One type of electronic device where high reliability is the most important design objective is medical instrumentation. Instruments such as defibrillators, which provide life-saving electrical stimulation of the heart, and portable ultrasound machines that can provide critical diagnoses such as mitral valve defects, are mission critical and must have a robust design to ensure that they maintain the necessary uptime for patient care. The markets for these instruments are both large, approximately $10 million for defibrillators and $7 million for ultrasound instruments and growing at a minimum of a compound annual growth rate of over five percent.1, 2
Population growth and the aging of the population in the developed world creates growth in the cardiac emergency care and diagnostic equipment markets. Fortunately, technical and medical innovation enables development of new products that enhance diagnostic and treatment capabilities, instrument portability, and communication capabilities. Lower power consumption and smaller size, for example, enable treatment and diagnostic assessment even in remote locations.
To maximize reliability, designers need to protect their product designs with components that provide overcurrent protection, overvoltage transient protection, and protection from electrostatic discharge (ESD). Designers also need to use the appropriate control components to ensure safe and efficient management of high-voltage discharge circuits. Furthermore, designs must incorporate appropriate sensors such as those that detect when an instrument is enabled for activation to protect both users and patients. This article presents component recommendations for overload protection, safe and efficient control, and protective sensing of defibrillators and ultrasound machines used in critical cardiac care.
Protection, Control, and Sensing Components for Defibrillators
Defibrillators generate high power to resynchronize the heart muscle and restore normal heart rhythm so that blood can be effectively pumped through the body. Defibrillators can supply millisecond pulses up to 360 J, at 1000 V, and up to 30 A. With these high output levels and with its critical function, protection and control components are essential for ensuring defibrillator performance. Figure 1 shows an example defibrillator with the recommended protection, control, and sensing components. Figure 2 shows a block diagram for a defibrillator and a table listing the recommended components for the circuit blocks that require protection, control, and safety sensing.
Power Supply. The power supply block connects to the AC power line, and the circuit is subject to overcurrent and overvoltage conditions. To ensure its reliability, the circuit must be protected against both conditions. Designers should consider use of slow-blow cartridge fuses. Cartridge fuses offer low resistance to minimize energy consumption and have high interrupting current ratings for protection against large overloads. These fuses develop a discoloration of the glass body to provide a visual indication of an overcurrent for easy identification of a problem. If the only problem was a transient overcurrent, users can replace a cartridge fuse quickly. Metal oxide varistors (MOVs) provide the transient surge protection against lightning and large inductive spikes that can arise on an AC power line. The MOVs can absorb peak currents as high as 10 kA that can last as long as 20 μs. In addition, an MOV can absorb up to 400 J of a square wave type of transient that can last over 2 ms. MOVs absorb the transient energy and keep it from entering and damaging the device circuitry. MOVs also have excellent temperature range; they can operate up to 85 °C without any derating of performance. Designers should place the MOV right after the fuse as close as possible to the AC power line input.
Battery Management Unit. The battery management unit that maintains battery performance is essential for ensuring reliable defibrillator performance in locations where line power is not available. The battery management unit monitors and controls a set of lithium ion batteries. Both the batteries and the charging control and battery output balancing circuitry need to be protected from overcurrents and overvoltages. A low-resistance, polymer resettable fuse can provide essential overcurrent protection and not require replacement if an overcurrent condition occurs. A transient voltage suppressor (TVS) diode array will protect the circuit from ESD voltage transients as high as 30 kV either from direct contact or through the air. In addition, TVS diodes consume little power during normal operation; standby leakage current is low, under 100 nA.
User Interface. The user interface is subject to the external environment including user contact and requires protection from ESD and other transients. If the display is a touchscreen, the integrated circuits (ICs) in the touchscreen are susceptible to ESD. Designers should protect these ICs and other user interface components with multilayer varistors. Multilayer varistors (MLVs) provide bidirectional clamping protection for low voltage circuits and can absorb surge currents up to 500 A. Furthermore, MLVs operate over a wide temperature range, from –40° to 125 °C and are available in space-saving, 0402 surface mount packages.
USB Interface. The USB interface, like the user interface, has exposure to the external environment. Designers need to protect the USB communication port from overcurrent and overvoltage conditions. A polymer positive temperature coefficient (PPTC) resettable fuse provides the necessary level of overcurrent protection while consuming a minimal amount of printed circuit board (PCB) space. A PPTC fuse with a holding current of 0.5 A in a 0402 pack age takes up a nominal area of only 1 × 0.5 mm. Versions of the PPTC trip in under 5 s for low overcurrents and have resistances in the milliohm range to minimize power loss across the fuse.
TVS diode arrays provide ESD protection of up to 10 kV for all data lines. Versions of the diode arrays minimize the performance impact on communication circuits with both low capacitance, typically 0.3 pF, and a typical leakage current of 10 nA. One package can house multiple diodes for space-efficient protection of data lines. Figure 3 shows a configuration for a four-channel TVS diode array.
H-Bridge Block. The H-bridge is the high-voltage–high-energy generator circuit block. For this circuit, designers should evaluate the use of IGBTs for the high voltage that the circuit needs to build up for discharge. IGBTs can support collector-emitter voltages of well over 1000 V and collector currents in the hundreds of amperes. These transistors have low on-resistance and fast switching to maximize defibrillator power efficiency. The low on-resistance and fast switching of IGBTs enable a lower heat signature than conventional transistors. This simplifies their cooling requirements. The designer should pair the IGBT transistors with specialty ICs, IGBT gate drivers, which manage the fast turn-on and turn-off of the IGBTs. The drivers eliminate the need for extra power supply circuitry to control the IGBTs.
Outer Selector. The outer selector block allows the high-energy pulse to be applied to the patient. A control on this high-energy output is an essential safety requirement for a defibrillator. Designers can use reed switch proximity sensors to prevent activation of the output unless the paddles are removed from their holders on the instrument’s case. One implementation for the reed sensor switch is to install a small magnet in each paddle and design a reed switch in each paddle holder. When an operator removes both paddles from their holders, the reed switch switches state and enables connection of the output to the paddles.
Wireless Communication Port. Today’s defibrillators often have a wireless communication port to transmit the patient’s electrocardiogram to a physician in a remote location. Designers can protect this port from ESD with surface mount polymeric ESD suppressors. With the capacity to absorb a 15 kV through-the-air discharge and with a response time of under 1 ns, the ESD suppressor fully protects a high data rate communication port. ESD suppressors with under 1 pF of capacitance and with less than 1 nA of leakage current have minimal impact on the high data rate protocol used by the communication port. Figure 4 shows one-port and two-port ESD suppressor diodes. In addition, the designer can select these suppressors in 0402 (0.5 × 1 mm) surface mount packages to consume the least amount of PCB real estate in the circuit block.
By protecting the key blocks of a defibrillator, particularly the blocks that interface with the external environment, designers can be confident that they have developed a robust instrument. Using gate drivers and IGBTs in the power circuit provides both reliability and improved efficiency to maximize defibrillator operating life when the instrument is operated on battery power. Finally, use of appropriate sensing devices, such as the reed sensor ensures both operator and patient safety.
Protection, Control, and Sensing Components for Ultrasound Scanners
Ultrasound machines drive piezoelectric transducers to create sound waves that bounce off interfaces between various density tissues and fluid. The reflections are imaged and used for diagnosis of various conditions including aneurisms and heart valve defects. Figure 5 illustrates an example of a portable ultrasound machine designed for use in offices and remote locations. Figure 6 shows the major circuit blocks of the instrument. The recommended protection components for the battery management unit, the user interface, the USB port, and the wireless interface are either identical or similar to the components described for protecting the defibrillator.
High-Voltage Pulse Generator. The unique block of the ultrasound machine is the high-voltage pulse generator which drives the piezoelectric transducer. The primary recommended component in this circuit block is a power MOSFET for use as a switch to generate high-frequency pulses. Power MOSFETS can support drain-source voltages up to 1200 V. Furthermore, power MOSFETS allow greater efficiency with a low RDS(on) combined with fast intrinsic diodes for fast switching transition times. Power MOSFETs offer both high power density in space-saving TO-263 surface-mount packages and TO-220 vertically leaded packages and excellent thermal performance, which reduces cooling requirements. Designers can use fast recovery diodes for rectification of the pulse output. Fast recovery diodes can have peak reverse voltages up to 1200 V and average forward currents up to 55 A. Versions of the fast recovery diodes have soft reverse recovery, which lowers EMI/RFI emissions and have low power dissipation.
Portable devices such as the ultrasound scanner must be rugged to withstand operation in many environments. Designers need to ensure that all connections to the external environment are suitably protected. Efficient power output circuits help to maximize battery life and reduce the chance that the battery pack will discharge below the instrument’s low battery turn-off threshold during a diagnostic test.
Design for Emergency Service
Cardiac equipment such as a defibrillator and an ultrasound scanner are life-saving devices. It is essential that these instruments be robust products and impervious to damage from overloads, electrical transients, and ESD. Designers should protect all the circuit blocks in their designs that are susceptible to these hazardous conditions. Attention to the details of both protecting key circuit blocks and controlling and sensing for maximum safety and efficiency will ensure highly reliable and robust medical instruments.
Due to the critical nature of these medical devices, it is recommended that designers collaborate with component manufacturers to ensure the selection of appropriate components for their specific designs. In an emergency, cardiac equipment can save a life. Every element of protection and efficiency designed into the device will help the instruments perform when they are urgently needed.
The following guides provide additional information:
Circuit Protection Selection Guide littelfuse.com/protectionguide
Sensing Products Selection Guide littelfuse.com/sensorsguide
Power Semiconductor Selection Guide littelfuse.com/powersemiguide
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- Defibrillator Market Size, Share and Industry Analysis. July 2019. Fortune Business Insights.
- Ultrasound Equipment Market Size, Share and Industry Analysis. June 2019. Fortune Business Insights.