Choosing the right medical power supply

This article originally appeared in the December'24 magazine issue of Electronic Specifier Design – see ES's Magazine Archives for more featured publications.

By Paul Heath, Account Manager, HMI and Power at the Milexia Group

Electric currents passing through the human body can have lethal consequences. As little as 40mA can be potentially deadly for a healthy person. For those under anaesthetic, or weakened by illness, the danger threshold is even lower. There is a good reason then why power supplies for medical devices and applications have to meet the most stringent requirements concerning leakage and stray currents.

There is a broad choice out there on the market, extending from readily available standard power supplies to semi-custom, user-configurable options. There is even the possibility of creating a unique and innovative power supply from scratch where the occasion demands.

For those involved with such a decision process, here are some considerations and issues to factor in:

Device size

It is crucial to ensure that the power supply’s physical footprint and mounting options are compatible with the design constraints of the medical device. Connecting and integrating the power supply with the device also means reviewing the connectors needed today and being mindful of possible modular connections for future expansions or modifications.

A comprehensive and dependable maker of power supplies will offer two categories of medical power supply, external and internal. The external option will be designed to sit on a desktop or be mounted on a wall. The internal alternative can come in a number of forms: an enclosed power supply within a metal case, or an open frame power supply without any case, frame, or housing. There is also the possibility of a configurable power supply with a metal case and modular design to allow for more flexible configurations.

Issues such as adjustability and programmability could be important, with some medical applications needing power supplies that have adjustable output parameters or programmable features so that they can be adapted to different operating modes or patient needs.

Heat dissipation and the patient environment

Thermal management should be top of any agenda, since overheating the area around the patient is clearly undesirable. A power supply’s cooling mechanisms should be evaluated to see if they are fan cooled, convection cooled or conduction cooled, checking also that the device can operate reliably without overheating and affecting the patient environment.

Even when a power supply meets leakage-current and isolation standards, it should still be evaluated for compliance within the patient environment. This is classed in a number of ways. Type B (Body) is where there is no direct physical contact with the patient, for example LED lighting in operating theatres and medical laser units. Classification changes to Type BF (Body Float) when there is physical contact with the patient or risk to the patient from device failure, such as with incubators, ultrasonic devices and diagnostic equipment. Type CF (Cardiac Float) involves direct contact with the patient’s heart, with a high risk of injury or death if the device fails. This would be the case with defibrillators and heart-lung machines.

Standards and regulations

Checks should be made to ensure power supplies meet certain regulatory standards. The 3rd edition of IEC 60601-1, for example, concerns the safety of both operating personnel and patients and introduces a classification system called MOP (Means of Protection) to evaluate medical electrical equipment.

Devices that do not come into direct contact with patients, and are only used by trained operators, fall into the Means of Operator Protection (MOOP) category. These must comply with IEC 62368-1 standards for Audio/Visual and Information Technology Equipment (ITE).

When devices do have direct physical contact with patients, they fall into the Means of Patient Protection (MOPP) category and need to meet even stricter safety standards, particularly concerning insulation. They must feature two separate insulation barriers to protect against electric shock. The permissible leakage current decreases as the contact with the patient increases, with devices classified based on their contact type with the patient.

Power supplies should meet international medical safety standards and certifications, such as IEC 60601-1, to ensure they are designed to protect both patients and operators. These standards cover crucial aspects like electrical safety, electromagnetic compatibility (EMC), and mechanical safety for life-supporting or lifesaving devices where faults due to electromagnetic or radio interference can be fatal. 

The IEC 60601-1-2 standard covers criteria for electromagnetic interference and immunity, essential in risk management and requiring equipment to withstand HF f ields up to 2.7GHz. It also sets limits for electrostatic discharge protection, from 6 to 8kV for contact discharge and from 8 to 15kV for air discharge.

Build quality, reliability, and efficiency

It is naturally desirable to check a power supply’s build quality. Whether a power supply is well-constructed will significantly impact the overall reliability and lifespan of the device it is powering. Power supplies should be sought that are built with high-quality components and durable designs. They should have standard protection features such as overvoltage protection (OVP), overcurrent protection (OCP), and short-circuit protection (SCP). These will safeguard both medical device and patient in case of a malfunction.

Where high-end medical equipment is involved, requirements will include low-noise variable speed fans, adjustable output voltage, active current sharing and low leakage current. For small to mid-sized devices, concerns are more likely to revolve around compact size, noiseless convection cooling, peak power and derating performance.

Lastly, remember that when you are trying to match the right power supply with the right application, there may not be an off the shelf solution that fits. Depending on the application, it might be the right choice to go for a custom product to meet your unique requirements. In this case, you’ll want to consider the level of technical support the manufacturer can deliver.