High-Current Measurement with Allegro Current Sensor IC and Ferromagnetic Core: Impact of Eddy Currents
High-Current Measurement with Allegro Current Sensor IC and Ferromagnetic Core: Impact of Eddy Currents
By Yannick Vuillermet,
Allegro MicroSystems Europe Ltd
介绍
The technique of measuring bus bar current with a surrounding ferromagnetic core is common knowledge. For high current measurement above 200 A, Allegro recommends using a linear IC of the A136x family, such as theA1367，与磁芯相结合（图1）。本文档侧重于对电流测量的交流（AC）效应。交流输入电流倾向于在磁芯中产生涡流。这些涡流改变了测量的磁场，从而降低了电流测量精度。
For more details about core design, refer to “Guidelines for Designing a Concentrator for High-Current Sensing Applications with an Allegro Hall-Effect Sensor IC”[1]，在Allegro网站上提供。
请注意，本文档中的所有结果都来自于ANSYS Maxwell软件中执行的电磁模拟。
magnetic core and Allegro A1367
Measurement Principle
理想情况下，磁场，H，在气隙中与输入电流完全成比例，I，在汇流条或电阻导线中。因此，通过线性磁场传感器测量该磁场足够，并且在输入电流和磁场之间表征系数以测量该输入电流。这个系数，sC，称为耦合因子或核心灵敏度。然而，该耦合因子在有限的电流和频率范围内仅是恒定的。该系数的任何变化都会导致输入电流测量误差。典型的精度要求在测量电流的百分比范围内。
Eddy Current Basics
Eddy currents are a direct effect of Lenz’s law which states that the direction and magnitude of the current induced in a conductor by a varying magnetic field is such that it creates a magnetic field
反对产生它的变化。在使用铁磁芯的AC电流传感器应用中，作为切向变化磁场的响应，在芯内引起涡流。图2示出了YZ横截面，其示意性地表示散装芯中的涡流。
These eddy currents generate an induced magnetic field, Heddy, opposite to the exciting magnetic field, Hexc. This is measured at the sensor level as a reduced core sensitivity, SC, or otherwise
said, a current measurement error.
in a bulk magnetic core
为了减少涡流,这是必要的to cut the current paths in the core. This is achieved by using a laminated core with thin sheets. These sheets must be electrically isolated from each
other.
通过堆叠片或在Z方向上可以通过堆叠或Z方向在Y方向上进行叠片（图3）。涡流仍然流动，但幅度差。
涡流：滚动（左）和堆叠（右）
典型应用使用Allegro A1367LKT线性传感器IC
A typical high-current application using an Allegro A1367LKT linear sensor IC is considered here. Maximum peak current in this application is 600 A. The geometry is as given in Figure 4. The core length along the Z axis is 6 mm. The core is made of ferromagnetic material such as grain-oriented silicon steel with typical magnetic characteristics as in Figure 5. Initial relative permeability is 10000 and magnetic polarization at saturation is 1.8 T. Note that for the sake of simplicity, magnetic hysteresis is not considered. The core electrical resistivity is 45 μΩ/cm.
直流核心敏感度，sC评估,从0到600。图6 t报告he expected measured field at the A1367 Hall plate location and the expected core sensitivity. The core magnetic sensitivity is constant up to the maximum current, as expected. The core sensitivity is around 2.36 G/A. In bipolar mode, the A1367 uses a ±2 V output span. Consequently, the IC sensitivity is ~1.4 mV/G, and the recommended A1367 part option would be A1367-LKTTN-2B-T. Figure 7 displays the core magnetization at maximum DC current; the magnetization
does not reach saturation.
现在，正弦电流用600峰值供应到汇流条。
Three cores are evaluated:
- 块
- Laminated with 0.375 mm sheets, along the Z direction
- Laminated with 0.250 mm sheets, along the Z direction
图8报告磁芯灵敏度衰减Δ过频率。百分比，频率f的衰减定义为：
SC_F.是频率的核心磁敏度f. SC_DCis the core magnetic sensitivity in DC and 10 A. In a bulk core, the sensitivity decreases very quickly versus frequency: at 100 Hz, this is already significant (>5%). Otherwise said, a bulk core is only suitable for near DC measurements.
Laminated cores can be used up to a few kHz, depending on required accuracy. As expected, thinner sheets result in better AC performances.
图9显示了在气隙中测量的输入电流和磁场之间的相移。图9表示IC测量的磁场落后于AC电流
在母线酒吧流动。在层压芯中，该滞后可以高于几kHz的电流频率的电视量。
As a direct consequence, an input current step could be measured with a significant delay because of its high harmonic content. Note that the attenuation and the lag are due to the eddy current
physics only. A perfect magnetic field sensor with infinite bandwidth would also see these effects.
对于0.375mm板叠层芯，图10中报告了衰减与输入电流。在这个剧情上可以看到一个非常有趣的现象。在低频，衰减
is constant over the current, whereas attenuation drops around 300 A at 5 kHz. This can be explained by an early saturation of the core which is induced by the eddy currents. Below 300 A,
the attenuation is only due to the eddy current in the concentrator which works in the linear area of Figure 5. At 300 A, the eddy currents locally generate a high magnetic field that saturates the
core. Hence, the core magnetic sensitivity is already reduced at 300 A, while the core normally saturates at more than 600 A in DC. This is clearly visible while comparing the core magnetization
从图7和图11.注意，图11的核心磁化映射上可见的“噪声”不是真实的，而是由于仿真网格。
Figure 12 represents the eddy current magnitude density inside a section of a 0.375 mm laminated core at 5 kHz and 600 A.
vs. frequency at 600 A AC current
层压芯，厚板0.375毫米
层压芯，厚板0.375毫米
Conclusions
分析显示：
- Eddy currents induce a current measurement error due to an altered core sensitivity and a phase shift between the input current and the generated magnetic field.
- 涡流通过核心层压减少：薄片较薄，频率的行为越好。
- 散装磁芯仅适用于直流测量或非常缓慢的AC，大约小于10 Hz。
- Laminated magnetic cores are recommended for AC measurements for frequency up to a few kHz, with sheets of a few hundred μm and a required accuracy around a few
百分。 - For a given application, the worst-case measurement error is at maximum application frequency and maximum application current.
Allegro engineers can assist customers to design the best magnetic core for their application depending on current and frequency range. Contact your local Allegro Microsystems technical
援助中心。
[1]“使用Allegro Hall效应传感器IC设计用于高电流传感应用的集中器的指导方针”，亚博尊贵会员
//www.wangzuanquan.com/en/insights-and-innovations/technical-documents/hall-ychect-sensor-publications/current-sensor-concentator..