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Figure 1: Medical power system with secondary DC-DC
Many medical products are likely to come into contact with patients or operators during normal use or may require a sensor or device to be directly applied to a patient to perform their function. These ‘applied parts’ must be suitably isolated from any power source and ground to prevent current flow and consequent injury to the patient.
The definition of an ‘Applied Part’ and a ‘Medical Device’ is given in medical standard IEC 60601-1, currently at its 3rd edition, published in 2005. The standard has been adopted around the world, for example as EN 60601-1:2006/A1:2013/A12:2014 in the EU and ANSI/AAMI ES60601-1: A1:2012, C1:2009/(R)2012 and A2:2010/(R)2012 in the USA.
The standard defines three categories of applied parts with increasing risk level:
Type B (Body): Applied parts which are generally not conductive and may be connected to earth.
Type BF (Body Floating): 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 with the heart.
Type CF (Cardiac Floating): Applied parts suitable for direct cardiac connection. This means connection to the heart of the patient including intravenous connection such as during dialysis. These applied parts must be floating and separated from earth.
MOOPs and MOPPs
Applied parts are prevented from causing electric shock to patients by Means Of Protection (MOPs) which limit dangerous voltage, current and energy. For example, suitable connection to protective earth or ‘Basic’ insulation provides 1 MOP and reinforced insulation provides 2 MOPs. Depending on the environment, Means Of Operator Protection (MOOPs) or Means Of Patient Protection (MOPPs) are defined.
For BF or CF connections, 2 x MOPPs are required from a power supply AC primary to secondary, 1 x MOPP from primary to earth and additionally 1 x MOPP from output to earth. Table 1 shows creepage and clearance needed to achieve this for a ‘system voltage’ of 250VAC along with the test voltages required.
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Table 1: Creepage, clearance and test voltages for MOOPs and MOPPs
Leakage currents must be limited
Touch current, patient auxiliary current and leakage current must also be limited. Touch current must not exceed 100µA in normal conditions and 500µA in a Single Fault Condition (SFC), effectively limiting leakage current also to 500µA in normal conditions. Table 2summarizes the limit values.
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Table 2: Patient auxiliary and leakage current limits
Electro-Magnetic Compatibility (EMC)
A medical device must also comply with the EMC requirements of IEC 60601-1-2, currently at its 4th edition. This latest revision widens the scope and tightens limits of immunity testing to include wireless communication devices which may be close to life-critical equipment. Devices include cell phones and other equipment which incorporates Blue-Tooth, Wi-Fi, Tetra, RFID or paging functions. The 4th edition also includes requirements for risk analysis, recognizing that medical equipment may be operated in a variety of environments including professional healthcare, home, and uncontrolled ‘special’ areas such as in ambulances at scenes of emergencies. The device manufacturer must consider these possibilities, specify what essential product operation is required and mitigate effects at appropriate immunity levels.
Power supply options
In BF- & CF-rated medical devices, the power system is a critical factor in complying with isolation, leakage current and EMC requirements. In home healthcare environments Class II or ‘earth free’ equipment is required and isolation to earth is therefore not relevant. However, limits for enclosure and patient leakage currents must still be met while complying with EMC standards, which can be difficult at higher powers, above around 300W, without a grounded chassis.
‘Medically approved’ power supplies are commonly available but most do not have 1 x MOPP output to ground isolation or sufficiently low patient leakage current for BF/CF applications, due to excessive isolation capacitance. To address this problem simply and at low cost, a medically-isolated DC-DC converter can be employed on a power supply output to provide power for just the patient connect circuitry, which is typically low power (Figure 1). DC-DCs are available with very low coupling capacitance for total leakage in the µA range and with 1 x or 2 x MOPPs isolation rating. Choice of 1 or 2 MOPPs depends on possible external signal input/output connections to the device and their isolation ratings to dangerous voltages or ground, known or unknown. A diagnostic port or ethernet connection would be an example.
Ranges of suitable medically-certified DC-DC converters are available at power levels from 1W upwards (Figure 2). Fully regulated parts can suit battery-operated equipment that might have a charger connected or include a signal interface, requiring MOPs, or if a regulated rail is already available, fixed input DC-DC converters are the lowest cost.
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Figure 2: Medically-certified DC-DC converters from XP Power - 1 watt to 20 watts
Multiple output solutions
If multiple AC-DC power supplies are used in a system to provide several power rails, even if certified to the highest medical grade, their touch and leakage currents add and can total more than the allowed limits. At low power, again a solution is to use one larger AC-DC with multiple connected medically-certified DC-DC converters with ultra-low coupling capacitance. If the power requirements are too high for this approach, AC-DCs are available in open-frame or U-channel formats with multiple outputs, meeting the leakage and touch current limits, up to around 300W. At higher powers still, a medically-certified modular AC-DC power supply can be considered such as the fleXPower series from XP Power (Figure 3).
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Figure 3: Up to 20 outputs are available from the XP Power fleXPower series, up to 2500 watts
Medical motor driven applications
Where higher power is needed in patient-connect applications, for example driving motors in hospital beds or for surgical tools and bone-shavers, adding an isolated DC-DC converter to a lower grade AC-DC power supply is not a practical approach due to limited availability of certified high-power DC-DCs and the inevitable loss in system efficiency with series connected converters. A preferable solution is an AC-DC which meets the requirements as standard. Design of such a power supply is a challenge with the requirements for low leakage current and low EMI conflicting, but low-noise topologies can be chosen which minimize EMI at source. Achieving spacings and required isolation voltage rating from output to ground can also be difficult, especially when output-referenced components such as power diodes have grounded heatsinks.
At these higher power levels, the use-category is often BF, with the less severe patient leakage current rating of 100µA rather than 10µA for the CF category. BF-rated power supplies are increasingly common, for example the CMP250 range from XP Power, rated at 250W, meeting IEC 60601-1-2 4th edition EMI requirements (Figure 4). The product has the added benefit of needing just convection cooling for full power rating, avoiding the need for noisy fans. The product is also particularly suitable for motor drive applications with a constant current characteristic in overload and a high peak power rating of up to 200% for one minute.
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Figure 4: XP Power CMP250 series: 250 Watts convection cooled with medical (BF) certification
Patient-connected medical devices are a challenge for power system designers when working to comply with medical safety and EMC standards. There are options however to use fully compliant power supplies or combinations of AC-DC and DC-DC converters which can minimize system certification and unit costs and provide low risk, fast time-to-market solutions.
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