This article explores how to select current-sense resistors for power management systems.
Current-sense resistors are precision components that produce a voltage drop proportional to the current passing through them. Since this voltage drop is typically in the millivolt range, it demands precise circuit design and sophisticated measurement techniques.
Although other current sensing techniques exist, the resistive-sensing approach offers several advantages. The linear relationship between current and voltage across the resistor simplifies measurement and interpretation of results. Current-sense resistor implementation is relatively straightforward and, therefore, cost-effective.
Applications in Power Electronics
Accurate current measurement is essential in power applications for several reasons. It enables overcurrent protection, helps maximise system efficiency, and ensures proper load distribution.
In battery management systems, precise current monitoring affects charging efficiency and lifespan. For motor control applications, accurate current sensing is crucial for maintaining torque control and preventing thermal damage. Hence, current-sense resistor applications include:
- Power supplies and converters.
- Battery management systems (BMS).
- Motor control applications.
- Renewable energy systems, including solar inverters and wind power monitors.
- Pulse protection circuits and overcurrent detection.
- Industrial power monitoring
- LED lighting.
Current-Sense Resistor Selection
The choice of resistance value in current-sensing applications depends on balancing measurement accuracy and power efficiency. High-current applications favour low-value resistors (milliohms or lower) to minimise power loss, while low-current applications can tolerate higher resistance values for better measurement accuracy.
Since power dissipation in a resistor is given by P = I²R, higher resistance values increase power dissipation. This can lead to thermal issues, requiring larger resistors or heat-sinking solutions. A lower resistance value reduces power loss but may generate a voltage too small for accurate measurement.
Choosing a resistor value is often a compromise. To minimise the impact of circuit and system noise on the measured voltage, the resistor value should be high. For a given current, the voltage drop will then be large, reducing the impact of noise. However, if the resistor value is large, the power loss will increase, causing resistor self-heating. This, in turn, causes the resistor to drift from its nominal value.
The resistance of a current-sense resistor varies with temperature. A higher temperature coefficient of resistance (TCR) means small temperature variations can cause significant resistance changes, leading to inaccurate measurements.
For high-current applications, selecting a low-resistance, low-TCR resistor is essential to maintain efficiency and precision. The lower the resistance, the higher the required resistor material metal content. This, in turn, has an adverse effect on the TCR.
In all applications, resistor tolerance is an important consideration. The higher the tolerance, the higher the measurement error. When selecting resistors, engineers must balance tolerance with cost and application requirements. For power applications, a 1% resistor tolerance is usually acceptable, but this depends on the specific application.
Resistor value, power rating, and tolerance all impact, to varying degrees, the resistor package size. In most applications, the designer should select the smallest possible resistor package. This must be balanced against application demands, including ambient temperature, potential mechanical damage, ease of manufacture, and long-term reliability.
Kelvin (4-wire) vs 2-wire Sensing
In very low-resistance current-sense resistor applications (typically <10 mΩ), it is important to consider the resistance and temperature dependence of solder joints and system board tracks. Combined, these can have a higher resistance than the sense resistor device itself.
Kelvin resistors employ separate paths for current flow (power connections) and voltage measurement (sense connections). The voltage measurement wires connect directly to the resistor terminals, avoiding parasitic resistance from PCB traces and solder joints.
Kelvin resistors are relatively expensive. If extreme precision is not required, a 2-terminal current-sense resistor is a more economical choice.
Resistor Manufacturing Technologies
For power applications, thick-film resistor technology is a common choice. This technology is robust, has good power-handling capacity, and is available in small package sizes. However, achieving both a low resistance value (<50MiliOhms) and an acceptable TCR can be a challenge. For very high-power applications, wirewound resistors may be a better option, albeit at the expense of larger package sizes and higher costs.
Thin-film resistor technology is generally unsuitable for power applications. Foil resistors offer good all-around performance but at a relatively high cost.
A variety of standard power resistors are available for current-sensing applications. When a standard resistor device fails to match specifications, custom resistor manufacturers can often offer suitable solutions.