High-precision Kelvin detection using dual pad sense resistors

Introduction

Current sense resistors are available in a variety of shapes and sizes to measure current in many automotive, power control, and industrial systems. When using extremely low value resistors (several mΩ or less), the resistance of the solder will occupy a large proportion of the resistance of the sensing element, resulting in a large increase in measurement error. High-precision applications typically use 4-pin resistors and Kelvin detection techniques to reduce this error, but these dedicated resistors can be expensive. In addition, the size and design of the resistor pad plays a key role in determining the accuracy of the measurement when measuring large currents. This article describes an alternative that uses a standard low-cost dual-pad sense resistor (4-pad layout) for high-precision Kelvin detection. Figure 1 shows the test board used to determine the error caused by five different layouts.

Figure 1. Detecting the resistance layout test PCB board.

Current sense resistor

Common current sense resistors in the 2512 package have a resistance of at least 0.5 mΩ and a maximum power dissipation of 3 W. To demonstrate the worst-case error, these tests use a 0.5 mΩ, 3 W resistor with a tolerance of 1% (model: ULRG3-2512-0M50-FLFSLT manufacturer: Welwyn/TTelectronics) its size and standard 4-wire package is shown in Figure 2.

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

Traditional package

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

In order to avoid increasing the resistance, it is necessary to place the voltage detection traces correctly at the sense resistor pads. The system current will cause a significant voltage drop at the upper solder joint, but the sense current will result in 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.

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