Current sensing is a critical component in the evaluation, control, and diagnosis of electronic systems. Accurate current measurement is essential when designing, testing, and maintaining electronic equipment. This article will delve into the fundamental principles of current sensing and the details that are often overlooked in practical operation.
Basic principle of current detection
The basic principle of current detection is mainly based on two methods: electromagnetic induction and resistance sampling.
1. Electromagnetic induction method
Electromagnetic induction utilizes Faraday's law of electromagnetic induction, which states that a changing magnetic field will induce an electromotive force (EMF) in a closed coil. Current transformers are a typical application of this method. When a current being measured passes through a conductor, it generates a magnetic field in the surrounding area. The magnetic core in the current transformer concentrates this magnetic field. When the current changes, the magnetic field also changes, thus inducing an EMF in the coil of the current transformer. This induced EMF is related to the magnitude and rate of change of the measured current. By measuring the induced EMF and performing appropriate conversions, information about the measured current can be obtained.
Another electromagnetic induction method uses a current probe. The magnetic core in the current probe concentrates the magnetic field generated by the current being measured. The changing magnetic field induces an electromotive force (EMF) in a coil on the core. By measuring the induced EMF, the magnitude of the current being measured can be determined. For direct current (DC) measurement, some current probes employ the Hall effect principle. A Hall element is placed in a magnetic field. When current flows through it, a Hall voltage is generated in a direction perpendicular to both the current and the magnetic field. This Hall voltage is proportional to the magnetic field strength generated by the DC current being measured, thus allowing the measurement of the DC current.
2. Resistance sampling method
Resistance sampling utilizes Ohm's law, which states that the voltage across a conductor is proportional to the current flowing through it. A low-resistance precision resistor (often called a shunt) is placed in the current path; the voltage across the resistor is proportional to the current flowing through it. Because the shunt resistor should not significantly affect the current, its resistance is typically very small, in the milliohm range or lower. Although the voltage across the shunt resistor is small, it can be amplified and measured, or sampled directly using a high-precision analog-to-digital converter (ADC).
Easily overlooked details
In actual current detection, there are several details that are often overlooked, but they are crucial to measurement accuracy and system reliability.
1. Measuring range selection and accuracy
The selection of the measurement range directly affects the measurement accuracy. Setting the range too large will reduce detection accuracy; setting it too small will easily trigger overcurrent protection. Therefore, the upper limit of the range needs to be set according to the peak value of the measured current, typically around twice the peak value. Furthermore, selecting an appropriate AD input voltage range and improving AD resolution are also crucial to ensuring sampling accuracy.
2. Op-amp problem
In current sensing, operational amplifiers (op-amps) are crucial for signal processing. However, op-amps introduce zero drift and temperature-related issues. Zero drift manifests as a small voltage remaining at the op-amp's output even when there is no input, affecting measurement accuracy. Temperature-related issues arise because the op-amp's zero drift varies with temperature, necessitating calibration at different temperatures.
3. Electrical isolation
Electrical isolation is a crucial means of ensuring equipment safety and improving circuit immunity. Hall effect sensors, current transformers, Rogowski coil methods, and TMR current sensor methods possess inherent isolation characteristics. For resistance sampling methods, isolation operational amplifiers or modulators are required for electrical isolation. Common isolation methods include linear optocoupler isolation, isolation via sampling chips, and isolation via isolation amplifiers.
4. Sampling delay and interference
There are multiple delay sources between current sampling and the driving switch, including PWM dead time insertion, optocoupler isolation delay, switching delay, amplifier delay, low-pass filter delay, and ADC delay. These delays, when combined, shorten the sampling time window, affecting the accuracy of current sampling. Furthermore, magnetic field interference and operational amplifier differential-mode and common-mode interference are also significant factors affecting current detection accuracy.
5. Special considerations for resistance sampling
Although resistance sampling is low-cost and unaffected by magnetic fields, the following points should be noted in practical applications:
The sampling resistor must be small enough (generally no more than 1 ohm), the accuracy must be high enough (generally within 1%), and the temperature drift must be small enough.
A non-inductive resistor should be selected to avoid voltage drop caused by inductive reactance.
After soldering, the resistance value of the sampling resistor needs to be remeasured. Kelvin connection is usually used to reduce the influence of lead resistance.
6. Differences between three-phase and two-phase testing
In a three-phase system, the sampling window of a phase can be determined by sector, and the current of that phase with the smaller sampling window can be recalculated by using the relationship that the sum of the three-phase currents is zero. However, two-phase detection must directly use the collected current value, so the sampling window must be large enough to avoid failing to detect phase loss faults.
in conclusion
Current sensing is a crucial aspect of electronic system design and maintenance, primarily based on electromagnetic induction and resistance sampling. In practice, attention must be paid to details such as range selection and accuracy, operational amplifier issues, electrical isolation, sampling delay and interference, and special considerations for resistance sampling. Only by fully considering these details can the accuracy of current sensing and the reliability of the system be ensured. With the continuous development of electronic technology, current sensing methods and techniques will be constantly optimized and expanded, providing stronger support for a wider range of electrical and electronic applications.
