Analysis of the advantages and disadvantages of six current measurement methods

Current sensing is used to perform two basic circuit functions. The first is to measure the "how much" current flowing in the circuit. This information can be used for power management in the DC/DC power supply to determine the basic external load to save energy. The second function is to make a judgment when the current is "too large" or a fault occurs. If the current exceeds the safety limit and the software or hardware interlock conditions are met, a signal will be sent to turn off the device, such as a motor stall or a short circuit in the battery. Therefore, it is necessary to choose a robust design technology that can withstand the extreme conditions in the fault process. Using appropriate components to perform the measurement function can not only obtain an accurate voltage signal, but also prevent damage to the printed circuit board.
  Measurement methods
There are a variety of different measurement methods that can produce a signal indicating "how big" or "too big", as follows:
Resistive type (direct)
current sense resistor
Magnetic (indirect)
  Current Transformer
Rogowski coil
Hall effect device
Transistor (direct)
RDS(ON)
ratio
Each method has its advantages, it is an effective or acceptable current measurement method, but also has its own advantages and disadvantages, which is very important to the reliability of the application. These measurement methods can be divided into two categories: direct or indirect. The direct method means that it is directly connected to the circuit under test, and the measuring element will be affected by the line voltage. The measuring element of the indirect method is isolated from the line voltage. It is necessary to use the indirect method when the safety of the product is required.
resistance type
current sense resistor
Using resistance to measure current is a direct method, which has the advantages of simplicity and good linearity. The current-sense resistor and the measured current are placed in a circuit, and the current flowing through the resistor converts a small portion of electrical energy into heat. This energy conversion process produces a voltage signal. In addition to its ease of use and good linearity, the current-sense resistors are also very cost-effective, and the temperature coefficient (TCR) is stable, which can reach below 100 ppm/℃ or 0.01%/℃, and will not be affected by potential avalanche multiplication or thermal runaway Impact. In addition, the low-resistance (less than 1mΩ) metal alloy current-sense resistor has very good surge resistance, and can achieve reliable protection in the event of short-circuit and over-current conditions.
Magnetic
  Current Transformer
current transformer (Figure 1) has three outstanding advantages: isolation from line voltage, lossless measurement of current, large signal voltage can resist noise well. This indirect method of measuring current requires the use of changing currents, such as alternating current, transient current or switched direct current, to generate a changing magnetic field that is magnetically coupled to the secondary winding. 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 resistance loss when the circuit current passes through the copper winding is very small. However, as shown in Figure 2, due to the load resistance, core loss, and the existence of primary and secondary DC resistance, the loss of the transformer will cause a small amount of energy to be lost.


Figure 1, an ideal current transformer circuit


Figure 2, the composition of current transformer loss


Rogowski coil
Rogowski coil (Figure 3) is similar to a current transformer, it will induce a voltage in the secondary coil, and the magnitude of the voltage is proportional to the electrical flow through the isolation inductor. The special feature is that the Rogowski coil uses an air core design, which is completely different from a current transformer that relies on a high permeability iron core such as laminated steel and the magnetic coupling of the secondary winding. The air core design has a smaller inductance, a faster signal response and a very linear signal voltage. Because of this design, Rogowski coils are often used on existing wiring like hand-held meters to temporarily measure current, which can be considered as a low-cost alternative to current transformers.


  image 3


  Hall Effect
When a current-carrying conductor is placed in a magnetic field (Figure 4), there will be a potential difference perpendicular to the direction of the magnetic field and the current flow. This potential is proportional to the magnitude of the current. When there is no magnetic field and current flowing, there is no potential difference. But as shown in Figure 5, when there is a magnetic field and current flowing through, the charge interacts with the magnetic field, causing the current distribution to change, thus generating the Hall voltage.
The advantage of the Hall effect element is that it can measure large currents and has low power dissipation. However, this method also has many shortcomings, which limit its use, such as compensation for nonlinear temperature drift; limited bandwidth; when measuring a small range of current, a large offset voltage is required, which will cause errors; Affected by external magnetic field; sensitive to ESD; high cost.


Figure 4, Hall effect principle, no magnetic field


Figure 5, Hall effect principle, with magnetic field


Transistor
RDS(ON) -Drain to source on resistance
"Because the transistor is a standard control device for circuit design, it does not require resistance or energy-consuming devices to provide a control signal, so the transistor is considered to be an overcurrent detection method with no energy loss. The transistor data sheet shows the drain-to-source on-resistance (RDS(ON)). The typical resistance of a power MOSFET is generally in the milliohm range. This resistance is composed of several parts. The first is the lead connected to the semiconductor die (Figure 6). This part of the resistance affects a lot of channel characteristics. Based on this information, the current flowing through the MOSFET can be calculated using the formula ILoad = VRDS(ON) / RDS(ON).
Due to the small change in the resistance of the interface area and the TCR effect, each component of RDS(ON) will cause measurement errors. By measuring the temperature, and correcting the measured voltage with the expected change in resistance caused by the temperature, the TCR effect can be partially compensated. In many cases, the TCR of the MOSFET can be as high as 4000ppm/°C, which is equivalent to a temperature rise of 100°C, and the resistance change reaches 40%. Generally speaking, the signal accuracy of this measurement method is about 10% to 20%. From the application's requirements on accuracy, this accuracy range is acceptable for providing overvoltage protection.


Figure 6, Simplified model of N-channel enhancement mode MOSFET


Ratiometric-Current Sensing MOSFET
MOSFET is composed of thousands of transistor cells connected in parallel to reduce on-resistance. The current-sense MOSFET uses a small number of parallel cells connected to a common gate and drain, but the source is separated (Figure 7). This creates a second isolated transistor, the "detection" transistor. When the transistor is turned on, the current flowing through the detection transistor is proportional to the main current flowing through other cells.
The accuracy tolerance range depends on the specific transistor product, the low is up to 5%, and the high is up to 15% to 20%. This method is generally not suitable for current control applications that generally require a measurement accuracy of 1%, but it is suitable for over-current and short-circuit protection.


Figure 7 

As can be seen from the above summary table, there are many ways to detect the current in the circuit, and the appropriate method should be selected according to the specific needs of the application. Each method has its advantages and shortcomings, and these factors must be carefully considered in the design.
Author: Bryan Yarborough
Bryan Yarborough is currently the product marketing engineer of Vishay Intertechnology's Vishay Dale brand, specializing in SMD Power Metal Strip Strip? and wire-wound products. He previously worked at TT Electronics IRC, Saft Batteries and Corning Cable Systems. Mr. Yarborough holds a Bachelor of Science in Computer Science and a Master of Business Administration.