A 30-nΩ Accuracy Low Power Two-Step Ratiometric Shunt Resistance Measurement System Using a Switching Regulator- Based Current Generator for Shunt- Based Current Sensors
Shunt-based current sensors are widely used in battery management systems (BMSs) for automotive applications. Their current ranges are up to a few kA. Their low offset and low gain error enable accurate estimation of the batteries state-of-charge (SOC). The shunt-based current sensor uses a shunt resistor (RS) in the order of tens of μΩ to reduce Joule heating and cover the range of kA. It has a gain error of approximately 1% due to long-term drift of RS. Measuring RS with tens of nΩ (~0.2%) accuracy is necessary to calibrate the drift.
A ratiometric resistance measurement using a known-reference resistor (RREF1) can be used to measure the drift of resistance, where the test current is applied to RREF1 and RS, and the resistance ratio is measured to determine RS. Long-term stable reference resistors are commercially available only in values ≥ 10 Ω, and the resistance ratio becomes ≥ 400,000:1. The required dynamic range (DR) is ≥ 175.5 dB, which is not feasible.
The researchers propose a two-step ratiometric resistance measurement technique to relax the DR. The proposed system measures RS with a 10 mΩ RREF2 in the following two steps. First, the ratio of RREF1 to RREF2 is measured using a DC test current of 4.5 mA. Then, the ratio of RS to RREF2 is measured using a 1.0 Arms, 162.8 Hz AC test current. A lock-in detection technique reduces the influence of the thermal electromotive force of the RS. Keeping the resistance ratio ≤ 1000:1 in each step relaxes the required DR to 126.5 dB. Ratiometric measurement with the synchronous measurement using two analog front ends (AFEs) eliminates the drift and noise of the test currents.
In the proposed system, the 1.0 Arms, 162.8 Hz AC test current is required for the 25 µΩ RS measurement to obtain a sufficient signal. A switching regulator-based current generator (SRCG) is generally used to generate the AC test current efficiently. Here, the supply voltage is 5.0 V, and the output voltage is ~0.22 V, considering a parasitic resistance of ~220 mΩ.
Thus, a step-down ratio becomes 23:1, and an inrush current of 2.8 A flows from the power supply at a duty cycle of 6.2%. The SRCG consists of two stages to address this issue: a charge sampling stage and a switching regulator stage, which operate complementarily. The charge sampling stage operates at a duty cycle of 93.8% and gradually charges a reservoir capacitor. Therefore, it serves as a current buffer for the subsequent switching regulator stage, reducing the inrush current from 2.8 A to 123 mA. The two-stage SRCG operates with a 15.6 kHz carrier signal modulated at 162.8 Hz to reduce the size of the LC components.
The prototype system consists of an evaluation board with a prototype ASIC, RREF1, RREF2, a current generator, and an FPGA. The measured resistance 3σ error is ≤ 30 nΩ for 25 µΩ RS. The two-stage SRCG consumes 43 mA from the power supply when generating the 1.0 Arms AC test current. It achieves superior resistance measurement accuracy and low power consumption, making it suitable for automotive applications.