Advanced power packaging enables medical isolation in DC/DC converters

Electronic equipment is used increasingly in different medical and home healthcare environments and product designers will be familiar with the electrical safety standards that must be complied with: IEC 60601-1:2005 with its collateral documents and national variants, including EN 60601-1:2006 in Europe.

When considering a product’s AC/DC power supply requirements, there has been plenty of guidance written about the application of the latest version of the safety standard, with its reference to ‘measures of protection’ (MOPs) in operator and patient environments. It might seem wise to simply specify a power supply with the highest level of safety - 2 Measures Of Patient Protection, (MOPPs), rated at the maximum supply voltage and with leakage current no more than specified for the closest patient contact, type CF (cardiac floating), where electrical connection to internal organs is expected for monitoring or stimulation. In battery-operated equipment, it could also be thought that the safety standards are not applicable. There are instances however where a fully certified AC/DC on its own is insufficient but in other cases could be over-specified and examples where battery-operated equipment needs internal safety isolation.

Taking the case of AC-powered equipment, there must be a minimum of 2 x MOOPs or MOPPs isolation from AC line to output, depending on whether the equipment is expected to be used in an ‘operator’ or ‘patient’ environment respectively. However, patient-connect outputs must also be isolated from ground by a minimum of 1 x MOPP, to cover the possibility that the patient could become electrically ‘live’ from other faulty equipment and lethal current could then flow through the patient to earth. An additional isolated DC/DC converter providing power to the patient-connect circuitry can help in this situation; if it has suitable medical-grade isolation, it can enable an AC/DC with only 1 x MOOP output isolation to ground to be used in a patient-connect application.

DC/DC converters have become commodity parts and are marketed from many sources with various levels of isolation, with and without agency certification. Designers should be aware that a high withstand voltage rating is often just an indication of transient immunity and without specific agency certification to the relevant standard, the product may not be suitable as any type of safety barrier. For a medical application, the DC/DC data sheet should indicate the number of measures of protection, either MOOP or MOPP with a system voltage specified, typically 250VAC. It is possible for a DC/DC to legitimately have 2 x MOPP certification but only at 30VAC for example, and be dangerous if the system voltage is ‘normal’ AC mains. Medical-grade DC/DC converters are not common because achieving high isolation rating inside the part is not easy; the standards mandate significant solid insulation material or a large separation distance between input and output, 8mm creepage in the case of 2 x MOPP/250VAC certification, making internal construction difficult.

Designers of the R05CT05S had to take a radical new approach to the product packaging to enable the functionality and high isolation ratings to be achieved. A traditional toroidal transformer could not meet the insulation requirements and a discrete E-core with bobbin or similar would be too big, so switching frequency was pushed up to 8MHz allowing far fewer winding turns and a small planar transformer core, while incorporating solid interwinding insulation to meet the medical requirements. Advanced circuit design using an embedded die maintains good efficiency, even at the high switching frequency. The die is wire-bonded to the DVE SOIC-16 leadframe and the whole product is then over-moulded.

DC/DC converters with high, medical-grade certified isolation can be valuable tools to achieve required overall isolation ratings for the most sensitive applications, including CF, patient connect. With careful application, systems can be designed, using the RECOM R05CT05S for example, to minimise costs without compromising safety. The advanced circuit and 3D power packaging techniques used in the product are typical of those used in the latest RECOM products continuing the company’s drive towards smaller, more efficient and cost-effective DC/DC and AC /DC converters in all applications including medical.