Berkeley Nucleonics Corporation
2955 Kerner Blvd., San Rafael, CA 94901 ° Tel (415) 453 9955 ° Fax (415) 453 9956 ° www.berkeleynucleonics.com
2.0 Overview
The PCO-6131 is a compact, OEM-style high power pulsed current source
designed to drive diode lasers, bars and arrays in pulsed, QCW or CW modes. It
delivers output current variable from 1 A to 125 A, pulse widths variable from <100
ns to DC, and pulse repetition frequencies variable from single-shot to 500 KHz at
duty cycles up to 100%.
The PCO-6131 is based on a hysteretic, average current, switch-mode regulator.
This type of regulator is a variable frequency, variable pulse width design which
maintains current in an energy storage inductor between a minimum and
maximum level. The ripple is limited to the minimum and maximum current
determined by the hysteretic controller. The controller turns on to charge the
energy storage inductor when the current drops to the lower limit and turns off
when the current reaches the upper limit and repeats this operation as necessary
to maintain the proper current. The time for these operations will vary dependent
on the load voltage and the input voltage, therefore the pulse width of the
controller will change as necessary to charge the inductor and the period will
change dependent on the rate at which the inductor discharges. The relationship
for this operation represented by V = L
di
/
dt
. When the output is shorted such as
when no pulse is being outputted, the voltage applied to the inductor is 24 Volts.
This results in a very fast increase in current so very short on times are necessary.
When the controller switches off then a very small voltage is across the inductor so
the current decays very slowly and therefore the off time is quite large. All this
reverses when the output is connected to a load. When the load is near the
maximum V, the time to charge the inductor is very large but the decay time is
very short. The advantage of this controller is that the current is controlled to an
upper and lower limit regardless of the pulse width. It can generate short or long
pulses and performs as a current source.
The current regulator is started when the TTL "enable" line is taken high and runs
as long as the enable is high. This happens when the control gate is taken high
and will continue until the gate control is taken low. It will take a finite time for the
current source to charge to the proper current (i.e., a ramp-up time), after which a
pulse can be generated. The use of the hysteretic regulator provides a large input
range and high efficiency.
The current source is combined with a crowbar (shorting switch). The shunting
crowbar switch shorts the output of the regulator until output current is needed.
The pulse is generated by opening the shunt switch for the length of the input
pulse. A pulse is generated when the control gate is high, the shunt switch is
opened for the length of the input control gate, forcing the current to flow through
the laser diode load. The pulse rise and fall times are then limited only by the
stray/parasitic capacitance and inductance of the shunting switch and output
leads.