Powering wireless medical instrumentation requires the right approach

January 30, 2017 // By Tony Armstrong
As with many applications, low power precision components have enabled rapid growth of portable and wireless medical instruments. However, unlike many other applications, many medical products typically have much higher standards for reliability, run-time and robustness. As you would expect, much of this burden falls on the power system and its associated support components.

Medical products must operate properly and switch seamlessly between a variety of power sources such as an AC mains outlet, battery backup and even harvested ambient energy sources. Furthermore, great lengths must be taken to protect against and tolerate faults, maximize operating time when powered from batteries and ensure that normal system operation is reliable whenever a valid power source is present.

One of the current key trends fueling growth in the potable and wireless medical instrumentation is patient care. Specifically, this is the increased use of remote monitoring systems within the patient’s own home. The key reason for this is trend is purely one of economics and that is the continuous increasing costs of keeping a patient in a hospital are simply too prohibitive. As a result, many of these portable electronic monitoring systems must incorporate RF transmitters so that any data gathered from the patient can be sent directly back to a supervisory system within the hospital for both immediate or later review and analysis by the governing physician.

Given this typical scenario, it is reasonable to assume that the cost of supplying the appropriate medical instrumentation to the patient for in-home use is more than offset by the cost of keeping the patient at the hospital of observation purposes. Nevertheless, it is of paramount importance that the equipment used by the patient be reliable and fool-proof. As a result, the manufactures and designers of such products must ensure that they can run seamlessly from multiple power sources while also ensuring the high reliability of their wireless data transmissions back to the hospital. This requires that the system designer takes great care when designing the power management architecture. It must be robust, flexible, compact and efficient.


Potential Pitfalls & Solutions

It is not unusual for a system designer to use linear regulators in a system that incorporates wireless transmission capability. The primary reason being that it minimizes EMI and noise emissions. Nevertheless, although switching regulators generate more noise than linear regulators, their efficiency is far superior. Noise and EMI levels have proven to be manageable in many sensitive applications as long as the switcher behaves predictably. If a switching regulator switches at a constant frequency in normal mode, and the switching edges are clean and predictable with no overshoot or high frequency ringing, then EMI is minimized. Moreover, a small package size and high operating frequency can provide a small tight layout, which minimizes EMI radiation. Furthermore, if the regulator can be used with low ESR ceramic capacitors, both input and output voltage ripple can be minimized, which are additional sources of noise in the system.

The main input power to today’s feature-rich medical devices is usually a 24V or 12V DC source from an external AC/DC adapter. This voltage it then further reduced to either 5V and/or 3.xV rails using synchronous buck converters. Nevertheless, the number of internal post-regulated power rails in these medical devices has increased while operating voltages have continued to decrease. Thus, many of these systems still require 3.xV, 2.xV or 1.xV rails for powering low power sensors, memory, microcontroller cores, I/O and logic circuitry. Furthermore, since the medical device operation is often critical, many of them often incorporate a battery back-up system to guarantee operation should the main power supply to the unit fail for some reason.

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