People and power don’t mix well, and this is particularly true when people are medical patients. Aside from the more usual environment of a medical facility, patients are also increasingly using medical devices at home. Medical equipment is therefore heavily regulated by standards-based requirements and subsequent product testing to ensure the safety of patients and healthcare professionals alike.

Fig. 1 - Using a DC/DC converter to achieve 2 x MOPP protection.

The most pressing safety concern related to medical devices comes into play when the patient is physically connected to an electrically powered device, such as the conductive pads of an electrocardiograph. These are defined as applied parts (AP).

The route to ensuring patient safety lies in the implementation of IEC 60601, “Medical electrical equipment,” the standard central to the safety of electronic and electrical medical equipment power supplies.

Understanding Key Definitions

To safeguard patients connected to an AP medical device, IEC 60601 dictates that all medical devices must contain at least one means of protection (MOP), which could include insulation, earthing systems, air gaps, minimum creepage distances, or other protective electrical impedances.

Since the release of the third edition, the standard makes a clear distinction between equipment that is designed to be used by operators and equipment that may come into contact with patients. Different levels of protection are required, defined as Means of Operator Protection (MOOP) and Means of Patient Protection (MOPP), respectively.

MOPP regulations are the stricter of the two because of the additional risk that a patient might be unconscious when an electrical fault occurs. In addition, more than one MOP may sometimes be required in case one fails.

Medical-grade DC/DC converters 1 ~ 60 W. (Credit: Traco Power)
Medical-certified AC/DC power supplies 10 ~ 450 W. (Credit: Traco Power)

There are three classifications for AP devices: cardiac floating (CF) for conductive devices that could come into direct contact with the patient’s heart; body floating (BF) for conductive devices that may have prolonged contact with the patient’s body; and lastly, body (B) for devices that are not normally conductive and can immediately be released from the body. Types CF and BF require a minimum of 2 × MOPPs.

Protecting Devices Against EMI

IEC 60601 has evolved over the past 40 years to keep up with advancements in technology. With wireless communication technology becoming more abundant in hospitals and homes, there is a need for these sensitive circuits to be impervious to electromagnetic interference (EMI). The fourth edition of the standard has, therefore, now increased the acceptance levels for electromagnetic compatibility (EMC).

Risk Reduction and QMS

Medical AC/DC 450 W, open frame or enclosed with fan. (Credit: Traco Power)

Two significant new introductions to the standard have been the requirements for the manufacturers of medical devices to undertake risk assessments, which must be undertaken in compliance with ISO 14971, “Medical devices – Application of risk management to medical devices,” to define best practice throughout the entire life cycle of an AP medical device.

Component manufacturers must also implement an ISO 13485 compliant quality management system (QMS) to demonstrate their ability to consistently meet both customer and regulatory requirements.

Practical Tips for Meeting the Standards

For AP medical devices to meet IEC 60601, the first step would usually be to select a medically approved AC/DC supply. Many designers will specify a power supply with 2 x MOPPs to simplify the qualification process and risk assessment, even if the highest level of protection is not actually required for the device’s application.

An operating room depicting where different power products are typically used. (Credit: Traco Power)

However, it can become challenging to source a medical-grade stand-alone AC/DC power supply with a BF-compliant 2 × MOPP rating, particularly when the DC voltages needed for the AP instrument are different to the main system DC voltage. In this case, a 2 × MOPP IEC 60601 DC/DC converter combined with an ITE 60950 rated AC/DC power supply can easily be used to obtain compliance instead of trying to source a custom AC/DC power supply for the task.

Specialized DC/DC converters are available that meet the standard 2 × MOPP requirements. With up to 5000 V AC of isolation, double insulation, and 8 mm of creepage distance through its galvanically isolating transformer, a DC/DC converter can provide protection in the event of a mains failure (see Figure 1). By providing this, the DC/DC converter avoids mains voltage levels appearing at any patient AP points.

An important part of the protection lies in a low coupling capacitance between the primary and secondary transformer windings to ensure that there is negligible transfer of current across the isolation barrier.

Traco Power, for example, offers both AC/DC and DC/DC solutions for medical applications, all of which fulfill the 2 × MOPP requirement. Compliant with the EMC requirements of IEC 60601-1-2 (4th edition), they are suitable for all patient-connected, AP medical devices (BF compliant). The AC/DC products range from small 5W PCB-mounting modules, to a range of mid-power open frame designs, and enclosed power supplies offering power levels of up to 450 W. The company’s medically approved DC/DC converters offer 5000 VAC rms of input-output isolation, rated for a 250 VAC rms working voltage. This, along with leakage currents below 2 μA, make them ideal choices for use in conjunction with non-approved AC/DC PSUs in safety-critical medical applications.

Conclusion

The healthcare market is growing rapidly, and the applicable standards that govern medical products will no doubt continue to evolve to keep safety standards high. It is especially important when the patient is physically connected to an electrically powered device, Understanding the differences between Means of Operator Protection (MOOP) and Means of Patient Protection (MOPP) is key to properly safeguarding patients.

Protecting against EMI but also applying ISO 14971 can ensure that devices are safe when the patient is physically connected to an electrically powered device. Partnering with reliable and experienced power supply manufacturers such as Traco Power can greatly simply the process.

This article was written by Florian Haas, Director of Marketing for Traco Power Group, San Jose, CA. For more information, click here .

Medical Design Briefs Magazine

This article first appeared in the January, 2020 issue of Medical Design Briefs Magazine.

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