Power Management for Medical Devices

The main requirements of OEM medical designs include the selection of low-power components, the ability to place devices in low-power states, a powerful CPU core to control and perform advanced calculations, and a large non-volatile memory to store both program images and user data. These designs also require a host of peripherals to connect various analog or digital systems. An efficient DC/DC or AC/DC power supply and a very low-power microcontroller (MCU) to maximize battery life are the key elements for proper power management in medical diagnostic devices.

Battery management for portable devices

Rechargeable lithium-ion batteries offer many advantages over other types of power sources. Their chemistry allows for the miniaturization of devices, adapting to the stringent demands of wearable medical devices. Moreover, these batteries provide excellent charge cycling and prolong the overall system life.

The lifespan extension of a medical device with an adequate rechargeable battery and low-power-circuit solutions prevents frequent maintenance interventions and protects the patient from harm in case of any complications from the rechargeable battery.

An energy management system for rechargeable batteries consists of cells, charging circuits, and battery management units (BMUs). The cells contain the electrodes to store energy and, thus, power the medical device. A charge management circuit makes it possible to assess the current and voltage conditions between the battery and the medical device. Monitoring the voltage of each cell inside a battery pack is essential to determine the general state of the device. Battery operation outside the voltage range significantly reduces cell life. A charger must monitor the status of the lithium-ion battery during the charging process to keep it within the safe operating area.

The BMU offers protection to the cell, limiting it to the maximum (and minimum) values of current and voltage. For implantable medical devices, such as cardiac pacemakers, cardioverter defibrillators, drug delivery pumps, and neurostimulators, further considerations arise from the need for sterilization, environmental conditions, and the potential for patient injury.

System protection is required to reduce the risk of cell failure. The grouping of the functional blocks of a simple BMU varies widely, from a simple analog front end that offers balancing and monitoring and requires an MCU to a highly integrated solution that operates autonomously.

A fuel-gauge functional block keeps track of the current entering and leaving the battery. Pairing a current sense amplifier and an MCU with an integrated low-resolution ADC is one method to measure this current. A high-resolution ADC offers a wide dynamic range at the expense of speed. For irregular loads, a successive-approximation-register (SAR) ADC proves to be a winning choice (Figs. 1 and 2).

Figure 1: Typical sensor circuitry for an ECG monitor. (Image: RECOM)
Figure 2: Different types of cell balancing: (A) Bypass cell balancing FETs are used for a slow charge and (B) active balancing is used during the discharge cycle. (Image: Renesas Electronics)

An example of an integrated solution is the ADP5350 BMU from Analog Devices Inc., which integrates a high-performance buck regulator, a fuel gauge, a boost regulator that operates at a 1.5-MHz switching frequency for LED lighting, and three 150-mA low-dropout (LDO) regulators. The device has an internal field-effect transistor (FET) that allows the battery to be insulated on the supply side of the system (figure 3), thus offering greater protection.

Figure 3: A typical application circuit with the ADP5350 battery management unit. (Image: Analog Devices Inc.)

An intelligent battery can also be equipped with an authentication key based on the SHA-1 standard to protect a battery-powered medical device. This key may be standard for all manufacturers’ batteries or specific to the manufacturer of the medical device. The specific custom authentication key can be combined with private battery labeling.

An example is Maxim Integrated’s MAX14663, a complete energy management solution for portable medical devices, including blood glucose meters. The device integrates a switching charger and Maxim’s proprietary ModelGauge fuel gauge to provide an accurate estimate of the available charge for rechargeable lithium-ion batteries. A boost converter and LED current sinks are also integrated to power the OLED display and provide an LED backlight.

Converters for high power

Healthcare devices are heavily regulated by industry-standard requirements and related tests to protect patients and healthcare professionals. The degree of isolation of commercial AC or DC devices is not adequate to meet medical standards. Many have only one protection measure, and the insulation capacity is normally too high to meet the requirements of low leakage current.

The relevant safety standard is “Medical Electrical Equipment” IEC 60601-1 and its national versions, EN 60601-1 in Europe and ANSI/AAMI ES60601-1 in the United States. The standard consists of a series of requirements for electrical and electronic equipment used in health care.

According to IEC 60601-1, medical devices must incorporate at least one means of protection (MOP) to ensure that both patient and operator are protected from the risk of electric shock, even in fault conditions. The standard treats operators and patients in slightly different ways, determining different classifications: means of protection of the operator (MOOP) and means of protection of the patient (MOPP). Medical equipment must, therefore, have two MOPs so that if one measurement fails, the second measurement will still provide adequate protection.

A possible solution is to use a 2-MOOP medical-grade AC/DC isolated power supply with an additional power isolation stage for the sensor electronics in the form of one or more medical-grade DC/DC converters (Figure 4).

Figure 4: Using a DC/DC converter to meet medical applications requirements. (Image: RECOM)

A variety of power-supply manufacturers, including Traco, offer a wide range of AC/DC and DC/DC devices that meet all MOPP requirements for medical applications. Traco’s AC/DC solutions range from small 5-W PCB mounting modules to designs offering power levels up to 450 W.

All high-efficiency DC/DC converters are switching power supplies and generate conducted and radiated electromagnetic noise. The 60601-1-2 medical standard has noise limits that must be suppressed as much as possible in the DC/DC converter or externally filtered.

A modular DC/DC converter is a simple solution. An example is RECOM’s 2-watt REM2 series products, which offer 2-MOPP isolation and 250-VAC operating voltage in a compact SIP8 package. The converter is pre-certified according to IEC/EN/ES 60601-1, making final compliance testing easier.

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