Electronics Protection - June/July 2011 - (Page 10)

Feature Designers Face a Potentially Confusing Assortment of Surge Protection Device Choices this, ESD is the most prevalent threat to the reliable, long-term operation of these devices. For example, a recent study reveals that American children aged eight to18 spend an average of 7.5 Surge protection devices suppress voltage transients and divert hours per day using a cell phone, computer, television or other placed in the optimal circuit locations. This can be a bit daunting at first, but observing a few excessive current away from sensitive electronic components. This electronic device. Every time they touch a screen, push a button crucial criteria can help survive harsh increases user safety and enables end products tosimplify the selection process. plug/unplug a data cable, there is an opportunity for potentially or operating conditions. For these reasons, and to comply with safety damaging ESD and transient surges. The same threats exist for any standards before an end product can be Threatmost electrical and consumer electronics including LCD/LED TVs, set top boxes, MP3/ Identify the sold, electronic equipment incorporate protection devices to damage from various external events, which are typically PMP players and many others. Electronic circuitry is prone in appropriate circuit locations. ESD is caused by the transfer of static electrical charge from a associated with the handling of an end product, or its connection to power and communications Properly selected and placed protection devices are of strategic body to an electronic circuit, and in environments where the relaimportance to a manufacturer, lines. Thesethey will: be understood further by identifying the likely source low, the ESD event may have a peak voltage as because threats can tive humidity is very of an over-voltage condition, and the circuit or reliability • Improve end-product uptime, sustainability and components that need to be protected. Table 1 provides an overviewan ESD event will be painful to high as 15,000 volts. At 6,000 volts, • Reduce warranty return, repair and replacement costs of these threats and the types of protective devices a person. Lower voltage discharges may go unnoticed, but can still that are typically used to avoid damage. • Minimize fire and shock risks and greater damage potential cause catastrophic damage to electronic components and circuits. • Minimize operating dangers and potential liabilities In less severe cases, ESD may result in faulty circuit operation or Table 1. Typical circuit threats, protection devices, and selection criteria To secure these benlatent defects that later Type of Threat Typical Applications Typical Protection Main Selection efits, circuit designers must progress to outright failure. Devices Used Criteria select the appropriate The reason for using ESD Lightning Equipment connected Protection thyristors, Operating voltage, protection devices from protection devices is to to external power and varistors, TVS* breakover/clamping communication lines diodes, gas discharge voltage, surge a list of several differhelp the application avoid tubes energy/power/current ent types that are comthese types of damage. ratings monly available. Then, ESD suppressors must Electrostatic Electronic equipment Polymer ESD Uni/bi directional, the selected devices must have very fast response Discharge (ESD) with a human suppressors, TVS ESD voltage rating, interfaces and data diode arrays, low dynamic be placed in the optimal times and repeatedly ports multilayer varistors resistance, low circuit locations. This can handle high peak voltages (MLVs), LED clamping voltage, be a bit daunting at first, and currents for short protectors capacitance but observing a few crucial durations. As shown in Electrical Fast Equipment that is TVS diodes, metal Fast response and Transient (EFT) connected to a system oxide varistors recovery for criteria can help simplify Table 1, the most approor circuit with repetitive pulses the selection process. priate protective devices switched inductive are generally polymer ESD loads or mechanical suppressors, silicon TVS relays Identify the Threat Inductive load Equipment associated Metal oxide varistors, Energy/peak current diode arrays, and multiElectronic circuitry is switching and with large motors, TVS diodes, gas rating, operating layer varistors. If the circuit prone to damage from commutative spikes pumps, compressors, discharge tubes voltage, surge current to be protected includes various external events, relays, and AC power rating, LEDs, then special protecdistribution which are typically associCurrent switching/ Wide range of Switching thyristors, Proper blocking tive devices designed speated with the handling of diversion electrical and LED protectors voltage, and current cifically for that application an end product, or its electronic circuits carrying capacity may also be needed. connection to power and *Transient Voltage Suppressor Table 1. Typical circuit threats, protection devices and selection criteria. Crowbar devices, such communications lines. as gas discharge tubes and These threats can be understood further by identifying the likely protection thyristors, can be used on communication lines where source of an over-voltage condition, and the circuit or compothe short circuit current of the system is less than the holding nents that need to be protected. Table 1 provides an overview of current of the crowbar device (protection thyristor), or where the these threats and the types of protective devices that are typically operating voltage of the circuit is much lower than the holdover used to avoid damage. voltage of the crowbar device (GDT). In either case, they are used In addition to the selection criteria in Table 1, the package size, for lightning surge protection, not ESD protection. mounting options, electrical characteristics and installed cost will Polymer ESD suppressors, TVS diode arrays, and MLVs are influence selection of a circuit protection device. Still, the primary clamping devices. A clamping device (Figure 1) essentially limits consideration is to ensure that the device protects the circuit and the transient voltage to a set value (above the operating voltage components of concern, and can do so repeatedly without failure. of the circuit being protected). When the applied voltage reaches Space doesn’t permit a full discussion of all these over-voltage the device’s breakdown value, it will switch to a much lower conditions in this article. However, the common threat to elecresistance value to minimize the transient energy that the circuit tronic equipment, ESD, will be covered in some detail. As with will experience. This value is typically referred to as the dynamic all transient overvoltage events, choosing the most appropriate resistance. A clamping device will stop conducting as soon as the suppressor technology requires a balance between equipment transient voltage drops below the device’s breakover voltage, and protection needs and operating requirements, and also taking into the circuit will then be able to operate normally. account the anticipated threat level. Polymeric ESD suppressors are small, fast-acting devices with good voltage clamping capabilities. Their ultra-low capacitance Protective Device Selection Criteria (typically around 0.05 pF) makes them ideal for ESD suppression Hand-held devices represent an installed base of multi-billions on high-speed digital I/O lines or high-frequency RF lines. of units, and this continues to grow at a brisk pace. Because of 10 June/July 2011 www.ElectronicsProtectionMagazine.com Jim Colby, Technology and Business Development Manager Littelfuse, Inc. http://www.ElectronicsProtectionMagazine.com

Table of Contents for the Digital Edition of Electronics Protection - June/July 2011

New Material Adapts to Strain
XPAL Power Debuts PowerSkin Battery-Boosting Smartphone Cases
Ruggedizing Enclosures: From Military to Seismic to Mobile Applications
The Benefits of Using Thermal Simulation Software in Electronics Design
Designers Face a Potentially Confusing Assortment of Surge Protection Device Choices
Military’s Demands Drive Innovative Thermal Management Solutions
Integra Enclosures Releases new Product to Genesis Line of Enclosures
New LCR COTS Filters Perform in Extended Temperature Range for Added Reliability
Thermacore Introduces VME64x/VPX Cold Plates for Military Cooling Applications
Laird Technologies Expands Thermal Gap Filler Line
ASI New Line of 13mm MCB’s offer UL 1077 Supplemental Protection
New Deutsch HDP Series Electrical Connector
Industry News
Calendar of Events

Electronics Protection - June/July 2011