When designing power supplies it is important to minimise the potential impact of overcurrent and short circuit conditions. Current sense resistors, combined with suitable measuring circuitry, detects overcurrent events. In this post, we concentrate on sense resistors in power applications, including power supplies, motors and high power switching.
Current sense resistors work by detecting and converting current to voltage. The current flow through the resistor is calculated based on the voltage drop across the resistor. Measured in millivolts, the voltage drop is detected using precision amplifier circuits.
Choosing A Current Sensing Resistor
When current sense resistors it is important to consider:
- Power rating.
- Limitations of the sensing amplifier circuit.
- Resistor tolerance.
- Temperature Coefficient of Resistance (TCR).
- Resistor package size.
To minimise the impact of circuit and system noise on the sensed voltage the resistor value should be relatively high. For a given current the voltage drop will then be relatively large and the impact of noise reduced. But if the resistor value is large the power loss will also increase and cause resistor self-heating. This, in turn, will cause the resistor to drift from its nominal value.
The average voltage present at the input terminals gives the voltage drop across the resistor. Given the voltage, it is important to choose a low sense resistor value (typically <1 Ohm) to minimise power loss and heat generation.
The accuracy of the voltage measurement is directly related to the tolerance of the resistor. Current sense resistors tend to have a tolerance of one per cent or less. To minimise errors due to drift in resistance with temperature sense resistors typically have a TCR value below 100 ppm/°C.
For most applications, the designer should select the smallest possible resistor package. This requirement must be balanced against the demands of the application. These include ambient temperature, potential mechanical damage, and the impact of resistor self-heating. Ease of manufacture and long term reliability are also key considerations.
Resistor Manufacturing Issues
As stated above, to minimise power loss and heat generation the resistance of the current sense resistor must be low. This means the resistor must have a sustained high current carrying capability. It must also be able to withstand high current pulses.
Manufacturing a thick film power resistor for current sensing applications presents many challenges including:
- Very low resistance values.
- High current carrying capability and load life stability.
- Low TCR.
- Tight tolerance values.
- Non-inductive non-capacitive.
- A High temperature rating for reliability
Optimising materials and manufacturing for one factor can have a negative impact on others. For example, the lower the resistance the higher the required metal content. This, in turn, has an adverse effect on the TCR. A special low Ohmic value thick film material may be ± 300 ppm/⁰C, and a pure conductor could be as high as ± 3,000 ppm/⁰C. The resistor design and manufacturing process often involves compromises.
Current Sense Resistors vs Kelvin Resistors
As discussed above, the resistance of the current sense resistor must be minimised, but this can lead to serious measurement errors. At low resistor values, 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.
The TCR of the system board tracks can be higher than that of the sense resistor. This introduces some temperature dependence into measurements made across the sense resistor device.
A Kelvin resistor can resolve this issue. It is a 4-terminal device that employs two terminals for the current measurement flow (the sensing element) and two independent terminals for the main current flow. However, Kelvin resistors are relatively expensive. Unless extreme precision is required, a 2-terminal current sense resistor is a more economical choice.
A variety of standard power resistors are available for current sensing applications. If a standard device fails to match the specification, a custom resistor manufacturer can offer solutions.