Current Sense Resistors
- Kevin Li
Introduction
Current-sense resistors are cost-effective components that improve efficiency and reduce losses due to their high measurement accuracy. Current sense resistors are particularly useful in precisely measuring current in the following types of designs:
Automotive
Industrial
Computer Electronics
Listed below are the key parameters of current-sense resistors with summarized descriptions:
Resistance: The resistance of current-sense resistors are very low so that they yield insignificant voltage drops and low power dissipation. The selection of these devices should be made based on the maximum differential voltage desired at the highest expected current and the need to satisfy a power-loss budget if necessary.
Wattage: The wattage of current-sense resistors describe the maximum power rating they can handle.
Tolerance: This value describes the deviation in the nominal resistance of the current-sense resistor. These devices typically have very low tolerances to achieve high accuracy measurements.
Temperature Coefficient of Resistance (TCR): This value defines the sensitivity of the resistance to varying ambient temperature. Current-sense resistors typically have low TCRs to decrease temperature dependencies for high accuracy measurements.
To sense current, a shunt resistor is placed in series with the electrical load of current sensing interest. In this way, they are directly able to measure current. By Ohm’s law, a voltage drop is produced across the resistor and can be quantified by:
Kelvin Principle Measurement
The contact resistances of the solder pads and the traces of the PCB for very low resistance 2-terminal SMT resistors are often unknown and usually higher than the resistance of the current sense shunt resistor itself. It is very clear that these resistances can create measurement inaccuracies.
Additionally, the TCR of PCB copper traces are much higher than the TCR of the shunt resistive element (~3900ppm/°C versus <50ppm/°C). This can cause the resistive element to be much more temperature dependent than intended to be.
To mitigate these effects, designers use current sense resistors that implement the 4-wire Kelvin principle. It uses additional leads for current measurement independently from the main current flow.
A 2-terminal resistor can also implement the 4-wire Kelvin method. By separating the current path through the resistor, directly sensing the voltage drop across the resistor improves the measurement accuracy. Unless extreme precision is required, the 2-terminal resistor is the more economical choice.
PCB Layout of the Current-Sense Resistor
To achieve accurate current measurements, there must be four connections to the current-sense resistor. Two connections should handle the current flow while the other two sense the voltage drop across the resistor. One of the most common mistakes in doing layouts for current-sense resistors is connecting the current-sense amplifier inputs to the current-carrying trace instead of directly to the pads of the current-sense resistor.
Some current-sense resistors feature independent four-wire (Kelvin) connections. This is commonly used when the value of the shunt resistor is below 0.5mΩ and the solder resistance in series with the resistor connections significantly adds to the overall shunt resistance.
It is difficult to know which layout technique will provide the best results. This is because the resistance accuracy depends greatly on the measurement location used when the resistor was manufactured. This location is typically unspecified and datasheets often lack layout recommendations.