High-Accuracy Kelvin Sensing Using a Dual-Pad Sense Resistor

Introduction

Current sense resistors are available in a variety of shapes and sizes and are used to measure current in many automotive, power control, and industrial systems. When using very low value resistors (a few mΩ or less), the resistance of the solder will become a large percentage of the resistance of the sense element, which will significantly increase the measurement error. High-precision applications often use 4-lead resistors and Kelvin sensing techniques to reduce this error, but these specialized resistors can be expensive. In addition, when measuring large currents, the size and design of the resistor pads play a key role in determining the sensing accuracy. This article will describe an alternative solution that uses a standard, low-cost, two-pad sense resistor (4-pad layout) to achieve high-precision Kelvin sensing. Figure 1 shows the test board used to determine the errors caused by five different layouts.

Figure 1. Sense resistor layout test PCB board.

Current Sense Resistor

Common current sense resistors in a 2512 package can have resistance values ​​as low as 0.5 mΩ and can dissipate up to 3 W. To demonstrate worst-case error, these tests used a 0.5 mΩ, 3 W resistor with a 1% tolerance (Part Number: ULRG3-2512-0M50-FLFSLT Manufacturer: Welwyn/TTelectronics). Its footprint and standard 4-wire package are shown in Figure 2.

Figure 2. (a) Dimensions of the ULRG3-2512-0M50-FLFSLT resistor; (b) standard 4-pad package.

Traditional packaging

For Kelvin sensing, the standard two-wire package pads must be split to provide separate paths for the system current and the sense current. Figure 3 shows an example of such a layout. The system current path is indicated by the red arrows. If a simple two-pad layout is used, the total resistance is:

To avoid adding resistance, the voltage sensing traces need to be correctly routed to the sense resistor pads. The system current will cause a significant voltage drop at the upper solder joint, but the sense current will cause a negligible voltage drop at the lower solder joint. This pad separation solution can eliminate the solder joint resistance in the measurement, thereby improving the overall accuracy of the system.

Figure 3. Kelvin detection.

Optimized Kelvin package

The layout shown in Figure 3 is a significant improvement over the standard two-pad solution, but when using very low value resistors (0.5 mΩ or less), the physical location of the sense point on the pad and the symmetry of the current flowing through the resistor become more significant. For example, the ULRG3-2512-0M50-FLFSL is a solid metal alloy resistor, so every millimeter that the resistor extends along the pad will affect the effective resistance. Using a calibrated current, the optimal sense layout was determined by comparing the voltage drop under five custom packages.

Test PCB board

Figure 4 shows five layout patterns built on the test PCB, marked as A to E. We try to place the traces to test points at different locations along the detection pads, represented by the colored dots in the figure. The individual resistor packages are:

1. A standard 4-wire resistor based on the 2512 recommended package (see Figure 2(b)). The pairs of test points (X and Y) are located at the outer and inner edges of the pad (x-axis).

2. Similar to A, but the pad is extended inwards longer to better cover the pad area (see Figure 2 (a)). The inspection points are located at the center and end of the pad.

3. Use both sides of the pad to provide a more symmetrical system current path. At the same time, move the detection point to a more central position. The detection points are located in the center and end of the pad.

4. Similar to C, except the system current pads are joined at the innermost point. Only the outer sense point is used.

5. A mixture of A and B. The system current flows through the wider pad, and the sense current flows through the smaller pad. The sense points are located at the outer and inner edges of the pad.

Figure 4. Test PCB layout.

Apply solder to the stencil and use reflow soldering in a reflow oven. The resistor used is ULRG3-2512-0M50-FLFSLT.

Test steps

The test design is shown in Figure 5. A calibrated current of 20 A is passed through each resistor while the resistor is maintained at 25°C. The resulting differential voltage is measured within 1 second of the current being applied to prevent the resistor temperature from rising more than 1°C. The temperature of each resistor is also monitored to ensure that the test results are all measured at 25°C. The ideal voltage drop across a 0.5 mΩ resistor is 10 mV at 20 A.

Figure 5. Test setup.

Test Results

Table 1 lists the data measured using the detection pad locations shown in Figure 4.

Table 1. Measured voltages and errors

* No Kelvin sensing. Voltage is measured across the high current main pad to demonstrate errors associated with solder resistance.

Observations

1. Packages C and D have the least error due to the comparability of the results and the fact that the individual resistor deviations are within tolerance. Package C is the preferred package because it is less likely to cause problems related to component placement tolerances.

2. In each case, the test points at the outer ends of the resistors provided the most accurate results. This indicates that these resistors were designed by the manufacturer based on the overall length of the resistor.

3. Note that when Kelvin sensing is not used, the error associated with solder resistance is 22%. This equates to a solder resistance of approximately 0.144 mΩ.

4. Package E shows the effects of an asymmetric pad layout. During reflow, the component passes through a large amount of solder to reach the pad. This package should be avoided.

in conclusion

Based on the results shown previously, the best package is C, with an expected measurement error of less than 1%. The recommended dimensions for this package are shown in Figure 6.

Figure 6. Optimal package size.

The layout of the sense traces also affects the measurement accuracy. For the highest accuracy, the sense voltage should be measured at the edge of the resistor. The recommended layout shown in Figure 7 uses vias to route the outer edge of the pad to another layer, thus avoiding cutting the main power layer.

Figure 7. Recommended PCB trace routing.

The data in this article may not apply to all resistors, and results may vary depending on the resistor material and size. The resistor manufacturer should be consulted. It is the user's responsibility to ensure that the layout dimensions and construction of the package meet all SMT manufacturing requirements. ADI is not responsible for any problems that may arise from the use of this package.