Experiments with Supersonic Beams as a Source of Cold Atoms

Figure 4.24: Photographs of the 64 stage coilgun coil driver board. At left, the entire
board is shown (except for the IGBT which mounts to the three square pads). At
right, a zoom-in view provides more detail.
for multiple channels is also unchanged, though many of the individual components of
the system are upgraded. The IGBTs are upgraded to a 1.2kV blocking voltage model
(Powerex CM200DY-24A) and the thyristor is likewise upgraded to a 1 kV voltage
model (Littlefuse SK055R). These changes allow the freewheel diode and resistor to
be taken out of the circuit, which decreases the switching time of the coil.
While the IGBT gate drive circuitry is unchanged, the method used to drive
the thyristor gate is also modified. The DC/DC converter used to drive the gate was
found to fail when subjected to the voltage spikes produced by the faster coil switching
at higher current. Rather than replace the DC/DC converter with a different model,
the solution implemented is simply to close the IGBT 20 μs before switching the
thyristor. Closing the IGBT grounds the cathode of the thyristor, allowing a 5 V
power supply, switched by an opto-coupler, to provide the necessary current to the
gate.
The change in the thyristor drive circuitry necessitates a change in the snubber
capacitor circuitry across the IGBT. While the snubber capacitor is still needed in its
initial role to absorb the continuing current from the coil after the IGBT is opened,
in the original configuration, the snubber discharges very quickly when the IGBT is
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