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Metal element resistors maximize heat dissipation

Posted: 09 May 2008 ?? ?Print Version ?Bookmark and Share

Keywords:metal elements resistor? open-air SMT? flameproof form factor?

OARS-XP

Developed to provide a current sense device with high current-carrying capability for power electronics circuits, the OARS-XP Series metal element resistors from TT electronics IRC deliver power ratings up to 5W in a flameproof open-air surface-mount form factor that promises the maximum heat dissipation for a PCB-mounted component.

The OARS-XP Series resistors consist of a 0.25-inch wide metal element resistance alloy, welded to compliant copper terminals designed to handle thermal coefficient of expansion (TCE) mismatch between the device and the PCB. Unlike other resistor technologies, such as thin film, thick film or foil devices, the open air metal element will not experience degradation of performance at extreme temperatures, even up to 500∼C (a conservative temperature rating for the alloy itself). However, since most PCB solder materials typically melt between 183∼C and 230∼C, the solder will reflow long before the device itself experiences any heat damage.

According to Bryan Yarborough, an IRC application engineer, the company provides specific recommendations to design engineers for maximizing the heat dissipation of the OARS-XP Series resistors when developing PCB layouts and traces for the devices. "As with any component, the practical power rating for the device is ultimately dependent upon the PCB design," Yarborough explained. "These factors include allowing adequate width and thickness of the traces for the device, maintaining adequate airflow across the PCB, and avoiding the placement of the device in close proximity to other heat-generating components, such as transformers, power components or microprocessors."

As an example, an OARS-XP Series device can achieve a 3W power rating if mounted on a standard FR-4 PCB with traces at least 0.25-inch wide with a minimum of 56.7g of copper. With wider and/or thicker traces, or multilayer traces designed to efficiently draw heat away from the component, the device dissipates up to 5W of power, Yarborough continued.

Applications for the OARS-XP Series resistors include surge and pulse circuits, AC power applications, and current sensing and feedback circuits.

Resistance value for the resistor ranges from 1m次 to 25m次 (.001次 to .0025次), with tolerances down to ㊣1 percent. Inductance values are less than 10nH, with operating temperature ranging from -40∼C to 125∼C. IRC will also produce devices outside these specifications to meet customer requirements. Pb-free RoHS-compliant and zero ohm jumper versions are also available.





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