Special considerations apply for power supplies when even small electrical currents pose a shock hazard.
Dermot Flynn, Chris Jones • Advanced Energy
The consequences of leakage current came to the forefront in the 1970s, and since then technical standards for the safety and effectiveness of medical electrical equipment have evolved to become the most stringent of any industry. 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 equipment is grounded, these currents will flow in the ground circuit and present no hazard. However, if the ground circuit is faulty, the current flows through other paths such as the human body.
Over the years, researchers have discovered that as little as 3 µA (0.000003 A) applied directly to a portion of the heart during a critical part of the cardiac cycle could cause lethal arrhythmia. To ensure patient safety, the International Electrotechnical Commission (IEC) created two main standards for power supplies including IEC 60950-1 for ITE (information technology equipment) and IEC 60601-1 for medical equipment. Both standards protect against electrical shock, but as medical equipment may come into contact with a patient, IEC 60601-1 requires a higher level of safety.
The classification of medical equipment for protection from electric shock is as follows:
Class I: Reliable protective earth is provided such that all metal parts cannot become live in case of insulation failure (three-pronged ac plug – live, neutral and electrical ground).
Class II: No protective earth. Double or reinforced insulation is used against electric shock (two-pronged ac plug – live and neutral only).
Class I or II when external power source is used. Classification does not apply if internally powered by battery.
Protective earth: Ground conductor in the power cord, also known as chassis ground.
Additional criteria within IEC 60601-1 defines three distinct categories requiring protection against electrical shock from an applied part (any part of the electrical medical equipment that comes into contact with a patient):
Type B = “Body” – Sometimes considered “patient vicinity.” No electrical contact with the patient and usually earth-grounded. Type B is the least stringent classification and is used for applied parts that are normally not conductive such as LED lighting, medical lasers, MRI body scanners, hospital beds and phototherapy equipment.
BF = “Body Floating” – More stringent than Type B but less stringent than CF, it is generally used for applied parts that have conductive contact with the patient but not directly to the heart. Examples are blood pressure monitors, incubators and ultrasound equipment.
CF = “Cardiac Floating” – Electrically connected to the heart of the patient, CF is the most stringent classification. It is used for applied parts that may come in direct contact with the heart, such as a dialysis machine.
Research in the medical field is accelerating, and the resulting advanced devices require advanced power conversion technology that also meets safety regulations. Choosing a power supply for a medical product, however, is no easy task. In addition, it is critical to determine if a standard “off-the-shelf” medical power supply or a custom medical power supply can meet the specific application requirements.
Custom vs. standard
Custom designs have many advantages including providing exactly the power needed for a specific application. On the flipside, there is development cost, non-recurring engineering (NRE) fees, and a development time typically spanning 8-12 months. The supply is also single-sourced unless another vendor is paid again for development.
The advantages of standard medical power supplies include the wide variety in industry standard voltages and packaging, immediate availability and safety pre-approval. They can also be modified by many manufacturers and distributors, and are more readily available from a supply chain perspective. In addition, there is no upfront cost for NRE or safety. However, COTS power supplies may not be ideal for medical products as they offer limited control over modifications and the lifecycle of the power supply. For multiple outputs, configurable power supplies offer great flexibility with millions of output voltage combinations with safety approvals.
Most medical equipment designs have multiple components with differing power needs, so designers often use a distributed power architecture. If a product requires low-power ac-dc, there are often options between a power supply embedded in the end-product or an external unit such as an adapter. In addition, many medical instruments, including mass spectrometers and scanning electron microscopes, need high-voltage dc sources. So it is important to look at power from an overall system perspective. Devices that require medium or high-power ac-dc, consider the following questions:
Is it offered in single-and three-phase input?
Is it available with or without intelligence?
Is it generally enclosed with fan cooling?
Fan cooling in particular is an important consideration. In medical applications having acoustic and vibration sensitivity, fanless power supplies offer advantages. Cooling is via the natural flow of air (passive natural convection) or the transfer the heat through direct contact with a cooler component (passive conduction). Passive natural convection in power supplies usually involves an open-air rack, where the natural movement of air across electronic components removes some excess heat. Passive conduction was once only possible with the most basic power supplies. But advances in thermal modeling, component design/selection and materials technology now enable high-performance units to be cooled without using fans.
Electromagnetic compatibility (EMC) is also critical in power supply design and selection. EMC requirements for medical electrical (ME) equipment and ME systems are stated in IEC 60601-1-2, a collateral standard to IEC 60601-1. Under this medical EMC standard, power supplies are classified as “non-Medical Electrical (ME) equipment” and therefore are technically required to comply only with IEC EMC standards applying to that equipment (e.g., ITE immunity standard EN 55024), provided the power supply will not result in the loss of basic safety or essential performance of the ME system in its intended environment. Essential performance is determined by the ME system manufacturer. Given that there are no specified performance criteria for the non-ME equipment itself, the ME system manufacturer should determine whether the EMC performance of a given power supply is adequate for the specified application.
Digital control now comprises the feedback loop for many power supplies, via communications channels such as I2C and PMBus, taking the place of older, analog feedback control. Advantages of digital control include reduced parts count, greater flexibility and improved productivity. During development, power supply design bugs can be fixed by firmware patches, eliminating multiple PCB re-spins. In addition, firmware and software can be upgraded in the field via the internet without physically attending the system, and modular code can be reused.
Digital power supplies contain a history log that enables backtracking of failures with failure modes including PMBus/IPMM-defined events such as over voltage (OVP) and over current (OCP). To assist in field failure analysis, additional events can be recorded such as the runtime of the power supply (power-on-hours), the actual maximum load, the actual maximum ambient, and the actual maximum input.
Digital control also enables the use of a graphical user interface (GUI) as an aid to engineers. It is a visual tool for debugging and modifying power supply issues such as input thresholds, output settings, delay times, switching frequency, loop response and other parameters. A universal PMBus GUI also enables enhanced power management features, including:
Monitoring and reporting power supply input parameters such as voltage, current, and power; self-diagnosis for potential failures or weakened parts by monitoring and comparing specific parameters against nominal values; event logging of conditions such as input voltage surge and sag, ac recycles, maximum ambient/internal temperatures, and failure logging to backtrack failures; fan speed optimization based on load and ambient temperature.
All in all, significant progress in the past few years has equipped the medical industry with smaller, more reliable power supplies that reduce cost, complexity and human error. Looking beyond the volts, amps and safety approvals, it is also critical to find a vendor of medical-grade power supplies that will be a trusted partner for many years to come.