Next-Generation Resettable Fuses Target Complex Protection Needs

Overcurrent protection is critical for the lifespan of electronic devices in a wide range of applications. Overcurrent conditions can lead to costly field failures, so ignoring circuit protection is not an option.

This article will detail some of the latest circuit-protection-technology advancements that have increased the performance of the resettable fuse, enabling improved resistance stability as well as higher current and voltage-handling capabilities all from a smaller footprint.

Resettable fuses have become an attractive circuit-protection solution to guard against overcurrent conditions due to their ability to cycle back to a conductive state after the current is removed, acting more like circuit breakers. This allows the circuit to function again without having to open the device or incur the time, cost and resources to replace it.

Next-generation resettable fuses meet the standards and requirements for secure and reliable overcurrent protection within rated limits. Once system requirements are defined and circuit analysis has determined the operating parameters, selecting the right fuse for a specific design is rather simple.

A wide range of speed, voltage and current limits among next-generation resettable fuses ensures that the ideal component is incorporated into a design and a smaller footprint is possible for applications and electronics that are increasingly shrinking in size.

Next-generation resettable fuses can find use in many applications:

Power over Ethernet for use in IP telephones
Embedded computers
Wireless LAN access points
Universal Serial Bus for PCs
PDAs
FireWire / IEEE-1394
Digital audio
Digital video
Automotive and aeronautics applications

RESETTABLE FUSE TECHNOLOGY

Many of the leading resettable fuses are now manufactured with a mixture of polymer, carbon, and other proprietary materials. The characteristics of a fuse can be customized by adjusting the quantity of each ingredient within the composition or by changing the area of the cutout. Conductive carbon chains embedded in plastic in the fuse design create an environment of low impedance — 3 mΩ to 20 Ω. The proximity of the carbon chains in this crystalline structure allows easy current flow and the component offers low resistance at operating current.

Resettable fuses exhibit a positive temperature coefficient (PTC) effect when heated. As the PTC increases due to current or ambient temperature, the material expands. This expansion increases impedance from low to high, effectively creating an open circuit with some leakage current.

Unique to resettable fuses is the exponential rather than linear increase in resistance with rising temperature. The black carbon impregnated in the plastic becomes isolated as temperature increases, turning the crystalline polymer amorphous to prevent current flow and increases resistance at a rate of I²R.

When the circuit trips, resistance increases by multiple orders of magnitude. It is at this transformation of the material from low resistance to high resistance that a resettable-fuse device trips, allowing the fuse to protect the load from overcurrent within rated limits. Once the current is removed, the material will cool and return to its highly conductive, low-resistance crystalline state.

NEXT-GENERATION DESIGNS

Manufacturers of resettable fuses are introducing new technologies and manufacturing processes for superior performance and smaller sizes as the trend in miniaturization continues. One such technology, dubbed freeXpansion^™ technology by Bourns, limits the mechanical restrictions during the expansion of materials at the trip state and the contraction of materials upon reset. This allows smaller SMT package sizes (0603 / 1608 metric) and the resettable fuse can handle larger currents and voltages with improved resistance stability.

These next-generation resettable fuses deliver reliable overcurrent and over-temperature protection — and meet the voltage and current requirements — for a wide range of applications. One example of a targeted application is USB 3.0, which was released in November 2008 and specifies higher operating currents than its USB 2.0 predecessor.

The operational life of a resettable fuse device depends on the surge, duration and spike of the current to which it is exposed in a given system. These next-generation devices have been designed and tested to meet industry standards for the most demanding environments and applications, making the selection of the appropriate device relatively simple.

It is important to note that because a larger chip package results in larger thermal mass, the material heats more slowly than in a smaller package. A smaller package trips faster, within a fraction of a second, and the proper component can be chosen to protect the load for a given design based on the maximum allowable trip time. With the increase of electronic devices and human interfaces with these products, creative overcurrent solutions are needed more than ever before to ensure the efficient use of all system resources.

Selecting the appropriate resettable fuse can be narrowed down to three short steps:

Verify the operating voltage of the application circuit (e.g. 5.5 V). With this value, refer to the data sheets of the resettable-fuse models and select a device that has a maximum voltage rating (often referred to as V_max) higher than the circuit voltage (e.g. a device of V_max 24 V would be >5.5 V). This ensures that the PTC can operate effectively within the operating voltage of the circuit.
Check the operating current of the circuit (e.g. 500 mA). With this value, refer to the Hold Current (I_hold) in the resettable-fuse data sheet and select a model with an I_hold value higher than the operating current (750-mA device is >500 mA). This helps ensure that the PTC will not nuisance-trip in the application.
RESETTABLE FUSE APPLICATIONS

Check the ambient temperature of the circuit (e.g. 40°C) and then, using the thermal-derating chart on the data sheet, select a PTC model that has a higher I_hold value than the circuit's operating current at that ambient temperature (e.g. 750 mA at 40°C >500 mA at 40°C).

These steps can help simplify the product selection process. To optimize the solution for a specific application other specifications should also be reviewed, such as device resistance, device time-to-trip, mechanical dimensions, etc.

These next-generation resettable fuses are designed to meet the demands of specific applications such as Power over Ethernet (PoE), USB, and FireWire. Selecting the appropriate component for these applications is simple based on the current observed at the various ports in the system.

For PoE overcurrent protection, the resettable fuse must meet the 350-mA interface at up to 57 V. Fig. 1 illustrates how one PTC placed at the power supply provides overcurrent protection in a circuit laden with transformers. This overcurrent protection is necessary only at the source.

USB is a four-wire interface with a pair of data lines — power and ground — and it is required that a resettable circuit is used for overcurrent protection. For any design using a USB interface, it is essential to provide overcurrent protection on the power line, and all three power and data lines will utilize overvoltage protection as illustrated in Fig. 2.

As seen in the general USB interface example, it is possible to use just one PTC device for several USB controllers or one PTC device per USB controller. This hypothesis is based on the resistance and current in each load and the board space available.

An additional consideration when choosing a next-generation resettable fuse device for a USB application is maintaining a low resistance, namely under 700 mΩ, with a voltage drop below 350 mV. With the introduction of USB 3.0 current requirements are even greater, which may lead to designing a dedicated PTC for each line.

Since the USB standard operating voltage is always 5 V, the device's minimum voltage of 6 V provides at least a 20% margin (1 V) over the USB required voltage for an additional cushion of protection. The short-circuit current as defined by the USB specification is 5 A.

After 60 seconds of operation, a maximum operating current of 5 A is allowed for a USB port. However, the normal operating current for low-power ports is 100 mA and only 500 mA for high-power ports, both of which are far beneath the 5-A short-circuit level.

A wide range of next-generation resettable parts is available for USB designs at a 6-V rating. Once the current and trip time for a device are determined, it is simple to select the appropriate part.

CONCLUSION

FireWire (IEEE-1394) is similar to USB in its design. All power sources of the interface require overcurrent protection per the IEEE-1394 standard.

Fig. 3 illustrates overcurrent protection on the voltage line and overvoltage protection on the voltage and data lines. As with USB, the fuse device is capable of handling overcurrent for one controller or multiple controllers based on the needs of the system and the number of loads.

Typically, up to four controllers can share a single fuse device. There are four power levels for devices in the FireWire standard, and the maximum of each is 30 V, which is significantly higher than that of USB at 5 V. Current ranges from 100 mA to 1.5 A based on 15 W, 30 W, 45 W, and 3 W maximum power for the four respective power levels.

Overcurrent protection is critical for the lifespan of electronic devices in a wide range of applications. Next-generation resettable fuses meet the standards and requirements for secure and reliable overcurrent protection within rated limits. Once system requirements are defined and circuit analysis has determined the operating parameters, selecting the right fuse for a specific design is rather simple.

A wide range of speed, voltage and current limits among next generation resettable fuses ensures that the ideal component is incorporated into a design and a smaller footprint is possible for applications and electronics that are increasingly shrinking in size.