Design considerations for power supply operation in medical equipment

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Power supplies designed for use in medical and healthcare equipment have to comply with several safety standards before they can be brought to market. The medical safety standard, ISO 60601, is well documented, and this article covers this topic in more detail.

Aside from the critical design features required by the standard, there are a couple of factors that impact how the power supply operates in the end equipment. In this article, we'll investigate two of these factors: keeping audible noise to a minimum; and enabling a control interface for the operation of the power supply.

Medical and healthcare equipment covers an enormous range, from hand-portable blood pressure monitors to trolley-mounted patient monitoring equipment, to fixed installation ultrasound scanners and CT scanners. Oral care systems, laser hair removal equipment, and UV light therapy are some examples of task-specific dental and beauty equipment. Then there are the items of equipment used within an operating theatre or laboratory such as aspiration pumps, anaesthesia machines, and blood chemistry analysers.
 

Medical and healthcare power supply design

Given the full range of equipment listed above, the power supply requirements are equally diverse. Some handheld equipment such as a patient-monitors may be battery-powered, using rechargeable or primary cells. For this type of application, ISO606061 compliant DC-DC converters may be used internally to provide an isolated single or dual output power rail. Low power DC-DC converters up to 15 - 20 watts are incredibly compact and offer high levels of efficiency.

At the other end of the scale, large fixed installation equipment such as a Philips MRI scanner consumes up to 22 kW during operation. With multiple subsystems, located in separate places, such as the operator's console, the control room, and the examination room, each has its specific power requirements. Most subsystems would feature a distributed power architecture, such as an intermediate bus architecture (IBA) that uses high power (> 1.5 kW) front end hot-pluggable AC/DC power supplies to feed point of load DC/DC converters. See Figure 1 (below).

Another example is a table-top autoclave used for sterilising surgery instruments suitable for use in a dentist's surgery. A unit with an 18-litre capacity consumes 1.7 kW from a 240 VAC mains supply. An autoclave generates steam to sterilise the instruments, so the majority of the power consumed will be used for a heating element within the unit's water chamber. A relatively low amount of energy is required for the controller and LCD panel, something a compact 2-inch x 4-inch 120 Watt AC/DC power supply could provide.

Figure 1: Example of an intermediate bus architecture 

A lot of the equipment and applications listed above may be mounted close to patients on hospital wards or used in an operating theatre. In both these situations, together with any equipment used for home healthcare, such as a ventilator, keeping audible noise to an absolute minimum is essential.

Unfortunately, many power supplies, typically mid-high-power AC-DC units, use fans to provide a forced airflow to remove waste heat from within the power supply. Fans, and the movement of air through cowlings and finger safety grills, generates high levels of background noise.

Another aspect of power supply design that is common across the above applications is the ability to control the operation and output of a power supply. There are a variety of reasons for such a requirement, but lowering the energy consumption profile, close monitoring of the PSU operation, and sequencing of the output voltage(s) are three popular drivers.
 

Managing a power supply with the PMBus protocol

Power supplies used to be architected in an analogue domain. However, the advent of digitally controlled switched-mode AC-DC and DC-DC supplies has brought them into the digital domain. This architectural change presented the opportunity to incorporate an analogue-to-digital converter with a microcontroller to provide monitoring and control capabilities.

Initially proposed in 2003, PMBus, a digital power management protocol, takes its roots from the IT system management standard SMBus. It initially served the needs of complex distributed power architectures, for example, carefully sequencing the multiple power rails required by sophisticated programmable logic devices such as FPGAs and ASICs.

PMBus has quickly evolved to provide a supervisory control function for the host system to control separate circuits and subsystems. PMBus uses a serial communication interface similar to I2C, but with the addition of an interrupt line SMBALERT that allows an individual slave power supply to alert the host rather than rely on regular slave polling from the host. See Figure 2 (right).

As with I2C, the PMBus host issues packets to an individually addressed slave power supply. With the current V1.3 protocol standard, there are up to 256 different control, configuration or monitoring commands possible. Not every power supply will use the complete list, some being not relevant, or the PSU not accommodating that particular feature. For example, the Murata DSQ series of DC-DC converters use 74 of the 256 commands. Power supply configuration parameters, such as voltage trim, over-voltage limit, and over-current limit are all held in Flash memory within the PSU, allowing the unit to configure itself at start-up.

With PMBus, for example, the central processor of a large PET scanner can sequence subsystems on in the correct order, and when not active, and enable a low saving standby mode by selectively switching off the output from individual power supplies. Similarly, the voltage rails of powerful computing devices such as GPUs and FPGAs used to render scanned images can be driven at the optimum voltage for a specific task.

Incorporating PMBus is equally useful for controlling the operation of compact monitoring equipment on a hospital ward, the host microcontroller turning on measurement functions at prescribed times and then putting those functions in a power-saving mode afterwards.
 

Eliminating forced-air cooling noise

As previously mentioned, audible fan noise is a nuisance for patients wishing to sleep. Hospital wards, day-care facilities and operating theatres aspire to be havens of calm, with unnecessary noise kept to a minimum. The design of many power supplies requires waste heat to be pushed or pulled out of the unit by a forced airflow. The heat equates to losses within the power conversion stages, typically the summation of many small losses rather than one or two major ones.

The airflow created by a cooling fan aims to bring the temperature of the PSU components within an acceptable temperature range. Increasing temperature harms reliability, a critical factor for medical equipment, so cooling is vital. Consider a 100-watt power supply that is 90% efficient. 10% of the energy, in this case 10 watts, is heat due to losses and needs to be dissipated.

An alternative approach to using forced air cooling is convection cooling. Most modern power supplies have a convection cooling rating that will allow operation up to, say, 80% of the full load forced air-cooled rating.

To achieve this, power supply design engineers need to maintain adequate space within the power supply so that free air convection currents can cool the unit. With power supplies now available with 94% to 96% efficiency ratings, the amount of waste heat to be dissipated can be managed using convection cooling heatsinks. Thermal management of the power supply and any other heat-generating devices within the medical equipment needs carefully reviewing by the product design team.

Another approach that is popular for equipment contained in sealed enclosures is conduction or baseplate cooling. Frequently used for externally mounted cellular base stations and rugged avionics equipment, the approach uses power supplies that are constructed with all the significant heat-generating components mounted together on a solid metal baseplate. By then mounting the baseplate against the external metal wall of an enclosure, heat can be conducted to the outside world.

Conduction cooling is now gaining adoption for a broad range of medical, industrial and consumer equipment. Thermal management techniques that do not require a fan, such as conduction cooling, is particularly important for medical and healthcare equipment. By removing the audible noise from diagnostic and monitoring equipment, patients gain an environment conducive to rest.
 

Conclusion

From medical imaging systems such as X-ray machines and ultrasound scanners to electrosurgery and digital radiography equipment, they all benefit from using convection or conduction-cooled power supplies. Also, to enable power saving and the comprehensive control of end-equipment, a medically certified, PMBus-equipped power supply is essential.

Avnet Abacus' industry leading linecard features products from the world's best power suppliers, designed and manufactured to medical safety standards for a range of applications: 

• Medical imaging: ultrasound scanners, MRI, CT, PET, X-Ray
• Surgical devices: robotics, electro surgery, laser surgery
• Medical devices: patient therapy, patient monitoring, patient transport, beds, ventilators, powered air-purifying respirator, anesthesia machines, aspiration and suction pumps, autoclaves, sterilisers, blood chemistry analysers, centrifuge and many more
• Dental equipment: oral care systems, CAD/CAM systems, digital radiography
• Wellness and beauty: laser hair removal, UV light therapy, laser therapy, ultra-sound

To view the linecard and explore solutions from our suppliers, visit our medical power solutions page. Alternatively, if you would like to discuss your power requirements in detail, get in touch with our team of technical specialists in your local language.

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