Compact current measurement design based on Hall effect

There are two main methods for measuring current in the 3-20A range: the traditional measurement method using a resistive shunt, and the use of a current sensor. Both technologies have limitations: the first method lacks galvanic isolation, and the second method has limited bandwidth. In addition, both methods have considerable calibration requirements. LEM current sensors have helped effectively solve these problems, but in order to fully meet the current needs for lower cost and smaller size, it is time to redesign the product.

LEM Current Sensor

In 2002, LEM acquired NANA Electronics KK, a Japanese company that produces Hall effect current sensors. The new company was renamed NANALEM KK and is headquartered in Machida, Tokyo. The new R&D team combines the expertise of the two companies and has redesigned Japan's best-selling SY series and developed it into the HX series.

Hall Effect Principle

The heart of the HX sensor is a Hall effect generator. The Hall effect, discovered by Edward H. Hall in 1879, occurs when a current flows through a thin sheet of conducting material (the Hall generator) and is placed in an orthogonal magnetic field. The electromagnetic Lorentz force then induces electrons to flow to the edge of the sheet according to 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 piece of conductive material, such as gallium arsenide (GaAs), which enables reliable and stable performance during use. At a control current of 5 mA, the obtained Hall voltage is about 1.25 mV/mT.

Hall Effect Open Loop Current Measurement

The magnetic field generated by the primary current generates a linear magnetic flux B in the gap of the magnetic circuit, which induces a proportional Hall voltage VH in the Hall generator. This voltage is then amplified by the electronic circuit to obtain an output analog signal proportional to the primary current. The HX series can measure DC and AC currents, as well as complex current waveforms in phase-controlled rectifiers, active power converters, PWM converters, and switch-mode power supplies. The output voltage is always a true image of the primary current.

Immunity to dv/dt noise

One of the issues engineers encounter when designing drive controls and switching devices is high dv/dt noise caused by rapid voltage changes during rectification.

Power semiconductor technology is constantly evolving. IGBTs with very high rectifier speeds can now be found in many semiconductor product catalogs. As a result, current general-purpose inverters generally operate at very high switching frequencies, usually above 20 kHz. The benefits of operating at such high frequencies include smoother waveforms, safer operation, and higher efficiency.

The high dv/dt values ​​generated each time the switching device switches will generate capacitive currents between the main cable and the sensor's electronic circuit. Most analog linear amplifiers are sensitive to such parasitic currents. As a result, dv/dt noise will be superimposed on the output signal. Depending on the amplitude and slope of the varying voltage, the initial spike and the subsequent oscillations can sometimes be so high that they activate the sensor's current protection circuit, which in turn stops the inverter. LEM's experience helped to ensure perfect immunity to critical noise during the HX series design phase without compromising bandwidth, so that the performance of the HX exceeds that of other similar sensors (Figures 2 and 3).

Ultra-fast response time to step current is essential for IGBT short-circuit protection. The HX series can accurately track current changes at a speed of more than 50A/?s, and the fastest response to step current is 3s.

Another thorny issue that design engineers often face is available space. Small sensors help solve this problem, with the HX sensor weighing only 8 grams and requiring a mounting area of ​​only 15 x 19 mm. But it is well known that when such sensors are placed side by side in a three-phase application, the primary current of each may affect the electronics of the other sensors. When mounted side by side in a three-phase application, the HX current sensors cause very little mutual interference (Figure 4).

The dedicated HX sensors have two primary coils that can be connected in series or in parallel (Figure 5). In some inverter applications, a pair of these sensors can be used to measure all three phases, 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 clearance/creepage distance is over 5.5 mm, making these sensors particularly suitable for isolated current measurements in the low and medium power range.