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

[6666]

It's in the sd2000 schematic, the circuit that controls the irf710.
It works as I built it ages ago on breadboard while testing some transmit circuit stuff.
This way you don't need to worry about using the avalanche diode in the mosfet to clamp the flyback.

Cheers Mick

Thanks I will try and dig it up.

[Mechanic]

Where do we find it ? thanks.

It's in the sd2000 schematic, the circuit that controls the irf710.
It works as I built it ages ago on breadboard while testing some transmit circuit stuff.
This way you don't need to worry about using the avalanche diode in the mosfet to clamp the flyback.

Cheers Mick

[6666]

There is in use a very nice auto snubber circuit, that hold the flyback lower than
200V... see in the schematic the irf710 act as resistor in parallel with the high
vltage (200vdc) electrolytic capacitor.

Where do we find it ? thanks.

[Xtrem]

for example the SD2000 which doesn't use any limiter and its TX is far below any Tau valeu with that 0.4ohm coil

There is in use a very nice auto snubber circuit, that hold the flyback lower than
200V... see in the schematic the irf710 act as resistor in parallel with the high
vltage (200vdc) electrolytic capacitor.

[Monolith]

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.

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.

[Mr.Jaick]

Current is constant for about 3 times the target Tau.

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

[Mr.Jaick]

waltr
I don't know what to say but this reply truly made my day.
YOU ARE simply AMAZING
I read your post like ten times and everytime a bigger smile on my face, absolutely loved it
you have always helped me , i won't forget that.

so it's quite hard to balance the performance / power dissipation ... hmmm

again thanks for having me, my generation is SO lucky to be able to get advice from top end industry engineers like this.

[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.

[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.

[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

[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.

[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.

[waltr]

Very good summary Eric.
Thanks.

[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.

[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.