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 account for a large proportion of the resistance of the sense element, resulting in a large increase in measurement error. High-precision applications often use 4-pin resistors and Kelvin sensing techniques to reduce this error, but these specialized resistors can be expensive. Also, when measuring high currents, the size and design of the resistor pads play a key role in determining the detection accuracy. This article will describe an alternative that uses a standard low-cost two-pad sense resistor (4-pad layout) for high-accuracy Kelvin sensing. Figure 1 shows the test board used to determine errors due to five different layouts.

Figure 1. Sense resistor layout test PCB board.

Current sense resistor
Common current sense resistors in the 2512 package have resistance values ​​as low as 0.5 mΩ and may dissipate up to 3 W. To demonstrate worst-case error, these experiments used a 0.5 mΩ, 3 W resistor with a 1% tolerance (Model: ULRG3-2512-0M50-FLFSLT Manufacturer: Welwyn/TTelectronics) its size and standard 4-wire package as shown in picture 2.

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

traditional packaging
For Kelvin sensing, the standard 2-wire package pads must be split to provide separate paths for system and sense currents. Figure 3 shows an example of such a layout. The path of the system current is indicated by the red arrow. If a simple two-pad layout is used, the total resistance is:

To avoid adding resistance, the voltage sense traces need to be correctly laid out 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. It can be seen that this pad separation scheme can eliminate the solder joint resistance in the measurement, thereby improving the overall accuracy of the system.

Figure 3. Kelvin detection.

Optimizing Kelvin Encapsulation
The layout shown in Figure 3 is a significant improvement over the standard two-pad scheme, however, when using very low value resistors (0.5 mΩ or less), the physical location of the sense point on the pad and the current flowing through the resistor are symmetrical The impact of sex will become more pronounced. For example, the ULRG3-2512-0M50-FLFSL is a solid metal alloy resistor, so every millimeter the resistor extends along the pad results in an effect on the effective resistance. Using the calibration current, the optimal detection layout can be determined by comparing the voltage drop across the five custom packages.

Test PCB board
Figure 4 shows the five layout patterns built on the test PCB, labeled A through E. As much as possible, we routed the traces to test points at different locations along the detection pads, represented as colored dots in the figure. The individual resistors are packaged as:

1. 1. Standard 4-wire resistor based on the 2512 recommended package (see Figure 2(b)). Detect point pairs (X and Y) at the outer and inner edges of the pads (x-axis).
2. 2. Similar to A, but the pad extends inward longer to better cover the pad area (see Figure 2(a)). The detection points are at the center and end of the pad.
3. 3. Utilize both sides of the pad to provide a more symmetrical system current path. Also move the detection point to a more central location. The detection points are at the center and end of the pad.
4. 4. Similar to C, except that the system current pad is bonded at the innermost point. Only external detection points are used.
5. 5. A mixture of A and B. The system current flows through the wider pads and the sense current flows through the smaller pads. The detection points are located on the outer and inner edges of the pads.

Figure 4. Test PCB layout.

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

test steps
The test design is shown in Figure 5. A calibration current of 20 A was passed through each resistor while keeping the resistors at 25°C. Within 1 second of applying the current, the resulting differential voltage was measured to prevent the resistance temperature from increasing by more than 1°C. The temperature of each resistor is also monitored to ensure that the test results are all measured at 25°C. At 20 A, the ideal voltage drop across a 0.5 mΩ resistor is 10 mV.

Figure 5. Test setup.

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

Table 1. Measured Voltage and Error

*No Kelvin detection. The voltage across the high current main pad is measured to demonstrate errors related to solder resistance.

Observation results

1. 1. Since the comparability of the results and the deviation of each resistance are within the tolerance range, it is concluded that the errors of packages C and D are the least. Package C is the preferred package as it is less likely to cause issues related to component placement tolerances.
2. 2. In each case, the detection point on the outer end of the resistor provides the most accurate results. This shows that these resistors are designed by the manufacturer based on the total length of the resistors.
3. 3. Please note that the error associated with solder resistance is 22% when Kelvin detection is not used. This corresponds to a solder resistance of about 0.144 mΩ.
4. 4. Package E demonstrates the effect of asymmetric pad layout. During reflow, the components pass through a large amount of solder before they can be landed. This encapsulation should be avoided.

in conclusion
Based on the results shown earlier, 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. Optimum package size.

The layout of the sense traces also affects the measurement accuracy. For maximum 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 edges of the pads to another layer to avoid cutting the main power plane.

Figure 7. Recommended PCB trace routing.

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