Advanced Power Packaging Enables 2MOPP Medical Isolation in DC/DC Converters

Mar 12, 2021
Test device and 3D Power Packaging logo with R05CT05S series from RECOM
DC/DC converters with medical-grade isolation can be used in combination with AC/DC power supplies or in battery powered equipment to minimize end-product cost in healthcare applications, while meeting the highest safety standards.

Table of Contents

Electronic equipment is used increasingly in a wide range of medical and home healthcare environments, and product designers will be familiar with the electrical safety standards that must be met: 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 extensive guidance on applying the latest version of the safety standard, with its reference to measures of protection (MOPs) in operator and patient environments. It might seem sensible to simply specify a medical 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 greater 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 assumed 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, as well as examples where battery-operated equipment still requires internal safety isolation.

Implementing 2MOPP Isolation Using Secondary DC/DC Converters

Medical device safety schematic
Fig. 1: One medical power supply scenario allowing the highest level of patient connection with economical parts
Taking the case of AC-powered equipment, there must be a minimum of 2MOOPs or MOPPs isolation from the AC line to the output, depending on whether the equipment is intended for an operator or patient environment respectively. However, patient-connect outputs must also be isolated from ground by a minimum of 1MOPP, to cover the possibility that the patient could become electrically live from other faulty equipment, allowing lethal current to 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 offers suitable medical-grade isolation, it can enable an AC/DC with only 1MOOP output isolation to ground to be used in a patient-connect application.

For unspecified signal connections such as communication interfaces, whether mains-powered or battery-operated, 2MOPP must be provided between patient connections and the signals in case an external fault causes the signals to become live. Here too, a separate DC/DC converter can provide the necessary additional isolation.

Figure 1 shows an example scenario using a Class I AC/DC power supply with 2MOOP and unspecified signal inputs, together with an additional DC/DC converter with 2MOPP that ensures the required overall isolation of the signal inputs. In this way, a more economical AC/DC power supply with operator protection can be used in an application involving patient connections.

The DC/DC converter typically only needs to deliver low power to the patient connection circuitry and can therefore be relatively small and cost-effective. Due to its small size, it also has very low coupling capacitance and consequently very low leakage current, providing better patient protection than using the AC/DC power supply alone.

Selecting DC/DC Converters for Medical Applications

DC/DC converters have become commodity parts and are marketed from many sources with varying levels of isolation, with and without agency certification. Designers should be aware that a high isolation or withstand voltage rating is often only an indication of transient immunity, and without specific agency certification to the relevant standard, the product may not be suitable as a safety barrier.

For a medical application, the DC/DC data sheet should clearly indicate the number of measures of protection, either MOOP or MOPP, with a specified system voltage, typically 250VAC. It is possible for a DC/DC converter to legitimately carry 2MOPP certification but only at 30VAC, for example, and be unsafe if used at normal AC mains voltage. Medical DC/DC converters are relatively uncommon because achieving high isolation voltage within the device is challenging; the standards mandate significant solid insulation or large separation distances between input and output, such as 8mm creepage for 2MOPP/250VAC certification, making internal construction difficult.
RECOM's RxxCT series
Fig. 2: Isolated DC/DC converter with 2MOPP/250VAC rating
An example of a DC/DC converter that achieves a high level of medical isolation is the recently released RxxCTxxS series from RECOM (Figure 2). This economical 0.5W device features a 5V nominal input and selectable outputs of 3.3V or 5V, alternatively 3.7V or 5.4V, to provide suitable headroom for low drop-out regulators (LDOs). The converter is supplied in a compact 10.3mm x 7.7mm SMD package, just 2.65mm high, making it suitable for space-constrained medical power modules.

The product’s key specification for medical applications is its 2MOPP/250VAC continuous rating according to IEC/EN 60601-1, with a 5kVAC test voltage. It also features only 3.5pF coupling capacitance, resulting in very low leakage current in 250VAC/50Hz applications. In non-medical applications, the ratings are even more notable, offering reinforced isolation at an 800VAC working voltage according to EN 62368-1. Operating temperature extends up to 140°C with derating, and the part includes enable, sync, and trim functions, along with undervoltage lockout.

Advanced Power Packaging for Medical Isolation and 2MOPP Compliance

The designers of the RxxCTxxS series had to adopt a radically new approach to product packaging to achieve the required functionality and high isolation ratings. A traditional toroidal transformer could not meet the insulation requirements, and a discrete E-core with bobbin or similar construction would be too large. As a result, the switching frequency was increased to 8MHz, allowing significantly fewer winding turns and the use of a small planar transformer core, while incorporating solid interwinding insulation to meet medical isolation requirements. Advanced circuit design using an embedded die maintains good efficiency even at the elevated switching frequency. The die is wire-bonded to the DVE SOIC16 leadframe, and the entire device is then over-molded.

Conclusion: High-Isolation Medical DC/DC Converters for Safe System Design

DC/DC converters with high, medical-grade certified isolation can be valuable tools for achieving the required overall isolation ratings in the most sensitive applications, including CF patient connections. With careful application, systems can be designed using devices such as the RECOM R05CT05S to minimize cost without compromising safety. The advanced circuit design and 3D power packaging techniques used in this product are representative of those employed in the latest RECOM DC/DC converters and medical power supplies, supporting the company’s ongoing drive toward smaller, more efficient, and cost-effective DC/DC and AC/DC converters across all application areas, including medical.
Applications
  Series
1 RECOM | RxxCTxxS Series | DC/DC, SMD, 0.5W, Single Output
Focus
  • Compact 10.3x7.5mm SMD package
  • 5kVAC reinforced isolation
  • 5V or 3.3V post-regulated, selectable outputs
  • Low EMI emissions