Agrowing array of electronic devices are available to healthcare providers, patients, and their families, including glucose meters, blood pressure monitors, automated external defibrillators (AEDs), and many others. To ensure safe, reliable performance of these devices, their designers must factor in circuit protection requirements from the earliest stages of the circuit design process. For example, a seemingly minor electrostatic discharge could easily render a portable medical device useless if it’s not properly protected, exposing the patient to the danger of misleading (or no) readings and the device’s manufacturer to legal liability if inaccurate results lead to improper treatment.
Comprehensive circuit protection is an essential part of medical device design to ensure device reliability, patient safety, and the security of patient information. A growing number are designed with built-in communication capabilities to provide continuous communication between the patient and caregivers located almost anywhere. This holds the promise of improved care at lower cost, but requires equipment designers to pay close attention to reliability and safety issues in their circuit designs and component selection. In fact, the U.S. Food and Drug Administration has recently decided to tighten security standards due to growing concern that devices ranging from fetal monitors used in hospitals to pacemakers implanted in people are vulnerable to cyber-security breaches with the potential to harm patients or reveal private information. See http://www.fda.gov/medicaldevices/safety/alertsandnotices/ucm356423.htm.
Even though overall design cycles keep getting shorter (as much as 13 percent shorter over the last three years according to one electronics industry survey), it is essential for circuit designers to weigh circuit protection options as early as possible during the board layout process. This makes it possible for the designer to choose both the optimal protection device and the optimal location for use with a particular chipset. To anticipate a device’s circuit protection needs accurately, the design engineer must also first understand how and where it will be used. This article outlines several overvoltage and overcurrent circuit protection approaches.
Multiple aspects of a medical instrument’s circuitry are subject to a variety of electrical threats over its lifetime. In short, any power or communication interface represents a potential gateway for an electrical transient and is also susceptible to damage over the lifetime of the device. Therefore, it’s important to pay attention to the power supply (battery pack, DC input, AC input), microprocessor, audio/speaker lines, communication interfaces (wired and wireless), sensors, LCD displays, keypads, and buttons.
Figure 1 shows a simplified electrical diagram for a generic handheld medical device. The green boxes indicate circuits that may be susceptible to overvoltage or overcurrent conditions. Because both patients and healthcare providers will handle this device frequently, the primary electrical threat is a simple static discharge. An electrostatic discharge (ESD) can transfer excessive voltage and current to internal circuitry.
Fortunately for circuit designers, a variety of circuit protection devices are available to address overvoltage (ESD and lightning surge) and overcurrent (short circuits and overload) situations. See section called “Circuit Protection Device Overview” below. For example, adding resettable positive temperature coefficient (PTC) thermistors to the USB port typically used to charge the batteries of portable instruments is one way to add overload current protection to portable equipment. These devices can limit current when an overload occurs, then reset to a low resistance value when the overload condition has passed, eliminating the inconvenience of having to replace a fuse.
For overvoltage protection, portable medical devices aren’t directly connected to the AC grid even during recharging, so they will be most likely to experience low level (remnant) surge events and ESD strikes. To protect against these events, space-efficient semiconductor- based devices can be used. These discrete and array-type diodes, which are available in a variety of form factors, provide low clamping voltages to protect modern chipsets, which are often easily damaged.
Circuit Protection Configurations
To ensure the security of patient data, protecting the communication interfaces of medical devices is a high priority. The Continua Health Alliance’s Design Guidelines define specific versions of Bluetooth and USB interfaces that act as wireless and wired transports to link health care instruments with caregivers. The following examples illustrate how various protection devices might be applied for portable equipment.
As illustrated in Figure 2 (top), a handheld instrument’s wireless radio frequency (RF) interface can be exposed to ESD surges induced through its antenna. This circuit shows a semiconductor- based solution that protects the RF amplifier input module of the Bluetooth circuit from ESD threats. The protection device is a 0.5pF (to maintain signal integrity) discrete diode that provides low clamping voltage to protect the equipment’s sensitive RF front-end.
An instrument’s sensor input (such as the input that connects a test strip for a glucose meter to its controller IC) is the interface between the user and the measurement circuitry. This input makes an instrument susceptible to ESD damage because the sensor makes contact with the user. As shown in Figure 2 (bottom), several discrete diodes can be used in this circuit (four lines are shown). These devices offer low clamping voltage, extremely low leakage current (<100nA), and can ensure the sensor circuit receives a strong enough signal to produce an accurate measurement.
Many medical product designers use the product’s USB port as both an input for recharging the on-board battery and as a way to transfer data. As a result, this port can inadvertently act as a gateway for ESD and overload currents. Furthermore, the ESD protection used in previous generations of USB may be incompatible with the newest version (USB 3.0) due to the increase in data transfer rate to 5Gbps and required decrease in channel capacitance in order to support the new data rate. As a result, designers are more challenged to find voltage transient protection solutions that can protect sensitive data lines without adding signal distorting capacitance.
The additional data pairs of USB 3.0 expose electrical systems to greater ESD threats because they provide more possible entryways for the electrical transients. New silicon array ESD protection devices, designed to be placed directly on data pairs, not only protect legacy USB 2.0 data lines but also these additional data signal pairs. For example, the Littelfuse SP3012 Series TVS Diode Array integrates either four or six channels of ultra-low capacitance rail-to-rail diodes and an additional zener diode to provide protection for electronic equipment that may be subject to ESD. They are designed to absorb repetitive ESD strikes above the maximum level specified in the IEC61000-4-2 international standard (±8kV contact discharge) safely without performance degradation. The extremely low loading capacitance (0.5pF) also makes it ideal for protecting high-speed signal lines.
Figure 3 shows how to combine technologies to provide more complete circuit protection for the legacy USB 2.0 port. First, a resettable PTC is used to protect against overcurrent conditions, in this case, a very low resistance device in a compact surface-mount package. This characteristic ensures that power dissipation and voltage drop are minimized when the device is being charged. Second, the power port and the I/O lines should be protected against ESD. This figure illustrates an array that protects all three lines and has a capacitance value low enough to ensure there is no data loss during high-speed (480Mbps) data transmission.
During regular use of a medical device, users make frequent contact with the on/off switch and other buttons. Space-efficient diodes in a 0201 package can protect these controls. These diodes are extremely robust against ESD (up to 30kV contact discharge) and their small form factor allows easy incorporation into the board design. The discrete form factor also offers the board designer high layout flexibility.
Circuit Protection Device Overview
A 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 having to replace a fuse.
The previous circuit protection examples highlighted some considerations for portable medical equipment. For higherend equipment (imaging, diagnostic, lab), additional protection devices are relevant because this equipment is much more complicated and is exposed to a greater number of electrical threats. Circuits such as AC mains and high-voltage DC are present, and they require solutions that can handle much higher energy than those found in portable devices. The following table and descriptions provide a complete overview to the protection options available for overvoltage and overcurrent events.
Overvoltage Suppression Devices
Table 1 lists the more common types of overvoltage suppression devices and performance characteristics.
Gas Discharge Tubes (GDTs) are typically used to protect telecom lines, datacom lines, and other instrument signal lines from surge voltages. Because they can handle surge currents up to 40,000 amps, they are a good choice for reducing lightning- induced transients.
Varistors (variable resistors) possess characteristics that divert current created by excessive transient voltage away from sensitive components. There are two major types:
- Multi-layer varistors (MLVs) provide protection from low to medium energy transients (0.05 to 2.5 Joules) in sensitive equipment operating at 0 to 120VDC. They are most commonly used for ESD protection.
- Metal oxide varistors (MOVs) are available with energy ratings from 0.1 to 10,000 Joules, allowing them to divert transient currents away from sensitive components. For low voltage DC power ports or signal ports, varistors combining high surge ratings with small disc sizes are ideal for designs where space is at a premium. For example, a 10mm varistor is available with a maximum surge current rating of 2,000A, four times higher than the maximum rating of a standard MOV of the same size. These devices can protect circuits from electrical threats such as indirect lightning strike interference, system switching transients, and abnormal fast transients from the power source.
Polymeric ESD suppressors are a good choice for highspeed digital I/O and RF lines because of their low capacitance (~ 0.05pF) and fast voltage clamping ability. The low capacitance helps ensure no signal loading or distortion occurs.
Transient Voltage Suppression (TVS) diodes protect a wide variety of circuits and components from an assortment of threats common 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, and clamping the transient voltage to a lower level than other technologies. Their transient protection ratings range from 400 watts to 15,000 watts, and up to 15,000A.
Silicon Protection Arrays (SPA diodes) 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, with clamping voltages lower than other technologies for the best possible protection.
Protection Thyristors are designed to suppress overvoltage transients in telecom and datacom equipment, and are able to divert currents as high as 5,000A to ground within nanoseconds of reaching their breakover voltage.
Overcurrent Protection Devices
Fuses are the most common overcurrent protection devices and are categorized as fast-acting or Slo-Blo (time-lag) types. The latter help minimize repeated replacements when a circuit experiences brief but recurring overcurrent “spikes.” For portable applications, fuses in small surface-mount form factors are commonly used for their space efficiency and ability to interrupt overload and short-circuit currents.
PTC thermistors are resettable alternatives to fuses. 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 versus temperature characteristics. Once the overload is gone, the PPTC cools and returns the circuit to normal operation. This avoids the need to replace fuses.
This article was written by James Colby, Manager, Business and Technology Development for the Semiconductor Division of Littelfuse, Inc., Chicago, IL. For more information, visit http://info.hotims.com/45607-164.