Current sensing is used to perform two essential circuit functions. First, it measures “how much” current is flowing in a circuit; information that may be used for power management in a DC/DC power supply to determine essential peripheral loads to conserve power. The second function is to determine when there is “too much” current, or a fault condition. If current exceeds safe limits, a software or hardware interlock condition is met and a signal is sent to turn off the application, as in a motor stall or short-circuit condition in a battery. It is essential to choose a technology with a robust design capable of withstanding the extreme conditions that exist during a fault. The appropriate component performing the measurement function will sustain an accurate voltage signal as well as prevent damage to the printed circuit board.
A signal to indicate the “how much” condition and the “too much” condition is available in a variety of different measurement methods, as listed below:
Each has advantages that make it an effective or acceptable method for current measurement, but also has tradeoffs that can be critical to the reliability of the application. They can also be classified into two main categories of measurement methods: direct or indirect. The direct method means that it is connected directly in the circuit being measured and that the measurement components are exposed to the line voltage, whereas the indirect method provides isolation that may be necessary for design safety.
Current Sense Resistor
The resistor is a direct method of current measurement that has the benefit of simplicity and linearity. The current sense resistor is placed in-line with the current being measured and the resulting current flow causes a small amount of power to be converted into heat. This power conversion is what provides the voltage signal. In addition to the favorable characteristics of simplicity and linearity, the current sense resistor is a cost-effective solution with a stable temperature coefficient of resistance (TCR) of
< 100 ppm/oC or 0.01 %/oC, and does not suffer the potential of avalanche multiplication or thermal runaway. Furthermore, low-resistance (< 1 mΩ is available) metal alloy current sense products offer superior surge performance for reliable protection during short-circuit and overcurrent events.
A current transformer (Figure 1) has three key advantages: it provides isolation from line voltages, lossless current measurement, and a large signal voltage that can provide noise immunity. This indirect current measurement method requires a changing current — such as an AC, transient current, or switched DC — to provide a changing magnetic field that is magnetically coupled into the secondary windings. The secondary measurement voltage can be scaled according to the turns ratio between the primary and secondary windings. This measurement method is considered “lossless” because the circuit current passes through the copper windings with very little resistive losses. However, as shown in Figure 2, a small amount of power is lost due to transformer losses from the burden resistor, core losses, and primary and secondary DC resistance.
Current transformer loss components