Medical device power supplies: How standards enable safety

Autor: Florian Haas, Director Marketing - Traco Power Group

It is not surprising, that medical device manufacturers are literally overrun in these extraordinary times. In order to build these devices and bring them to hospitals as quickly as possible, good employees, efficient processes, good logistics and, not to forget, hundreds of components are needed.

Even though the urgency of having such medical devices ASAP is enormous, the safety of patients and hospital staff should however never be overlooked. One of these sources of danger are surges from these medical equipment. To ensure the safety of patients and healthcare professionals, the healthcare business is regulated by a range of standards-based requirements and associated product testing.

IEC 60601 is central to assuring patient and provider safety, providing a suite of requirements specifically for the electrical and electronic equipment used in healthcare. Originally published some 40 years ago, IEC 60601 has kept pace with industry changes.

So, what are some of the key principles of IEC 60601 as they relate to the implementation of the power supply? What are some of the new requirements, such as the need for risk assessment? Also, what are practical ways of achieving compliance? What kind of support is available for medical device manufacturers?

Introduction

A key medtech safety concern is that the patient is often electrically connected to the device, such as by the conductive pads of an electrocardiograph. IEC 60601 defines these pads as applied parts (AP) — an important definition within the standard for a medical product’s overall requirements.

Medical devices must incorporate at least one means of protection (MOP) to ensure that both the operator and the patient, if connected via an AP, are protected from the risks of electric shock, even under fault conditions. A MOP can be achieved through safety insulation, protective earth, a defined creepage distance, an air gap, other protective impedances, or by implementing a combination of these techniques.

The standard treats operators and patients differently, resulting in the classifications “means of operator protection” (MOOP) and “means of patient protection” (MOPP) (Figure 1). MOPP requirements are more stringent because the patient may be physically connected via an AP and unconscious when the fault occurs. Each term is defined in terms of an isolation voltage, creepage distance, and insulation level.

Evolving standards

Although power supplies and modules are not medical devices, medtech manufacturers must design power solutions with medical applications in mind to meet the standards.

Hospitals and clinics provide dedicated, clean power for use by their most sensitive medical devices. Issues such as electromagnetic compatibility (EMC) become more important for home-based medical devices because of the prevalence of technologies such as Bluetooth and Wi-Fi. IEC 60601’s latest version (4th edition) has updated the testing procedures and acceptance levels for EMC accordingly.

Medtech manufacturers must also conduct risk assessments according to ISO 14971, which defines best practices for all stages of a medical device’s life cycle. The EU’s Medical Device Directive further adds to the compliance burden, requiring manufacturers to implement a quality management system (QMS) that complies with ISO 13485.

The typical approach to achieving IEC 60601 compliance is to use an AC/DC power supply that is approved for medical use. However, body-floating (BF)-rated applications also require the AP instrument to meet the 2 x MOPP rating. Many of the medically approved AC/DC power supplies on the market today do not have a 2 x MOPP rating and are not appropriate as a stand-alone power solution for applications where BF compliance is required. In these cases, an IEC 60601-approved, 2 x MOPP-rated DC/DC converter will support BF compliance for the AP. Another common example is medical appliances that have battery back-up capability and must fulfill the 2 x MOPP rating during AC failure.

Medical appliances often require various DC voltages for the AP instrument that differ from the main system DC voltage provided. To avoid sourcing custom AC/DC power supplies, this can be resolved by combining IEC 60601, 2 x MOPP rated DC/DC converters with, for example, an ITE 60950-rated AC/DC power supply. Alternatively, using 2 x MOPP, medical-grade DC/DC converters for the AP instrument may suffice, even when the selected AC/DC power supply has IEC 60601 approval.

The primary requirements for 2 x MOPP (Figure 2) are 4000 V AC isolation, 8 mm creepage distance and double insulation. Most commonly available DC/DC converters (including those that carry EN60950 approval) offer between 500 V DC to around 1600 V DC of isolation and are, therefore, not suitable for medical applications. However, specialized DC/DC converters will meet the requirements for AP when used in conjunction with such standard off-the-shelf power supplies.

By providing 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 still protect the patient in the event of a failure of the main AC/DC power supply, thereby avoiding mains voltage levels appearing at any patient AP points.

Traco Power’s approach to medically safe power solutions

Traco Power’s transformer technology was designed to ensure the medtech industry’s required separation and isolation while achieving sufficient coupling to allow the DC/DC converter to operate efficiently. Coupling capacitance values as low as 10-15pF between the primary and secondary transformer windings ensure negligible transfer of current across the isolation barrier, providing patient protection in line with IEC 60601 requirements.

Traco Power also implements its QMS in accordance with ISO 13485, which covers both the design and manufacturing processes. Because Traco Power manufactures power solutions rather than medical devices, it is not required to provide risk assessment data. However, the company is compliant with ISO 14971 and provides customers with risk assessment files covering crucial areas, including insulation breakdown, use while inverted, effects of fan failure, flammability, and mechanical shock. Having this data can save customers time and expense during the design process.