Optimizing High Current Sensing Accuracy 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 significant 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 approach 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.

Uploaded on 2012-9-5 16:49:27

Download attachment (167.27 KB)

 

Figure 1. Sense resistor layout test PCB.

Current Sense Resistor

Common current sense resistors in 2512 packages 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 No. ULRG3-2512-0M50-FLFSLT Manufacturer: Welwyn/TTelectronics) with dimensions and a standard 4-wire package as shown in Figure 2.

Uploaded on 2012-9-5 16:49:28

Download attachment (52.48 KB)

 

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

Traditional Package

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 follows the path 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 properly routed to the sense resistor pads. The system current will cause a significant voltage drop in the upper solder joint, but the sense current will cause a negligible voltage drop in the lower solder joint. This pad splitting scheme can eliminate the solder joint resistance from the measurement, thereby improving the overall accuracy of the system.

Uploaded on 2012-9-5 16:49:29

Download attachment (68.36 KB)

 

Figure 3. Kelvin sensing.

Optimizing Kelvin Packages

The layout shown in Figure 3 is a significant improvement over the standard two-pad approach, but the physical location of the sense point on the pad and the symmetry of the current flowing through the resistor become more significant when using very low value resistors (0.5 mΩ or less). 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 drops across five custom packages. [page]

Test PCB

Figure 4 shows five layout patterns constructed on a test PCB, labeled A through E. Whenever possible, we routed the traces to test points at different locations along the sense pad, represented by the colored dots in the figure. The individual resistor packages are:

Uploaded on 2012-9-5 16:49:38

Download attachment (89.23 KB)

 

1. Standard 4-wire resistor based on the 2512 recommended footprint (see Figure 2(b)). The sense point pairs (X and Y) are located at the outer and inner edges of the pad (x-axis).

2. Similar to A, but the pad extends inward longer to better cover the pad area (see Figure 2(a)). The sense points are located at the center and ends of the pad.

3. Use both sides of the pad to provide a more symmetrical system current path. Also move the sense points to a more central location. The sense points are located at the center and ends of the pad.

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

5. A hybrid 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.

Uploaded on 2012-9-5 16:49:33

下载附件 (178.22 KB)

 

Figure 4. Layout of the test PCB.

Solder was applied to the stencil and reflow soldering was performed in a reflow oven. The ULRG3-2512-0M50-FLFSLT resistor was used.

Test Steps

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

[attach]87555[/attach]
Figure 5. Test setup.

Test Results

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

Table 1. Measured Voltages and Errors

Footprint	Sense Pad	Measured (mV)	Error (%)
A	Y	9.55	4.5
	X	9.68	3.2
B	Y	9.50	5
	X	9.55	4.5
C	Y	9.80	2
	X	9.90	1
D	X	10.06	0.6
E	Y	9.59	4.1
	X	9.60	4
	Top pad*	12.28	22.8

*No Kelvin sensing. The voltage across the high current main pad was measured to demonstrate the error 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 variations are within tolerance. Package C is the preferred package as it is less likely to cause issues associated with component placement tolerances.

2. In each case, the sensing points at the outer ends of the resistors provide the most accurate results. This suggests that the resistors were designed by the manufacturer based on the total length of the resistors.

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

4. Package E demonstrates 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.

Conclusion

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

2012-9-5 16:49:35 上传

下载附件 (63.63 KB)

 

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 plane.

2012-9-5 16:49:36 上传

下载附件 (59.67 KB)

 

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.