Explore Modern Isolation Strategies for Medical Device Designs

Electrical safety is the cornerstone of medical device design, where industry and governments are aligned on protecting patients and healthcare operators from electrical, mechanical, and thermal hazards that can arise from the direct and often critical interaction between electronics and the human body. Since patients are frequently in vulnerable states, a failure in electrical isolation could result in serious, even fatal, injuries. Designers can simplify safety compliance by taking advantage of modern components that can replace complex discrete parts.

The international standard IEC 60601-1 provides a framework for safety and essential performance, primarily through the concept of Means of Protection (MOP). This framework distinguishes between Means of Operator Protection (MOOP) and Means of Patient Protection (MOPP), with the latter significantly more stringent due to the heightened vulnerability of patients, who are often directly connected to the equipment via conductive electrodes or probes.

The IEC 60601-1 standard specifies different parameters based on whether the device needs to conform to MOOP or MOPP classification (Figure 1). For basic insulation, MOOP typically demands 1,500 V_AC, while double insulation requires 3,000 V_AC. In terms of creepage, MOPP necessitates 4 mm for basic insulation and 8 mm for double insulation, twice what is required for MOOP.

Figure 1: MOP classification levels based on IEC 60601-1, Third Edition. (Image source: Analog Devices, Inc.)

Meeting the rigorous safety and essential performance requirements of IEC 60601-1 demands sophisticated isolation techniques while managing leakage currents and adhering to precise PCB layout requirements. Those who adhere to the standard can find an easier path to global regulatory approval.

Aligning safety design and risk management

In addition to the IEC standard, designers should follow the risk management framework in ISO 14971 to identify hazards, evaluate risks, and determine which protective measures are adequate for the device's intended use. With these guidelines, designers should begin with a comprehensive risk assessment of key considerations:

• Determining the operational environment, specifically whether the device will be used in clinical, home, or mobile settings
• Identifying if the device interfaces with the patient invasively (such as with catheters) or noninvasively (such as electrocardiogram (ECG) electrodes)
• Maintaining safety even if one protective measure fails

Unintended leakage current flowing through a patient should be severely limited, often kept below 100 µA, and for applications with direct cardiac connections, as low as 10 µA. PCB layout plays a vital role, as carefully spacing high-voltage and low-voltage traces prevents arcing and electrical faults.

Designers should not view MOPP and MOOP as, respectively, higher- and lower-quality safety measures (Figure 2). Rather, they are application-specific protection strategies based on the expected user and the associated level of risk. Importantly, the actual creepage, clearance, and dielectric withstand requirements for each protection category vary depending on factors such as the required working voltage, pollution degree, material group, and altitude.

Figure 2: MOOP and MOPP are application-specific categories, reflecting the distinction between patient and operator protection strategies. (Image source: Analog Devices, Inc.)

Coping with design constraints

Electrical isolation is only as effective as its implementation. Even when the appropriate isolation technology has been selected, PCB layout, leakage current management, and patient connection requirements ultimately determine whether a design satisfies IEC 60601-1. Designers must ensure the entire isolation barrier can provide the required levels of protection under both normal operating conditions and single-fault conditions.

Adequate creepage and clearance distances between conductive elements are essential to prevent dielectric breakdown and arcing across the isolation barrier. Creepage is measured along the surface of the insulating material, while clearance is the shortest distance through air. In space-constrained designs, routing slots and other layout techniques can increase creepage without expanding the board area.

Even during normal operation, small currents can flow through insulation and parasitic capacitance. While these currents are usually negligible in industrial equipment, they can pose a safety risk in medical devices, especially when patients are electrically connected to the system. Consequently, IEC 60601-1 sets stringent limits for earth leakage, touch leakage, patient leakage, and patient auxiliary leakage. These limits influence various decisions, including power supply architecture, filter design, and grounding.

The IEC 60601-1 standard also classifies applied parts according to how they interact with the patient:

• Type B applied parts are intended for general patient contact.
• Type BF applied parts provide additional isolation for conductive body contact.
• Type CF applied parts require the highest level of protection for applications involving direct cardiac connection.

These classifications directly affect allowable leakage current, insulation requirements, and ultimately the selection of isolation components.

Shifting to modern components

Designers employ a variety of isolation technologies to establish galvanic separation between high-voltage and low-voltage domains while meeting safety, performance, and regulatory requirements.

Isolation transformers are commonly used in power supplies to provide high dielectric strength while minimizing leakage current between primary and secondary circuits. Isolated communication interfaces prevent hazardous ground loops and maintain safety barriers in connected medical equipment. For example, isolated USB interfaces can enable patient-connected devices such as monitors and infusion pumps to meet applicable safety requirements.

Optocouplers transmit digital or analog signals across an electrically nonconductive barrier using light, providing signal isolation without a direct conductive path. While optocouplers are widely used, they rely on LEDs and photodetectors to transmit signals using light. This can lead to slower signal speeds, limited bandwidth, and performance degradation as the LED ages.

Among these approaches, digital isolation can provide the signal speed, integration, and reliability required by increasingly connected medical devices, using capacitive, magnetic, or RF coupling technologies to transfer data across an isolation barrier. Unlike isolation transformers, which transfer power while maintaining galvanic separation, digital isolators are designed primarily for high-speed signal communication.

The iCoupler approach

Analog Devices, Inc. (ADI) has developed an alternative to optical transmission. The company's iCoupler technology spans a broad portfolio of components that use chip-scale transformer/magnetic coupling to transmit data across the isolation barrier.

Integrating microscopic transformers directly onto an integrated circuit, iCoupler technology can reduce many of the constraints of traditional isolation products, such as cost, size, performance, and reliability. ADI's portfolio of digital isolators and isolated interface devices features highly integrated solutions that offer reliability, longevity, speed, and data integrity, compared to discrete isolation components.

Once the required protection level and isolation barrier have been established, component selection depends on what must cross that barrier. ADI's isolation solutions address different system-level requirements, including power conversion, signal integrity, communications, and high-voltage switching:

• Integrated DC-to-DC converters, such as the ADUM5020 series (Figure 3), provide regulated, isolated power and minimize radiated emissions.

Figure 3: The ADUM5020 is a fully integrated isolated DC-to-DC converter that supports high working voltage in medical device applications. (Image source: Analog Devices, Inc.)

• Digital isolators such as the ADUM2201 family provide reliable, high-speed data transmission up to 10 Mbps, depending on the variant, while supporting designs requiring reinforced isolation for medical applications.
• ADUM4120 isolated gate drivers enable safe and reliable control of high-voltage switching devices by separating low-voltage control circuitry from high-voltage domains.
• The ADUM4160 provides isolated USB communication, allowing data transfer across an isolation barrier while maintaining galvanic separation between connected systems, while the LTM2884 combines signal isolation, an isolated DC/DC converter, and serial communication functions in a single package, simplifying designs that require both isolated data and power.

Conclusion

Isolation in medical devices begins by identifying who must be protected, how the device interacts with the patient, and the MOOP/MOPP requirements that apply to each barrier. From there, designers can select integrated isolation components appropriate for power, data, communications, and switching functions while accounting for leakage current, creepage, clearance, and single-fault operation. ADI's portfolio of digital isolators and integrated isolation solutions can help simplify these designs by providing reliable galvanic isolation, high-speed communication, and compact implementation.