As power-supply technology advances, so does the need for accurate current-sense methods. For low- and medium-power applications, low-value current-sense resistors are used almost exclusively. In higher-power applications, often more exotic methods of current sensing are used. These include Hall effect devices and other devices that use the principle of the magnetic field developed around a current-carrying conductor.
However, due to the simplicity, ease of use and overall low cost of resistor-type current-sensing methods, designers often prefer to use them over other technologies. In recent years, advancements in resistor technology, packaging and lower resistance values with increased current-handling capability have allowed their use in increasing numbers in power-supply applications.
Resistors in current-sense applications use the basic principle embodied in Ohm's law, namely that current flowing through a resistor generates a voltage across it. To minimize power losses, engineers select resistance values that are as low as possible yet still generate a sufficient voltage to provide immunity from background and stray noise. This noise can be significant in many environments and may even be self-generated in switching-type power supplies.
Traditionally, surface-mount (SMT) current-sense resistors were relegated to 1 W to 2 W of power dissipation. This prevented their use in all but lower-power and lower-current-sense applications. But as newer techniques are developed to manage the heat generated on the board by a SMT component, designers are getting more options in their power-supply designs.
Resistor Types and Technologies
Resistors are relatively simple devices. In their simplest form, they can consist of a copper trace of a given size and shape on a pc board. Although quite cost effective for applications with low power dissipation, this technique of creating a resistor isn't widely used except in very low-precision applications.
Copper material used in the manufacture of pc boards has a high temperature coefficient of resistance (TCR), approximately 0.4%/°C. Other factors relating to circuit-board etching and manufacturing result in resistors made with this technique typically being capable of no better than ±40% to 50% of the correct value. In addition, for resistors with large power ratings, the amount of board area needed to minimize TCR effects, and to dissipate the power needed, make this technique impractical.
There are typically four basic types of precision resistor: thick film, wire wound, thin film and composition. For current-sense applications, the first two methods are overwhelmingly used. (Thin-film and composition types, with few exceptions, are incapable of achieving the resistance values in the milliohm range that are needed for most current-sense applications.)
A typical thick-film current-sense resistor (1-W power rating) is manufactured in a manner similar to that of a standard-value chip resistor (Fig. 1). A flat ceramic base or substrate is screen-printed with a paste made of fine particles of metal and glass powder. The conductor material is typically a silver alloy containing palladium or platinum, and the resistor element (for very low-resistance values) is often an alloy containing approximately 60% palladium and 40% silver. After printing, the thick-film resistor is fired at a high temperature to sinter the metal and glass particles.
This type of construction is capable of providing resistance values down to a few milliohms and is low cost, but is usually limited to 1 W or 2 W of power dissipation. Because the construction is basically a leadless (non-compliant) termination, scaling this method up to larger sizes (and higher power dissipation) is limited as sizes larger than the 2512 configuration can experience problems with solder-joint failures if the board is subject to flexing or temperature cycling.
Power dissipation, however, can be increased by using a side termination, as is the case for resistors like IRC's LRF3W style component (Fig. 2). In this case, the side termination raised the power rating to 3 W in what is traditionally a 1-W-sized package. Part of the increased power-handling capability can be attributed to the resistor's internal construction. This device uses copper conductors and copper/nickel alloy resistors.
This configuration is also much less affected by TC mismatch of the resistor to the board, and the wider solder terminations allow much larger connections to the resistor. The latter feature is beneficial when conducting larger amounts of current. These materials and the internal construction serve to spread the heat generated internally, allowing more efficient power dissipation in a smaller-sized package.
Wire-wound resistors, the other main type of current-sense resistors are used in applications down to submilliohm values (at very low-resistance values, these resistance values aren't really wound, but instead are typically stamped from a flat-metal resistor alloy). The metals used for wire-wound resistors are usually made from one of the following resistance alloys: cupron (an alloy of copper and nickel), manganin (nickel, copper and manganese) or nichrome (nickel and chromium). These metal alloys have characteristics of low-resistance change with temperature, high stability even at elevated temperatures and cost effectiveness.
Packaging for SMT Sense Resistors
The traditional SMT package for wire-wound resistors basically consists of a flat-stamped resistor alloy with solderable terminations attached to each end. This configuration is simple and reliable, but is also limited to relatively small package sizes and correspondingly low power dissipation.
One variation of this type, the OARS series from IRC, is formed so that the resistor element is elevated off the board, significantly reducing the amount of heat transferred by conduction to the board (Fig. 3). With an outline size of 0.81 in. ÷ 0.275 in., this arrangement takes better advantage of any airflow that might be across the board, further reducing its operating temperature, and offers an inherent degree of compliancy with the pc board. A companion part, the OARS-XP (extended power), is available with power ratings to 5 W.
Another common package is the molded SMT wire-wound resistor, consisting of a wire-wound or metal element molded into an epoxy package with compliant metal leads for soldering (Fig. 4). This package has the advantages of power dissipation up to 3.5 W, low-resistance values (typically 1 mW or less) and compliant terminations. However, beyond 3.5 W, this construction makes it difficult to dissipate the extra heat generated.
For high-power current-sense applications, there are additional package configurations that may be considered (Fig. 5). The TO-263 (or D2PAK) is available for resistance values down to approximately 10 mW and typically has a power-dissipation rating of 20 W when used with a heatsink. Extremely high-power dissipation can be had in a SOT-227 package. With this package, power ratings up to 100 W or more are available and usually in a Kelvin connection, also known as a four-terminal connection. Of course with this part, a heatsink also is necessary.
Four Terminal, Two Terminal
Because of their very low-resistance values, current-sense resistors are susceptible to errors in voltage measurement due to the resistance of the connection to the board. As a result, minute variations in the amount of solder used or trace layout can affect the measured voltage when using any two-terminal resistor. The solution to this problem is to use a four-terminal connection. This type of connection minimizes voltage measurement error by separating the power and voltage-sensing terminals.
Special four-terminal resistors are available for optimum accuracy, but acceptable performance from two-terminal resistors can usually be obtained if circuit traces are designed properly (Fig. 6). A modified Kelvin connection for pc-board traces can significantly increase current-sense accuracy when using two-terminal resistors. The four-terminal connection separates the current-carrying traces from those used for voltage sensing. By eliminating current flow in the voltage-sensing terminals, the associated voltage drop in it is eliminated, increasing the accuracy of the measured voltage.
In current-sense applications, the sense voltage is often quite low (typically millivolts). As a result, the materials used in current-sense resistors often are selected by their manufacturer to exhibit a low thermal electromotive force (EMF). This thermal EMF is the thermocouple effect caused by different metals used in the resistor's construction. Unequal temperatures found across the resistor, which are always present to some degree, coupled with the different resistor materials, produce a thermal EMF error, which adds to the total error in sensing the voltage across the current-sense resistor.