Helium-neon lasers are useful in a variety of tasks, but they're difficult loads for a power supply to handle. Powering a laser usually involves some form of startup circuitry to generate the initial breakdown voltage and a separate supply to sustain conduction. Typically, almost 10 kV is needed to start conduction, and about 1500 V is required to maintain conduction at their specified operating current. The circuit shown considerably simplifies driving the laser (see the figure). The startup and sustaining functions are combined into a single closed-loop current source with over 10 kV of compliance.
When power is applied, the laser doesn't conduct and the voltage across the 190-Ω resistor is zero, and the LT1170 switching-regulator "FB" pin observes no feedback voltage. Therefore its switch pin (VSW) provides full duty-cycle pulse-width modulation to L2. Current flows from L1's center tap through Q1 and Q2 into L2 and the LT1170. This flow causes Q1 and Q2 to switch, alternately driving L1.
When the 0.47-µF capacitor resonates with L11, it supplies boosted sine-wave drive. L1 provides substantial step-up, which causes about 3500 V to appear at its secondary. The capacitors associated with L1's secondary form a voltage tripler, producing over 10 kV across the laser. The laser breaks down and current begins to flow through it.
The 47-kΩ resistor ballasts the laser, which limits current. That current flow causes a voltage to appear across the 190-Ω resistor. A filtered version of this voltage appears at the LT1170 FB pin and subsequently closes a control loop. The LT1170 adjusts its pulse-width drive to L2 to maintain the FB pin at 1.23 V, regardless of changes in operating conditions. The laser can then see constant current drive, which in this case is 6.5 mA. Other currents are obtainable by varying the 190-Ω value.
The 1N4002 diode string clamps the voltage when laser conduction begins, protecting the LT1170. The 10-µF capacitor at the VC pin frequency compensates the loop. The MUR405 maintains L2's current when the VSW pin isn't conducting. The circuit starts and runs the laser over a 9- to 35-V input range with about 75% electrical efficiency.