Desired specs for a PI coil driver MOSFET

[DFansler]

I am just starting on a project using the latest revision of Gary's PI. The goal is to use multiple small coils mounted in a linear array. The output from each coil would be displayed on an LCD graphics display as a scrolling image. So passing ove an object the display would create a 2D image of the object. My main goal is to use this underwater looking for lost boat anchors. Of course since Blackbeard was married and lived in Bath,NC - which is only 30 miles from where I keep my sailboat, I am obligated to go look for his booty :-)


In reading Joseph J. Rogowski paper "Making a Fast Pulse Induction Mono Coil", he mentions that one of the items that slows down a coil's pluse is the output capacitance of the MOSFET driving the coil. The MOSFET I have seen used by designers here is the IRF840. It appears to have an 8A continuous capacity, 32A pulse, reverse breakdown voltage of 500 volts and a COSS of 310pF. In reading threads on Geotech1. I have seen mention of reverse spikes of over 600v. In doing a little searching, I found the STD6N95K5 which has 9A continuous capacity, 36A pulse, reverse breakdown voltage of 950 volts, on resistance of 1 ohm, and a COSS of only 30 pF. Is there something I am missing, but would this not make a better coil driver?


kind regards,
David
S/V Night's Beautiful Daughter
www.dv-Fansler.com

[Carl-NC]

It's difficult to determine a good PI driver MOSFET from the specs, you just about have to try it and see. A "fast" PI is mostly needed for trying to detect sub-gram gold nuggets. In your case, fast is not needed.

Your idea for an array has been discussed before. Unfortunately, resolution and depth tend to be about equal. That is, for a 1-foot resolution you'll get about 1 foot of depth. If you want 1-inch resolution, you'll get about 1-inch of depth. If you want 10 feet of depth, then expect 10 feet of resolution. In the end, you'll probably find that induction-based imaging just doesn't work, and a plain old metal detector is good enough.

- Carl

[Tepco]

In doing a little searching, I found the STD6N95K5 which has 9A continuous capacity, 36A pulse, reverse breakdown voltage of 950 volts, on resistance of 1 ohm, and a COSS of only 30 pF. Is there something I am missing, but would this not make a better coil driver?

Please note that STD6N95K output capacitance data is taken at 100V D-S bias voltage, this capacitance is highly nonlinear with applied voltage, at 0V D-S will be much higher, not significantly (10 times) better than IRF 740. This is why datasheet parameters alone can be misleading.