Powering patient-connected medical devices with DC-DC converters
Jun 12, 2019
Professional and home healthcare is a rapidly expanding market and patient-connected devices are increasingly being used for anything from vital signs indication to wireless charging of implants. Each connected device though needs DC power, either from batteries or derived from AC mains with low power modular DC-DC converters often used as part of the safety isolation system. This article describes how DC-DC converters are used in this application with an explanation of the relevant safety certification needed. A commercial converter range is introduced which has been designed exactly for this application.
In professional and home healthcare, it is common for medical devices to be attached to the human body to measure an increasing number of health indications. These range from surface parameters such as skin resistivity, body temperature and blood oxygen saturation levels to heartbeat monitoring using electrocardiograms (ECG) and many more (Figure 1).
Figure 1: Typical sensor circuitry for an ECG monitor
Sometimes sensors are more invasively connected, internally to the body, such as during surgery. Most sensors are electronic and clearly their design needs to ensure that voltages and currents are low enough so they do not present an electrical shock hazard to the patient. There is also though a requirement that the sensor does not provide a route for damaging current to flow from other faulty equipment through the patient and back through the sensor. This means that the sensor and its power supply must not only be sufficiency isolated from hazardous high voltages but also from ground. Even battery-powered hand-held devices may have a route to ground through a connected printer, USB port or laptop for example.
In electronics, the relevant safety standard is ‘Medical Electrical Equipment’ IEC 60601-1 and its national versions such as EN 60601-1 in Europe and ANSI/AAMI ES 60601-1 in the USA. In the standard, ‘isolation’ has a very specific meaning, effectively requiring double- safe electrical separation between hazardous voltages or high energy sources and the patient. Medical equipment must therefore have two ‘Measures of Protection’ (MOPs) so that if one measure fails, the second measure will still provide adequate protection. The MOP requirements have two frades, MOOP for operators and the higher-grade MOPP for patients.
To provide power for the sensors or to read data across the isolation barrier, miniature safety transformers or optocouplers with reinforced insulation are a practical solution. These give high withstand voltage isolation but still have a coupling capacitance across the barrier which could allow a leakage current to flow. The coupling capacitance must therefore be kept as low as possible.
If the sensor power supply is derived from an AC-DC converter, then all power supplies have safety isolated outputs as standard. However, the grade of isolation for commercial or IT AC-DC’s is not adequate to meet the medical standards. Many have only one measure of protection and the isolation coupling capacitance is normally far too high to meet the low leakage current requirements. Additionally, the outputs may be ‘floating’ but with insufficient isolation to ground. Some ‘Medical grade’ AC-DC’s are available that have ‘Measures of Operator Protection’ (MOOPs) but are not suitable for direct patient connection because input/output and output/ground isolation is insufficient and the AC-DC barrier coupling capacitance not low enough. AC-DC’s with the required Two Measures of Patient Protection’ (2MOPPs) are available but are limited in range and expensive. Multi-channel sensors will often require multiple, mutually-isolated power rails and it is certain that anything other than a fully custom 2MOPP AC-DC will not have all the necessary power rails needed.
DC-DCs can form part of the isolation system
A solution is to use a standard 2MOOP isolated medical grade AC-DC and add a further stage of power isolation just for the sensor electronics in the form of one or more medical grade DC-DC converters. It is possible that these converters are needed anyway to provide the particular sensor voltage rails needed and if they are specified with high isolation and low capacitance, they can form part of an isolation system that meets even the most stringent ‘Cardiac Floating’ (CF) application. Figure 2 shows the typical connections for a system in an earthed casing or electrical ‘Class I’ arrangement.
Figure 2: Using a DC-DC converter to meet medical ‘CF’ applications.
In the example of Figure 2, the DC-DC converter requires 2MOPPs even though the AC-DC has 2MOOPs because there is unspecified equipment (SIP/SOP) connected to the DC-DC input, which could generate hazardous voltages in a fault condition. If the equipment were electrical ‘Class II’, that is, with no ground connection and in a plastic case with no unspecified external connections, and if the AC-DC is minimum 2MOOP, then the DC-DC converter could have just 1MOPP grade of isolation (D), for direct patient connection applications. The DC-DC will often only need to supply low power and can be physically small and low cost.
A typical application using DC-DC converters might be as shown in Figure 3. Here an instrumentation amplifier is used as an ECG sensor to provide digital data for analysis. Another DC-DC powers a driver that provides a stimulus to the patient, isolated from the sensor. The data is isolated with opto-isolators. The instrumentation amplifier requires typically +/-5V and the driver perhaps +12V. Two, 2W board-mount DC-DC converters powered from a regulated system voltage provide the isolated voltage rails. Because the input is regulated and load constant, the DC-DCs can be a simple, low cost ‘ratio’ converters with a few percent load regulation for more than adequate performance.
Figure 3: DC-DC converters provide isolated rails for sensors and stimuli
Further DC-DC converters can provide more isolated rails for other measurement or stimulus channels. The small size and design of the DC-DC converters naturally gives low coupling capacitance so leakage current is kept extremely small.
DC-DC converters can generate noise
All high efficiency electronic DC-DC converters are ‘switched-mode’ power supplies and as such can generate conducted and radiated EM noise. In real circuits, this noise can interfere with other measurements, especially when signals are in the mV range such as in ECG equipment. There are also given limits to noise in the medical standard 60601-1-2 so levels must be suppressed as much as possible in the DC-DC or filtered externally. Unfortunately, external filters often have to bridge the isolation barrier to reduce ‘common-mode’ noise which potentially increases leakage current. DC-DCs therefore with low inherent noise levels are preferred.
A DC-DC converter with 2MOPP isolation as in Figure 2 needs to meet the creepage and clearance distances specified in the standards for the ‘system’ voltage in the equipment. In practice, for 240VAC, this means greater than 8mm creepage and clearance, if the part can be used at altitudes greater than 2000m. Connected medical devices must be small, so the low power DC-DC often has just a few square centimetres of board space available. This means that the isolation transformer is a complex design and has to use triple insulated reinforced wiring and added insulation and epoxy encapsulation to reduce the ‘pollution degree’ of the local environment. Implementing the DC-DC as a discrete solution is therefore not easy and will require sourcing of an expensive custom transformer in many applications.
Modular DC-DC converters are a cost-effective solution
A modular DC-DC converter is an easy solution and products such as the new, low cost, 2W REM2 series from RECOM[1] (Figure 4) provide 2MOPP isolation and 250VAC working voltage rating in a compact SIP8 package. The converter is pre-certified to IEC/EN/ES 60601-1 which makes final conformity testing much simpler. Single and dual outputs are available to power sensors or instrumentation amplifiers with five nominal input voltage options. The parts operate from -40°C to +95°C with derating from 80°C and up to 5000m altitude. Efficiency is high, peaking at 85% and noise is low meeting the requirements of IEC/EN 60601-1-2 along with EN 55011 for EMC. Isolation capacitance is just 25pF typical.
Figure 4: 2W DC-DC converter from RECOM with 2MOPP/250VAC rating
Isolated DC-DC converters are an essential tool to achieve the level of safety isolation required by medical standards for patient-connected devices. Certified off-the-shelf parts solve all application problems in one compact package.
References
[1] RECOM: www.recom-power.com/medical
RECOM: We Power your Products
Figure 1: Typical sensor circuitry for an ECG monitor
Sometimes sensors are more invasively connected, internally to the body, such as during surgery. Most sensors are electronic and clearly their design needs to ensure that voltages and currents are low enough so they do not present an electrical shock hazard to the patient. There is also though a requirement that the sensor does not provide a route for damaging current to flow from other faulty equipment through the patient and back through the sensor. This means that the sensor and its power supply must not only be sufficiency isolated from hazardous high voltages but also from ground. Even battery-powered hand-held devices may have a route to ground through a connected printer, USB port or laptop for example.
In electronics, the relevant safety standard is ‘Medical Electrical Equipment’ IEC 60601-1 and its national versions such as EN 60601-1 in Europe and ANSI/AAMI ES 60601-1 in the USA. In the standard, ‘isolation’ has a very specific meaning, effectively requiring double- safe electrical separation between hazardous voltages or high energy sources and the patient. Medical equipment must therefore have two ‘Measures of Protection’ (MOPs) so that if one measure fails, the second measure will still provide adequate protection. The MOP requirements have two frades, MOOP for operators and the higher-grade MOPP for patients.
To provide power for the sensors or to read data across the isolation barrier, miniature safety transformers or optocouplers with reinforced insulation are a practical solution. These give high withstand voltage isolation but still have a coupling capacitance across the barrier which could allow a leakage current to flow. The coupling capacitance must therefore be kept as low as possible.
If the sensor power supply is derived from an AC-DC converter, then all power supplies have safety isolated outputs as standard. However, the grade of isolation for commercial or IT AC-DC’s is not adequate to meet the medical standards. Many have only one measure of protection and the isolation coupling capacitance is normally far too high to meet the low leakage current requirements. Additionally, the outputs may be ‘floating’ but with insufficient isolation to ground. Some ‘Medical grade’ AC-DC’s are available that have ‘Measures of Operator Protection’ (MOOPs) but are not suitable for direct patient connection because input/output and output/ground isolation is insufficient and the AC-DC barrier coupling capacitance not low enough. AC-DC’s with the required Two Measures of Patient Protection’ (2MOPPs) are available but are limited in range and expensive. Multi-channel sensors will often require multiple, mutually-isolated power rails and it is certain that anything other than a fully custom 2MOPP AC-DC will not have all the necessary power rails needed.
DC-DCs can form part of the isolation system
A solution is to use a standard 2MOOP isolated medical grade AC-DC and add a further stage of power isolation just for the sensor electronics in the form of one or more medical grade DC-DC converters. It is possible that these converters are needed anyway to provide the particular sensor voltage rails needed and if they are specified with high isolation and low capacitance, they can form part of an isolation system that meets even the most stringent ‘Cardiac Floating’ (CF) application. Figure 2 shows the typical connections for a system in an earthed casing or electrical ‘Class I’ arrangement.
Figure 2: Using a DC-DC converter to meet medical ‘CF’ applications.
In the example of Figure 2, the DC-DC converter requires 2MOPPs even though the AC-DC has 2MOOPs because there is unspecified equipment (SIP/SOP) connected to the DC-DC input, which could generate hazardous voltages in a fault condition. If the equipment were electrical ‘Class II’, that is, with no ground connection and in a plastic case with no unspecified external connections, and if the AC-DC is minimum 2MOOP, then the DC-DC converter could have just 1MOPP grade of isolation (D), for direct patient connection applications. The DC-DC will often only need to supply low power and can be physically small and low cost.
A typical application using DC-DC converters might be as shown in Figure 3. Here an instrumentation amplifier is used as an ECG sensor to provide digital data for analysis. Another DC-DC powers a driver that provides a stimulus to the patient, isolated from the sensor. The data is isolated with opto-isolators. The instrumentation amplifier requires typically +/-5V and the driver perhaps +12V. Two, 2W board-mount DC-DC converters powered from a regulated system voltage provide the isolated voltage rails. Because the input is regulated and load constant, the DC-DCs can be a simple, low cost ‘ratio’ converters with a few percent load regulation for more than adequate performance.
Figure 3: DC-DC converters provide isolated rails for sensors and stimuli
Further DC-DC converters can provide more isolated rails for other measurement or stimulus channels. The small size and design of the DC-DC converters naturally gives low coupling capacitance so leakage current is kept extremely small.
DC-DC converters can generate noise
All high efficiency electronic DC-DC converters are ‘switched-mode’ power supplies and as such can generate conducted and radiated EM noise. In real circuits, this noise can interfere with other measurements, especially when signals are in the mV range such as in ECG equipment. There are also given limits to noise in the medical standard 60601-1-2 so levels must be suppressed as much as possible in the DC-DC or filtered externally. Unfortunately, external filters often have to bridge the isolation barrier to reduce ‘common-mode’ noise which potentially increases leakage current. DC-DCs therefore with low inherent noise levels are preferred.
A DC-DC converter with 2MOPP isolation as in Figure 2 needs to meet the creepage and clearance distances specified in the standards for the ‘system’ voltage in the equipment. In practice, for 240VAC, this means greater than 8mm creepage and clearance, if the part can be used at altitudes greater than 2000m. Connected medical devices must be small, so the low power DC-DC often has just a few square centimetres of board space available. This means that the isolation transformer is a complex design and has to use triple insulated reinforced wiring and added insulation and epoxy encapsulation to reduce the ‘pollution degree’ of the local environment. Implementing the DC-DC as a discrete solution is therefore not easy and will require sourcing of an expensive custom transformer in many applications.
Modular DC-DC converters are a cost-effective solution
A modular DC-DC converter is an easy solution and products such as the new, low cost, 2W REM2 series from RECOM[1] (Figure 4) provide 2MOPP isolation and 250VAC working voltage rating in a compact SIP8 package. The converter is pre-certified to IEC/EN/ES 60601-1 which makes final conformity testing much simpler. Single and dual outputs are available to power sensors or instrumentation amplifiers with five nominal input voltage options. The parts operate from -40°C to +95°C with derating from 80°C and up to 5000m altitude. Efficiency is high, peaking at 85% and noise is low meeting the requirements of IEC/EN 60601-1-2 along with EN 55011 for EMC. Isolation capacitance is just 25pF typical.
Figure 4: 2W DC-DC converter from RECOM with 2MOPP/250VAC rating
Isolated DC-DC converters are an essential tool to achieve the level of safety isolation required by medical standards for patient-connected devices. Certified off-the-shelf parts solve all application problems in one compact package.
References
[1] RECOM: www.recom-power.com/medical
RECOM: We Power your Products
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