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/o C or 0.01 %/o C, 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.
The Rogowski coil (Figure 3) is similar to a current transformer in that a voltage is induced into a secondary coil that is proportional to the current flow through an isolated conductor. The exception is that the Rogowski coil is an air core design, as opposed to the current transformer that relies upon a high-permeability core, such as laminated steel, to magnetically couple to a secondary winding. The air core design has a lower inductance, providing a faster signal response and very linear signal voltage. Because of its design, it is often used as a temporary current measurement method on existing wiring such as a handheld meter. This could be considered a lower-cost alternate to the current transformer.
When a current-carrying conductor is placed in a magnetic field (Figure 4), a difference in potential occurs perpendicular to the magnetic field and the direction of current flow. This potential is proportional to the magnitude of the current flow. When there is no magnetic field and current flow exists, then there is no difference in potential. However, as shown in Figure 5, when a magnetic field and current flow exist, the charges interact with the magnetic field and cause the current distribution to change, which creates the Hall voltage.
The advantage of Hall effect devices is that they are capable of measuring large currents with low power dissipation. However, there are numerous drawbacks that can limit their use, such as non-linear temperature drift requiring compensation; limited bandwidth; low-range current detection requiring a large offset voltage, which can lead to error; susceptibility to external magnetic fields; ESD sensitivity; and high cost.
RDS(ON) – Drain-to-Source On-Resistance
Transistors are considered a lossless overcurrent detection method since they are standard control components to the circuit design and no further resistance or power-dissipating devices are required to provide a control signal. Transistor datasheets provide the on-resistance for the drain-to-source (RDS(ON)) with a typical resistance in the mΩ range for power MOSFETs. This resistance consists of several components that begin with the leads (Figure 6) connecting to the semiconductor die through the resistance that makes up the numerous channel characteristics. Based on this information, the current passing through the MOSFET can be determined by ILoad = VRDS(ON) / RDS(ON) .
Each constituent of the RDS(ON) contributes to measurement error that is due to minor variations in the resistances of the interface regions and TCR effects. The TCR effects can be partially compensated for by measuring temperature and correcting the measured voltage with anticipated changes in resistance due to temperature. Often times the TCR for MOSFETs can be as large as 4000 ppm/o C, which is equivalent to a 40 % change in resistance for a 100 o C rise. Generally, this measurement method provides a signal with approximately 10 % to 20 % accuracy. Depending on the accuracy requirements, this may be an acceptable range for providing overcurrent protection.
Ratio Metric – Current Sense MOSFETs
The MOSFET consists of thousands of parallel transistor cells that reduce the on-resistance. The current sensing MOSFET uses a small portion of the parallel cells and connects to the common gate and drain, but a separate source (Figure 7). This creates a second isolated transistor; a “sense” transistor. When the transistor is turned on, the current through the sense transistor will be a ratio comparable to the main current through the other cells.
Depending on the transistor product, the accuracy tolerance range can vary from as low as 5 % or as wide as 15 % to 20 %. This is generally not suitable for current control applications that typically require 1 % measurement accuracy, but is intended for overcurrent and short-circuit protection.
The summary table above shows that there are a variety of methods for detecting current in a circuit; the method selected will depend on the specific needs of your application. Each has its own advantages and tradeoffs that must be considered in your design.