Power supply safety for home medical devices

The growing market for home medical devices presents new design challenges to ensure that people with no medical training, or who are elderly or disabled, can use equipment properly and safely, by Ron Stull of Bel Fuse.

The sophistication of medical technologies used in non-clinical settings ranges from those giving basic assistance to complex equipment for delivering advanced treatment. However simple or complex the functionality, they all have one design element in common, they require a power supply.

Risk factors to consider

Whether mains or battery powered, home medical devices must comply with IEC 60601-1, which comprises a stringent series of standards that address the basic safety and essential performance requirements of medical electrical equipment. For example, IEC 60601-1-2 is focused on EMC, 60601-1-3 covers radiation protection for diagnostic X-ray systems and 60601-1-11 covers home healthcare equipment.

There are several risk factors introduced by the home environment that IEC 60601-1-11 seeks to mitigate when designing a product for use outside the clinical environment. These include the equipment being operated by untrained users, the lack of protective earth, increased EMI, power fluctuations and the shock and vibration of portable equipment.

Medical devices, such as an electric-powered blood pressure monitor and continuous positive airway pressure (CPAP) systems, illustrate additional concerns that affect the power supply and other relevant design criteria.

Blood pressure monitoring

Blood pressure is a critical vital sign guiding acute and long-term clinical decision-making. Measurement is relatively simple. The patient puts on a cuff containing pressure sensors, which is then inflated. The sensors measure systolic (peak arterial) and diastolic (minimum arterial) pressures as the cuff deflates. These measurements are used to calculate the mean arterial pressure.

Ideally, home blood pressure monitors should be designed to be intuitive and support the untrained user, so that accurate readings are obtained repeatedly without the need for professional supervision. One example is the wheelchair-accessible blood pressure measurement device by South Korean medical analysis company InBody. This incorporates the cuff and an elbow sensor inside the unit, enabling it to automatically adjust to ensure that the patient is in the correct position for highly accurate results every time.

The system also requires an ac-dc power supply that complies with the medical IEC 60601-1 electrical safety requirements and the additional requirements for home use laid out in the IEC 60601-1-11 collateral standard for 2 x MOPP (means of patient protection).

A primary concern for power supplies is the inconsistency of protective earthing in homes. Medical power supplies are often rated with an IEC protection Class I, meaning they rely on a protective earth connection to provide the necessary level of protection. The earth connection cannot be guaranteed in a home or relied on to provide protection.

IEC 60601-1-11 requires that power supplies be rated with IEC protection Class II. Class II power supplies do not rely on protective earth ground and provide the proper level of protection by applying double or reinforced insulation. A protection Class II rated internal ac-dc power supply series or an external series of wall plug adapters address these medical at-home power requirements and safety concerns.

Assisted breathing

Those suffering from respiratory conditions such as apneas, hypopneas and severe snoring require support to regulate their breathing.

Continuous positive airway pressure devices can provide consistent air support for nighttime breathing difficulties. These sophisticated devices require direct contact with the patient via a facemask and regulate the patient’s oxygen supply by monitoring on a breath-by-breath basis.

CPAP devices also offer climate control, which automatically adjusts the humidity and temperature of the delivered air. This protects the patient from condensation on the mask and tubing, and provides optimum comfort during treatment.

CPAP devices are life-critical and therefore automatic monitoring features are essential.

Devices designed for use in a home environment must also take into account that at-home healthcare operators can be untrained and may want to eliminate the need to adjust settings manually. The product design must also consider that home environments may contain a range of devices emitting electromagnetic noise that could interfere with the functioning of medical equipment and sensors. As a result, home medical devices must meet stricter requirements than traditional medical equipment to limit emissions and potential interference. Traditional medical equipment must meet CISPR 11 Class A emissions, while home medical equipment must meet the more stringent Class B limits.

Like a blood pressure monitor, the CPAP device requires a power connection that complies with the medical IEC 60601-1 and its collateral safety standards. CPAP devices with wall power should use a compliant isolated ac-dc power supply.

AC-DC power supply is required to comply with IEC 60601-1 electrical safety requirements and IEC 60601-1-11 collateral standard

The diagram, right, illustrates the circuit location of the IEC 60601-1 isolated ac-dc power supply in a CPAP device (on the far upper left) and specifies the input voltage range. Power supplies used in home medical applications must allow for a +10/-15% mains supply variation to account for the less predictable residential power infrastructure. This typically equates to an input range of 85Vac to 264Vac for universal operation (100Vac to 240Vac nominal).

Some portable CPAP devices run on battery power, which is particularly helpful for patients in transit. The ac-dc battery charger configuration for such devices still requires a connection that complies with the medical IEC 60601-1-11 safety standard. Portable medical devices must also meet harsher shock and vibration tests, and some added mechanical stress tests.

High levels of safety

The use of home medical electronic health devices is likely to grow significantly, driven by complex social factors including an ageing population, the desire for more people to be treated at home in order to alleviate stress on health services and a growing awareness of personal wellbeing. It is crucial that home medical devices meet the highest levels of safety while being as foolproof and as easy to use as possible.

Power supplies are integral to the design and operation of medical equipment. The intrinsic nature of an ac-dc power supply or a dc-dc converter means the voltages and currents they provide could be hazardous if not properly managed. Selecting a pre-certified model can lessen the designer burden of ensuring compliance for the medical device.