Portable, hand-held medical devices are improving patient care, both at home and in medical facilities. However, reliability and safety demand proper use of electronic circuit protective devices. This article reviews a number of available options to ensure protection of portable electronic medical devices and covers the factors in making the right selection.By James Colby
James Colby is manager, business and technology development, for the Electronics Division of Littelfuse Inc. He is responsible for identifying and developing strategic growth markets as well as introducing new products into those markets. Colby can be reach at 847-391-0530 or firstname.lastname@example.org.
A key design element is the use of overvoltage and overcurrent protection components, which help prevent circuit damage due to electrostatic discharge (ESD), overload currents, and other surges. In some types of portable equipment, protective devices also provide an extra margin of electrical safety for the users. The selection criteria for protective devices varies according to instrument subsystem. The major subsystems that must be considered include communication interfaces, DC input/charging circuits, battery packs, sensors, LCD displays, keypads, and buttons.
Major Threats to CircuitsFigure 1 is a simplified electrical diagram for a generic handheld instrument. The green text on interconnecting lines lists typical devices used to protect portable instrument circuits that are shown in the boxes. The primary threat is static electrical charge, which is easily generated by users as they walk across carpets, and is transferred from clothing or skin into instrument circuitry through electrical terminals, cables, instrument case, etc. This results in excessive voltage or current being applied to electrical components. There may be other sources of static charge build-up, but in most cases the charge transfer mechanism is commonly referred to as electrostatic discharge, or ESD.
Another source of overvoltage and overcurrent conditions is the battery charger or power supply circuit when an instrument is plugged into the AC wall outlet. Such conditions may occur due to sudden electrical load changes on power lines. In addition, nearby lightning strikes during a storm can induce large voltage and current surges on the power lines, which can be transferred into the instrument. Regardless of the origin, the user and instrument circuits should be protected from excessive voltage and current.
Therefore, protection from ESD and other forms of overvoltage and overload currents ensure reliable instrument operation, and prevent premature failures. Circuit protection devices also help prevent the creation of electrical current paths due to circuit failures arising from an overvoltage condition. Unintended current paths may pose a threat of electrical shock to the instrument user. Ultimately, premature failures and dangers to users have serious economic consequences for instrument manufacturers.
Overview of Circuit Protection DevicesA wide variety of circuit protection devices are available, but it’s easy for designers to fall into the habit of using only one or two types as a matter of familiarity and convenience. For example, overload current protection in portable instruments is often limited to a small fuse. However, a positive temperature coefficient (PTC) thermistor can limit current when an overload occurs, but then “resets” to a low resistance value when the overload is gone. This eliminates the inconvenience of having to replace a fuse.
The different types of overvoltage protection devices and all their variations are too numerous to mention. The more common ones are listed in Table 1, along with some of their important characteristics.
Gas Discharge Tubes—GDTs are typically used to protect telecom lines, datacom lines, and other instrument signal lines from surge voltages. They are a good choice for reducing lightning induced transients because they can handle surge currents up to 40,000 A.
Varistors—Variable resistors possess characteristics that divert current created by excessive transient voltage away from sensitive components. There are two major types:
MLV (multi-layer varistor)—These devices provide protection from low to medium energy transients (0.05 to 2.5 Joules) in sensitive equipment operating at 0-120 VDC. They are most commonly used for ESD protection.
MOV (metal oxide varistor)—With energy ratings from 0.1 to 10,000 Joules, these varistors can divert transient currents away from sensitive circuit components in a wide range of applications. They are most commonly used on AC lines for lightning protection.
Polymeric ESD Suppressors—Polymeric ESD suppressors are a good choice for high-speed digital I/O and RF lines because of their low-capacitance (~0.05 pF) and fast voltage clamping ability. The low capacitance ensures that no signal loading or distortion occurs.
TVS Diodes—Transient Voltage Suppression diodes protect a wide variety of circuits and components from an assortment of threats that are experienced on DC power lines. Their p-n junctions have a much larger cross-section than normal diodes, allowing them to conduct large currents to ground without sustaining damage. Their transient protection ratings range from 400 to 15,000 W.
Silicon Protection Arrays (SPAs)—Silicon diode arrays are designed to protect analog and digital signal lines from ESD and other overvoltage transients. They offer space-efficient ESD protection in multi-channel arrays.
Resettable Devices—An alternative to a fuse is a positive temperature coefficient (PTC) thermistor. As current increases, self-heating increases PTC resistance and automatically limits current. Polymer-based (PPTC) materials are typically used, which have a pronounced knee in their resistance vs. temperature characteristics. Once the overload is gone, the PPTC cools and returns the circuit to normal operation. This avoids the need to replace fuses.
Taken as a whole, several mounting options are available for circuit protection devices, including radial leaded, axial leaded, surface mount, etc. Being able to purchase the necessary device in the appropriate mounting style (perhaps for tape and reel application) can have a major impact on circuit assembly cost. Other important selection criteria are indicated by the column headings in Table 1.
Examples of Circuit Protection Schemes
As alluded to in Figure 1, a handheld instrument’s wireless (RF) interface can be exposed to ESD and other voltage surges induced through its antenna. The circuit in Figure 2 illustrates a Polymer ESD solution (see Table 1) that protects the RF amplifier input module from ESD threats.
Besides the Continua standards, the IEC 61000-4-2 standard applies to the protection circuit in Figure 2. This standard defines a test method for verifying that the end product is not susceptible to ESD events.
An instrument’s sensor input is the interface between the user and the measurement circuits. This input also makes an instrument prone to ESD damage, since the sensor is attached to, or comes in contact with the user in some fashion. A Polymeric ESD suppressor or diode array can be used to protect the sensor input (Figure 3). The benefits include ESD protection as well as very low leakage current.
ESD can severely damage an instrument’s LCD module. For typical displays, the data transfer rate is less than 20 Mbps, so protection devices with capacitance values less than 40 pF will work without causing significant signal distortion. This allows the use of multi-line silicon diode protection arrays (Figure 4).
The USB port of portable medical devices may be used as a power source for charging the on-board battery pack, as well as data interchange to the care giver. The port can be an entryway for ESD as well as overload currents. A combination of technologies can provide a complete circuit protection solution (Figure 5).
In addition to these examples, an instrument may need protection for its battery pack, microprocessor, audio speaker lines, or keypad and various buttons. Protection device manufacturers typically offer a wealth of information on the selection and application of individual devices. Working with a supplier that offers a broad spectrum of protection technologies helps ensure that the best solution is ultimately selected.