faster than the single transistor drive, but this causes the avalanche situation to occur sooner.

Eric.

isn't that better so we can use less power

[waltr]

Originally Posted by Ferric Toes
The wide, 350uS, Tx pulse is there to give good excitation to larger good conductors e.g silver and gold coins, as well as small objects.
I'm a bit confused here
2. is it the flyback that excites the targets OR the pre magnetic field from the coil ON moment? presumably both in different ways

During TX ON, Current flowing thru coil Builds a magnetic field then should level off to a steady state. The length of time at this steady state allows the eddy currents in the target to die out. High conductive targets require a longer time for the eddy current to die out. A silver US quarter needs at least 200usec for this to happen.
This is the first requirement.
When the coil current is switched OFF, the magnetic field in the coil collapses. Remember it is the changing magnetic field that induce current to flow in a conductor (target). This then causes Eddy currents in the target. These eddy current then create a magnetic field that induces current in the coil and are what we are trying to 'detect' in the RX sampling.
Also, the coil's own magnetic field collapsing creates current flow, the 'flyback' which is what we do not want the sample with RX.

3. why saturating the coil is important?
we want high flyback voltage but lower than avalanche
flyback depends on inductance, peak coil current and how fast it shuts down
4. so what's wrong with just using enough pulse width to reach desired flyback voltage?

We do not 'want' high flyback Voltage. The flyback is simply a consequent of applying a magnetic field to a target then turning off that field (nothing in physics is free). Ideally we want NO flyback Voltage but the Laws of Physics states this is not possible therefore we Dampen the Flyback with a resistor to get rid of the flyback Voltage as quicky as possible without causing other problems.
We try to apply the largest magnetic field to a target so the target can produce higher eddy current when the coil's magnetic field is removed. However, due to real world parts (MOSFETs, etc) too high of a flyback Voltage is not desired, Avalanche, etc).

We also do not Saturate the coil...What we want is the current in the coil to obtain a steady state, Current is constant for about 3 times the target Tau.

[Ferric Toes]

My comment -Changing to the MUR460 seemed to make a small difference, but will look into that later.

The difference appears to be in the reverse leakage current with the HER208 being higher. The voltage on the drain/cathode junction settles down between pulses at least 50V lower than the MUR460. I haven't yet confirmed this by looking at the data sheets. Nothing else seems to change, so it matters little. I have, though, put a 1Meg bleed resistor to ground so that with either diode the voltage is brought to zero before the next Tx pulse.

Eric.