What does the future hold for the power supplies of medical devices?
Shane Callanan, director of engineering technology, Advanced Energy Industries, reflects on the evolution of medical device power supplies – and the new requirements shaping their future.
According to the New England Society of Medical Engineering, up to 10,000 people per year were electrocuted in American hospitals during the late 1960s and early 1970s as a result of leakage current from defective electronic medical equipment. Defined as the flow of electric current in an unwanted conductive path under normal operating conditions, leakage current is a direct function of the line-to-ground capacitance value. As long as the equipment is grounded, these currents will flow in the ground circuit and present no hazard. If the ground circuit is faulty, however, the current flows through other paths such as the human body. With staggering statistics prompting the healthcare industry to conduct a wide range of studies to examine the risk of ventricular fibrillation by electric shock, researchers eventually determined that as little as three micro amps (0.000003 amps) of voltage applied directly to a portion of the heart during a critical part of the cardiac cycle could cause lethal arrhythmia. It is a small amount, considering a lightning strike – which many people survive – contains a minimum of 5,000,000,000 micro amps (5000 amps). But for patients with compromised health, even the smallest voltage can be fatal.
Medical devices, which require a consistent and safe power supply, have some of the tightest leakage current requirements of any industry. And it is understandable, given the high risk involved and a history of tragic outcomes. Up until the 1970s, when the Switch Mode Power Supply (SMPS) began gaining popularity, the backbone of power conversion was the linear Direct Current (DC) power supply, a design requiring a large transformer to raise or lower the Alternating Current (AC) voltage and produce a clean DC voltage. Because the transformer size is indirectly proportional to the frequency of operation, the power supply is typically large and heavy but best for sensitive analog circuitry.
Alternatively, a SMPS not only converts the AC line power directly into a DC voltage without a transformer but also converts raw DC voltage into a higher frequency AC signal that is used in the regulator circuit to produce the desired voltage and current. The result is a transformer that is considerably smaller and lighter than a linear power supply—by as much as 80 percent—and best suited for portable devices. Generating less heat, the SMPS is often more efficient than linear but transient response times can be up to 100 times slower, which presents challenges—and the need for expertise—in manufacturing.
Particularly as medical devices increasingly use sensitive analog electronics, wireless technologies and microprocessors, it is imperative that during the manufacturing process, power supplies meet customer needs as well as essential standards, including electromagnetic compatibility (EMC) compliance. For example, health professionals performing cosmetic procedures with medical-grade lasers want and need high-power density and reliability incorporated into a portable power supply that has longevity, black box recording capabilities, seamless connectivity and firmware that is easy to upgrade. With the amount of allowable leakage current specified to ensure that direct contact between a patient and any medical equipment is highly unlikely to result in electrical shock, manufacturers must deliver a power supply that not only meets design specification, but also includes appropriate isolation barriers, to minimize leakage current and ensure the safety of both patient and clinician.
Even with the challenges of design and compliance ever present, as new equipment can take years to develop and refine before reaching the market, advances in power supplies have enabled remarkable improvements in surgical procedures alone. Common knee replacement surgeries, the first of which were performed in 1968, are one of the most successful medical procedures today. Many well-equipped surgeons are now operating with the help of 3D models, which are fed into the power supply. In the event the surgeon tries to remove too much or too little of the knee in respect to the 3D model, the power supply shuts off immediately to avoid any errors. The impact of this new approach has been tremendous, with success rates jumping from approximately 50 percent to 90 percent or higher. Advanced power supplies have also enabled life-saving thermal solutions. Among the many applications utilising heating and cooling therapies, doctors are using helmets with automatic and interval temperature adjustment capabilities designed to prevent brain damage in newborns.
Though invisible, energy is ubiquitous in every facet of life and nowhere more critical than the field of healthcare, where research is accelerating, and more advanced devices and equipment are launching to market at a rapid pace. Driven by a gradual shift in focus to patient care and speed of recovery, the need for more accuracy in terms of treatment requires new diversity in supply, from Radio Frequency (RF) amplifiers that power ultrasonic equipment to thermal solutions for heating and cooling. While change has been slow and significant progress achieved only in the last ten years, advanced power supplies have undoubtedly provided the medical industry with scalable solutions enabling smaller, more reliable and powerful devices that reduce cost, complexity and human error while heralding a new era in medical innovation.