Power supply designers must minimize the effects of overcurrent and short-circuit conditions. A common and relatively low-cost solution is to combine a current sense resistor with a suitable measurement circuit to detect an overcurrent event.
The precision measurement circuit measures the voltage (in millivolts) across the resistor that is connected in series with the load. The current flowing through the sense resistor is calculated based on the voltage drop across the resistor.
In this article, we will focus on sense resistors in power supply applications. These include power supplies, motors, and high-power switches.
Select the current sense resistor
When selecting a current sense resistor, it is important to consider:
Rated power. Resistance tolerance.
Temperature coefficient of resistance (TCR).
Resistor package size.
Detect the limitations of the amplifier circuit.
The choice of rated power resistor is usually a compromise. In order to minimize the effect of circuit and system noise on the measured voltage, the resistance value should be relatively high. For a given current, the voltage drop will be large and the effect of noise will be reduced.
However, if the resistance value is large, the power loss will also increase, causing the resistance to self-heat. This, in turn, causes the resistor to deviate from its nominal value.
The sense resistor value is typically <1 ohm to minimize power loss and heat generation. This means that the resistor must have a consistently high current-carrying capacity. It must also be able to withstand high current pulses.
Resistance Tolerance Voltage measurement accuracy is directly related to resistance tolerance. Current sense resistors tend to have tolerances of 1% or less.
The average voltage of the TCR input terminals gives the voltage drop across the resistor. To minimize errors due to temperature sense resistance drift, resistors tend to have TCR values below 100 ppm °C.
The low resistance values of the sense resistors (see above) present design and manufacturing challenges. The lower the resistance, the higher the metal content of the resistive material required. This, in turn, adversely affects TCR.
Package size In most applications, designers should choose the smallest possible resistor package. This requirement must be balanced with the needs of the application. These factors include ambient temperature, potential mechanical damage, and the effects of resistor self-heating. Ease of manufacture and long-term reliability are also key considerations.
Detection Circuit Limitations The resistance of the current sense resistor must be low, but this can lead to significant measurement errors. At low resistance values, it is important to consider the resistance and temperature dependence of the solder joints and system board traces. Together, these devices can have a higher resistance than sense resistor devices.
The TCR of the system board traces can be higher than the TCR of the sense resistor. This introduces some temperature dependence to the measurement of the sense resistor device.
Kelvin resistors can solve this problem. It is a 4-terminal device with two terminals for the current measurement current (sensing element) and two independent terminals for the main current. However, Kelvin resistors are comparatively expensive. If extreme accuracy is not required, a 2-terminal current-sense resistor is a more economical option.
A variety of standard power resistors are available for current sensing applications. When standard resistor devices are not up to specification, custom resistor manufacturers can provide a solution.