Medical Electronics
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 electro-medical 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. For some world-class power supply manufacturers, Risk Management is part of the Quality Standard processes though not in the same format or level as described above. In regards to design controls, DFMEA (Design Failure Mode and Effects Analysis) is a method for evaluating a design for robustness against potential failures. Mean-time-between-failure (MTBF) calculations and component de-rating are guidelines used to select appropriate components that meet the required application conditions and product life. For purchasing controls, the rigorous and disciplined use of a preferred parts list and approved vendor list ensures that only those parts and suppliers that meet performance and quality metrics are considered and
used. To improve production and process controls, full functional, Hi-Pot and safety tests are completely done in tandem with lot sampling for burn-in and accelerated life tests based on a standard quality plan. In addition, traceability, field failures and failure analysis data are correlated back to production metrics performance and specific component lots or date codes when needed.
Collaboration
Discussion between the OEM and power supply vendor is crucial. This collaboration can help identify any obstacles in the power supply and how to properly adjust. Looking at the specifications together can go a long way in minimising the work done to make the medical device including the power supply component comply to the IEC 60601-1 3rd edition. Important discussion points include: • MOOP or MOPP classification • Classification of Applied Parts: Type B, BF or CF
• Main Fuses and over-current releases • Elevation • Leakage Current • Safety insulation for transformers • Y1 &Y2 Capacitors • Hipot voltage • Creepage and clearance
Classifying the particular medical device application into MOOP, MOPP, Type B/BF/CF are key questions since these determine the degree of evaluation the power supply and medical device need to undergo. MOOP is less stringent than MOPP. Within MOPP, Type B has the least stringent requirements, followed by BF.
Type CF has the strictest specifications. These classifications have to be determined by the medical device OEMs by properly considering the device’s intended use and particular application environment. On several occasions, forcing an existing power supply to comply with the demands of the 3rd edition, especially on MOPP, requires a handful of component changes and a complete printed circuit board (PCB) re-spin. That entails practically repeating the power supply qualification (i.e. DVT build and QA verification), if not an abbreviated one. This in turn may force verification and validation on the medical device level as well using the upgraded power supply.
Also, if some of the requirements listed in the bullets above are not met by the power supply, the medical device design engineers may look at the system level if such requirements are considered outside of the power supply, but within the medical device like having redundant fuses, capacitors and insulation.
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 setting, 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, de-rate to approximately 50% at 70 ºC before eventually shutting down. Some PSU designs run cooler and do not de-rate 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 signalling 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.
Conclusion
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. Also, a closer look at the medical device application and environmental conditions that match a power supply’s features and performance is important.
SL Power Electronics |
www.SLPower.com
Cochise Mapa is Director Global Product Management, SL Power Electronics
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Components in Electronics October 2012 25
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