How to select and apply the right components to protect medical devices, users, and patients
Figure 13: GDTs do not have to look like the big spark-gap devices seen in movies; the GTCS23-750M-R01-2 is a 75 volt, 1 kA GDT
Standards guide the design
Medical devices must meet multiple safety standards, some of which apply to all consumer and commercial products, and some of which are for medical devices only. Many of these standards are international in scope. Among the many standards and regulatory mandates are: ■ IEC 60601-1-2, “Medical electrical equipment – Part 1-2: General requirements for basic safety and essential performance - Collateral Standard: Electromagnetic disturbances - Requirements and tests.” ■ IEC 60601-1-11, “Medical Electrical Equipment Part 1-11: General requirements for basic safety and essential performance – Collateral standard: Requirements for medical electrical equipment and medical electrical systems used in the home healthcare environment.” ■ IEC 62311-2, “Assessment of electronic and electrical equipment related to human exposure restrictions for electromagnetic fields (0Hz to 300GHz).”
in an SMT package that measures just 4.5mm in length and 3mm in diameter. Image source: Littelfuse, Inc.
with more general applicability. Each component brings a set of attributes that makes it a best fit – or at least a better one – in the different circuit and system locations requiring such protection. No single device will fit the multiple diverse system requirements and so designers will end up using multiple protection approaches. In most cases, the many decisions regarding which devices to use and how best to do so are inherently complicated and also subject to regulatory review. Designers should strongly consider asking for help from knowledgeable application engineers at the protection device vendor or their designated supplier (distributor). Their experience and expertise can reduce time to market, ensure a more thorough design, and ease the path to regulatory approval.
Figure 11: In a defibrillator, the SSR allows the low- voltage electronics to drive the high-voltage paddles while allowing the “floating” upper-side drivers of the H-bridge arrangement to remain isolated from system ground (area 5). Image source: Littelfuse, Inc.
Figure 9: In this portable ultrasound scanner block diagram, a TVS diode such as the SMCJ33A with a 53 volt clamping voltage can be used for protection against transients at USB ports as well as at the LCD/LED display (areas 2 and 3). Image source: Littelfuse, Inc.
■ IEC 62133-2, “Secondary cells and batteries containing alkaline or other non-acid electrolytes – Safety requirements for portable sealed secondary lithium cells, and for batteries made from them, for use in portable applications – Part 2: Lithium systems.” Being careful about circuit protection device selection and how they are used goes a long way toward meeting these safety mandates. Using accepted,
approved techniques and components can also speed up the approval process.
conductor to be protected – usually an AC power line or other “exposed” conductor and system ground – to provide a near-ideal mechanism for diverting higher overvoltages to ground. Under normal operating conditions, the gas inside the device acts like an insulator and the GDT does not conduct current. When an overvoltage condition (called the sparkover voltage) occurs, gas inside the tube breaks down and conducts current. When the overvoltage condition exceeds the parameters of the sparkover voltage rating, the GDT turns on and discharges, diverting the damaging energy. GDTs are available as two-pole devices for ungrounded lines and three-pole devices for grounded lines, both in small SMT
packages for ease of design-in and board assembly (Figure 12). GDTs are available for sparkover values rated as low as 75 volts and can handle hundreds and even thousands of amperes. For example, the GTCS23-750M-R01-2 is a two-pole GDT with a 75 volt sparkover and a 1kA current rating, housed in an SMT package measuring 4.5mm long and 3mm in diameter, allowing it to be placed almost anywhere to provide protection (Figure 13).
Conclusion
The requirements of where, why, what, and how to use circuit protection devices in general, and in medical units in particular, is a complicated design challenge. There are many suitable protection components, some specific to a given circuit function and others
Figure 12: GDTs are offered as (left) two-pole devices for ungrounded circuits and (right) as three-pole devices for grounded circuits (the GDT symbol is the “Z-like” graphic to the right of each schematic diagram). Image source: Littelfuse, Inc.
Figure 10: A diode array such as the SP3019-04HTG provides ESD protection for multiple high-speed I/O lines. Image source: Littelfuse, Inc.
we get technical
14
15
Powered by FlippingBook