THEORY OF OPERATION
4-27
Versapulse Select Service Manual
0621-499-01 01/94
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4.5 FLASH LAMP POWER CIRCUITS
Refer to the Isolated Trigger Board PCB schematic in Section 8, and the HVPS & PFN Interface Circuits
Simplified Block Diagram in topic 4.4.5. The flash lamp power circuits include the HVPS, simmer supply,
isolated trigger circuits, main charging capacitor circuit, flash lamp SCR's and the flash lamps. These circuits
operate together to start and simmer the flash lamps between firings; charge the PFN capacitor before firing;
and then trigger the discharge of the main charging capacitor through the selected flash lamp.
Simmer - The flash lamps are simmered at a low current between pulses simply to maintain the electrical
path to the SCR anode side - i.e., if the lamp was not conducting (open circuit on lamp cathode side) the SCR
could not be gated on. The lamp is simmering but the rod is not simmering. The simmer board provides a
separate simmer supply current for each flash lamp, as well as suppling a transformer coupled RF field
around each flash lamp to promote ionizing of the flash lamp at turn on. The RF field is established by
connecting a transformer secondary to the cavity housing, which is electrically isolated from the chassis. The
Simmer board operates off of 400 VDC supplied by the HVPS. C7, R11 and DS2 provide an indication of the
presence of the 400 VDC supply voltage to the simmer supply board.
Charging the PFN Capacitor - Prior to each YAG pulse the HVPS charges the PFN cap to the level specified
by the Cap charge command voltage signal from the Controller PCB. The range of charge is from 700 to 1400
VDC. The level of charge determines the pulse energy. R1, C1 and DS1 form a charge level indicator. DS1
flashes faster as the level of charge on the capacitor increases.
Discharging the PFN capacitor - There are four identical isolated trigger circuits on the Isolated Trigger PCB
- one for each of the four flash lamp circuits. Each has an optical fiber input from the controller PCB, and an
output to its associated flash lamp SCR. All four circuits share a common 30 VDC supply developed by zener
CR1 from the main charging capacitor voltage. All four circuits are functionally the same - the Trigger I
circuit operation will be described in the next paragraph.
To discharge the PFN capacitor through the I flash lamp, the Controller PCB sends a pulse of infrared light
through the optical fiber connected to RECV 1. RECV turns on, turning on its associated FET Q1. Q1 on
connects the 30 VDC through R2 to the gate of an SCR connected between capacitor ground and the cathode
of the flash lamp. The SCR turns on, providing a discharge path for current from the main charging capacitor
through the flash lamp and SCR to capacitor ground.
4.6 OPTICS
The optics include all components that act on the aiming and/or treatment beam. This includes the Ho:YAG
cavity module; Ho:YAG combining optics; folding mirrors; the energy sample optics; shutters; attenuators;
aiming beam laser and combining optics; the fiber focusing lens and blast shield.
Ho:YAG CAVITY - The Ho:YAG cavity provides pulsed output of 2.1 micron wavelength light
energy. The cavity is pumped with a xenon flash lamp. The lasing medium is a Ho:YAG rod. The rod
is positioned between an HR and OC (≈ 15% leakage) mirror. Up to four such cavities can be housed
in a single Versapulse Select system.
MULTIPLEXING OPTICS - The multiplexing optics consist of an imaging and flat mirror for each
cavity, and the servo positioned imaging mirror. For each cavity, an imaging mirror directs the OC
output off a flat mirror to the surface of the servo positioned imaging mirror. The servo positioned
mirror can be rotated to line up with each of the up to four YAG beam paths, so that each output is
directed down a common beam path.
12/95
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