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How to Decide Way to Sense Current

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Update time : 2021-09-13 11:08:44
High-precision current sensor is key to improve the efficiency of closed-loop control systems. 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 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. The pros and cons of different approaches to isolated current sensing are summarized in this article.
  • Shunt resistors are used in a wide range of industrial applications and offer relatively high accuracy at low-temperature drift. However, their use is limited by the power dissipation caused by their own resistance value. In applications with high common-mode voltages, shunt resistors require isolated amplifiers such as the AMC1200 or, for the highest-performing systems, an isolated delta-sigma modulator like the AMC1304L05. This device offers a low input voltage range of ±50mV allowing you to use smaller resistance shunts without compromising performance.
  • Rogowski coils measure alternating current (AC) only and are wrapped around a conductor that distributes the current to be sensed. They deliver a voltage proportional to the rate of change of the AC current and therefore require an integrator before being processed using an analog-to-digital converter (ADC). A voltage will be induced 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 coils use 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.
  • The current transformer (CTs) has three outstanding advantages: isolation from line voltage, lossless measurement of current, and 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, 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. CTs are widely used for sensing currents in power grids.
  • Magnetoresistive sensors change their resistance with the presence of a magnetic field, direct current (DC) or AC. Magnetoresistive sensors are small in size and are typically used for the position and angle sensing. They are cost-effective alternatives for low-current applications that don’t require high accuracy. Depending on the material used, there are two types of magnetoresistive sensors. Anisotropic magnetoresistance (AMR) sensors use ferromagnetic materials in which a magnetic field influences the electrical resistance. The resistance variation is very small; therefore, Wheatstone bridges are often used to sense it. Giant magnetoresistance (GMR) sensors rely on a significantly higher impact of the magnetic field on the resistance of a structure built of alternating ferromagnetic and nonmagnetic layers. Compared to AMR sensors, the production process is more complex and expensive.
  • When a current-carrying conductor is placed in a magnetic field, a potential difference is generated 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. 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 sensor 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 current, a large offset voltage is required, which will cause errors; easily affected by external magnetic fields; sensitive to ESD; high costs, etc.  
  • Fluxgate sensors deliver the highest level of sensitivity, widest dynamic range, and lowest noise and temperature-drift performance compared to other current-sensing methods. The design of an external fluxgate sensor is complex and requires low mechanical tolerances. HANGZHI has developed high-performance fluxgate analog current transducer and digital current transducer characterized by high accuracy, cost-effectiveness, low zero drift and low-temperature drift.

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