A current sensor is a device that senses the current being measured. Small power supply devices have incorporated an increasing number of new technologies, such as switching power supplies, hard switching, soft switching, voltage regulation, linear feedback voltage regulation, magnetic amplifier technology, digitally controlled voltage regulation, PWM, SPWM, and electromagnetic compatibility. Practical needs directly drive the continuous development and progress of power supply technology. To automatically detect and display current, and to provide automatic protection and more advanced intelligent control in the event of overcurrent, overvoltage, or other dangerous conditions, power supply technologies with sensing detection, sensing sampling, and sensing protection are becoming increasingly popular. Sensors for detecting current or voltage have emerged and are gradually gaining favor among power supply designers in my country. A current sensor is a detection device that can sense the current being measured and transform that information into an electrical signal or other required form of information output that meets certain standards, satisfying requirements for information transmission, processing, storage, display, recording, and control. Current sensors, also known as magnetic sensors, are used in many applications, including household appliances, smart grids, electric vehicles, and wind power generation. Magnetic sensors are widely used in our daily lives, such as in computer hard drives, compasses, and other household appliances.
Amidst countless hopes and endless dreams, humanity has entered the 21st century. On this beautiful and bountiful planet, in pursuit of survival and development, humankind has built monuments to civilization through its diligence and wisdom. However, while humanity revels in its glorious achievements, our home has been shrouded in shadow.
As a high-energy-consuming product in daily life, automobiles generate a large amount of carbon dioxide every day. The advent of electric vehicles has solved the problem of high energy consumption of ordinary gasoline vehicles. Charging stations are the most important peripheral equipment for electric vehicles, providing them with a continuous power supply. Current sensors are the most important core detection components in electric vehicle charging stations, ensuring that the amount of electricity delivered to electric vehicles by the charging station is accurately measured.
2. Principle of Current Sensor
1) Principle of Hall effect closed-loop current sensor
The Hall current sensor is based on the magnetic balance Hall principle, also known as the closed-loop principle. When the magnetic flux generated by the primary current IP is concentrated in the magnetic circuit through a high-quality magnetic core, the Hall element, fixed in the air gap, detects the magnetic flux. A multi-turn coil wound on the magnetic core outputs a reverse compensation current to cancel out the magnetic flux generated by the primary current IP, ensuring that the magnetic flux in the magnetic circuit remains zero. Through special circuit processing, the sensor's output can accurately reflect the current change of the primary current.
2) Characteristics of Hall effect sensors
Regardless of whether a Hall sensor operates on an open-loop or closed-loop principle, the basic performance differences are minimal, with the fundamental advantages being:
A. Fast response time; B. Low temperature drift; C. High measurement accuracy; D. Strong overload capacity; E. Wide measurement bandwidth; F. Strong anti-interference capability.
3. Basic parameters of current sensors
There are many domestic manufacturers of Hall current sensors. The underlying principles are the same; the differences lie in the selection of components, manufacturing processes, and process control during production. Taking the JCE308-TS7 current sensor, commonly used in the charging pile industry, as an example, it has been tested and certified by the China Aerospace 513 Research Institute. Its basic technical parameters are as follows:
Main technical parameters:
Model: JCE308-TS7
Rated measuring current IPN: 300A
Measurement range (IP): 0~±500 A
Rated measurement output IM: 150mA
Power supply voltage (±5%): ±12V~±15V
Conversion ratio KN: 1:2000
Current consumption IC: ≤28 (@±12 V) + 15 mA (output measurement current)
Precision X: — Accuracy XG (@ VPN, TA=+25℃): ±0.6%
—Nonlinearity εL (@ VPN, TA = +25℃): <0.1%
—Zero-point offset current IO: ≤±0.15mA (@+25℃)
—Zero-point temperature offset (IOT): ≤±0.44mA (@-10℃~+70℃)
—Response time tr (@90% of VP max): ≤1us
—Operating frequency f: DC-100KHz
Withstand voltage Vd: Withstand voltage between primary and secondary circuits: 6kV/50Hz/1min
Operating temperature (TA): -40°C to +85°C
Storage temperature TS: -40°C ~ +85°C
Secondary side internal resistance RS: 21Ω (TA=70℃)
4. Issues to consider when selecting a current sensor
1) When selecting sensors, pay attention to the operating temperature of the product. Industrial-grade sensors cannot meet the basic requirements; the operating temperature must be between -40°C and 85°C.
2) The altitude requirement is above 3500 meters;
3) When selecting a sensor, the measurement range can be slightly larger to allow for some overload tolerance. Note that closed-loop current sensors should not be subjected to prolonged overload measurements.
4) When selecting terminals, railway-specific shock-resistant terminals should be used as much as possible to prevent them from falling off due to strong vibrations during transport or operation of the frequency converter;
5) Do not use the sensor in humid conditions to avoid condensation or moisture causing the product to malfunction.
Measurement principle of AC zero flux current sensor (winding detection type)
The AC zero flux method improves the low-frequency characteristics of the CT method by using a current transformer (winding detection type).
Measurement principle:
● In order to eliminate the magnetic flux Φ generated inside the core caused by the alternating current flowing through the measuring conductor (primary side), a secondary current corresponding to the turns ratio of the feedback winding on the secondary side flows through it.
● Magnetic flux in the low-frequency band cannot be completely eliminated, leaving residual magnetic flux.
● The residual magnetic flux is detected by the detection winding, and a secondary current flows through it to eliminate the magnetic flux Φ via the AMP circuit.
● The current flowing through the shunt resistor generates a voltage across the shunt resistor.
● This voltage is the output voltage, which is proportional to the current flowing through the measuring conductor.
Features compared to other methods:
● Because it is a negative feedback operation that eliminates the magnetic flux inside the core, it is not affected by the BH characteristics of the core and has excellent linearity.
● The phase error is also very small at low frequencies, making it suitable for power measurement.
● The operating magnetic flux level is low, therefore the insertion impedance is low.
● It operates as a CT scanner in the high-frequency band, thereby achieving wide bandwidth.
● The detection is performed using windings, therefore it is limited to measuring alternating current and cannot measure direct current.
