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The principle and application of current sensors

Issuing time:2022-11-15 11:35

With the development of power electronics technology, the existing components for detecting and measuring cur𝐆rent can no longer m⛄eet the transmission requirements of medium, high frequency, high di/dt, and wide spectrum current waveforms. The two Hall current sensor is now the main current detection component that fills this gap.

1、 Hall effect

The chip semiconductor shown has a pair of current terminals and a pair of voltage terminals. When the constant DC current I is connected from the current terminal and the Hall element is placed in a magnetic field, the charge in the Hall element will be affected by Lorentz force, and the positive and negative charges will be shifted to both sides of the voltage terminal, so an electric field will be formed inside the Hall element. When th♎e electric field force and Lorentz force phase equilibrium are applied to the charge inside the Hall element, The amount of charge accumulated on both sides of the voltage terminal also stabilizes, forming an electric potential between the two voltage terminals, known as the Hall potential. The value of the Hall potential UH is related to the constant DC I and the external magnetic field strength H. In the equation, R is ♍the Hall constant, which is related to the semiconductor material properties and carrier concentration, and d is the thickness of the Hall element. After the Hall element is made, d and R are both constants. The constant DC I is limited by the operating temperature of the Hall element and also has a specified value, so the Hall potential UH is only proportional to H. By utilizing the magnetic field H around the carrier fluid, the corresponding Hall potential U is generated on the Hall element, which proportionally reflects the value of the current in the carrier fluid.

2、 The basi🐼c working principle of Hall🗹 current sensor

The magnetic circuit of the shown structure concentrates the magnetic flux around the carrier fluid through the Hall element, while the current terminal of the Hall element is energized with the specified DC I. A Hall potential is generated at the voltage terminal of a Hall element, which is amplified and input into the compensation coil of the magnetic core. When the magnetic ꦑflux generated by the compensation coil completely compensates for the magnetic flux generated by the measured current, the cur𝄹rent of the compensation coil proportionally reflects the value of the measured current. The measured current in the figure is I, the number of ampere turns on the original side is I * l ampere turns, and the compensation coil is set to N turns. The compensation current is I. When the ampere turns are balanced, I. The sampling resistor R can be converted into a voltage output, resulting in a voltage U at both ends of R. For obvious U. It is proportional to the measured current I, as long as U is measured. It can be converted to I, which is the working principle of the Hall current sensor.

传感器5.jpg

3、 The characteristics of Hall current sensors (hereinafter referred to as sensors) can be summarized as follows based on the above workin𓂃g principles:

(1) The input and output of the sensor have good electrical isolation like ordinary cur꧋rent transformers, with insula﷽tion and voltage resistance better than 3kV;

(2) The sensor is a large closed-loop system, so it can transmit any waveform current in the frequency band from DC to 100kHz without distortion. The current pulse waveform trans🐟mitted by the Hall current sensor is displayed. The waveform is the measured current, with an amplitude of 32A, a width of 100us, a rising edge gradient of ≤ 0.6us, a repetition frequency of 10Hz, and a unidirectional current pulse.

(3) Because it is a compensated measurement, it has good accuracy (±🌞 1) and linearity (≤ 0.1% * rated current).

(4) The magnetic circuit of the sensor ꦿoperates with al🌺most zero magnetic flux and is quickly compensated during dynamic changes, so the sensor is a non inductive component.

4、 Comparison between Ha✃ll current sensor🍒s and other detection components

The commonly used components for detecting current in the past were diverters and current transformers. The main problem with using a splitter is the lack of electrical isolation between the input and output. In addition, when using a splitter to detect high-frequency or large currents, there is inevitably electrical inductance, so the connection of the splitter not only affects the measured current waveform, but also cannot truly transmit non sinusoidal waveforms. A current transformer has high accuracy at the specified operating frequency, but it can adapt to a narrow frequency range, especially when it cannot transmit direct current. In addition, there is excitation current 🐭during the operation of the current transformer, so it is an inductive component with the same drawbacks as a shunt.

5、 The usage method of Hall current sensor

The wiring method of the Halꦏl current sensor requires an external 15V regulated DC power supply and an external sampling resistor at the M-end. The shell of the Hall current sensor is printed with an arrow indicating the positive direction of the measured current on the original side. When the current being measured on the original side flows in the direction of the arrow, the sensor M terminal outputs a positive voltage, and vice versa, outputs a negative voltage.

6、 The Application of Hall Current Sensors

The current in power electronic circuits often has large di/dt, non sinusoidal, DC components, et൲c. To truly detect this current waveform, Hall elements are currently appropriate components. Utilizing the wide operating frequency band of Hall current sensors, it can be used to detect non sinusoidal currents

With the increase of the single unit capacity of power electronic equipment, the current harmonics absorbed by the equipment from the power grid also increase. Methods can be use☂d to detect harmonics i❀n the AC incoming current.

Example ② The technology of using PWM (pulse width modulation) control♕ to change the input or output current waveform of power electronic equipment has been widely used. To analyze the ꦕslight current waveform distortion, the only way to detect the waveform is to use a high fidelity Hall current sensor, and the method is the same.

Example ③ In spot welding, pulse electroplating and other equipment, Hall current sensor can be used to detect the current waveform, and peak voltmeter can be used to measure the sensor output voltage, and then the💟 measure♈d peak current can be converted.

Example ④ Using the non inductive characteristics of Hall sensors to measure di/dt in power electronic circuits will not change the original di/dt value due to the insertion of sensors. The fast response of Hall curr🏅ent sensors is suitable for detecting current signals in overcurrent or short circuit protection circuits of power electronic devices, achieving fast protection.

In a voltage source inverter, due to commutation failure, it is easy to damage the semiconductor devices in the upper and lower bridge arms of one phase due to overcurrent. This type of inverter requires fast overcurrent protection. Hall current sensors can be used to detect the current of semiconductor devices in each arm. If T1T4 conducts electricity simultaneously due to commutation failure, t📖he corresponding two sensors detect the current signal at the same time, which is converted into a square wave by a voltage comparator, making both inputs of the "AND" gate "1", thus the "AND" gate outputs "l". With this method, all inverter trigger pulses are used to cut o🅘ff the short-circuit path. The advantages of this method are:

(1) As long as there is a small current in T1T4 at the same time, the protection will act immediately because the pro♏tection action is early and T1T4 will not wit🌳hstand a large current;

(2) The protection action speed is fastꦦ. Because the Hall current sensor is a non inductive component, it will not generate overvoltage when T1T4 is turned off, so the design of the RC absorption circuit does not need to consider the influence of the sensor. The Hall current sensor also has electrical isolation in DC detection, thus expanding its ✃application range.

Example 6: In power electronic devices that output DC, Hall current sensors can be used to obtain ionized DC current signals, which are used for current feedback, cut-off control, current 💃stabilization regulation, DC side overcurrent, short circuit protection, etc.

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