A power supply designer must minimise the impact of overcurrent and short circuit conditions. A common and relatively low-cost solution is to combine current sense resistors with suitable measuring circuitry to detect overcurrent events.
A precision measuring circuit measures voltage (measured in millivolts) across a resistor in series with the load. The current flow through the sense resistor is calculated based on the voltage drop across the resistor.
In this post, we concentrate on sense resistors in power applications. These include power supplies, motors and high-power switching.
Choosing A Current Sensing Resistor
When choosing current sense resistors, it is important to consider:
- Power rating.
- Resistor tolerance.
- Temperature Coefficient of Resistance (TCR).
- Resistor package size.
- The limitations of sense amplifier circuits.
POWER RATING – The choice of a resistor is often a compromise. To minimise the impact of circuit and system noise on the measured voltage the resistor value should be relatively high. For a given current the voltage drop will then be large and the impact of noise reduced.
However, 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.
Sense resistor values are typically <1 Ohm to minimise power loss and heat generation. This means the resistor must have a sustained high current-carrying capability. It must also be able to withstand high current pulses.
RESISTOR TOLERANCE – Voltage measurement accuracy is directly related to the resistor tolerance. Current sense resistors tend to have a tolerance of one per cent or less.
TCR – The average voltage at the input terminals gives the voltage drop across the resistor. To minimise errors due to drift in resistance with temperature sense resistors tend to have a TCR value below 100 ppm/°C.
The low resistance value (see above) of sense resistors presents design and manufacturing challenges. The lower the resistance, the higher the required resistor material metal content. This, in turn, has an adverse effect on the TCR.
PACKAGE SIZE – In 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.
SENSE CIRCUIT LIMITATIONS – The resistance of the current sense resistor must be low, 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 with 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. If extreme precision is not required, a 2-terminal current sense resistor is a more economical choice.
A variety of standard power resistors are available for current sensing applications. When a standard resistor device fails to match the specification, a custom resistor manufacturer can offer solutions.