Virtually all electronics in existence have a universal requirement in order to function, namely a power source.  Along with this power source comes a need to protect the electronics from overcurrent conditions, which may result from a fault, or from any harmful operating condition that the circuitry or any of its interfaces may be exposed to.  Consequently, circuit protection has become a multi-billion dollar industry as companies such as Bourns, Inc. continue to meet and anticipate the needs of designers, especially as the consumer electronics market continues to grow.  Bourns provides a comprehensive line of circuit protection solutions to the industry which includes its wide range of Multifuse® polymer PTC resettable fuses. These devices are designed to protect a broad range of applications including computer, battery, automotive, telecommunications, industrial and consumer electronics. They offer a small footprint to meet the needs of compact applications and particularly as the trend towards miniaturisation continues with consumer electronics. 

What is a Polymer PTC resettable fuse?
Bourns® Multifuse® devices are made from polymer positive temperature coefficient (PTC) materials.  They are also known as thermistors as they share the behaviour that an increase in temperature results in an increase in resistance.  While a thermistor has a linear relationship of resistance to temperature, the Multifuse® device’s resistance remains stable as it rises slightly with temperature until a breakover trip point, where the resistance begins to increase exponentially as an open circuit is created.  The Multifuse® device will trip even when no current is flowing through it as a reaction to temperature, such as in a reflow process.  This characteristic is an advantage, as the Multifuse® device can be used to protect a circuit from both overcurrent and overtemperature conditions. 

When a short circuit occurs or an adjacent component overheats, the current in the Multifuse® device will increase, thus increasing the power dissipation.  Once the tripping current is reached or the temperature of the Multifuse® device increases to approximately 120 °C, the resistance increases by multiple orders of magnitude in the tripped state to protect the electronics.  

It is often incorrectly assumed that a Multifuse® device cools down and resets itself independently.  On the contrary, the proper method to reset the device is to cycle power to it once the fault has cleared.  In order to reset the device, the current through the Multifuse® device must be less than the leakage current.  Typically, the resistance will return to its normal operating range within one second.  If power is not cycled, then the device will not cool off on its own.  Unlike some metallic switches, the Multifuse® device will not oscillate by tripping and resetting continuously but will stay tripped until the power has been cycled.

Though capable of withstanding greater than 6000 trip cycles, which is what the standard UL tests perform, the surge or duration may reduce the expected life of a device if exposed to continuous trip cycles.  Based on a specific design or application some variation is expected in the life cycle of a Multifuse® device.  The life cycle is dependent on the surge spikes in addition to the duration of the surges. 

 Polymer PTC Resettable Fuse Fundamentals
The key to the operation of the Multifuse® resettable fuse is the device’s construction.  A mixture of polymer, carbon, and proprietary additives are extruded into a plaque and punched out to create the device.  The resulting carbon chains are conductive and have low impedance, which is maintained at normal operating current.  As the PTC heats, either due to high current or the ambient temperature, the materials in the Multifuse® device expand, breaking the conductive carbon chains.  Once separated, the carbon chains are no longer conductive and the material becomes high impedance in this tripped state.  The circuit then resembles an open circuit, though a nominal amount of leakage current flows to the other device.  While this is not ideal for all circuits, it is preferable to the surge current that would flow through in the absence of a Multifuse® device.

Next Generation Designs
Bourns has developed a new technology called freeXpansion™that was introduced in the surface mount polymer PTC Multifuse® product line.  The expansion of materials during a trip and the contraction of materials upon reset occur with a minimum amount of mechanical restrictions in Multifuse® freeXpansion™ devices.   A benefit of Multifuse® freeXpansion™ devices is their ability to handle larger currents and voltages with improved resistance stability.  Additionally, smaller package sizes in surface mount parts are possible. 

Several series of Multifuse® devices have taken advantage of the new technology to meet the voltage and current requirements to protect from overcurrent and overtemperature in consumer electronics, automotive electronics, USB interface, and video interface applications.  The Multifuse® freeXpansion™technology has allowed Bourns to release the presently smallest PTC device generally available in the market, the 0603 (1608 metric) sized Bourns® Model MF-FSMF series, as well as the high-powered Bourns® Model MF-LSMF series which is capable of holding 3 A, 24 V.

Multifuse® devices also offer design flexibility for customers.  A wide range of package sizes covering a broad spectrum of resistance and holding current ratings are available. In addition, a Multifuse® device can be modified to meet the specific needs of a customer.  Two examples of slightly different feature sets for the Multifuse® device are the ability to create one with a lower resistance rating and another with a higher holding current.

 Bourns’ Circuit Protection Expertise
Bourns has a history of excellence in the field of circuit protection.  For over 60 years, Bourns  continues to grow because of the company's continued commitment to providing value for its customers in markets as diverse as computers and peripherals, telecommunications, point of sale equipment, and industrial and medical hardware. Bourns continues to invest in innovative new technologies and products through internal research and development.  The company’s engineering expertise and superior customer service set Bourns apart from the competition and are the reasons that the company consistently ranks among the top-rated suppliers in the world.

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This news article was originally published in July 2011.