Medical Electronics
Figure 2
The required output power needed for lamp-pumped solid-state or high- powered pulsed-excimer lasers is usually given in terms of joules per second, which is a function of charge time, repetition rate, output voltage, and component characteristics. During a charge-discharge cycle, the rate of change in voltage is not constant, which differs very much from conventional applications where usually the peak-current and charge rate are pretty well defined. Designing such types of power supplies requires very close cooperation with the equipment manufacturer to test the power solution in real conditions. It is very common for medical laser manufacturers to split the power solution into two parts. These are the power supply itself (Figure 2) and the high-voltage capacitor bank, which for safety reasons could be in a sealed tank.
In terms of technology, modern power supplies use digital control techniques, not only improving efficiency but in the case of medical lasers, also improving the reliability of the equipment due to its operating principle being based on pulsed energy which is known to be stressful for electronic components.
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Using digital control offers huge benefits in terms of energy management. It is possible to control the power unit to a single bit and to adjust all parameters cycle by cycle. For example, during surgery the surgeon could request more power or longer pulses for tumor ablation. Controlled by the embedded computer, the power supply can be programmed in between two pulses to change the charging voltage and/or the amount of energy required by the emitting element.
What else should power designers consider? Safety - There is no doubt that dealing with high voltages and significant amount of energy requires close attention to safety. Usually built into a final equipment, obviously the power supply must comply with overall safety and environmental regulations, but during the design process power designers must pay special attention to all risks related to hazards inherent to high voltages.
Risk - Regarding power supplies included in final equipment and not medical equipment as such, certain customers are requiring a full risk assessment analysis
e.g. ISO 14971, which must be considered from day one.
EMI - High-peak energy switching generates electromagnetic emissions and line disturbances which may interfere with other medical equipment. Filtering and power factor correction requires special attention during the design phase to not only comply with standards and regulations at the time of certification, but to take cognizance of the aging of filtering components e.g. electrolytic capacitors during the lifetime of the equipment. Noise & thermal – In addition to local regulations, hospital, medical and para- medical institutions are requiring electronic equipment to operate without audible noise. Considering that laser equipment includes a number of dissipating elements, forced cooling is often required. To achieve good cooling with the lowest audible noise, manufacturers are using large fans rotating at low speeds to cool down their systems. For safety, capacitor-banks and power supplies are housed in sealed boxes, limiting cooling to conduction through the chassis. This is an important point to consider during the design of power supplies for medical lasers.
How can new power technologies benefit medical lasers?
For many years the size of a power supply for medical laser use hasn’t been a real concern - but things are changing. Medical laser manufacturers are considering a new generation of ‘portable’ lasers for homecare and to increase the mobility of medical services. Research to develop more powerful, and with combined wavelengths, LED lasers are showing good progress. Operational control is easily performed on a tablet (no more built-in displays) but of course parts of the equipment will require a serious diet to achieve portability. In the case of LED lasers, supercapacitors based on nanotechnologies are offering impressive capacity levels to store high energy and using Wide Band Gap semiconductors e.g. Gallium Nitride, Silicon Carbide, the size of the power supply could be shrunk by a factor of x3. This is very promising and I have no doubt that LED medical lasers will benefit from the latest technologies and innovations happening in the power supply industry.
www.prbx.com Components in Electronics May 2022 47
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