This blog discusses two approaches for power systems within medical equipment for patient connected applications. The part of the equipment which comes into contact with the patient is known as the applied part. The applied part is defined as the part of a medical device which, in order for the medical device to perform its function, deliberately comes into direct contact with a patient. This also applies to parts which are likely to come into contact with the patient.
Applied parts are classified in the 60601-1 suite of standards according to the type of patient contact and the type or nature of the medical device. The latest version of 60601-1 is the 3rd edition which was first published in December 2005. The standard has been adopted in the major countries and regions of the world and published as the following latest versions:
IEC 60601 1:2005 (Third Edition) + CORR. 1:2006 + CORR. 2:2007 + A1:2012
Europe: EN 60601-1:2006/A1:2013/A12:2014
USA: 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 has differing requirements for protection against electric shock. The classifications are outlined below, in order of the least to the most stringent: - Type B (Body). Type B classification is given to applied parts with are generally not conductive and may be connected to Earth. Type BF (Body Floating). Type BF classification is given to applied parts which are electrically connected to the patient and must be floating and separated from Earth. This classification does not include applied parts which are in direct contact to the heart. Type CF (Cardiac Floating). Type CF classification is given to applied parts suitable for direct cardiac connection (connection to the heart of the patient including intravenous). These parts must be floating and separated from Earth. Power systems for type BF & CF medical devices are designed to provide additional isolation from the secondary output to Earth normally rated at 1 x MOPP (Means of Patient Protection) at the AC line voltage.
Additionally, the power system must be designed to limit the touch current, patient auxiliary current and patient leakage current. The maximum allowable values for the touch current are 100μA in normal conditions and 500μA in a Single Fault Condition (SFC), which effectively limits the system earth leakage current to 500μA in normal operation. The maximum allowable patient auxiliary current and patient leakage currents are defined in the table below.
The challenge for the power system designer in patient contact equipment, where an electrical connection is required, is to ensure that the system minimises the leakage currents under normal operation and protects under fault conditions by isolating the patient from ground. In these systems the power system is the critical factor in meeting these important requirements.
The majority of medically approved standard power supplies are not suitable for direct connection to the patient because they do not have the required isolation from output to ground, nor do they meet the requirements for patient leakage current. While they may offer the required 2 x MOPP from input to output and 1 x MOPP from input to ground, most of these units employ operational isolation from output to ground, often rated around 500VAC/VDC, when patient connect applications require a minimum of basic insulation at mains voltage, where the test voltage required is 1500VAC and the creepage and clearance distances must be adhered to. Additionally, the isolation capacitance from input to output is too high, allowing excessive leakage current from output to ground.
A simple and low cost solution for lower power systems is to employ a second isolation stage in the form of a medically approved DC/DC converter which provides basic isolation at mains voltage and minimal input to output capacitance (20 - 50pF) reducing the potential patient leakage current to single digit μA. This solution also accounts for system input and output signals which may be connected to uncontrolled external equipment such as a computer or monitor.
Figure 1: Medical power system with secondary DC/DC Isolation
DC/DC converters from 1W up to 20W are readily available, with the required input to output isolation and very low internal capacitance, designed specifically for these applications at a competitive cost. When used in conjunction with a standard medically approved mains input power supply they can reduce patient leakage current to levels as low as 2 μA suitable for use in both BF & CF applications. Where the supply to the DC/DC converter is from a regulated AC/DC power supply a fixed input, semi regulated output device can be used making it a very cost effective solution. Wide range input DC/DC products, offering a tightly controlled output over a wide DC input and output load range, with up to 2 x MOPP isolation and equally low internal capacitance are also readily available for DC input or battery powered portable devices.
Figure 2: XP Power's JHL and JHM Series
In higher power applications such as surgical equipment or motor driven applications, it is undesirable to employ an additional isolation stage due to both the poor availability of suitably isolated DC/DC devices with high power ratings and the inefficiency of dual conversion of the power. In these applications a power supply designed with the necessary isolation, spacing and patient leakage current is required.
As these are typically BF rather than CF applications, the patient leakage current requirement is less severe (100uA rather than 10uA) and there are increasing numbers of standard products available which allow patient contact in BF rated applications. XP Power's new 250W, BF rated, CMP250 series is a good example with the isolation and leakage current requirements catered for and the added benefits of convection cooling to eliminate noisy system fans and up to two times peak power for up to one minute for motor driven applications such as bone shavers, surgical tools and electrically powered tables, beds & chairs.
Figure 3: XP Power's CMP250 complies with BF-type medical requirements
This combination of high isolation and low leakage currents presents its own design challenges in an AC/DC supply; the internal spacing requirement on the secondary side is greatly increased and must be implemented with system integration in mind. The requirements for low emissions and low leakage current are in conflict, requiring a low noise topology and care in minimising differential and common mode noise throughout the product while minimising the line frequency ripple in primary circuits to reduce patient leakage current for the same input to output capacitance.
The requirements for patient connected medical equipment are challenging for the medical device power system designer. Utilising standard, approved, suitably rated products or combinations of products with proven EMC performance such as those discussed above, provides the best value, lowest risk and fastest time to market for the end equipment simplifying both safety and EMC compliance of the end medical device.