Board-mount DC-DC converters in medical applications
2019. 5. 15
Professional and home healthcare is a rapidly expanding market. Increasingly, electronic monitoring is used for anything from critical care vital signs monitoring to mundane bed occupancy sensors. Power for the electronics normally needs safety galvanic isolation. This article explains why power isolation is needed in typical situations and describes the safety standards that need to be applied. Example board-mount DC/DC converters are presented with an explanation of some of their published specifications.
In the European Union, medical devices must meet the essential requirements of directive MDD:93/42/EEC, Article 3 and products placed on the market must meet the essential requirements of risk analysis in Article 3, Annex I, ER1, Annex II, Annex III and Annex VII. This is to ensure safety and basic performance of the devices, particularly, protection of operators and patients against electric shock. Compliance is presumed if a product conforms to the 60601-1 safety standard, currently in its 4th edition. The MDD will be replaced by the Medical Device Regulation (MDR) by 2024, which is even more stringent.
The requirement for isolation in AC/DC converters is clear
It’s clear why an AC/DC converter used in medical applications needs high grade protection against electric shock – if the power supply fails you might have a direct and lethal connection from high voltage AC to a conductor in intimate contact with a patient, maybe even to an internal organ for invasive sensors. The reverse is also true; medical grade equipment must have protection against earthing a “floating” patient that is subjected to a external high voltage, for example during cauterisation or defibrillation procedures. The medical safety standards are clear and define the levels of isolation or Measures Of Protection (MOPs) necessary for both operator and patient. For each category, there must be a minimum of two MOPs overall, 2 x MOOPs and 2 x MOPPs respectively. For electrical connections there are three further definitions of ‘applied parts’:
Type B: No electrical contact with patient and may be earthed
Type BF: Electrically connected to patient but not directly to heart, floating
Type CF: Electrically connected to the heart of the patient, floating
As with all safety standards, there are also environmental factors which affect the required isolation characteristics: system voltage, pollution degree, over-voltage category of the supply voltage and altitude. The presence or not and connection method of functional earth (FE) or protection earth (PE), ‘ground’ is also an important factor.
DC/DCs may also need isolation
In medical equipment there is often a need for DC/DC converters which may take a battery- or AC-derived DC input and convert voltage levels to practical values needed in the circuit or for powering peripherals to the equipment such as sensors. In the majority of cases, the DC/DC input is a low voltage such as 5V or 12V, with a low voltage output such as +/-5V or 3.3V. Functionally, isolation is not always required but may be included to break ground loops, to help control cross-talk between sensors, to reduce electromagnetic interference (EMI) or simply to generate a negative rail voltage from a positive supply. Where isolation is not needed functionally, it still may be absolutely necessary though for safety reasons, even though input and output voltages are low and well within the definition of a ‘safe’ voltage. There are different reasons why this is so.
DC/DCs along with AC/DCs may form part of an overall medically approved system
AC-powered medical equipment can use a ‘medical-certified’ power supply with the required isolation system to meet the application such as 2 x MOOPs or 2 x MOPPs according to IEC 60601 and its regional versions EN 60601 in Europe and ANSI/AAMI ES 60601 in the US. These power supplies have to be specially designed to meet the statutory medical safety requirements and therefore are generally more expensive than industrial or commercial supplies which have more relaxed safety standards. A way to use a cheaper mass-market supply with certification to industrial safety standards such as EN 60950-1 or its successor EN 62368-1 is to follow it with a further level of isolation – the safety standards allow ‘serial’ levels to be added together (Figure 1). The extra level could be a DC/DC converter between the AC/DC output and just the areas of circuitry that are accessible to operators or patients. The overall cost could be much less than purchasing a dedicated certified medical supply but there is also a specification for leakage current in medical environments which the IT rated supply will almost certainly exceed. This can be reduced though by reduction or removal of ‘Y’ type suppression capacitors from line to ground in the AC/DC but at the expense of EMI, which then has to be reduced in other ways. An AC/DC meeting requirements for IEC 60950-1 may be suitable for means of operator protection on its own but in the medical environment, both AC line and neutral must be fused, each with a breaking capacity of 1500A. Also, the latest edition of IEC 60601 (4th) has amendments to EMC specifications which the system must comply with, so use of IT grade power supplies is not necessarily a simple substitution.
Figure 1: ‘Measures of Protection’ can be added
DC/DCs prevent fault current back from the patient
Part of the safety requirements for a medical system is that failure of other equipment must be considered in a risk analysis. The worst case is that the patient or operator becomes ‘live’ from faulty connected equipment, the ‘single fault condition’ (SFC), and could be electrocuted or suffer burn injuries if current flows from this live source through the person to ground. It is therefore necessary that connections to a patient for example, are isolated from ground to a specified level. ‘Isolation from ground’ is a relative measure – there will always be some stray or residual capacitance from patient to ground through perhaps multiple channel equipment, allowing some 50/60Hz current to flow. The currents add through different paths, so the standards set a very low level for each source. (Figure 2).
Figure 2: Maximum allowed leakage currents in different medical applications. NC = Normal Condition
DC/DC converters are often used to provide the extra level of isolation to achieve protection from an external single fault condition. Equipment may be AC line-powered, class I or II, battery operated, hand-held, and any type may have unspecified other connections such as to a network by ethernet or via USB to a data logger or printer. Cables may or may not have screens which may or may not connect to ground. Specification of the isolation level of the DC/DC is therefore not as easy task. An example of use of a DC/DC converter is shown in Figure 3 for class II equipment (earth free, plastic case). Here the main AC/DC isolation (B) has 2 x MOOP protection and the DC/DC providing power to an applied part need only have 1 x MOPP for overall 2 x MOPP in a BF or CF application. Note that the DC/DC isolation must be rated for the system voltage (240VAC in this case), the plastic case must be minimum 1mm thickness with suitable electrical rating and there are no other external connections to the low-voltage circuitry.
Figure 3: A possible connection scenario requiring a high isolation DC/DC converter, Class II equipment
Another example could be in Figure 4 where class I (earthed), metal cased, AC input equipment provides power to a BF or CF ‘applied part’ patient connection. There are also unspecified input/output signal connections to other external equipment from the low voltage side of the equipment power supply (SIP/SOP).
Figure 4: A possible connection scenario requiring a high isolation DC/DC converter, Class I equipment
In the example, note that even though the main isolation from 240VAC (B) is 2 x MOOPs, 2 x MOPPs are still required in the DC/DC converter (D) because there are unspecified connections, SIP/SOP to the 48V rail which must be considered to be at system voltage (240VAC) under fault conditions. Isolation D therefore must also be rated for 240VAC. The DC/DC output must also be isolated from the metal casing (E) with 1 x MOPP. If the SIP/SOP connection is specified as having 2 x MOOP isolation to the system voltage, and cannot be reasonably substituted accidentally, the barrier D can reduce to 1 x MOPP. ‘BOP’ in the diagrams is ‘Basic Opposite Polarity’ isolation between AC line and neutral.
Other DC/DC specifications are important
Isolation and leakage current in normal and single fault conditions are the headline requirements for DC/DC converters in these medical applications but there are also limits for product and internal transformer temperatures, material flammability and all possible hazards have to be considered and documented in risk analysis such as fire, smoke, burn, and mechanical hazards.
Practical design of DC/DCs for medical applications
Including the necessary creepage and clearance distances in miniature DC/DC converters can be very challenging, particularly around and inside the transformer. There is little chance of using physical spacing in the transformer to provide the correct isolation level and specialist techniques are necessary including the use of triple insulated wire (TIW) which forms three overlapping layers of wire insulation with a continuous helical wrap. Leakage current through stray capacitance is normally low anyway but DC/DCs may require some additional external input-output capacitance to meet regulations for suppression of system common-mode noise. If fitted, these capacitors cross the isolation barrier and must be the correct, safety-rated ‘Y’ types and limited in value to keep leakage current low for the intended application. DC/DCs with inherently low EMI are therefore preferred.
Commercial solutions
Manufacturers such as RECOM[1] have addressed the challenges of designing DC/DC converters for medical applications with their low cost REMxE range of parts. Power ratings are 3.5W, 5W and 6W in DIP24 through-hole packages with SMT parts planned. The DC/DCs have 2:1 input ranges with 5V, 12V, 24V and 48V nominals and a selection of fully-regulated single and dual outputs. All parts have 2 x MOPP rating at 250VAC with internal/external creepage and clearance of >8mm and have maximum 1µA leakage current. Operating temperature range is -40°C to +85°C with no derating for the 3.5W part with the 5W and 6W parts derating from 80°C and 75°C respectively. Board-mount isolated DC/DC converters are an essential aid to achieve safety ratings for medical devices – commercial solutions from RECOM have all the regulatory and performance credentials at attractive prices.
------------------------------------
References RECOM: www.recom-power.com/medical
Please refer to the DC/DC Book of Knowledge,
RECOM: We Power your Products
The requirement for isolation in AC/DC converters is clear
It’s clear why an AC/DC converter used in medical applications needs high grade protection against electric shock – if the power supply fails you might have a direct and lethal connection from high voltage AC to a conductor in intimate contact with a patient, maybe even to an internal organ for invasive sensors. The reverse is also true; medical grade equipment must have protection against earthing a “floating” patient that is subjected to a external high voltage, for example during cauterisation or defibrillation procedures. The medical safety standards are clear and define the levels of isolation or Measures Of Protection (MOPs) necessary for both operator and patient. For each category, there must be a minimum of two MOPs overall, 2 x MOOPs and 2 x MOPPs respectively. For electrical connections there are three further definitions of ‘applied parts’:
Type B: No electrical contact with patient and may be earthed
Type BF: Electrically connected to patient but not directly to heart, floating
Type CF: Electrically connected to the heart of the patient, floating
As with all safety standards, there are also environmental factors which affect the required isolation characteristics: system voltage, pollution degree, over-voltage category of the supply voltage and altitude. The presence or not and connection method of functional earth (FE) or protection earth (PE), ‘ground’ is also an important factor.
DC/DCs may also need isolation
In medical equipment there is often a need for DC/DC converters which may take a battery- or AC-derived DC input and convert voltage levels to practical values needed in the circuit or for powering peripherals to the equipment such as sensors. In the majority of cases, the DC/DC input is a low voltage such as 5V or 12V, with a low voltage output such as +/-5V or 3.3V. Functionally, isolation is not always required but may be included to break ground loops, to help control cross-talk between sensors, to reduce electromagnetic interference (EMI) or simply to generate a negative rail voltage from a positive supply. Where isolation is not needed functionally, it still may be absolutely necessary though for safety reasons, even though input and output voltages are low and well within the definition of a ‘safe’ voltage. There are different reasons why this is so.
DC/DCs along with AC/DCs may form part of an overall medically approved system
AC-powered medical equipment can use a ‘medical-certified’ power supply with the required isolation system to meet the application such as 2 x MOOPs or 2 x MOPPs according to IEC 60601 and its regional versions EN 60601 in Europe and ANSI/AAMI ES 60601 in the US. These power supplies have to be specially designed to meet the statutory medical safety requirements and therefore are generally more expensive than industrial or commercial supplies which have more relaxed safety standards. A way to use a cheaper mass-market supply with certification to industrial safety standards such as EN 60950-1 or its successor EN 62368-1 is to follow it with a further level of isolation – the safety standards allow ‘serial’ levels to be added together (Figure 1). The extra level could be a DC/DC converter between the AC/DC output and just the areas of circuitry that are accessible to operators or patients. The overall cost could be much less than purchasing a dedicated certified medical supply but there is also a specification for leakage current in medical environments which the IT rated supply will almost certainly exceed. This can be reduced though by reduction or removal of ‘Y’ type suppression capacitors from line to ground in the AC/DC but at the expense of EMI, which then has to be reduced in other ways. An AC/DC meeting requirements for IEC 60950-1 may be suitable for means of operator protection on its own but in the medical environment, both AC line and neutral must be fused, each with a breaking capacity of 1500A. Also, the latest edition of IEC 60601 (4th) has amendments to EMC specifications which the system must comply with, so use of IT grade power supplies is not necessarily a simple substitution.
Figure 1: ‘Measures of Protection’ can be added
DC/DCs prevent fault current back from the patient
Part of the safety requirements for a medical system is that failure of other equipment must be considered in a risk analysis. The worst case is that the patient or operator becomes ‘live’ from faulty connected equipment, the ‘single fault condition’ (SFC), and could be electrocuted or suffer burn injuries if current flows from this live source through the person to ground. It is therefore necessary that connections to a patient for example, are isolated from ground to a specified level. ‘Isolation from ground’ is a relative measure – there will always be some stray or residual capacitance from patient to ground through perhaps multiple channel equipment, allowing some 50/60Hz current to flow. The currents add through different paths, so the standards set a very low level for each source. (Figure 2).
Figure 2: Maximum allowed leakage currents in different medical applications. NC = Normal Condition
DC/DC converters are often used to provide the extra level of isolation to achieve protection from an external single fault condition. Equipment may be AC line-powered, class I or II, battery operated, hand-held, and any type may have unspecified other connections such as to a network by ethernet or via USB to a data logger or printer. Cables may or may not have screens which may or may not connect to ground. Specification of the isolation level of the DC/DC is therefore not as easy task. An example of use of a DC/DC converter is shown in Figure 3 for class II equipment (earth free, plastic case). Here the main AC/DC isolation (B) has 2 x MOOP protection and the DC/DC providing power to an applied part need only have 1 x MOPP for overall 2 x MOPP in a BF or CF application. Note that the DC/DC isolation must be rated for the system voltage (240VAC in this case), the plastic case must be minimum 1mm thickness with suitable electrical rating and there are no other external connections to the low-voltage circuitry.
Figure 3: A possible connection scenario requiring a high isolation DC/DC converter, Class II equipment
Another example could be in Figure 4 where class I (earthed), metal cased, AC input equipment provides power to a BF or CF ‘applied part’ patient connection. There are also unspecified input/output signal connections to other external equipment from the low voltage side of the equipment power supply (SIP/SOP).
Figure 4: A possible connection scenario requiring a high isolation DC/DC converter, Class I equipment
In the example, note that even though the main isolation from 240VAC (B) is 2 x MOOPs, 2 x MOPPs are still required in the DC/DC converter (D) because there are unspecified connections, SIP/SOP to the 48V rail which must be considered to be at system voltage (240VAC) under fault conditions. Isolation D therefore must also be rated for 240VAC. The DC/DC output must also be isolated from the metal casing (E) with 1 x MOPP. If the SIP/SOP connection is specified as having 2 x MOOP isolation to the system voltage, and cannot be reasonably substituted accidentally, the barrier D can reduce to 1 x MOPP. ‘BOP’ in the diagrams is ‘Basic Opposite Polarity’ isolation between AC line and neutral.
Other DC/DC specifications are important
Isolation and leakage current in normal and single fault conditions are the headline requirements for DC/DC converters in these medical applications but there are also limits for product and internal transformer temperatures, material flammability and all possible hazards have to be considered and documented in risk analysis such as fire, smoke, burn, and mechanical hazards.
Practical design of DC/DCs for medical applications
Including the necessary creepage and clearance distances in miniature DC/DC converters can be very challenging, particularly around and inside the transformer. There is little chance of using physical spacing in the transformer to provide the correct isolation level and specialist techniques are necessary including the use of triple insulated wire (TIW) which forms three overlapping layers of wire insulation with a continuous helical wrap. Leakage current through stray capacitance is normally low anyway but DC/DCs may require some additional external input-output capacitance to meet regulations for suppression of system common-mode noise. If fitted, these capacitors cross the isolation barrier and must be the correct, safety-rated ‘Y’ types and limited in value to keep leakage current low for the intended application. DC/DCs with inherently low EMI are therefore preferred.
Commercial solutions
Manufacturers such as RECOM[1] have addressed the challenges of designing DC/DC converters for medical applications with their low cost REMxE range of parts. Power ratings are 3.5W, 5W and 6W in DIP24 through-hole packages with SMT parts planned. The DC/DCs have 2:1 input ranges with 5V, 12V, 24V and 48V nominals and a selection of fully-regulated single and dual outputs. All parts have 2 x MOPP rating at 250VAC with internal/external creepage and clearance of >8mm and have maximum 1µA leakage current. Operating temperature range is -40°C to +85°C with no derating for the 3.5W part with the 5W and 6W parts derating from 80°C and 75°C respectively. Board-mount isolated DC/DC converters are an essential aid to achieve safety ratings for medical devices – commercial solutions from RECOM have all the regulatory and performance credentials at attractive prices.
------------------------------------
References RECOM: www.recom-power.com/medical
Please refer to the DC/DC Book of Knowledge,
RECOM: We Power your Products
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