For medical devices, power supplies are required to meet exacting standards as defined in the IEC 60601-1 3rd edition, that were effective as of June of 2012 for Europe. The IEC 60601-1 standard is a globally recognized standard for electro-medical equipment safety. The next evolution in the IEC 60601-1 third edition requires a risk management file and process conforming to ISO 14971, the international standard for application of risk management to medical devices.
Medical device OEMs are required to meet this standard for their end products and are responsible for the risk management and applications categorized as means of operator protection (MOOP) or means of patient protection (MOPP). Even though the third edition standard really does not apply directly to components, power supply manufacturers are working closely with their medical device customers to comply with this new regulation. For next-generation medical device products, it is easier to start requiring power supplies certified to the third edition. For existing electromedical devices especially those in the EU market, complying may mean re-qualifying another power supply through end-device verification and validation or doing risk assessment of the continued use of current power supply.
Risk management covers risk analysis and evaluation followed by risk control to bring overall risk to an acceptable level, with a continuous monitoring and feedback process. Below is a table typically used to identify hazards and conditions affecting safety and critical performance of the medical device, to gauge the severity of harm done, identify the cause of the condition, to assign probability of occurrence and to define activities to mitigate the risks. To include the power supply in the risk assessment, the medical device product team (i.e. staff coming from regulatory affairs, engineering, marketing, quality, etc.) should consider feedback from their power supply partner in identifying the hazard conditions as well as the probability of those events occurring. The four items listed below are common failure modes recorded in the FDA adverse events database as well as causes in product recalls. The probability of occurrence can be provided by the power supply partner based on their field failure data (parts per million failures on the corresponding product or family). The nature of the clinical or therapeutic function of the electromedical device affects heavily the severity and hence the risk of each failure mode. Typically, a power failure merely causes postponing the use of the medical device until another unit comes along. However, when the need for the medical device is urgent and essential to administering therapy quickly, then any delay due to non-operation can be a real issue. The rest of the information below are generic examples and require more detail in a true risk assessment documentation.
Hazard Causing Harm
“S” x “P” = “R”
Risk Control Measures
Device stops working
Component or circuit failure
Device will not start
Device stops working or Thermal shock
Overload or low input voltage
Leakage current too high
Choosing the Right Design Approach
Many considerations are involved in defining the optimum power supply design including the specific medical device application. Patient monitoring equipment like pulse oximeters, ECG or ICU monitors can be sensitive to common-mode noise resulting from the output of a power supply where the high-impedance high-gain amplifier sensors can malfunction in the presence of common-mode noise. Another consideration is fan cooling within the medical device. In most hospital and operating room settings, internal fans are avoided because they circulate contaminants, they have shorter operating life, and they tend to be noisy. Some convection cooled power supplies operate at their full load power rating from zero to 50ºC, derate to approximately 50% at 70ºC before eventually shutting down. Some PSU designs run cooler and do not derate as much. So power convection rating is a key factor to consider. Another factor comes from recent findings arising from medical device adverse events reports showing PSU overheating and shutting down. An over temperature signaling feature from the PSU to trigger the medical end device to regulate non-essential power consumption would be an advantage. PSU features like remote inhibit can also support the self-diagnostic testing function done by the medical device.
The implementation of the IEC60601-1 3rd edition helps to ensure performance and safety throughout the lifetime of a given device. Forward planning and upfront collaboration with suppliers will help mitigate problems later down the road.
By Cochise Mapa, director global product management, SL Power Electronics