Many medical products may come into contact with patients or operators during normal use, or may require sensors or devices to be directly applied to patients to perform their functions. These “applied parts” must be properly insulated from any power source and ground to prevent current flow and injury to the patient.
The definitions of “applied parts” and “medical devices” refer to the third edition of the medical standard IEC 60601-1 published in 2005. This standard has been adopted all over the world. For example, the European Union adopts EN 60601-1:2006/A1:2013/A12:2014, and the United States adopts ANSI/AAMI ES60601-1:A1:2012, C1:2009/(R)2012 and A2: 2010/(R)2012.
This standard defines three types of application components, with increasing risk levels:
• Class B (main body): normally non-conductive and groundable application parts.
• Type BF (body floating): Applied parts that are electrically connected to the patient must be floating and insulated from the ground. This classification does not include applied parts that are in direct contact with the heart.
• CF type (heart floating): suitable for application parts directly connected to the heart. This means the connection to the patient’s heart, including the venous connection during dialysis. These application parts must be floating and insulated from the ground.
MOOPs and MOOPs
Protective measures (MOPs) that limit dangerous voltages, currents, and energy prevent applied parts from causing electric shocks to patients. For example, a proper connection to protective ground or “basic” insulation provides 1 MOP, and reinforced insulation provides 2 MOPs. According to the environment, operator protection methods (MOOPs) or patient protection methods (MOPPs) are defined.
For BF or CF connection, AC power supply requires 2 x MOPP from primary to secondary, primary power supply to ground requires 1 x MOPP, and output to ground requires 1 x MOPP. Table 1 shows the creepage and clearance required to achieve this at a “system voltage” of 250VAC and the required test voltage.
Table 1: Creepage, clearance and test voltage of MOOPs and MOPPs
Leakage current must be limited
The touch current, patient auxiliary current, and leakage current must also be limited. Under normal conditions, the contact current should not exceed 100µA, and under single fault conditions (SFC) should not exceed 500µA, effectively limiting the leakage current under normal conditions to 500µA. Table 2 summarizes the limits.
Table 2: Patient assistance and leakage current limits
Electromagnetic compatibility (EMC)
Medical equipment must also comply with the EMC requirements of IEC 60601-1-2 (currently the 4th edition). This latest version expands and strictly limits the scope of immunity testing to include wireless communication devices that may be close to life-critical equipment. Devices include mobile phones and other devices, including Blue TootTM, Wi-Fi, Tetra, RFID or paging capabilities. The fourth edition also includes requirements for risk analysis, recognizing that medical equipment may operate in a variety of environments, including professional healthcare, homes, and uncontrolled “special” areas, such as ambulances in emergencies. Equipment manufacturers must consider these possibilities, specify what basic product operations are required, and mitigate the effects at an appropriate level of immunity.
Power options
In BF-&CF grade medical equipment, the power supply system is a key factor to meet the requirements of insulation, leakage current and EMC. In the home medical environment, Class II or “ungrounded” equipment is required, so it has nothing to do with ground insulation. However, while complying with EMC standards, it is also necessary to meet the limits of enclosure and patient leakage current, which may be difficult if the power is higher than 300W and there is no grounded enclosure.
“Medical certified” power supplies are common, but due to excessive insulation capacitance, most power supplies do not have 1 x MOPP output to ground isolation or low enough patient leakage current for BF/CF applications. In order to solve this problem simply and at low cost, a medically insulated DC-DC converter can be used on the power output to only provide power to the patient connection circuit, which is usually low power (Figure 1). DC-DC has a very low coupling capacitance, the total leakage is in the μA range, and the insulation class is 1 x or 2 x MOPPs. The choice of 1 or 2 MOPP depends on the possible external signal input/output connections of the device and its insulation rating to hazardous voltage or ground (known or unknown). For example, a diagnostic port or an Ethernet connection.
Figure 1: Medical power supply system using secondary DC-DC.
The range of DC-DC converters that meet the medical certification can be found in products with power above 1W (Figure 2). Fully regulated components can be applied to battery-operated devices. These devices may be connected to a charger or include a signal interface, require MOPs, or if there is already a regulating track, the cost of a fixed input DC-DC converter is the lowest.
Figure 2: XPPower’s medically certified DC-DC converter