DC/DC converters with medical-grade isolation can be used in combination with AC/DC power supplies or in battery powered equipment to minimise end-product cost in healthcare applications, while meeting highest safety standards.
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.
In another scenario where there are unspecified signal connections such as communication ports to equipment, either AC or battery powered, there must be 2 x MOPP between the patient connections and the signals, in case an external fault makes the signals ‘live’. Again, an extra DC/DC converter can provide the additional isolation needed. Figure 1 shows one particular situation, a class I AC/DC power supply 2 x MOOP-rated with unspecified signal inputs, with an additional 2 x MOPP-rated DC/DC converter providing the necessary total isolation to the signal inputs and allowing a lower cost ‘operator-rated’ AC/DC to be used in a patient-connect situation.
The DC/DC converter will typically only need to provide low power for a patient-connect interface and will therefore be relatively small and low-cost. It will also typically have very low coupling capacitance due to its physical size and consequently very low leakage current, again enabling a higher level of patient connect than the AC/DC alone might allow.
DC/DC converters in medical applications must be selected with care
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.
An example of a DC/DC converter that achieves a high level of medical-grade isolation is the recently released R05CT05S from RECOM (Figure 2). This is an economical 0.5W part with 5V nominal input and selectable outputs of 3.3V or 5V, alternatively 3.7V or 5.4V, to provide head voltages for low drop-out regulators (LDOs). The converter is in a compact 10.3mm x 7.7mm SMD package just 2.65mm high for space-constrained applications. The product’s highlight specification for medical applications is its 2 x MOPP/250VAC continuous rating according to IEC/EN 60601-1 with 5kVAC test voltage. It also has just 3.5pF coupling capacitance, for negligible leakage current in 250VAC/50Hz applications. In non-medical applications the ratings are even more impressive – reinforced isolation at 800VAC working voltage according to EN 62368-1. Operating temperature is up to 140°C with derating and the part features enable, sync and trim functions along with an undervoltage lockout.
Advanced power packaging enables medical isolation
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.
Conclusion
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.
