Board-mount DC/DC Converters in Medical Applications

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 verify the safety and basic performance of the devices, particularly for the protection of operators and patients against electric shock. Compliance is often demonstrated when a product conforms to the 60601-1 safety standard, currently in its 4th edition. The MDD was replaced by the Medical Device Regulation (MDR) in 2024, which is even more stringent.

It’s clear why an AC/DC converter used in medical applications needs a high grade of 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 direct 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 an 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, 2MOOPs and 2MOPPs 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 absence) and connection method of functional earth (FE) or protection earth (PE), ‘ground’ is also an important factor.

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 equipment peripherals 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 can still be absolutely necessary for safety reasons, even though input and output voltages are low and well within the definition of a ‘safe’ voltage. Let's look at why this is the case.

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, for instance, 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).


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 an easy task.

An example of the 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 2MOOP 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.


Another example could be in Figure 4 where class I (earthed), metal cased, AC input equipment that 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).


In the example, note that even though the main isolation from 240VAC (B) is 2MOOPs, 2MOPPs 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 1MOPP. If the SIP/SOP connection is specified as having 2MOOP isolation to the system voltage, and cannot be reasonably substituted accidentally, the barrier D can reduce to 1MOPP. ‘BOP’ in the diagrams is ‘Basic Opposite Polarity’ isolation between AC line and neutral.

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 potential hazards have to be considered and documented in risk analysis such as fire, smoke, burn, and mechanical hazards.

Achieving the necessary creepage and clearance distances in miniature DC/DC converters can be very challenging, particularly around and inside the transformer. There's often 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/DC converters 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.

Manufacturers such as RECOM have addressed the challenges of designing DC/DC converters for medical applications with their cost-effective REMxE range of parts. Power ratings are 3.5W, 5W and 6W in DIP24 through-hole packages with SMT versions planned. These 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 component for achieving safety ratings for medical devices – commercial solutions from RECOM offer all the regulatory and performance credentials at attractive prices.