Coil switch

[Anonymous]

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

 

[Anonymous]

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.

 

[Anonymous]

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.

 

[Anonymous]

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

 

[Anonymous]

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 <I>very long</I> power-efficient PI idea...
Happy New Year to all,
Carl