How Current-Sense Resistors Enable Accurate Power Measurement & Management

The ongoing demand for higher efficiency and better power management requires accurate current measurement. This need cuts across diverse industrial and consumer applications and power electronics, including battery management systems (BMSs), switched-mode power supplies (SMPSs), and motor drives.

While there are several ways to measure current and thus determine power, employing a current-sense resistor (often called a shunt resistor) and a differential amplifier is one of the most technically suitable and cost-effective solutions.

A current-sense resistor is much more than “just another resistor” with the desired value. For precise sensing, it must have high absolute accuracy, offer superior dissipation performance for reliability, be stable with respect to temperature changes despite self-heating and ambient temperature shifts, and have minimal thermoelectric-contact effects.

To size the sense resistor, you first determine a suitable resistance value by assuming an acceptable maximum voltage drop (V = IR) across the current-sense resistor at full current load. A good starting point is a nominal maximum of around 100 millivolts (mV), which is often a good compromise among factors such as dynamic range, sensitivity, noise, impediment to current flow, and wasted power due to dissipation.

Then, you look at the maximum current through the resistor to calculate its highest value, where R = V/I. This works out to 1 milliohm (mΩ) or less in many cases. Using the selected resistor value and maximum current, you can calculate the required dissipation rating for the resistor using the formula I²R.

Connection topology is critical

Equally important, the physical sensing arrangement must minimize any voltage measurement errors. Due to the extremely low resistance value and low voltage drop, subtleties such as contact resistance between current connections, sensor wires, and the sense resistor become important considerations.

In the basic two-wire voltage-sensing arrangement, the contact points at the resistor for the present current-flow path and voltage connection to the resistor are the same (Figure 1, left).

Figure 1: Two-wire sensing (left) and four-wire Kelvin sensing (right) have a small but significant difference in their physical connection of current and voltage contact points; the latter minimizes errors due to losses in lead wires. (Image source: Wikipedia, modified by author)

However, the apparent two-wire arrangement may compromise measurement accuracy at the low voltage levels across the resistor. To overcome the relatively small but significant errors induced by two-wire sensing connections, it’s common to use a four-wire Kelvin sensing arrangement (Figure 1, right).

The current flow and voltage-sensing connections are independent contact points in this topology. Although the electrical connection schematics may appear the same, the physical implementations are quite different.

By separating the current-flow contacts and path from the voltage-sensing points, four-wire sensing ensures that a voltage drop across the lead wires and the current-flow contacts will not affect the measurement accuracy. This is especially problematic when making precision readings where the sense resistor value is roughly the same as that of the lead wires used to measure it.

Four-wire sensing greatly minimizes this problem by moving the voltage measurement points immediately adjacent to the target impedance, thus bypassing any voltage drop that may occur in the high-current path.

The right resistor technology is also important

In addition to having a low resistance value of 1 mΩ or less, the sense resistor must have a low temperature coefficient of resistance (TCR) to prevent drift caused by ambient temperature changes and I²R-induced self-heating. As a result, these resistors' design, materials, and fabrication are highly specialized undertakings.

The CSI Series of metal strip shunt resistors from Bourns, Inc. helps designers meet these requirements. Members of this family are available in a wide range of resistance-value combinations as low as 0.2 mΩ and power dissipation ratings up to 15 watts (continuous).

The resistors are manufactured using electron-beam welded (EBW) resistive material and copper alloy and are available in two and four-terminal options. The two-terminal models are offered in three footprint sizes: 5930, 3920, and 2512. The four-terminal devices are intended for the more precise four-wire Kelvin resistance measurement and come in a 4026 footprint.

Their unique metal-alloy current-sensing element is designed explicitly for shunt resistor use, with low thermal electromotive force (EMF) and TCRs as low as ±50 parts per million per degree Celsius (ppm/°C) in the +20°C to +60°C temperature range.

Note that some counterintuitive materials-science insight is being used to fabricate these resistors. You usually would not want high-TCR copper (about 3900 ppm/°C) in any low-TCR component. However, copper also has excellent thermal conductivity, so it is carefully blended into the resistor design to increase its power-handling performance.

A representative example of a two-wire resistor in the CSI Series is the CSI2H-2512R-1L00J (Figure 2), a 1 mΩ, 5 watt resistor with a tolerance of ±5% and a TCR of ±75 ppm/°C. Other versions are available with a tighter tolerance of ±2% and even 1%.

Figure 2: The CSI2H-2512R-1L00J is a 1 mΩ, 5 watt resistor intended for two-wire sensing. (Image source: Bourns)

This resistor is fabricated using Bourns’ Type R material and features an extremely low self-inductance of under 2 nanohenries (nH). Self-inductance is an essential but often overlooked parameter that can be problematic if the resistor is in a high-speed switching circuit.

If you need four-wire Kelvin sensing, the CSI4J-4026R-1L00F current-sense resistor is a 1 mΩ component rated at 8 watts (Figure 3). This ±1% resistor (also available in 2% and 5% versions) has a TCR of ±75 ppm/°C. Self-inductance is under 3 nH. Note the different contact configuration; it is designed to enable four-wire functionality.

Figure 3: With its extra split-off connection points, the 1 mΩ CSI4J-4026R-1L00F is designed explicitly for four-wire Kelvin current sensing. (Image source: Bourns)

Due to the impact of TCR on sense-resistor accuracy, the datasheets for these components include multiple graphs showing the change in resistance with respect to performance at 25°C.

Conclusion

Sense resistors may seem like simple components, but dig deeper. Given what they are expected to do and need to provide in their application niche, you’ll find they have subtleties and considerations that can only be met by a vendor with experience, materials expertise, and fabrication know-how, all supported by a detailed datasheet.

Reference

1: Maxim/Analog Devices, Application Note 5761, “Lord Kelvin’s Sensing Method Lives On in the Measurement Accuracy of Ultra-Precision Current-Shunt Monitors/Current-Sense Amplifiers”