28 Mar Power Up- Design for Power Management
A majority of electronic diagnostic, life science, and medical devices are still wall-powered, despite the leaps taken in battery technology over the last two decades. Inevitably these devices will employ some sort of AC/DC converter and likely further DC/DC conversion to power all the circuits inside. At the beginning of a medical device development program, you may need to make early decisions about power management, which can produce significant challenges down the road if not made judiciously. At Key Tech we focus on three things to align power management with user needs and a device’s use environment.
#1 Electrical safety
Safety of a device will be heavily scrutinized by a chosen third-party product safety lab to achieve regulatory approvals. One of the best investments you can make is to purchase quality medical (IEC 60601-1) or non-medical (IEC 60950/62368) AC/DC power supplies from a reputable manufacturer like XP Power or TDK Lambda, which will hold up to this scrutiny. In many cases these supplies will come with CB scheme preapprovals that speed up a lot of regulatory testing. As a bonus, you’ll save engineering effort that would otherwise be spent designing and documenting the electrical safety of a semi-custom solution.
Want to make the electrical safety argument even easier? For smaller devices, consider an external (a.k.a. “desktop” or “wall”) AC/DC adapter to power the device. At the expense of a few minor tradeoffs, having just low DC voltage enter the device further simplifies the electrical safety and compliance path. Such power supplies can be found below a few hundred watts, suitable for smaller devices like Mi-Helper but insufficient for larger instruments like ePlex.
#2 EMC
Electromagnetic compatibility (EMC) testing can be arduous and expensive, often coming late in a project when deadlines are tight and design changes are expensive. Plus, adverse findings can require costly retesting. Nearly half of the relevant tests are direct measures of a device’s AC/DC power supply. The same quality power supplies that meet safety requirements are almost always pre-tested by their original manufacturer for all EMC tests, and while you will still need to retest your product, you are very likely to pass those tests. EMC testing is always challenging, but a quality power supply will mitigate that challenge significantly.
#3 System integration
Power supplies and power distribution are usually architected early in a program, yet power needs inevitably evolve throughout development. When architecting, overestimate the required capacity. Come up with a detailed power budget that is easy to recalculate later as needs evolve. Consider abnormal conditions when power draw may peak. Key Tech has seen cases of compressors stalling or motors overloading, which overdraws a supply and causes the device to reboot; these occurrences can be difficult to reproduce and troubleshoot.
To simplify system integration, limit the number of supply voltages needed in a system. Each new voltage rail adds complexity to the power management of the device. For example, Key Tech often works with clients whose devices integrate an industrial computer. Their feasibility prototypes will often make use of commercial computer hardware because it is cheap and readily available. As a marketable product begins to be designed and power management is considered, the ATX power supply standard which requires 5 (!) different voltages is always inconvenient. Sometimes we will “roll our own” ATX supply from a combination of modular medical supplies, but finding an industrial or commercial 12V-only motherboard greatly simplifies power management, reducing cost and size of the overall product.
Power management can also be simplified by avoiding expensive and inefficient power conversion steps for high-power busses in the system. These busses are usually best fed directly from the AC/DC converter, then stepped down for lower power voltage supplies. Large diagnostic instruments will often use 48 V as the system voltage, suitable for driving strong servos and stepper motors. 24 V is also a common system voltage because it will accommodate medium-sized motors and actuators. Occasionally, a 12V system voltage can be used for smaller devices that need less powerful actuators.
Getting power management right requires strong systems thinking to weigh the pros and cons of many options. Sometimes focus will be on cost, which guides one set of decisions, and other times focus will be on size or speed to market. At Key Tech we specialize in engineering with product development and the whole system in mind. Interested in learning more? Drop us an email at TalkToUs@keytechinc.com.
Ward received his B.S. in Electrical Engineering from Bucknell University where he co-authored publications on both a river water quality monitoring system and an animatronic vulture. In his free time, he enjoys cycling, backpacking, and baking artisan bread.
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