Tutorial: How to select the right components to protect medical devices, users & patients

Author : Rolf Horn | Applications Engineer | Digi-Key Electronics

01 August 2021

Digi-Key_How to select the right components to protect medical devices, users & patients
Digi-Key_How to select the right components to protect medical devices, users & patients

Use of non-laboratory, patient-contact diagnostic & life-sustaining medical equipment, such as ventilators, defibrillators, ultrasound scanners & electrocardiogram (EKG) units, continues to increase. Reasons include an ageing population, heightened care expectations among patients & improvements in medical electronics technology which make such systems more practical.

The full version of this article was originally featured in the August 2021 issue of EPDT magazine [read the digital issue] and the Digi-Key Article Library. And sign up to receive your own printed and/or digital copy of EPDT each month.

But as Rolf Horn, Applications Engineer at electronic component distributor, Digi-Key explains here, such equipment needs protection against multiple types of electrical issues that can harm the equipment, hospital staff and patients…

However, full circuit protection takes much more than just a thermal fuse, and implementing protection is not a matter of finding the single best device for a given design and application. Instead, it involves first understanding which circuits need protection, and then determining the best mode of protection. In general, multiple passive components are needed to provide protection, and typical systems may need a dozen or more of these specialised protection devices. Protection devices are like insurance: while it may only rarely, or even never, be needed, the cost of not having it far exceeds the cost of having it.  

This article looks at where protection is needed in such medical systems, including patient-facing signal/sensor I/O, power supply, communication ports, processing core and user interfaces. It also discusses the various types of circuit and system protection components, using devices from Littelfuse by way of example, and examines the role and application of each.

The role of protection in medical systems
For most engineers, the phrase “circuit protection” immediately brings to mind the classic thermal fuse, which has been in use for over 150 years. Its modern embodiment is largely due to the work of Edward V Sundt, who, in 1927, patented the first small, fast-acting protective fuse designed to prevent sensitive test meters from burning out. He then went on to found what eventually became Littelfuse.

Since then, circuit protection options have expanded significantly, in recognition of the many potential circuit failure modes. These can be:
•  Internal failures that may result in a cascade of damage to other components
•  Internal failures that may put the operator or patient at risk
•  Internal operational issues (voltage/current/thermal) that may stress other components and lead to their premature failure
•  Voltage/current transients and spikes which are an inherent and unavoidable part of the circuit’s functionality and must be carefully managed

Many of these issues apply to battery-powered units, not just those that are AC line powered. The function of many, but not all protection devices is to suppress unacceptably large voltage transients. There are two major categories of transient suppressors: those that attenuate transients, thus preventing their propagation into the sensitive circuit; and those that divert transients away from sensitive loads, and so limit the remaining voltage. It is critical to study device datasheets carefully for thermal and performance derating curves, as some are specified for transient protection of various durations, bounded by defined voltage, current and time limits, rather than steady-state protection.

Among the many electrical parameters that must be considered are clamp voltage, maximum current, breakdown voltage, reverse working maximum or reverse stand-off voltage, peak pulse current, dynamic resistance and capacitance. It is also important to understand under what conditions each of these is defined and specified. Device size and number of channels or lines protected are also considerations. The choice of the best protection device to use in a given part of a circuit is a function of these factors, and there are often the inevitable trade-offs among the various parameters. There will almost certainly be preferred or “standard” approaches, but there are also choices that must be judged, assessed and made.

Circuit protection options are many: choose wisely
There are a variety of protection options. Each has a unique functionality and set of characteristics that makes it a suitable—or only—choice for implementing protection against specific classes of faults or unavoidable circuit characteristics. The main protection options are:
•  The traditional thermal fuse
•  Polymeric positive temperature coefficient (PPTC) devices
•  Metal oxide varistors (MOVs)
•  Multi-layer varistors (MLVs)
•  Transient voltage suppression (TVS) diodes
•  Diode arrays
•  Solid state relays (SSRs)
•  Temperature indicators
•  Gas discharge tubes (GDTs)...  

Read the full article in EPDT's August 2021 digital issue...

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