【Introduction】Professional and home healthcare is a fast-growing market, with more and more medical devices in contact with patients, from vital sign indicating devices to wireless charging implants. Whether battery-powered or AC-powered, DC power is required for every connected device, and low-power modular DC/DC converters are often used as part of a safety isolation system. This article describes the application of DC/DC converters in this field, explains the relevant safety certifications required, and introduces a commercial converter family designed specifically for this application.

In professional and home healthcare, more and more medical devices come into direct contact with the human body to measure various health indicators, including body surface parameters such as skin resistivity, body temperature and blood oxygen saturation levels, and the use of electrocardiograms (ECGs). ) device monitors the heartbeat (Figure 1).

Figure 1: Typical sensor circuit for an ECG monitor

Sometimes, such as during surgery, the sensor needs to be placed inside the body. Most sensors are Electronic devices, so they need to be designed to ensure low enough voltage and current to avoid shocking the patient. There is also a requirement that the sensor never provide a loop through which destructive current from the faulty device can flow back to the sensor through the patient. This means that the sensor and its power supply must not only be adequately isolated from dangerous high voltages, but must also be isolated from “ground”. Even a battery-powered handheld device may be grounded through an attached printer, USB port, or laptop.

In medical electronics, the relevant safety standards are “Medical Electrical Equipment” IEC 60601-1 and various national standards: eg EN 60601-1 in Europe and ANSI/AAMI ES 60601-1 in the United States. In the standard, “isolation” has a very specific connotation of requiring effective double electrical isolation between a hazardous voltage or high energy source and the patient. Medical devices must therefore have dual “measures of protection” (MOPs) in case one fails and the other still provides adequate protection. There are two levels of MOP requirements: the operator’s MOOP and the higher-level patient MOPP.

To power sensors or read data in isolated isolation conditions, miniature safety transformers or optocouplers with reinforced insulation construction are a practical solution. These methods provide high withstand voltage isolation, but there is still coupling capacitance at both ends of the isolation, and there is leakage current, so the coupling capacitance must be reduced as much as possible.

If the sensor power comes from an AC/DC converter, a safe isolated output is standard on all power supplies. However, the isolation level of AC/DC for commercial or IT use is not sufficient to meet medical standards. Many devices have only one measure of protection, and the isolation coupling capacitance is often too high for low leakage current requirements. Also, the output may be “floating” but not sufficiently isolated from “ground”. Some “medical grade” AC/DCs have measures of operator protection (MOOP), but are not suitable for direct patient connection due to insufficient isolation of input/output and output/ground, and the AC/DC isolation coupling capacitance is not low enough. Although AC/DC with dual patient protection (2MOPP) is also available, the power range is limited and expensive. Multi-channel sensors often require multiple isolated power rails, but it is certain that only a fully custom 2MOPP AC/DC will have all the power rails required.

DC/DC can form part of an isolated system

One solution is to use a standard 2MOOP medical-grade isolated AC/DC converter, then add another level of isolation to the sensor with one or more medical-grade DC/DC converters. These converters provide the required sensor voltage rails, and if they have high isolation and low coupling capacitance they can further be part of an isolation system for the most stringent “heart floating” (CF) class device applications.Figure 2 shows typical connections for a system with a grounded enclosure or “Class I” electrical configuration

Figure 2: Using DC/DC Converters for Medical-Grade CF Applications

In the example of Figure 2, even though the AC/DC has 2MOOP, the DC/DC converter requires 2MOPP because there are unspecified devices (SIP/SOP) connected to the DC/DC input, which may result in the event of a fault Hazardous voltage. If the device is a “Class II” electrical device, i.e. has no ground connection, and uses a plastic enclosure with no external connection specified, but if the AC/DC has at least 2MOOP of isolation, then the DC/DC converter may only require a class of isolation of 1MOPP ( D), for applications in direct patient connection. DC/DC usually only needs to provide low-power power supply, which is not only small in size but also low in cost.

A typical application of a DC/DC converter is shown in Figure 3. An instrumentation amplifier is used here as an ECG sensor, providing digital data for analysis. Another DC/DC powers the driver, which provides some kind of stimulation to the patient and is isolated from the sensor. Data is isolated with optocouplers. Instrumentation amplifiers typically require +/-5V, and drivers require +12V. Two 2W onboard DC/DC converters powered by the regulated system voltage provide isolated voltage rails. With a regulated input and a constant load, DC/DC can be a simple, low-cost “unregulated” converter that requires only a few percent load regulation to obtain adequate performance.

Figure 3: DC/DC converters provide isolated power rails for sensors and drivers

DC/DC converters can provide more isolated power rails for other measurement or stimulation channels. The small size and design of the DC/DC converter results in extremely low coupling capacitance and therefore minimal leakage current.

DC/DC converters generate noise

All high-efficiency DC/DC converters are “switched” power supplies and therefore generate conducted and radiated electromagnetic noise. In real circuits, this noise can interfere with other measurements, especially when the signal is in the mV range, such as in ECG equipment. The medical standard 60601-1-2 also has limits on noise, so the noise level inside the DC/DC must be suppressed as much as possible, or filtered externally. However, external filters often have to bridge the isolation barrier to reduce “common mode” noise due to leakage currents. As a result, DC/DC converters with lower inherent noise levels should be preferred.

As shown in Figure 2, a DC/DC converter with a 2MOPP isolation rating is required to meet the creepage distances and clearances specified in the equipment “system” voltage standard. In fact, if the components can be used at altitudes above 2000 meters, for a 240VAC power supply system, the creepage distances and clearances need to be greater than 8mm. The connected medical equipment must be small, so circuit boards for low-power DC/DC typically have only a few square centimeters of space. This means that the isolation transformer is a complex design and must use three layers of reinforced insulated wire, with added insulation and epoxy encapsulation to reduce the “pollution level” of the local environment. So implementing a discrete DC/DC solution is not easy, and in many applications requires purchasing an expensive custom transformer.

Modular DC/DC converters are a cost-effective solution

Modular DC/DC converters are a simple solution such as RECOM[1]The new, cost-effective 2W REM2 series (Figure 4) offers a 2MOPP isolation rating and 250VAC rated operating voltage in a compact SIP8 package. The converter is certified to IEC/EN/ES 60601-1, making final EMC testing easier. The converters are available with single or dual outputs to power sensors or instrumentation amplifiers and are available with a choice of five different nominal input voltages. The operating temperature range of the module is -40°C to +95°C, derating from 80°C, and it can be used for altitudes up to 5000m. The converters are 85% efficient, low noise, and comply with IEC/EN 60601-1-2 and EN 55011 EMC requirements. The isolation capacitance is only 25pF.

Figure 4: RECOM’s 2W DC/DC converter with 2MOPP/250VAC rated voltage

Isolated DC/DC converters are important components that meet medical isolation level requirements and are used in connecting patient equipment. Certified, off-the-shelf modules in tiny packages solve all application problems.

references

[1] RECOM: www.recom-power.com/medical

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