Parameters such as battery current and voltage are measured to monitor the battery in the vehicle. The electronics use these parameters to calculate the residual energy stored in the battery, among other variables. Based on this, a control unit can decide which loads may be switched on or must be switched off to ensure that vital assistance functions remain available.
To measure the current, an important component in any battery sensor is the sensing element, such as shunt resistors, which are the focus of this article.
The advantage of shunt resistors is that they provide a simple linear relationship between the measurand and the output signal. According to Ohm’s law, the voltage drop across the shunt is proportional to its constant resistance and the current flowing through it. This makes shunts suitable for DC and AC currents as well as for both current directions. The sensitivity to interference with respect to temperature is known from the data sheet and can be calculated if the actual temperature value is known.
One of the disadvantages is that the measurement signal of the shunt resistor – unlike current sensors based on the measurement of the magnetic field around a conductor – is at the potential of the current being measured. Therefore, current measurement in high-voltage systems still requires galvanic isolation of the measurement signal, which is an additional requirement. In addition, shunts dissipate power loss according to the formula Ptot = R·I2 and heat up and transfer the heat to their surroundings.
Types of shunt resistors
Shunt resistors come in a variety of designs depending on power and resistance; examples include thick film surface mount resistors (SMD) or metal surface mount resistors such as the Vishay Power Metal Strip resistor. Compared to thick film resistors, metal resistors offer a higher pulse load capability.
Sizes for surface mount resistors range from 0603 to 5931. The higher the currents, the more the design changes toward metal brackets with screw connectors. This is especially true in the range up to several hundred amps.
The sensing element is made of a special alloy with the smallest possible temperature coefficient which is welded to two connecting elements made of copper. The position of the sensing taps is important: They should be as close as possible to the sensing element with the smallest possible current path in the copper to ensure the copper path with its higher temperature coefficient does not nullify the Wrende element and falsify the measurement result.
Tandem shunt from Vishay
Automotive-qualified shunts with screw connectors and power loss up to 36 W have long been offered by Vishay. These are the WSBS and WSMS series. Available are the low-inductance power metal strip versions with resistance values and temperature coefficients from 50 µΩ and 10 ppm respectively. In addition, other values are also available and can be requested from Rutronik.
New to Vishay’s portfolio are the WSBE and WSBR series. The former extends the resistance range down to just 15 µΩ while maintaining a low temperature coefficient of up to ±10 ppm and a small thermoelectric voltage of up to 1.25 µV/°C. Since the power loss is proportional to the resistance of the shunt, the smaller resistance values of the WSBE shunts also allow higher currents to be measured.
The special feature of the new WSBR8518/8536 tandem shunt (Fig. 1) is its robust design with two independent measuring sections, which ensure high functional safety.
Verification can take place by comparing the voltage drops across the two independent measuring sections. In combination with appropriate evaluation electronics, applications with up to ASIL D classification can be achieved.
Evaluation electronics with Infineon’s Automotive PSoC 4 HVPA
This shunt configuration is particularly well suited to evaluation electronics with a two-channel analog front end, such as Infineon’s Automotive PSoC 4 HVPA (Fig. 2). Its two differential input pairs can be interconnected with the WSBR in such a way that they provide measurement results of the same magnitude but with reversed sign. This provides advantages in compensating for the offset.
The Automotive PSoC 4 HVPA has an internal voltage regulator (LDO) that can be connected directly to a 12V battery. Its analog front end (AFE) with high-resolution delta-sigma ADCs is particularly suitable for input signals in the millivolt range, such as those that drop at a shunt resistor. The Automotive PSoC 4 HVPA communicates with a higher-level control unit via a LIN interface. The LIN transceiver is already integrated on the chip. Developed in accordance with ISO 26262, the Automotive PSoC 4 HVPA 144K model meets the requirements for a Safety Element out of Context (SEooC) in accordance with ASIL B.
The combination of shunt and Automotive PSoC 4 HVPA can be tested with the evaluation board CYHVPA-128K-32-001 from Infineon (Fig.3).
Summary
The designs show: Vishay’s tandem shunt WSBR and Infineon’s Automotive PsoC 4 HVPA are an excellent pair for implementing a smart battery sensor.
For more information and a direct ordering option, please visit our e-commerce platform at www.rutronik24.com.
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