Power Architecture for Patient-Connected Medical Devices

Overview

This article discusses approaches to power architecture in medical devices intended for patient-connected applications. Key factors to consider include insulation, leakage/contact current, electromagnetic compatibility (EMC), the number of required power outputs, and the intended operating environment.

Patient-contacting components

Components of a medical device that contact the patient are referred to as application parts. An application part is a portion of the medical device that directly contacts the patient or may contact the patient during normal use, and is necessary for the device to perform its function.

Applicable standards

IEC 60601-1 classifies application parts based on the type of patient contact and the nature of the medical device. The current third edition was originally published in December 2005 and is adopted in major jurisdictions in the following versions:

• IEC 60601-1:2005 (3rd edition + CORR. 1:2006 + CORR. 2:2007 + A1:2012)
• Europe: EN 60601-1:2006/A1:2013/A12:2014
• United States: ANSI/AAMI ES60601-1: A1:2012, C1:2009/(R)2012 and A2:2010/(R)2012
• Canada: CSA CAN/CSA-C22.2 No. 60601-1:14

Each classification in the standard has specific protection-against-electric-shock requirements. The three classifications, listed from least to most stringent, are:

• B type (B): for application parts that are generally non-conductive and may be grounded.
• BF type (BF): for application parts that are electrically connected to the patient and must be floating and not grounded; this classification excludes application parts in direct contact with the heart.
• CF type (CF): for application parts that connect directly to the heart, including venous connections such as dialysis. These application parts must be floating and not grounded.

Insulation requirements

Medical devices that connect to patients must provide two means of protection (MOP) to prevent application parts and other accessible parts from exceeding voltage, current, or energy limits. A protective earth connection provides 1 MOP; basic insulation provides 1 MOP; reinforced insulation provides 2 MOP. Protection measures are further classified as means of operator protection (MOOP) or means of patient protection (MOPP). For patient-connected devices, 2 x MOPP is required.

Power architectures for BF & CF class medical devices must provide one or two levels of 2 x MOPP, one level to earth of 1 x MOPP, and additional safety insulation from a secondary output to earth rated as 1 x MOPP, all specified at the highest rated mains voltage.

Leakage current

Power systems must be designed to limit touch current, patient auxiliary current, and patient leakage current. The maximum allowable touch current is 100 μA under normal conditions and 500 μA under a single-fault condition, effectively limiting chassis leakage during normal operation to 500 μA. Maximum allowable patient auxiliary current and patient leakage current are specified in the standard.

For patient-contact medical devices that require electrical connections, the designer of the power system faces the challenge of providing the required safety insulation while minimizing leakage current to earth in both normal operation and fault conditions by isolating the patient from earth and providing appropriate protection.

Electromagnetic compatibility (EMC)

Medical devices must also meet the EMC requirements in IEC 60601-1-2. The latest 4th edition, published in 2014, revised the standard with two main objectives.

The first objective is to increase immunity, in part because of the proliferation of wireless communication devices operating near potentially life-critical equipment. These wireless devices include mobile phones, Bluetooth, WiFi, paging systems, RFID, and similar products.

The second objective introduced a risk analysis element to determine the appropriate immunity levels for a device, its intended operating environment, and foreseeable interference. The 4th edition recognizes that devices may operate outside professional healthcare facilities, in environments with less control of electromagnetic phenomena and less supervision. Manufacturers must therefore understand the device's basic operation and select suitable immunity levels to reduce the risk of malfunction or unintended operation.

The 4th edition defines three environments: professional healthcare facilities, home healthcare, and special environments (which include heavy industrial or medical equipment that intentionally generates high-power fields). Required immunity levels are related to these environments rather than to the device type; consequently, the term "life-support equipment" is no longer used.

Power solutions

In BF & CF class medical devices, the power system is a key element for meeting insulation, leakage current, and EMC requirements.

For home healthcare environments, a practical approach is to use a naturally floating Class II insulation scheme that does not require protective earth, provided the device still meets chassis and patient-to-earth leakage current limits. This approach is practical for products up to about 300 W; above 300 W, EMC requirements become more difficult to manage.

Most standard AC-DC power supplies that meet general safety standards are not suitable for direct patient connection for several reasons:

1. They lack the required output-to-earth insulation.
2. They do not meet patient leakage current requirements.
3. Although they may provide 2 x MOPP from input to output and 1 x MOPP from input to earth, many devices use functional insulation from output to earth rated around 500 VAC/VDC with a required test voltage of 1500 VAC, which must comply with creepage and clearance distances when basic insulation at mains voltage is required for patient-connected applications.
4. Input-to-output capacitance is often too high, resulting in excessive output-to-earth leakage current.

For low-power patient-connected devices, a simple and low-cost solution is to use a secondary insulation stage in the form of a medically-rated DC-DC converter. Such a converter provides basic insulation at mains voltage, minimizes input-to-output capacitance (typically 20–50 pF), and reduces potential patient leakage current to single-digit μA levels. This approach also accounts for system-level low-integrity input and output signals that may connect to uncontrolled external equipment such as PCs or monitors.

Figure 1: Medical power system with secondary DC-DC converter

In a simplified model of the power system, the patient leakage current path is represented by C4 and C5 in series. C5 represents the DC-DC converter's input-to-output capacitance, which is very small and presents a high impedance to reduce leakage current. C4 typically has a larger value.

Figure 2: Simplified model of a power system with secondary DC-DC isolation

Medically approved DC-DC converters with outputs from about 1 W to 20 W are available that provide the required input-to-output insulation and very low internal capacitance, purpose-built for these applications and cost-competitive while complying with IEC 60601 requirements. When used with a medically approved power input stage, patient leakage current can drop below 2 μA, making these solutions appropriate for BF and CF applications. If the DC-DC converter is powered from a regulated AC-DC supply and the required power is small (under about 2–3 W), a fixed-input, unregulated-output device can provide a very cost-effective solution.

There are DC/DC products available that maintain tightly regulated output across a wide input and output load range, provide 2 x MOPP insulation, and have similarly low internal capacitance. These are suitable for battery-powered or DC-input portable devices.

Multi-output power solutions

For devices that require multiple outputs for patient-connected functions, adding additional DC-DC converters can achieve patient leakage currents as low as about 2 μA per output, providing a straightforward way to meet patient auxiliary and patient leakage current requirements.

If an AC-DC front end must provide multiple outputs, system complexity can increase due to chassis leakage constraints driven by touch current limits, which may make multiple supplies impractical. In such cases, a multi-output AC-DC supply may be required.

For low-power systems in the 2–300 W range, a variety of medically compliant bare-board or U-chassis output modules are available; alternatively, additional voltage rails can be created using insulated or non-insulated DC-DC converters powered from a single-output AC-DC supply.

For high-power applications, modular configured solutions are available that provide high-power multi-output capabilities and comply with medical safety approvals. These solutions can offer hundreds to thousands of watts and numerous outputs as needed.

Motor-driven applications

In high-power equipment and motor-driven applications, such as bone saws, powered surgical tools, electric tables, beds, and chairs, it is often impractical to use additional insulated DC-DC stages because suitable high-power DC-DC devices with the required insulation are less available and double-conversion reduces efficiency. These applications require power supplies designed with the necessary insulation, creepage/clearance, and patient leakage performance.

The combination of high insulation and low leakage current creates significant design challenges in AC-DC supplies. Required internal spacing on the secondary side increases substantially and must be implemented with system integration in mind. Low emissions and low leakage current requirements can be conflicting, driving the need for low-noise topologies and careful management of differential and common-mode noise while minimizing mains-frequency ripple in primary circuits to reduce patient leakage through input-to-output capacitance.

These high-power applications are typically BF class rather than CF class. BF class leakage current requirements are less restrictive (for example, 100 μA rather than 10 μA). An increasing number of standard AC-DC supplies are suitable for BF-class applications. For example, some 250 W BF-class supplies meet insulation, leakage, and 4th edition EMC requirements, support convection-cooled operation to avoid system fan noise, and offer constant-current overload behavior with short-duration peak power capability suitable for motor-driven applications.

Design considerations and conclusion

Designing power systems for patient-connected medical devices is challenging. Using standard, approved, and appropriately rated power modules or combinations of products verified for EMC performance can simplify compliance and reduce development risk. Such an approach supports safety and EMC compliance while offering a practical path to system-level implementation with minimized risk and time to market.