Automotive Shunt-Based Isolated Current Sensor Reference Design

August 01, 2019 by Paul Shepard

The TIDA-020018 reference design from Texas Instruments (TI) describes high-side primary current sensing for dc-dc and onboard charger (OBC) applications. Although an isolated shunt approach has advantages such as high accuracy, linearity, and robustness, the shunt still experiences higher power dissipation because of the larger input range of isolated amplifiers.

This reference design helps solve the problem with a lower input range isolated amplifier, resulting in lower power dissipation. All advantages of the shunt plus the benefit of lower power dissipation can be achieved. The image above shows the block diagram of the TIDA-020018 reference design.

OBCs and dc-dc converters need high-side current sensing on the high voltage battery side for accurate monitoring of charging and discharging currents and for control loop operations. To measure high-side current on a high-voltage battery (400V, 600V, 800V) there must be an isolation presence because of higher common-mode voltages. In general, isolation is provided through magnetic isolation or reinforced isolation.

TIDA-020018 Automotive shunt-based isolated current sensor reference design for DC/DC and OBC applications board

According to TI, instead of using expensive magnetics, LEM modules, and getting severe non-linearities because of the temperature effects on the magnet, a shunt approach has several advantages such as high linearity, high accuracy, robustness, and so forth.

However, the shunt presents issues with higher power dissipation. Higher power dissipation is due to the larger values of the shunt resistor. Larger values of the shunt resistor were required because of the limit of the front-end stages of the isolated amplifier portfolio in the market.

This design addresses the problem with lower input range analog front-end isolated amplifiers. With the lower input voltage front-ends, it is possible to get the lower shunt values, resulting in lower power dissipation. This design shows the isolated-shunt-based approach for dc-dc and OBC applications for current sensing.

The power levels of conventional dc-dc converters in automotive applications is 3kW, and they need about 10A at full operational mode. Similarly, OBC converters operate at 6.6kW, and current of approximately 32A flows. The input range of isolated amplifier chosen is 50mV, meaning shunt values are designed in such a way that there is 5mOhm for the dc-dc application and 1.5mOhm for the OBC application. This implies lower power dissipation.

The ac or dc current flows through the shunt, resulting in a linear voltage drop across the shunt which appears on the front-end terminals of the isolated amplifier. which is amplified and converted into single ended stage and given to the ADC for further signal processing. In dc-dc converters and OBCs, analog and digital control loop must determine faults at a faster rate in charging and discharging states.

Lower delay at the isolation barrier helps the control loop to react at a faster rate on several kind of faults. The AMC1302-Q1 provides a less than 3-us delay.

The AMC1302 is a precision isolated amplifier with a capacitive isolation barrier that has high immunity to magnetic interference. Conventional isolated amplifiers in the market have the larger input range which makes the selection of shunt value higher, implying higher power dissipation. TI's AMC1302-Q1 with the ±50mV input voltage range allows significant reduction of the power dissipation through the shunt.

The input stage of the AMC1302 is optimized for direct connection to shunt resistors. Additionally, the low high-side supply current and voltage of the AMC1302 allow use of low-cost isolated power-supply solutions. The integrated input common-mode overvoltage and missing high-side supply voltage detection features of the AMC1302 simplify system-level diagnostics.