Compact current measurement design based on Hall effect

Generally, there are two main methods for measuring current in the range of 3-20A: the traditional measurement method using resistance shunts, and the use of current sensors. Both technologies have limitations: one method lacks galvanic isolation, and the second method has limited bandwidth. In addition, these two methods have quite high requirements for calibration. The LEM current sensor used to help effectively solve these problems, but in order to fully meet the current demand for cost reduction and size reduction, it is time to redesign the product.

    LEM current sensor
    In 2002, LEM acquired NANA Electronics K.K., a Japanese company producing Hall-effect current sensors. The new company was renamed NANALEM K.K., headquartered in Machida, Tokyo. The new R&D team combines the experience of the two companies and has redesigned the Japanese product SY series and developed it into the HX series.

    Hall effect principle
    The HX sensor is a Hall effect generator. In 1879, Edward H. Hall discovered the Hall effect, which occurs when current flows through a thin conductive material (Hall generator) and is placed in an orthogonal magnetic field. Then the electromagnetic Lorentz force will induce electrons to flow to the edge of the sheet according to the polarity.
    The Hall voltage VH generated between these two edges is directly proportional to the control current IC and the magnetic flux B (Figure 1). The Hall generator is made of a thin conductive material, such as gallium arsenide (GaAs), which can achieve reliable and stable performance during use. Under the control current of 5 mA, the Hall voltage obtained is about 1.25 mV/mT.

    Hall effect open loop current measurement
    The magnetic field generated by the current will generate a linear magnetic flux B in the gap of the magnetic circuit, and the magnetic flux B will induce a proportional Hall voltage VH in the Hall generator. Then this voltage is amplified by the electronic circuit to get an output analog signal proportional to the current. The HX series can measure DC current and AC current, as well as complex current waveforms in phase-controlled rectifiers, active power converters, PWM converters and switching power supplies. The output voltage is always the true image of the current.

    Anti-dv/dt noise ability
    When designing drive control and switching equipment, one of the problems that engineers encounter is high dv/dt noise caused by rapid voltage changes during rectification.
    Power semiconductor technology has been constantly evolving. Now, IGBTs with very high rectification speed can be seen in many large samples of semiconductor products. Therefore, current general inverters generally work at a very high switching frequency, usually above 20 kHz. The benefits of working at such a high frequency include smoother waveforms, safer operation, and higher efficiency.
    The high dv/dt value generated every time the switchgear is switched on and off will generate a capacitive current between the main cable and the electronic circuit of the sensor. Most analog linear amplifiers are very sensitive to this parasitic current. Therefore, dv/dt noise will be superimposed on the output signal. Depending on the amplitude and slope of the fluctuating voltage, the initial spikes and subsequent oscillations are sometimes so high that they will activate the current protection circuit of the sensor, which will cause the inverter to suspend operation. LEM's experience helped to ensure perfect immunity to critical noise during the design phase of the HX series without compromising bandwidth. Therefore, the performance of HX exceeds other similar sensors (Figure 2 and Figure 3).

    The ultra-fast response time to the step current is essential for IGBT short-circuit protection. The HX series can accurately track current changes at a speed above 50A/?s, and the response to step current is as fast as 3s.
    Another thorny issue that design engineers often face is available space. A small sensor helps solve this problem. The HX sensor weighs only 8 grams and requires only 15 x 19 mm of installation area. But it is well known that when such sensors are placed side by side in a three-phase application, their respective currents may affect the electronics of other sensors. When installed side by side in a three-phase application, the HX current sensors cause very little mutual interference (Figure 4).

    The dedicated HX sensor has two coils, which can be connected in series or in parallel (Figure 5). In some inverter applications, a pair of such sensors can be used to measure all three phases, with two phases per sensor (Figure 6). This eliminates the need for a third unit and helps reduce costs. The AC test voltage (50 Hz, 1 minute) is 3 kVRMS, and the gap/leakage distance exceeds 5.5mm, making these sensors particularly suitable for isolated current measurement in the low and medium power range.