Drive a Mosfet...

[vistac2000]

Hi to all friends,

For drive Tx pulse in a PI metal detector, we can connect AVR Micro pin directly to a mosfet for example IRF740 like this picture ? or we must
use a resistor ? what value is nice ?
What is the best way to drive a mosfet in PI MD?
What is the advantage of Mosfet driver ICs ?

thanks.... vistac2000

[kt315]

vistac,

you must more learn this matter in the forum... it is not a simple way like to put a one resistor or to put nothing...
read gurus. maybe it will help for your PI start, I hope...

Coil switch
Posted by: Carl-NC (---.ipt.aol.com)
Date: December 25, 2001 01:43PM
Merry Christmas tech heads!
Question for all, but mainly for Eric. The main switching transistor (I'll call it Tr1) for the coil needs to turn off as quickly as possible. On some designs this transistor is driven directly by a logic gate, on others there is additional circuitry between the logic and Tr1.
Sometimes the additional circuitry is a CE amp with Tr1 driven off the collector, thus a collector resistor passively turns Tr1 on and the CE transistor actively switches it off. This is fine because turn-on time is not tremendously important. Call this method 1.
Sometimes there is a follower stage driving Tr1, thus the follower transistor turns Tr1 on and a pull-down resistor passively turns it off. Now turn-off time depends on the time constant set by the pull-down resistor and all the parasitics. Call this method 2.
I originally built my proto-PI with method 1. For funsies, I flipped things around to use method 2. My turn-off transient on the gate of Tr1 went from 0.5us (method 1) to 10us with a 1K pull-down and 3us with a 100R pull-down. This mostly just shifted out the exponential decay in time, but there was also a slight degradation in the decay as well. Not much, but it is observable on the o-scope.
The only design I've seen (that I can think of) that uses method 1 is Mark Stuart's Microcontroller PI project. Method 2 is used in other project designs, the SMPI, CS-7, and one (or more) of Eric's designs. My question is: How much does this matter? Is the actual turn-off of Tr1 dominated by charge transfer internal to the device?
Happy holidays,
Carl
Re: Coil switch

Posted by: Eric Foster (---.ipt.aol.com)
Date: December 26, 2001 03:34PM

Hi Carl,
Even the method of switching the power transistor is interesting, as there are so many inter-related factors. Assuming for most applications nowadays that Mosfets are used, there are, as you say, various methods of driving them. Strangely, although Mosfets are devices with a very high input impedance, the drive to them is recommended to be from a low impedance source. This is because the parasitic input capacitance is quite high, at around 1500pf, for the IRF740 types, and you need a low impedance to charge and discharge this capacitance quickly for efficient switching operation. As you have outlined, there are various ways to do this, and the question, at the end of the day, does it matter?
In a PI detector, we are concerned with driving a current though a coil for a short period of time, and then switching it off cleanly and rapidly. Any switch-on delays do not really matter, as the growth of the current is limited by the coil’s inductance. This delay, before the current reaches its maximum, will likely be several orders of magnitude slower than any delay caused by the capacitance and drive impedance of the Mosfet. At switch-off, we need to examine things in a bit more detail. To get the best eddy current response from an object, the switch-off of the field from the coil, needs to be at least 5 times faster than the object time constant. Obviously, if the switch-off is the same length of time as the time constant of the object, then there will be no signal to observe. At the point where the drive pulse to the gate is terminated, nothing much happens initially, as the gate retains the voltage on its capacitance. This capacitance will then start to discharge through the drive circuit impedance. On many of my circuits, including the Aquastar, this is simply a 100R resistor to the source. The turn-on is achieved by a PNP transistor that connects the gate to the drain supply via a 47R resistor. The resistive divider pots down the drive voltage so that no more volts than is necessary for proper switching is applied to the gate, otherwise it will take longer to discharge the gate capacitance. The discharge time constant is 0.15uS, giving a total discharge time 0.75uS, so this is more than enough speed for a treasure hunting PI. However, the gate voltage does not have to fall all the way to zero, but only to the gate threshold voltage, where the Mosfet just ceases to conduct; about 4V max for the IRF740. Using a 15V supply to the Mosfet and using the above circuit arrangement to maximum gate turn-on voltage is 9.5V, hence the time to reach the turn-off volts is only about one time constant, which takes us back to 0.15uS. Now, if we look at what happens in the drain/coil circuit at the point of cut-off of the Mosfet. Inductors don’t like a change of current, so the energy stored in the magnetic field will collapse and try to induce a voltage to keep the current in the coil flowing at the value it had just before switch-off. This is why we get a high voltage appearing across the coil at this point in time. The Mosfet is now a very high resistance and, if unrestrained, the coil voltage could reach over 1000V. However, the Mosfet has a breakdown, or avalanche voltage, which in the case of the IRF740, is about 400V. The coil voltage quickly rises to this level and then the Mosfet drain/source junction breaks down, and further voltage rise is prohibited. The Mosfet is then a low impedance across the coil during this high voltage period, which gives another, rather longer time constant. If you insert a small (0.1R) current monitoring resistor in the coil circuit and look at the waveform on a scope, you can see the current ramping down linearly for the duration of the high voltage pulse. As soon as the coil voltage falls below the 400V, then the Mosfet becomes a high impedance again, and it is now the duty of the damping resistor to absorb the rest of the energy: yet another time constant. Only after everything has settled down, including the recovery time of the first receiver amplifier, can we then look at the microvolt signal levels from objects at some distance from the coil. The delays from whichever type of Mosfet drive circuit used, only make a very small contribution, in my experience.
The things that have a major effect on the field switch off time are 1) the coil inductance, 2) the coil current. 3) the breakdown voltage of the Mosfet, and 4) the capacitance of the coil/cable circuit. However, these will keep for another time.
Eric.
Re: Coil switch

Posted by: Dave Johnson (---.tnt2.prescott.az.da.uu.net)
Date: December 26, 2001 11:14AM

Dear Carl,
When the collector turns off, you're expecting to get this whopping voltage spike (unless it's a Fisher type pulse unit). C-B Miller effect will stretch out the off-transition unless you keep the base as close to a short circuit as possible.
Back when I was doing PI I generally preferred bipolar switching transistors to MOSFETS because for a given on-resistance, they have a lot less capacitance. This makes for a quicker decay when the flyback pulse finally collapses. However, during the last 10 years, MOSFETS have gotten a lot better, whereas bipolars are about the same, so I suppose that the bipolar advantage may not be so great any more, and may have vanished entirely.
Most PI topologies (again, the Fisher is different) have a preamp that "looks into" a damping resistor shunted by clamp diodes, without any active circuit elements. Thus, the damping resistor dissipates energy during the transmitter on-time. By switching the damping resistor out of the circuit during the on-time, this waste of power can be eliminated.
Another problem with the conventional topology is that when the receive voltage drops below the forward bias voltage of the diodes, the damping resistor is no longer in parallel with the coil (i.e., no longer performs its damping function) but is still in series with the coil, impairing the preamp noise figure. Using some combination of fixed and/or actively-timed switching, it is possible to keep a damping resistor in the circuit during the receive period, and to switch the preamp directly to the coil, improving noise figure.
In many cases the extra circuitry won't produce results that make it worth the trouble. But, in some cases it may.
The Fisher topology does keep the damping resistor in the circuit and does switch the preamp directly to the coil; however, in the Fisher design that's easy to do with a CMOS transmission gate because the Fisher system has no voltages higher than a diode drop beyond the 5 volt power supply rails. Because the preamp is seeing only small signals, it can be operated in the linear region without saturation-- in fact, the preamp can be a Class A common-emitter transistor, allowing lower noise and power consumption than can be achieved with integrated circuit preamps.
By the way, this is not a sales pitch for the Fisher topology. That topology does have the advantage of power efficiency (as explained in the Fisher patent), with battery drain typically 15-30 mA. However, this (nearly) VLF-like efficiency is effective only over a narrow range of target conductivities. The Impulse, for example, is hot on nickels, but is downright muddy on a silver dollar-- the consequence of keeping the on-time short enough to have the energy stored in the field rather than dissipated in the coil as heat. On the other end of the conductivity scale, the low rate of change of voltage during the flyback period means that there is poor sensitivity to the lowest conductivity targets (gold chains, etc.) which is a disadvantage in many applications. I mention the "Fisher topology" because it's what I'm most familiar with, it is notably different from the other PI machines, is easy for an experimenter to build, and is described in a patent so an experimenter has a starting place. It is also possible that a PI design which was intermediate between the conventional topology and the Fisher system might prove to have worthwhile benefits.
Although the Fisher system probably cannot serve as the basis for a modern high-performance PI-only machine, the topology lends itself well to hybrid PI/VLF/MF designs using an induction balance searchcoil, as explained in the Fisher patent. This could possibly give the Fisher system a new lease on life.
--Dave J.
Impulse

Posted by: Eric Foster (---.ipt.aol.com)
Date: December 26, 2001 03:31PM

Hi Dave,
I guess you must be the designer of the Impulse electronics? I didn't think it had a triangular current waveform, but the other day I connected a small resistor in series with the coil on a Impulse, and sure enough, it has. All in all, it is a design with lots of innovative features and it is a pity that it wasn't developed further.
Eric.
Re: Coil switch

Posted by: Carl-NC (---.ipt.aol.com)
Date: December 29, 2001 10:56PM

Thanks guys... took me a while to thoroughly read your replies.
Eric, one thing I might debate is the MOSFET threshold. This is a DC parameter, when you're slewing the gate hard (even with active pull-down) there is a delay from getting the carriers into or out of the channel region. This is not the gate capacitance, but I believe more like a bipolar's forward-bias diffusion capacitance. I've always thought it might even be desirable to slew the gate to a negative voltage for faster turn-off but, as you said, this is all probably excessive.
The reason I asked is that I am laying out the PCB for my everything-adjustable platform PI design. In the true spirit of this design I am making the switch work either way.
Dave, I've also thought about switching out the diode clamp resistor. As you say, it's just a noise source on the input to the preamp. With my current design it's no big deal, but there are some hotrod amps that are less than 1nV/rtHz input-referred noise that I may use in the future. A 1k resistor is 4nV/rtHz and would swamp out the benefit of a true LNA.
Now, on to Dave's very long power-efficient PI idea...
Happy New Year to all,
Carl

[Max]

Hi to all friends,

For drive Tx pulse in a PI metal detector, we can connect AVR Micro pin directly to a mosfet for example IRF740 like this picture ? or we must
use a resistor ? what value is nice ?
What is the best way to drive a mosfet in PI MD?
What is the advantage of Mosfet driver ICs ?

thanks.... vistac2000

Hi,
you can... but it's not so elegant solution.

The driver circuits are to provide sharp driving pulse edges and fast commutation. You can make discrete or using dedicated ICs made just for that... driving mosfets. Often driver IC will provide also some kind of separation from e.g. driving logic... that's anotehr interesting issue.

Kind regards,
Max

[softsx]

...What is the best way to drive a mosfet in PI MD?...

IMHO, the best way in this case is TLP250 or other components, which was designed for driving MOSFETs.

[porkluvr]

LTC1693-1,2,3 (-1 and -2 come in SO08. LTC1693-3 comes in smaller MS8 package.)
TC4426,7,8

The circuits below are a variation on some from Aziz. I think BS170 and BS250 were pre-drivers in the original circuits but BJTs (2N3904, 2N3906) are also good.

LM555 drives a MOSFET pretty well all by itself, but if you use a cmos version then these circuits would probably help with a snappy turn-off.

You will get faster response with one of the mosfet driver ICs but these discrete circuits work OK.

[Carl-NC]

Q3/D5 are not necessary... just use a Darlington for Q1. Done right, this is a blazing fast driver. My coil switch MOSFET is actually running cool to the touch when driving several amps of pulsed current.

[kt315]

there is especial class of mosfet driver IC's as people has wrote... I know the one... I have met of... if you feel some necessity you can use it.

[hobbes_lives]

there is especial class of mosfet driver IC's as people has wrote... I know the one... I have met of... if you feel some necessity you can use it.

I tried to get samples of LM5112 from National Semiconductors, but that didn't work out. I may order some the next time I place an order with Mouser. The specs look pretty good.

Here's a MOSFET driver circuit that looks good in spice, turns off quite fast, but hasn't been built and tested yet. Use it at your own risk.