With the 3rd Edition of IEC 60601-1 impacting U.S. design engineers in June, it is critical they are aware of the implications to their medical device designs. For home healthcare devices, there is a collateral standard that will have a specific effect. This article focuses in on powering these products and the items in the standard of significance for that aspect.
The world is faced with an aging population and rising healthcare costs. Technology offers an opportunity to enhance people’s lives while addressing economic challenges. Widespread trends include the ability to make medical devices smaller and more efficient. Advances in battery chemistries along with more efficient power solutions offer a specific method to reduce the footprint and provide a more portable solution. The healthcare system trend is to facilitate home healthcare. This saves on costs by reducing the time in the hospital. The convergence of these trends has led to new design challenges for the power systems engineer.
In 2012, the IEC based 60601-1 3rd Edition standard became mandatory in the European Union and in June of 2013, it will be required in the United States. The 3rd Edition standard introduced several new concepts to the design engineer. Terms like means of patient protection, means of operator protection, risk management, and essential performance have now become part of the specification and design process. In addition to these changes to the base standard, there were new concepts introduced. One of the new additions is 60601-1-11.
“Requirements for Medical Electrical Equipment and Medical Electrical Systems Used in Home Care Applications,” a collateral standard 60601-1-11 was specifically targeted for the home healthcare market. As a collateral standard, it is applied along with the General Standard 60601-1 for a particular application. The movement from controlled clinical settings with trained personnel to the home with users not specifically educated requires the medical device to have additional protection measures. Much of this is covered in the labeling and documentation, which is designed to clearly identify the operating conditions and hazards (Figure 1).
In addition to the educational effort, consideration for AC power was also a factor. In clinical settings, the infrastructure is a controlled factor. Earth ground is tested to code and can be relied upon in that setting; however, it was determined this is not the case for homes. Many homes do not have a reliable Earth ground and, therefore, a condition of the standard requires the power device to be operated as a Class II device. A Class II AC input does not rely upon earth ground as a protection path. It provides a higher degree of isolation while still meeting EMC requirements, which poses unique design challenges.
Traditional power supplies utilize a Class I input often referred to as a “three wire” AC input. The third wire is the earth safety ground and is used as a safe path in the event of a fault, protecting the user from hazardous shock. The typical AC input circuit of a power supply is shown in Figure 2.
The capacitors connected between the line or neutral and ground are specifically rated to fail safe. They also provide a path to harness and control conducted emissions that are present in switching power supplies due to the voltage being switched at high frequencies. A Class II device does not have this capacitive path and, therefore, great care must be taken by the design engineer in circuit design.
The result is that the power supply design engineer must pay careful attention to the circuit to ensure isolation and EMC compliance. The block diagram in Figure 3 illustrates the isolation barriers for the system.
The optimized power solution will take into consideration many factors, including power, input voltage, and output voltage. These factors will influence the topology selected and switching frequency. Regardless of those choices, the key component will be the transformer.
Transformer design is crucial for all power designs. In isolated converters, it becomes an important barrier from primary to secondary. Topology, switching frequency, power, and form factor will all influence the design decision. Techniques to optimize required isolation and tight magnetic coupling include approved triple insulated wire, multi cavity bobbins and split winding techniques. In all cases, the goal is to maintain the required creepage and clearance distances while achieving tight magnetic coupling.
Technology advances provide the opportunity to enhance people lives. Standards are designed to support the use of technology in a safe manner. Power supply designers and integrators must pay particular attention to ensure the design provides the isolation voltage, leakage current, and all the elements necessary to support the medical device market. The introduction of the 60601-1-11 collateral standard is an important step to facilitate the introduction of advanced technology in a safe manner.
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