xmit power

[bugwhiskers]

Can somebody who knows tell me please what will be the effect of changing the Mosfet to one that allows a higher back EMF. Will going from 400V to 800 V double the xmit power and perhaps the detection depth ?


ALSO
Page 4 of the HH notes has the following which I do not understand.

2. However, the exponential
“charging” of the coil current
does induce a target field, which
can degrade performance if it is
not settled out.

[bbsailor]

The voltage rating of the MOSFET will clamp the peak flyback voltage at that MOSFET voltage rating. On a scope look to see that the voltage spike has a flat-top at the MOSFET voltage rating. If so then the natutal voltage spike would be higher and a higher MOSFET would help. If there is no flat-top, indicating that the peak flyback voltage is below the MOSFET rating, then a change of the MOSFET will not help. If your lower voltage MOSFET is getting warm at higher pulse widths, and if the MOSFET is flat-topping, then you may minimize or eliminate the warming by using a higher voltage MOSFET. When the MOSFET starts to conduct as seen by the flat-top, then it is generating heat.

What will help is a MOSFET that has a lower output capacitance (COSS) to allow the damping resistor to be a little higher while a little faster delay can be achieved for detecting gold.

Changing the MOSFET will not double the depth of detection. Doubling the depth requirs 64 times the power.

The coil needs a minimum of about 3 times the coil time constant so the full field grows and target response is obtained before the field changes with the next pulse. The coil time constant is calculated by the coil inductance divided by the total resiatance of the coil, MOSFET (on-resistance) any series resiator and resiatance of the coil lead wire. Lets say you have a 300uH coil with a total coil circuit resistance of 10 Ohms, the your coil time constant will be 30uS. In about 3.3 time constants (a 100uS pulse width) the field would have achieved about 90 percent of it's full value. In theory it takes 5 time constants to reach full value but between 3 and 5 the additional field growth is very small.

bbsailor