to figures 1 - 4 below we can see that, compared to a MOSFET switched
coil with closed loop control and suckback reduction, the thyristor
fired coilgun falls a little short in terms of peak velocity and
efficiency. Of particular significance is the more rapid fall off
in velocity and efficiency as the launch position is advanced beyond
the VPD, especially at higher voltages. Again, note the reorientation
of the axes between velocity and efficiency graphs.
natural pulse width, as set by the choice of capacitor, has a strong
effect on the sensitivity to delta for a given voltage. This is
evidenced by the poor velocity developed with the 33,000 uF capacitor
as delta is increased. In fact, suckback can stop the projectile
dead in the tube under certain conditions. The extended current
tail produced when using badly matched parameters can be clearly
seen in fig 5. For this particular combination of voltage, capacitance
and delta, the thyristor switch produces a peak velocity and efficiency
of 5.1 m/s and 0.65 % respectively. Contrast this with the MOSFET
switching arrangement that produces a peak velocity and efficiency
of 11.3 m/s and 4.1 % respectively. Notice that the peak current
is slightly greater when using the thyristor. This is probably due
to the slightly lower dynamic resistance of the thyristor.
the thyristor fired coilgun exhibits a smaller global maximum for
both velocity and efficiency, it must be remembered that there was
no attempt to reduce suckback. In favour of the MOSFET we can say
that this switch type allows much more flexibility in the choice
of capacitance than is possible with this simple thyristor setup.
In fact, there's no theoretical upper limit to the capacitance that
can be employed when using a MOSFET switch.
summary it can be said that, due to the high pulsed current rating
for its physical size, and the modest triggering requirements, the
thyristor is an effective switching device for use in coilgun applications.
If suckback control measures are implemented then the thyristor
can compete with MOSFET switches under certain coil-capacitor combinations.
1. Velocity surface - MOSFET
2. Velocity surface - Thyristor
Fig 3. Efficiency surface - MOSFET
Fig 4. Efficiency surface - Thyristor
5. MOSFET and thyristor-fired current pulses.