Technical Advantages of Automotive-Grade Alloy Resistors and Design Guidelines for Current Sampling in New Energy Vehicles

In key systems such as the Battery Management System (BMS) and motor drives of new energy vehicles, the precision and long-term reliability of current sampling directly impact battery safety management, torque control accuracy and system efficiency. Such applications impose stringent requirements on sampling resistors: maintaining extremely low resistance drift over a wide temperature range, withstanding high pulse currents and harsh chemical environments, and delivering excellent long-term stability. Alloy resistors based on nickel-chromium-copper-manganese and other materials have become the preferred solution to these challenges due to their inherently low Temperature Coefficient of Resistance (TCR) and stable thin-film structure.

Core Technologies of Automotive-Grade Alloy Resistors

Compared with conventional thick-film resistors, alloy resistors provide optimized solutions for automotive applications in the following key parameters.

1. Low Temperature Coefficient (TCR) and Wide-Temperature Stability

An important source of current sampling error is the change in resistance value with temperature. The TCR of traditional thick-film resistors is typically in the range of ±100 to ±500 ppm/℃, which may cause significant resistance drift within the operating temperature range of automotive electronics. Through optimized alloy material formulations and thin-film preparation processes (such as magnetron sputtering), automotive-grade alloy resistors can control the TCR within ±50 ppm/℃, with high-performance models reaching ±25 ppm/℃ or even lower.

For example, the TCR of Hekotai's HKT-AR25 series alloy resistors is ±15 ppm/℃, which means the theoretical maximum resistance change is only about 0.285% over the full temperature range of -40℃ to 150℃, laying a foundation for high-precision sampling.

2. High Power Density and Heat Dissipation Capacity

In high-current applications such as BMS main circuits, sampling resistors need to dissipate considerable power at milliohm-level resistance values. Thanks to the good electrical and thermal conductivity of their materials, coupled with optimized electrode design and package heat dissipation paths, alloy resistors can achieve a higher rated power in a compact package. For instance, the alloy shunt of the 5931 package (Hekotai's HKT-AS59 series) can reach a rated power of more than 9W, meeting the sampling requirements of continuous current of several hundred amperes.

Power derating calculation must be performed during selection: Under the actual operating ambient temperature, the allowable power consumption of the resistor must be higher than the actual power consumption calculated by the formula P = I_rms² × R, with a sufficient margin reserved.

3. Sulfur Resistance and High Reliability

Corrosive gases such as sulfur and chlorine present in the automotive environment can react with the silver terminal electrodes of resistors, leading to failure. Automotive-grade alloy resistors enhance sulfur resistance by adopting precious metal alloy terminal electrodes and applying protective coatings. Reliability tests related to H₂S environment tests at 85℃/85%RH are the key to ensuring compliance with automotive standards such as ISO 16750-5.

In addition, long-term stability can be evaluated through the High Temperature Operating Life (HTOL) test. The resistance drift rate of Hekotai's automotive-grade alloy resistors after a 1000-hour test at the rated power of 70℃ can be less than ±0.05%, which is superior to conventional thick-film resistors.

Key Design Points of Current Sampling Circuits

1. Resistor Selection and Parameter Calculation

• Resistance selection: A trade-off must be made between the sampling voltage amplitude and power consumption. The sampling voltage drop is usually controlled between 10mV and 100mV, with the resistance value calculated by the formula R ≈ V_sample / I_max.
• Precision and TCR: Allocate the error share of the resistor according to the overall system accuracy budget. Note that the nominal precision usually refers to the initial precision at 25℃, while the actual operating error is mainly determined by TCR and long-term drift.
• Package and heat dissipation: For high-current applications, large-area packages (such as 2512, 5931) should be prioritized, and sufficient heat dissipation copper foil should be designed on the PCB. Hekotai's HKT-AS59 series product documents provide recommended pad and heat dissipation copper foil layouts.

2. PCB Layout and Anti-Interference Design

• Kelvin connection (four-wire system): For milliohm-level resistors, the Kelvin connection must be used to eliminate the influence of parasitic resistance of the wiring between the sampling points and the resistor terminals. The Sense wiring should be fine, symmetrical and as short as possible.
• Minimize the sampling loop area: Place the resistor close to the sampling amplifier and keep the differential wiring parallel and adjacent to reduce magnetic field coupling interference.
• Noise filtering and shielding: An RC low-pass filter can be configured at the input of the sampling amplifier (note that the dynamic response should not be affected). If necessary, perform ground wrapping on sensitive sampling lines or place them in the inner layer of the PCB.

3. Software Compensation for Temperature Drift

To achieve higher precision, software compensation for the temperature drift of the resistor can be adopted, which requires the following steps:

1. Obtain accurate TCR data: The supplier should provide the TCR characteristic curve or data points of the resistor within the application temperature range. For example, Hekotai can provide customers with detailed TCR test data of key models.
2. Measure resistor temperature: Place a temperature sensor (such as NTC) near the resistor, or use its own TCR characteristics (if supported) for indirect temperature estimation.
3. Establish a compensation model: Implement a temperature-based resistance correction algorithm in the code: R_actual = R_25℃ × [1 + TCR × (T - 25)].

Selection Verification and Supplier Considerations

1. Key Verification Tests

In addition to conventional electrical performance tests, the following tests are recommended for automotive applications:

• Full-temperature precision test: Measure the resistance value at temperature points such as -40℃, 25℃, 85℃ and 125℃ to verify TCR consistency.
• Pulse load capacity test: Verify the resistor's ability to withstand short-term high currents (such as motor starting current) to ensure the operating point is within its Safe Operating Area (SOA).
• Environmental reliability test: Conduct temperature cycle, high temperature and high humidity, sulfur resistance and other tests in accordance with AEC-Q200 requirements, or review the supplier's complete test reports.

2. Key Supplier Evaluation Points

When selecting a supplier of automotive-grade resistors, the following aspects should be focused on:

• Quality system certification: Whether the supplier has passed the IATF 16949 certification and whether the production process has complete traceability.
• Product certification and data completeness: Whether the resistor has passed the AEC-Q200 certification, and whether the data sheet provides detailed TCR curves, power derating curves, long-term stability data, etc.
• Technical support capability: Whether the supplier can provide application-specific design support, failure analysis and reliable technical data. Based on its IATF16949 system and automotive-grade product line, Hekotai Electronics can provide corresponding technical documents and support.

Conclusion

In the design of automotive current sampling, the core advantage of using automotive-grade alloy resistors lies in their excellent temperature drift characteristics and long-term reliability. Effective design not only requires accurate calculation of basic parameters such as resistance and power, but also ensures sampling accuracy through optimized PCB layout, and considers combining software temperature compensation to further improve the full-temperature precision. When selecting a supplier, its quality system, completeness of product certification data and technical support capability are considerations equally important to the performance of the device itself.