Billed as the first "true" power surface-mount resistor device in the industry, Stackpole Electronics, Inc. has introduced a new surface-mount power resistor capable of handling up to 12 watts at 85°C. Available in 5-, 10- and 12-watt ratings, the new HPC series offers a resistance value range from 0.1Ω to 100 KΩ. All devices are available in a 0.5 x 0.5 x 0.5-in. package size.
While this new power resistor technology will never be able to demonstrate the type of power handling of a large, leaded TO-220 power resistor with a heat sink, this new technology targets DPAK- or D2PAK-type surface-mount devices at less than 10 watts. A key challenge in the power resistor market has been achieving power levels at above 1-watt in a surface-mount package, although there are numerous 1-watt and 2-watt surface-mount wirewound resistors that sort of solve the problem.
A Stackpole engineer explains. Power designers typically will have to use multiple surface-mount power resistors in parallel to achieve 3, 5, 8 or 10 watts, or they have no choice but to use a larger through-hole device, which complicates the manufacturing process in part due to the mixed-assembly technology required to process both through-hole and surface-mount devices on the same board.
One of the issues with surface-mount wirewound resistors is that many of them are specified at 2-, 3- or 4-watts that technically speaking will run at 3 or 4 watts but they'll be running at such a high temperature that it will violate the UL PC-board temperature (typically 105°C) regulations. Therefore, they must be derated down to 1-watt. Upshot: existing surface-mount wirewound resistors are truly only useful up to 1 or 1 1/2-watts.
In other words, even if the resistor itself, in free air, could tolerate 4 watts and get to 200°C or more without the resistor wire burning open, the same resistor power, in a surface-mount condition, would elevate the temperature of the PC-board under the resistor body to well over 150°, violating the UL approval requirements, explained a Stackpole engineer. It is possible to run such resistors at their theoretical maximums by using PC-boards rated at extremely high temperatures such as expensive Teflon or polyamides, however, 95% of the electronics industry uses G10 or FR4 boards (or lesser capability) with a 105°C maximum temperature.
This new HPC power resistor keeps the heat off the board so the heat is not transferred down to the heating pads as compared with a conventional wirewound resistor where the heat is transferred directly to the PC-board. What this means is that the HPC series is different from conventional surface-mount resistors that depend almost exclusively on the termination pads for heat removal, and must be derated to about 50% of maximum power.
The new power resistor distributes heat from the resistive element through metallic pathways to all the package surfaces which means that the PC-board surface-mount pads experience only a fraction of the temperature rise as the resistive element itself. While at maximum power, even though the resistive element can be at 150°C, the PC-board solder pads will remain well under 105°C.
Unlike conventional 1-, 3- and 5-watt surface-mount wirewound resistors, the HPC series uses conventional thickfilm chip resistors bonded to the flat surface of a subminiature aluminum extruded housing, which has been designed so the heat is spread to all of the surface area of the aluminum structure to be cooled by air. This means the thermal resistance from the resistive element to the package is extremely low.
There is a second part to this equation. Over the past five years or so, TO-220 power resistor makers have typically emulated the semiconductor IC market by offering DPAK and D2PAK surface-mount devices. What they've done, explained a Stackpole engineer, is to take their 20- or 30-watt TO-220 devices, and trim the tab and form the leads to offer a 20- or 30-watt surface-mount version (DPAK or D2PAK) power resistor.
Here's the problem: A conventional 20- or 30-watt device needs a heat sink to get the heat out, and when the leads are trimmed and formed and then surface-mounted to a PC-board, there is no way to get more than 2-watts out of the device. "It's not physically possible to take a TO-220 D2PAK adaptation and get more than 2-watts out of it on a PC-board, contrary to the claims by makers of DPAK products," said a Stackpole engineer. "You can get 20 watts if you can magically attach some heat sink to the surface-mounted part but we don't know what that method is and they don't either."
Caveat: Virtually all SMD wirewound devices need to be derated at about 50% of maximum power when used in most non-automotive or military applications. In addition, even if the 20-watt device is operated only at 2 watts, it will work, but with nearly a zero safety factor.
Available in tape and reel or bulk packaging, typical pricing for the HPC resistors is approximately $0.75 each. Delivery is six to eight weeks.
Stackpole Electronics Inc., 1-919-850-9500, www.seielect.com