Diode Commutation
Results
Conclusions
Why
might this method be used instead of a dioderesistor? If we look at the muzzle
speed obtained using each method then we find that, for a similar overvoltage,
the series diode method yields the best results. Figs 1 and 2 show the voltage
and current data for a 6 segment series diode branch and dioderesistor branch
(R ~ 70m).
This comparison is based on equal peak turn off voltages.
FIg
1. Voltage drop across a series diode and a dioderesistor branch.
Fig
1 shows the commutating branch voltage as the diodes go from reverse blocking
to forward conduction, notice that the peak forward voltage for both methods (which
gets added to the supply value) is about 10~11V *. There is a slight difference
between them but it's not important.
Fig
2. Comparison of current decay using the two methods.
Fig
2 shows that the series diode method is faster at reducing the commutating current
and the subsequent retarding impulse. The decay time for the dioderesistor method
suffers more from the projectile induced current boost. Table 1 lists the muzzle
speeds for the different projectile types resulting from each method. It is interesting
to note that the current decay rate is initially similar for both methods but
a divergence occurs as the projectile begins to interact with the decaying current.
Projectile Type

Method

Speed (m/s)

Solid 
6 Series Diodes 
1.22E+1

Solid 
DiodeResistor 
1.15E+1

4mm Core 
6 Series Diodes 
1.25E+1

4mm Core 
DiodeResistor 
1.18E+1

6mm Core 
6 Series Diodes 
1.27E+1

6mm Core 
DiodeResistor 
1.21E+1

Table
1. Muzzle speed comparison for commutation methods.
All
the projectiles have a slightly greater muzzle speed when running with series
diode commutation. The differences are about 5% in terms of speed and around 10%
in terms of energy. This is by no measure a huge difference so either method could
be used to provide similar gains.
Table
2 show an efficiency comparison between a single commutating diode and a series
combination of 10 diodes.
Coil

Coilgun Efficiency 1 Diode

Coilgun Efficiency 10 Diodes

Overall Efficiency 1 Diode

Overall Efficiency 10 Diodes

A

3.11E+0

3.81E+0

2.92E+0

3.58E+0

B

3.62E+0

4.69E+0

3.16E+0

4.09E+0

C

3.39E+0

4.64E+0

2.65E+0

3.63E+0

D

2.84E+0

3.87E+0

1.95E+0

2.66E+0

Table
2. Efficiency comparison using optical triggering.
Conclusions
These
results show that a significant amount of kinetic energy is gained by collapsing
the current more quickly. Since diodes are cheap this method offers a simple means
of improving the performance of closed loop DC current switching. One downside
of increasing the overvoltage is that the switching device must be rated to at
least the supply voltage plus the initial conduction voltage of the diode array.
This may push up the price of the switching device. As always, there is a costperformance
compromise which is hard to crack.
Further
study is needed at higher voltages in order to establish how this effect is related
to the ratio of the supply voltage and the diode array conduction voltage. If
the relationship turns out to be anything like proportional then a huge number
of diodes would be needed in systems with supply voltages of 100s of volts. This
would effectively place an upper limit on the useful range of the 'MultiDiode'
method. Also, it is not clear for a given overvoltage whether the MultiDiode method
is always better than the dioderesistor method. In fact, if the small
performance difference between these methods is a universal feature then it will
probably be more cost effective to employ the dioderesistor method even though
the resulting muzzle speed is slightly smaller. More detailed experimentation
and mathematical modeling is needed to resolve this.
*
In both cases the actual voltage rises until the mosfet undergoes avalanche breakdown.
This lasts for a few microseconds before the diode(s) begin to conduct sufficient
current. The avalanche capacity
of the MOSFET allows it to cope with this overvoltage. The 11V value I'm using
above is therefore not the true peak value but it represents a peak voltage as
seen on a 'millisecond time frame'. This is an important distinction since the
MOSFET, under 'coilgun' conditions, can cope with avalanche breakdown for a few
tens up to perhaps a hundred or so microseconds.
