The third edition medical safety standard using DC/DC converters

Any electrical medical appliance, healthcare or dental product must comply with specific safety standards that are based on IEC60601-1. Now in its third edition, the standard includes the general requirements for the provision of basic safety and e

As part of the European Union Medical Device Directive, end equipment must also have a risk analysis management package provided as per the requirements of ISO 14971. However, the IECEE Medical Electrical Equipment Task Force decided that component AC/DC and DC/DC power supplies are not subject to the requirements of risk management.

The operating environment, location that the end-product will be used in, the potential for presence of contaminants such as conductive or non-conductive pollution and likelihood of moisture ingress are all factors that need to be considered in the design of the power supply and end-product. Other critical parameters include the mechanical strength of the enclosure, accessibility to hazardous voltages, temperatures and access to mechanical hazards.

The medical case

The third edition standard stipulates two categories of protection, each with their own requirements. These are Means of Operator Protection (MOOP) and Means of Patient Protection (MOPP).

For both MOOP and MOPP the key requirement is reducing the risk of any unintended electrical current from passing through the human body. The need for electrical isolation is vital in the form of physical spacing and/or insulating material. With hazardous voltages present in a system, a robust and reliable approach to isolation is needed such that multiple and un-related insulation system failures would need to occur before an operator or patient is put at risk. To achieve this, two independent measures of protection (MOOPs or MOPPs, as appropriate) are required, or a single measure of physical isolation such as creepage/clearance or solid insulation deemed equivalent. Previous iterations of IEC60601, a power supply for example, that meets the second edition requirements for reinforced isolation are likely to comply with the third edition for two MOOPs. However for patient protection (two MOPPs) the requirements are more stringent. Notably the creepage and clearance distances need to be greater as does the thickness of the insulation used. Also for patient protection, there is a higher dielectric high potential (HiPot) test value and leakage currents need to be a lot less.

Design to achieve the required category of insulation/isolation will vary depending on factors such as system voltage (i.e. the mains supply), the over-voltage category of the system, the material group type for creepage surfaces, pollution degree category and operating altitude. Altitude is often neglected, but it should be remembered that, for example, equipment that could be operated at 2,850m altitude should increase clearance distances for operator protection by 15%.

Isolation methods

Isolation methods can vary (Figure 1 illustrates isolation from electrical shock between a live part and a patient.) Methods can differ and can be a combination of physical distance, solid insulation or multiple layers of thin insulation. For example, for patient protection in 250V rms systems, the isolation could be 8mm physical separation across one barrier or two barriers each with 4mm separation.

Figure 1: Isolation from electrical shock with two measures of protection required

A practical example would be where the first measure of protection is provided by an IT-grade AC/DC converter meeting the requirements of IEC60950 for reinforced isolation followed by a DC/DC converter meeting basic or one MOPP requirement for IEC60601, the combination being equivalent to a two-MOPPs insulation system.

The AC/DC converter that meets IEC60950-1 Reinforced Insulation would meet the IEC60601-1 requirements of two MOOP or one MOPP. Other factors, however, such as fusing and overall leakage current, need to be considered for full patient protection compliance.

Figure 2: An isolated DC/DC converter provides two means of protection

In some applications, isolated DC/DC converters provide two means of protection. Figure 2 illustrates a mains powered circuit generating an intermediate non-isolated 12V which is passed through an isolated DC/DC converter to produce 5V. The DC/DC is powered from a seemingly safe 12V but with a direct connection to high voltage AC so the converter must have two means of protection for the 5V to be deemed safe.

Another example where a DC/DC converter may need to have a defined level of insulation is shown in Figure 3. Here, full patient protection against mains voltage is provided by the AC/DC converter under normal conditions (barrier B). However, the DC/DC converter, one MOPP barrier E, is providing protection from current flowing through the patient back through the DC/DC to ground from other faulty equipment energising the patient. If there are unspecified input and output lines connected to the equipment (shown as SIP/SOP), the DC/DC must have two MOPP. If the SIP/SOP lines are specified with a minimum of one MOPP, the DC/DC only requires one MOPP.

Figure 3: DC-DCs protecting against external equipment faults

Sometimes a high level of DC/DC converter isolation provides a convenience in product testing. Figure 4 shows a typical application where a low isolation rating DC/DC with low capacitance and high insulation resistance follows a full safety rated AC/DC supply. If an end-to-end hi-pot test is performed, most of the test voltage drops across the DC/DC by a simple pot-down effect perhaps way above its rating. If it happens not to fail, the main safety barrier in the AC/DC has not been verified. If the DC/DC is fully safety rated this instills confidence in the equipment.

Figure 4: Safety testing of combined AC/DC and DC/DC systems

When selecting power converters in medical and other safety rated applications, the working voltage must be specified. For AC/DC converters this is normally the AC mains input. For DC/DC converters, the working voltage can vary depending on the application. For example the Murata MEJ1 series is rated for one MOOP at 200V rms up to a maximum ambient of 85°C while the Murata NCM6 series is rated for two MOOPs at 250V rms. These are combinations that the manufacturer has elected to have agency tested. The same products can qualify for different ratings, for example the MEJ1 series could be rated for two MOOPs but at lower working voltages. The manufacturer will be able to advise what combinations are possible.