While resistors come in many flavors, the two most commonly used for power applications are metal film and wirewound. The biggest challenge for resistor manufacturers is packing more power into the same package size, whether it's for use in small electronic devices or large industrial equipment. And while a higher power rating is desirable, it can't cause an undesirable amount of heat generation.
Many engineers will still swear by outdated carbon compound resistors (despite availability issues and high price tags) for specific power applications because they are inductance-free. But for this discussion, the focus is on metal-film and wirewound resistors.
In the resistor world, the bigger the package, the more power the resistor can handle. But today's market increasingly demands smaller electronic products, which translates into a need for smaller components. Add to this mix requirements for better heat dissipation and tighter tolerance--which go hand-in-hand with more power--and you've got a big challenge.
Resistor manufacturers are beefing up their power lines to offer more power and better heat dissipation in same-size or smaller packages. In addition, several offer resistors that can operate at 275°C to withstand harsh environmental conditions, including those found in automotive and industrial applications. Others have developed heat-sink resistors and surface-mount packaging for these devices.
"The downsizing of standard electronic devices is forcing higher power to be dissipated in a smaller board space, so you need power resistors that can handle the higher power per square inch," said Kory Schroeder, product-engineering manager for Stackpole Electronics (Raleigh, N.C.).
Power rating vs. heat rise is also critical for characterization, Schroeder said. The key concern with power resistors is meeting the UL limit of 105°C for circuit board temperatures. Designers start to get nervous if the board heat approaches that temperature limit, he said.
In addition, more-efficient power supplies mean resistors need to have tighter tolerances and better temperature coefficients of resistance (TCR). Before the miniaturization trend, tolerance wasn't critical, but now designers regularly request 5 percent, Schroeder said.
On the flip side, industrial equipment is getting bigger and offering greater functionality, a trend that requires higher-power resistors, Schroeder said.
In general, the decision to use a metal-film or wirewound power resistor is determined by the power rating. If the requirement is below 5 watts, the typical choice is a metal-film resistor; above 5 W, designers go with wirewound. That means the overlap applications are typically found at 5 W.
These trends have led to new product developments over the past year.
Manufacturers such as Stackpole and BI Technologies have addressed the need for better heat dissipation. Stackpole (Raleigh, N.C.) retooled its HPC series with surface-mount devices in a 0.5 x 0.5-inch footprint capable of 5 W of continuous power in free air. The parts use the same board space as typical 3-W wirewound surface-mount devices. In addition, with 200-lfm or 400-lfm air movement, the power ratings improve to 10 W and 12.5 W, respectively, without exceeding a circuit board temperature of 105°C, Stackpole said.
Stackpole also introduced its RHC high-power, thick-film chip resistors in a 2512 package that can handle 2 W. Designed with specialized materials and processes, the chip resistors run 30°C cooler than standard 2512 chips and exhibit solder joint temperatures well below 105°C.
Stackpole also added 30-W and 50-W offerings to its TR series of TO-220 power film resistors. This type of resistor is typically mounted along with other power ICs to a heat sink maintained at 25°C, which allows these small parts to dissipate large amounts of power.
The BHP75 series of 75-W power resistors from BI Technologies (Fullerton, Calif.) uses three heat-dissipation methods. The series is housed in a TO-220 open-screened substrate package and features an insulated tapered venturi that's bonded to the substrate for maximum heat dissipation. The design forces hot air up the venturi and away from the resistor.
Still, more power is the name of the game.
KOA Speer Electronics (Bradford, Pa.) has released the WK73 wide-terminal flat chip resistor, which offers higher power ratings and enhanced heat dissipation. The WK73S3A in a 1225 case size offers a 1.5-W rating, compared with the company's RK73B3A in a 2512 case size with a 1-W rating.
For accuracy and stability, MHP TO-247 power resistors from TT electronics' IRC Advanced Film Division (Corpus Christi, Texas) are available in 100-W and 140-W packages. They exhibit very low inductance and low thermal resistance.
Stackpole now has 20-, 25- and 30-W power film resistors in a surface-mount package. The company also added 100- and 250-W offerings to its KAL aluminum-housed wirewound power resistors.
Resistors with the ability to withstand higher temperatures include offerings from TT electronics' IRC Wire and Film Technologies Division (Boone, N.C.). The CCW axial wirewound, CAW wirewound and CAF film power resistors operate at up to 275°C. The IRC CMO metal oxide resistors, in standard and miniature sizes with power ratings from 0.25 W to 9 W at 70°C, offer a maximum operating temperature of 240°C.
Among the vendors achieving higher resistance and voltage ratings is Vishay Intertechnology (Malvern, Pa.), whose RoHS-compliant D2TO20 20-W thick-film power resistor has a resistance range of 0.010 ohm to 550 kohms in a compact, TO-263 package.