Essential Power Supply Considerations for Medical Equipment Design

­This article explores a variety of factors that must be considered when choosing the power supply for medical applications, from the fundamental requirements of voltage and current specifications to more nuanced aspects like daily usage patterns, and more. 

Volts, amps, and efficiency

Although it is important to ensure that voltage and current specifications suit the intended medical application, attention must be paid to the input voltage range where the lower end is determined by the category for equipment with life support equipment needing to operate as low as 80VAC and non-life support needing 85VAC.

Through iterations of equipment design, products are generally becoming smaller and more portable. Therefore, the demand for smaller power supplies is increasing. However, a reduction in size brings challenges to power supply designers as efficiency must not compromised. When this happens, removing the waste heat effectively from a reduced volume becomes increasingly difficult without moving air.

The critical nature of medical equipment and its intended purpose underlines the importance of EMI immunity and emissions. The power supply’s emissions mustn’t interfere with other parts of the circuit, particularly control circuits. Similarly, the operating condition of the power supply should not be affected by any electrical noise generated within the application. Designers should also ensure that output noise and ripple are low enough that they do not affect sensitive circuitry that could give false triggers or readings.

Cooling options and noise considerations

From a mechanical perspective, managing the thermal path is of paramount importance. Many engineers recognize an electrolytic capacitor’s (e-cap) lifetime as a fundamental reliability factor for a power supply due to the changes in electrolyte property affecting its value from the effects of heat.  Depending on the quality of the e-cap, the design lifetime can vary significantly. As a rule of thumb, an e-cap lifetime doubles for each 10°C reduction in operating temperature; an e-cap rated at 20,000 hours at 105°C, for example, would have a service life of 40,000 hours at 95°C and 80,000 hours at 85°C, which is equivalent to more than 9 years in continued use.

Medical equipment engineers may wish to over-rate the power supply so that it doesn’t run at full power thereby improving reliability. If designing fully enclosed equipment, choosing a high efficiency power supply will minimize that waste heat, but you still need to get it out of the enclosure. Designers must, therefore, consider their cooling options to minimise the heat rise seen by the e-caps.

With forced cooled power supplies, the airflow is provided by an integral fan, which by its nature produces audible noise. However, excessive noise in hospital and clinical settings caused by multiple equipment can result in patient discomfort and, in a quiet bedside environment, it can be particularly irritating. In the lab setting, for medical test and analysis applications such as microscope cell discoverers, centrifuges, and other benchtop lab equipment, long-term operator exposure can cause fatigue and other stress-related injuries.

The second cooling option is convection cooling. As with fan-based power supplies, you must add open vents to the equipment to allow heat to escape. Depending on the type of medical equipment and the environment in which it operates, vents may be undesirable as they reduce ingress protection making it more difficult for the hospital staff to disinfect and keep the equipment clean. Easy-to-wipe surfaces with no ventilation are considered better. An alternative thermal mechanism is conduction cooling which allows for completely sealed designs.

Medical safety standards and applied parts

Depending on where the equipment is and its intended purpose, even in a hospital you may not necessarily need medically approved power supplies. For instance, if the equipment is more than two meters away from a patient, you may be able to choose a standard ITE-approved power supply. However, if the DC output of the power supply is connected to an applied part which touches a patient, then the power supply must be compliant with the 60601-1 standard.

There are three classification types for applied parts (AP) – Type B, BF, and CF. Type B (body) is for an AP that is not electrically connected to the patient and may be grounded. Type B equipment includes hospital beds, operating room lights, and MRI scanners. Type BF (body floating) devices, such as ultrasounds, incubators, and blood pressure monitors, are directly connected to the patient through sensors. Surgical equipment has the most stringent Type CF (cardiac floating) rating as the applied part electrically connects with the patient’s heart or bloodstream.

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It must be noted that you cannot safety approve an embedded power supply to BF rating as only the end equipment can get that safety approval. The power supply manufacturer, however, can design it to be suitable for BF applications. The output-to-ground must be 1xMOPP (means of patient protection) of basic isolation, and the maximum leakage current must be less than 100µA. If the power supply fulfils these safety requirements, it can be used in Type BF applications and non-patient connected medical applications.

For the applied part, you often have a separate DC-DC converter, which may also have 1xMOPP input-to-output isolation. If you have a 2xMOPP power supply then, depending on the application, you may not need the DC-DC converter; in practice, many applications have isolation requirements that necessitate extra protection provided by medically approved DC-DC converters. For applications where you have multiple Type BF-applied parts, it may be easier to use multiple 1xMOPP DC-DC converters.

Understanding peak power

The peak power of a power supply is limited by several factors. The first is whether the design can provide the extra output, and the second is the temperature of the components which might heat up to the point of failure. So, whatever the peak power you take, it needs to be within the thermal rating of the components. Another consideration is where you set the current limit. Usually, you want to set the current limit tightly but a power supply that allows peak loads will need to either have a much wider setting or a delayed protection circuit.

The output voltage may drop with peak power, but it must still be within its specified tolerance. For certain applications, like driving a DC motor to adjust the height of a dental chair, it is unlikely that you will need that peak power very often and only in short bursts. Under those circumstances, as the equipment designer, you may have enough leeway to choose a power supply with a slightly lower power rating. For example, you may be able to use a 400W rated power supply in such an application that requires a 550W to 600W peak power, but only if the current limit is set appropriately and the components, such as the e-caps, are within the thermal rating at that peak power.

Conclusion

Designers need an off-the-shelf power solution that can be successfully implemented and verified by regulators. A power supply that incorporates the specifications and features demanded by medical device applications will, in turn, minimize development costs and approval time. A low profile, small footprint, high efficiency, high density, conduction- or convection-cooled power supply with EMC and safety certification for global healthcare applications, such as the CCP550 from XP Power, is an appealing option. 

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The data sheet provides service life curves based on the average operating temperature of key e-caps. This data can be used to evaluate the product service life when installed in the individual application based on the equipment’s component temperatures, daily usage, and thermal mission profile in its unique cooling environment.

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