Design Features February 3, 1997

For want of circuit-protection devices...

Bill Travis, Senior Technical Editor

A variety of devices protects sensitive circuits against overvoltage or overcurrent conditions,
thereby preventing overheating, avalanche breakdown, and device burnouts.

  Ever-shrinking device geometries are bringing enormous performance benefits to circuit design in speed/power trade-offs and circuit density. On the other hand, these submicron geometries make the ICs susceptible to damage from voltage and current surges. Thus, circuit-protection devices are not a luxury, but a necessity in many applications. You can choose from a variety of protection devices designed to shield circuits and devices from the scourges of overcurrent and overvoltage conditions.

  Overcurrent conditions can arise from a variety of sources. Short circuits can occur in supply lines, output loads, and filter capacitors, for example. Without protection devices, these conditions can produce overheating (even flames) or downright destruction of the output stage of the circuit that valiantly tries to supply infinite current to the shorted path. Several types of protection devices are available either to open the shorted circuit or to keep the current to a safe level. The three principal classes of overcurrent-protection devices are electromechanical breakers, one-time fuses, and positive-temperature-coefficient (PTC) current limiters.

  The venerable electromechanical circuit breaker has been with us since the days of Thomas Edison. These devices' tripping mechanisms rely on either internal heating (of a bimetallic strip) or magnetic flux. The breakers you'll find in your home's junction box use bimetallic strips. They're susceptible to nuisance tripping on hot days or if they're mounted next to heat-generating equipment. Magnetic breakers, available since 1932 (Reference 1), use the flux in a solenoid for their tripping mechanism.

  Circuit breakers must automatically open the protected circuit if current overloads occur, but they should ignore transient current surges. To avoid nuisance tripping, breaker manufacturers design delay mechanisms that allow brief surges. It's possible to set precise delay responses in solenoid-based breakers. Another requirement for electromechanical circuit breakers is that they serve as manually activated on/off switches.

  A magnetic breaker senses current through a solenoid rather than heat in a bimetallic element. Thus, its trip point is more precise. It also offers more accurate and repeatable trip-delay characteristics. A special type of breaker uses a hydraulic-magnetic principle to provide an inverse delay response. It contains a damping fluid that regulates the solenoid core's travel speed. As the level of overcurrent increases, the viscosity-related delay decreases for quicker action and greater protection to threatened circuits. At lower percentage overcurrents, the delay lengthens. A family of trip-delay curves in Reference 1—for example, the one in Figure 1— gives delay time as a function of percentage of rated current. At 200% rated current, for example, a typical breaker might trip in 10 sec; for 1000% rated current, it could trip in 10 msec. Some recent introductions from Eaton Corp and E-T-A Circuit Breakers typify hydraulic-magnetic breakers.

  Eaton's AM/S Series is a UL-489-listed device that's rated for currents as high as 100A at 250V ac or 80V dc. It includes midtrip alarm, remote trip, and a low-silhouette toggle to prevent accidental turn-off. Prices range from $7.05 to $18.60 (500). The 8300 Series from E-T-A offers current ratings from 50 mA to 100A at 440V ac or 125V dc. Configuration options include as many as six poles; panel, snap-in, and DIN-rail mounting; paddle, baton, rocker, and toggle handles; and illumination. Unit prices range from $4.50 to $30.

  A device from Schweitzer Engineering Laboratories extends the life of dc contacts in electromechanical breakers and relays. The SEL-9501 is an arc suppressor that dissipates inductive energy as heat, thereby preventing contact vaporization. It smoothly interrupts the breaking current over a few milliseconds, thus absorbing the inductive energy of the circuit. The device can interrupt 10A of trip-coil current as many as four times per second.

  Another type of magnetic circuit breaker use no solenoid coil, but rather two concentric coils with a reed switch. The Circuit Saver from Inresco Inc uses a sensing coil, connected in series with the switch, to provide the magnetic flux to open the switch at a specified current. When the switch opens, a second coil, connected in parallel with the switch, provides the flux to hold the switch open. The second coil also inserts a large resistance in the circuit to protect it. When power decreases and reduces the flux in the holding coil, the device resets.

  Recent Circuit Savers provide protection for 3.3V circuits. The CS130 Series devices are available in versions for 3.3, 5, 12, and 24V circuits. They offer current ratings from 50 mA to 1A. Their trip currents are tightly controlled; for instance, 50-mA devices are guaranteed to trip with trip currents from 60 to 85 mA. The breakers need no derating, because trip points change less than 10% over the –40 to +85° C operating range. Trip times are fast: Figure 2 shows typical time to trip vs percent of rated current. At 200% rated current, a device typically trips in 60 m sec. The CS130 Series comes in a pc-mountable 0.35´ 0.36´0.87-in. package and costs $2.77 (1000).

  Throwaway fuses, like circuit breakers, have been protecting circuits for more than a century. They use a metallic element that melts and opens the circuit in the event of an overcurrent condition. Fuses are available in a range of current ratings and form factors, from tiny surface-mount devices to large cylindrical batons. References 2 and 3 give guidelines on selecting the appropriate fuse for an application. Reference 2 provides a quick checklist, using overload opening time, interrupting rating, and I2t rating. I2t, measured in amperes squared seconds, is the amount of energy required to melt the fuse element. You choose an I2t rating that's high enough to avoid nuisance openings, but low enough to ensure circuit protection.

  Reference 3 is a similar guide for selecting from AVX Corp's line of Accu-Guard surface-mount fuses, but it introduces two kinds of I2t: pre-arc and total. The total I2t is the total energy the fuse passes until total cessation of current flow. The pre-arc I2t is the energy that causes irreversible damage to the fuse element. You select a pre-arc I2t that's just larger than the Joule integral of the anticipated switch-on waveform; otherwise, nuisance blowing occurs. The Accu-Guard Series comes in 0805 and 1206 sizes. The smaller devices have current ratings from 250 mA to 2A; the larger fuses are rated from 250 mA to 3A. At double the rated current, the devices fuse in less than 5 sec. A recent surface-mount fuse from Bussmann uses thick and thin films on a ceramic substrate with a fused-glass coating. The 3216LV suits 125V applications; its package provides the metric equivalent to the classic 1206 surface-mount form factor. The cost is $0.22 (OEM).

  Most of the classic fuse suppliers offer surface-mount fuses that use thin-film fusible elements. The problem with surface-mount devices is the annoyance and expense of desoldering and resoldering if a fuse blows. Wickmann USA offers a surface-mount fuseholder assembly that allows you to replace blown surface-mount fuses. Its vertical orientation allows fuse replacement without tools. High-temperature surface-mount soldering processes prevent the installation of many time-lag fuses, so your choice of a surface-mount fuse is usually limited to fast-acting devices. The socket assembly permits the use of any type of fuse and costs $0.18 (OEM). Speaking of time-lag fuses, a recent radial-leaded device from Bel Fuse satisfies UL, CSA, and IEC standards. Type MRT offers current ratings from 63 mA to 6.3A at 250V ac. Its current-time characteristics are: 150%, greater than 1 hour; 210%, less than 2 minutes; 275%, 0.4 to 10 sec; 400%, 0.15 to 3 sec; and 1000%, 20 to 150 msec. Its price is $0.15 (5000).

  One-time fuses are reliable, time-proven devices that are easy to understand and design into a circuit. However, they have their disadvantages. Their throwaway nature makes nuisance blows annoying, and diagnostics and replacement time make one-time fuses costly. They can also increase system costs in more subtle ways. In an automotive application, for example, assume that one fuse protects a multiwire harness. Every wire in that harness must be rated to carry the full current rating of the central fuse, regardless of the current that wire normally carries. It would be desirable to fuse each wire, but an automobile's underpanel is already overcrowded. The use of a one-time fuse, therefore, results in cost and weight penalties in the wiring harness. A resettable or, rather, self-resetting current-limiting device in each harness wire would be preferable in this application.

Self-resetting polymer fuses

  A class of current-limiting devices exploits a sharp PTC to provide an abrupt increase in resistance during overcurrent conditions. Reference 4 gives an overview of the design and selection considerations for polymer-based PTC devices. Ceramic-based PTC limiters exist, too, but they're relatively large, have high thermal mass (therefore, a high thermal time constant), and exhibit higher on-state voltage drop than do polymer-based units. The time to trip for polymer PTC fuses is higher than that for one-time fuses, so you should carefully consider this parameter in your protection scheme. You need to know the time to damage in the protected circuit at various overcurrent levels and then consult PTC-device curves to ensure that the chosen device trips well within this time.

  Polymer-based PTC fuses, originally developed by Raychem Corp, are now available from Raychem, Littelfuse, and Bourns. In an effort to expand the market for polymeric devices, Raychem and Littelfuse have concluded an agreement for the manufacturing and marketing of certain devices in the Raychem line. The devices are available in various radial-leaded and surface-mount form factors. A new line of surface-mount devices from Raychem uses a package configuration that allows visual inspection of solder fillets. The miniSMDC020/ 050/075/110 fuses have hold currents of 0.2, 0.5, 0.75, and 1.1A, respectively, and trip times of less than 0.5 sec. Prices start at $0.30 (OEM). Another new line from Raychem provides both overcurrent and overtemperature protection for NiCd, NiMH, and lithium batteries. The VTP210 trips with low currents at ambient temperatures of 60 to 70° C, thus eliminating the need for thermal fuses in many batteries. Its price is $0.40 (OEM). Bourns also offers a line of polymeric PTC protectors in radial-leaded, tabbed, disc, and surface-mount form factors.

  In designing with polymeric PTC fuses, you should take into account various derating factors. First, these devices have a PTC of resistance (TCR) in the on state; you should note this TCR in your voltage-drop calculations. Second, the rated hold current is a strong function of temperature; you have to derate it to accommodate the maximum anticipated operating temperature. Third, the devices exhibit irreversible increases in on-state resistance after tripping. A typical spec sheet shows that a device whose initial on-state resistance is 3 ohms can exhibit 7.5 ohms post-trip resistance.

Protecting against overvoltage

  Overcurrent conditions can damage or destroy devices through overheating; overvoltage conditions can provoke the overcurrent situations and can also maim or kill devices through avalanche breakdown and arcing. A variety of protection devices exists for suppressing voltage surges. Gas-discharge tubes, for example, provide lightning protection in telecommunication equipment. Metal-oxide varistors (MOVs) are available in both single-layer (disc) and multilayer versions. Transient-voltage-suppression (TVS) diodes are essentially zener diodes.

  The TransGuard family of voltage suppressors from AVX is a group of zinc-oxide devices with a nonlinear voltage-current characteristic, like that of a pair of back-to-back zener diodes. The zinc-oxide material is similar to that of single-layer disc MOVs but AVX claims to use multilayer construction and a process that yields finer and more consistent grain structures. TransGuards are available in 0603, 0805, 1206, and 1210 surface-mount form factors, as well as in axial-leaded cylindrical versions. They handle working voltages from 3.6 to 60V and spec response times of 200 to 700 psec. Typical price for an 0805, 1J TransGuard is $0.162 (50,000). The devices are also available in four-element, surface-mount arrays called MultiGuards. Bussmann al-so uses a proprietary process that combines film-on-ceramic techniques with polymer-based active materials, to produce SurgX ESD (electrostatic discharge) protectors that suppress voltage spikes in less than 1 nsec.

  A product called ChipGuard from Bourns Inc uses a thin and flexible voltage-variable material to provide ESD protection. The material mounts in a connector in a gap separating the pins from the ground plate. It acts as an insulator during normal operation but conducts in the presence of ESD, thus shunting ESD transients to ground. Clamping voltage is 100V, and clamping speed is faster than 2 nsec. Price is $6.80 (10) for protection of nine signal lines.

  ESD protection is also the forte of Semtech Corp's LCDA line of TVS arrays. The LCDA05, for example, meets all the criteria for protection of USB ports: low capacitance for minimal attenuation at the 12-Mbps data rate, fast response time, low clamping and operating voltages for protection of the Universal Serial Bus (USB) ASIC, low leakage current, immunity to damage by repetitive ESD strikes, and the capability to handle ESD currents as high as 30A. Figure 3 shows the LCDA05 in a typical USB application. The LCDA Series is available in versions for 5, 12, 15, and 24V operation. Prices start at $0.90 (OEM). Another family from Semtech, the SMDA Series, provides five to seven bidirectional lines of protection in SO-8 packages. The devices meet protection mandates of the IEC 1000-4-2 and 1000-4-4 standards and cost $1.74 (1000). The SM16LC Series from ProTek Devices also provides protection for as many as eight data lines, meets the same IEC standards, and costs $2.31 (1000).

IC makers quash overvoltage

  Some overvoltage protectors do not protect by shunting voltage surges to ground, but rather by opening a signal path. Triple and octal signal-line protectors from Analog Devices and Maxim open a switch in a signal line when the input voltage exceeds a preset supply-voltage setting. The ADG-466 and ADG467 from Analog Devices and the MAX366 and MAX367 from Maxim use MOSFETs in the signal lines that resemble series resistors during normal operation. They protect from transients as high as ±40V.

  An array of SCR-connected transistors (Figure 4)from Harris Semiconductor provides ESD protection for as many as six lines. The SP723 satisfies the IEC 1000-4-2 standard for ESD suppression. Switching in less than 6 nsec, the device can handle 7A peak current, according to the IEC 8/20-m sec peak pulse curve, and one 100-?sec peak transient pulse of ±4A. The array clamps itself to the supply voltage (2 to 20V) and turns on whenever the voltage level exceeds one diode drop above the supply level. The SP723 costs $1.10 (1000).

  A hot-swap controller from Linear Technology allows you to safely insert and remove boards from hot backplanes. Using external n-channel pass transistors, the LTC1421 ramps up board-supply voltages at a programmable rate. A programmable circuit breaker protects against short circuits; warning signals indicate if a circuit breaker trips, if a power failure occurs, or if the n-channel pass elements' drivers turn off. The LTC1421 costs $4.90 (1000).

  The devices described so far are general-purpose protection arrays. An application-specific, surface-mount device from SGS-Thomson targets the automotive market. The RBO40-40M protects automotive circuits against reversed-battery connection and load-dump overvoltages (high-energy transients that occur when the battery is disconnected while the alternator is generating). It handles average forward currents to 40A and has maximum clamping voltages of ±40V. The device contains a series diode for reverse-battery protection, a bidirectional breakdown diode for clamping negative overvoltages, and a power-breakdown diode that protects against load dumps. The cost is $1.54 (1000).

  Transient protection—both current and voltage—is serious business, worthy of serious consideration early in the design process, not just as an afterthought. ICs are sensitive, expensive, and easy to zap. A variety of protection devices, such as the ones described here, make it easy and inexpensive to bulletproof your designs.

You can reach Senior Technical Editor Bill Travis at (617) 558-4471, fax (617) 558-4470, e-mail b.travis@cahners.com.

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