Medical Electronics
When efficient power solutions meet light in medical lasers
By Patrick Le Fèvre, chief marketing and communication officer for Powerbox M
edical lasers have found their way into a wide variety of medical applications: there are many examples of laser
treatments in ophthalmology, oncology and other forms of surgery that we have all either benefited from or heard about. If the nature of the laser source is specific to the targeted treatment, i.e. generating light emission in the range of 193 nanometers (Excimer ArF) to 10.600 nanometers (CO2) (Figure 1) and pulses from 5 nanoseconds to 1 millisecond, they all have something in common - a power supply. Besides powering the embedded computing and other electronic equipment, medical lasers require very specific power systems able to deliver repetitive high peak energy (voltage or current), with safety and reliability. A function of the final application, each type of laser requires a different type of power supply which can vary from a current generator for continuous-wave diode laser, to complex power solutions in the case of gas-lasers or lamp-pumps using flash-lamps as a light generator.
We could probably identify as many power supplies as there are types of lasers used in the medical space, although as a power supply manufacturer we simplify it to two: n Constant current types to power Light- Emitting Diode (LED) laser type n High voltage types to power flash- lamps and discharge tubes
Powering LED lasers
Originally limited in their power, diode lasers were not very common in medical applications, however with the development of a wide range of diodes generating wavelengths from 405 nanometers to 2200 nanometers, they become popular in the field of photodynamic therapy where the wavelength is more crucial.
As it is for other applications using LEDs (e.g. lighting) the power supply is often defined as an LED Driver. Used both in the new generation
46 May 2022
process’. In this type of application, the design of the power supply requires specific knowledge in high voltage switching and energy storage.
Lamp-pumped solid-state lasers and gas-laser power supplies have complex specifications, requiring two elements: a power supply converting the AC line voltage to the high voltage required by the emitting element, and a high-voltage capacitor energy bank for energy storage. Voltage will depend on the level of energy required to activate the pumping, but in conventional medical applications it is often between 600VDC and 3,000VDC.
Figure 1
of solid-state lasers or as a generator as such, laser LED drivers require particular attention to the stability of the current and compensation of the energy delivered in terms of the temperature of the LED element. Modern current generators for LED lasers are based on digital technology with an input/output (I/O) interface making it possible to monitor and control the power supply to meet application requirements. Using predictive algorithms, the power-stage can be programmed to operate safely and to deliver the specific energy required by a single pulse.
An LED laser could operate in the range of few milliwatts to more than 100 watts when using an LED matrix such as the ones used in LED solid-state pump-lights. With the development of supercapacitors, LED drivers for lasers often use them as energy storage. In such cases the power supply includes special circuitry that controls the energy stored in the supercapacitor to optimize, cycle-by-cycle, the level of energy delivered to the load.
Seen from a power supply designer’s viewpoint, powering and LED laser applications are very similar to conventional current generators, which is not the case
Components in Electronics
when designing power solutions for gas lasers or lamp-pumps using a discharge tube.
Powering gas and high energy solid-state lasers
Gas and high-energy solid-state lasers use flash-lights or discharge tubes that require high voltages to generate the necessary energy levels needed to initiate the ‘pumping
Similar to your flash camera, the power supply charges a capacitor, which then delivers the energy to the flash lamp. However, while we can accept a small delay in charging the capacitor of our personal camera, in the case of a medical laser the energy needs to be available without delay, requiring a capacitor-bank to store high amounts of energy.
For power designers not used to dealing with high energy transfer topologies, it can be difficult to estimate the size of the energy envelope and preferred control method to optimize the power stage.
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