Foreword

In applications such as battery management systems, servo drives, and industrial power supplies, the accuracy of current sampling directly determines the control performance and reliability of the system. When the sampling resistance drops to the milliohm level and the operating current rises to tens of amperes, the measurement error of ordinary 2‑pin chip resistors may deteriorate from the nominal 0.5% to 3%–5%. The reason for this accuracy degradation is that factors such as PCB trace resistance, solder contact resistance, and parasitic inductance are significantly amplified at low resistance values.

Kelvin connection physically separates the power loop from the sensing loop, thereby cutting off the above interference paths in principle. Taking Heketai RMS series 4‑pin alloy resistors as an example, this paper analyzes the accuracy guarantee mechanism of Kelvin connection in high‑current sampling, and demonstrates the quantitative improvement of system performance brought by high‑precision sampling through measured data and engineering cases.

Principle and Error Analysis of Kelvin Connection

Error Sources of Pin Resistance

In milliohm‑level measurements using traditional 2‑pin chip resistors, the error consists of the following components:PCB trace resistance: approximately 0.5–2 mΩ;solder contact resistance: approximately 0.2–1 mΩ;resistance tolerance: ±0.5%–±5%.

For a 2 mΩ sampling resistor, PCB trace resistance alone may introduce an additional deviation of 25%–100%. This means that a theoretical 2 mΩ resistor may exhibit an equivalent resistance of 3–4 mΩ in an actual circuit, causing the measured current value to deviate from the true value.

Principle of Kelvin Pin Connection

Kelvin connection adopts a four‑pin structure that separates the power loop from the sensing loop.The power pins carry high current for energy transmission;the sense pins carry only a tiny current and are dedicated to voltage measurement.

This separation physically eliminates the influence of voltage drops caused by high current on PCB traces and solder joints on the measurement signal. Taking a 2 mΩ resistor from Heketai as an example, its power pins and sense pins are arranged independently, ensuring that the detected voltage only reflects the real voltage drop across the resistor body.

Influence of Parasitic Inductance

In high‑frequency switching applications (e.g., 100 kHz DC‑DC circuits), parasitic inductance becomes an additional accuracy interference factor.The typical lead inductance of an ordinary 2‑pin resistor is 2–5 nH. At 100 kHz, the resulting interference impedance is close to the resistor value itself, leading to a significant increase in the actual measured impedance.A resistor with a nominal accuracy of 0.5% can then show an actual deviation of more than 30%.

Heketai’s 4‑pin package greatly reduces pin inductance through symmetrical and short‑path design, keeping parasitic effects within an acceptable range.

Key Parameters of Heketai Alloy Resistors

Temperature Coefficient (TCR)

The temperature coefficient determines the degree to which resistance changes with temperature.Heketai RMS series alloy resistors can achieve a temperature coefficient controlled within ±50 ppm/℃ in the typical resistance range.

This means:when the temperature changes by 100℃, the resistance drift is no more than 0.5%, which is on the same order as the resistor accuracy, ensuring measurement consistency over a wide temperature range.

Long‑Term Stability

The core value of the AEC‑Q200 automotive‑grade qualification lies in long‑term reliability.Heketai alloy resistors have passed the following tests:

• High‑temperature and high‑humidity storage: 1000 hours at 85℃/85% RH, with very small resistance change
• Temperature cycling: 1000 cycles between -55℃ and +155℃, with very small resistance change
• Load life: 1000 hours operation at rated power and 70℃, with very small resistance change

The above data show that within a 10‑year service life in automotive applications, the sampling accuracy degradation is less than 1%, far superior to ordinary thick‑film resistors.

Thermal Resistance and Power Derating

Thermal resistance determines the temperature rise under high‑current operating conditions.Taking the RMS2512 package as an example, its thermal resistance is approximately 40℃/W, maximum power rating is 3 W, and instantaneous overload current can reach the hundred‑ampere level.

Note:When a continuous current of 50 A is applied, the power dissipation is 5 W, which exceeds the rated power.Based on thermal resistance estimation, at an ambient temperature of 25℃, the chip temperature will far exceed its maximum operating temperature.

Therefore, the continuous current must be strictly controlled within the rated range. Instantaneous overload is acceptable but with a limited duration.

Selection and Design Considerations

Engineering Key Points of Kelvin Layout

There are 4 key points for Kelvin layout:

Symmetry PrinciplePower pins and sense pins must be placed symmetrically to eliminate discrepancies caused by geometric asymmetry.

Single-Point GroundingGrounds of all sense pins are connected at a single point to avoid inconsistent ground potentials.

Thermal Isolation DesignSense pins are placed away from heat sources such as power traces, inductors, and MOSFETs.

Shielding ProtectionGround shielding traces are placed on both sides of sensitive sensing paths.

Even when 4‑pin Kelvin resistors are used, system‑level errors still need to be compensated by methods including zero calibration, gain calibration, temperature compensation, and long‑term drift tracking.

Summary

Kelvin connection physically separates the power loop from the sensing loop, thereby eliminating the effects of PCB trace resistance, solder contact resistance, and parasitic inductance on sampling accuracy in principle.With AEC‑Q200 qualification, stable temperature characteristics, and long‑term reliability data, Heketai RMS series 4‑pin alloy resistors provide a reliable hardware foundation for high‑current sampling circuits.In applications such as battery management systems, servo drives, and photovoltaic inverters, high‑precision sampling can be translated into quantifiable system performance improvements, including enhanced state‑of‑charge estimation accuracy, reduced torque ripple, and optimized power generation efficiency.