Medical Electronics Power supplies in medical equipment
Hafiz Khalid, product marketing director at XP Power, outlines the top five design factors engineers must consider when selecting power supplies for medical applications.
M
edical equipment design is subject to stringent regulations, with safety as the paramount concern. The IEC 60601-1 standard
governs a broad spectrum of equipment types and use environments, from battery- powered portable monitors to MRI scanners that consume multiple kW of power, and from operating theatres to point-of-care services. Despite commercial pressures to minimize costs and incorporate advanced features such as automation and AI, safety standards must be meticulously adhered to.
1. Risk assessment
Recent editions of IEC 60601-1 mandate risk assessment and management as the foundation of medical product design. Manufacturers must evaluate the likelihood of patient contact and apply suitable protection levels. The standard distinguishes between a medical device (MD) that may come into contact with a patient and an in vitro diagnostic medical device (IVD), such as a centrifuge or blood analyzer, which does not.
The risk management process must comply with the ISO 14791 standard, and the ‘Application of risk management to medical devices’ document covers the entire product lifecycle. It also considers various other risks, including biocompatibility, data security, radiation, and usability. While these additional considerations may not apply, increasing digital control and monitoring within power supplies represents a new risk. Data and system security must be closely scrutinized for potential data corruption or tampering risks, making it crucial for engineers to be cautious and alert.
It is important to note that ISO 14971 simply provides a framework and does not specify dangers, acceptable risk levels, or risk quantification methods. Figure 1 illustrates a typical risk management workflow.
2. Prioritizing safety
Safety is the foremost requirement in medical equipment, encompassing protection against electric shock, fire, and mechanical hazards. The IEC 60601-1 employs ‘Means of Protection’ (MoP) terminology, requiring at least two MOPs
22 November 2024 Components in Electronics
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standards that require reinforced isolation, for example, follow this same theory – two separate levels of protection or one level with enhanced protection like think, solid isolation.
The standard distinguishes between operator and patient environments with differing isolation creepage and clearance distances. Two Means of Patient Protection (2 x MoPPs), for example, necessitates 8mm creepage and a 4kVAC test voltage for up to 250VAC system voltages. When connection to the patient is intended or probable, the medical safety standard demands consideration of the possibility of other equipment failing, leaving the patient ‘energized’ with mains voltage. In this instance, there should be no passage between equipment operating normally and the ground, allowing a fatal current to flow. In practice, this entails providing at least 1 x MoPP isolation between patient connections and ground in the medical equipment. Power supplies that comply with IEC 62368-1 (the standard for audio/ video, information, and communication technology equipment) typically do not meet this requirement, where the output may only have functional isolation to ground if any at all.
Figure 1: A typical risk management process according to ISO 14971
to prevent shock. This prerequisite follows the principle that two failures occur before the equipment becomes unsafe. Other safety
The same rule applies to undefined external signal connections to medical equipment that is connected to patients, as well as their power supplies. In such instances, 2 x MoPP isolation is mandated from these lines to patient connections, as one must presume the signal lines may be charged with AC mains voltage due to other malfunctioning equipment. Conversely, if
the connected equipment is specified, then 1 x MoPP to the interface is deemed sufficient. To fulfill these stipulations, additional low- power DC-DC converters with either single or double MoPP isolation and exceptionally low coupling capacitance are occasionally employed within power lines (Figure 2). The requirements for isolation for operator protection (Means of Operator Protection or MoOP) are like those for IEC 62368-1. Still, slight differences exist, for example, in fusing arrangements and allowed leakage current.
3. Leakage current considerations There will always be some leakage current from the AC mains through to the power supply chassis or output due to the inherent stray coupling capacitance or EMI suppression capacitors. In medical applications, acceptable leakage current levels vary depending on the equipment’s proximity to the patient’s heart. For normal and single-fault conditions, the specified leakage current can be as low as 10µA when the equipment directly connects to the heart. This is known as the ‘Cardiac Floating’ (CF) use case. The allowable levels are shown in Table 1.
For medical equipment that only requires 1 x MoOP, such as in labs, the overall earth leakage is 0.5mA in normal conditions (Note: this figure drops down to 0.3mA in the US). The Europe-wide limit is comparable to the Energy Source 2 (ES2) limit of up to 5mA at 50/60Hz, according to IEC 62368-1.
4. Compliance with EMC requirements
Medical devices must adhere to EMC requirements as outlined in IEC 60601-1-2,
Figure 2: An
arrangement of power supply and specified connected equipment with MoPP levels
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