Managing External Magnetic Field Interference When Using ACS71x Current Sensor ICs

理查德·狄金森和威廉·本特利

摘要

The highly compact Hall effect current sensor ICs without concentrators that are now available make it possible to enhance performance of small-current differentiation by simple layout steps and shielding the device from extraneous magnetic flux along its primary sensing axis.

Introduction

基于霍尔效应的电流传感器集成电路ACS71x系列通过感应电流通过霍尔元件附近时产生的磁场来测量电流（见图1）。他们直接测量这个磁场，而不使用磁集中器，这是其他磁性装置（例如快板）的一个共同特点^®微系统CA和CB封装，用于电流传感器IC的ACS75x系列）。

图1。ACS71x设备中的电流路径。电流沿任一方向通过U形回路并绕霍尔元件（X）流动。U型环安装在SOIC8封装的芯片下方。

缺少集中器的优点是几乎可以消除磁滞，这是IC中的一个误差源。然而，这也使得ACS71x设备对可能扭曲电流测量的外部磁场的屏蔽程度较低。在可能存在大磁场的应用中，必须注意霍尔元件相对于这些磁场的对准和间距。在某些情况下，屏蔽设备也是可取的。雷竞技最新网址

对齐

磁通线在与流过导体的电流方向正交的平面内围绕导体形成圆。霍尔元件只对垂直于其表面的磁通分量作出响应，并且只对该方向的磁场敏感。如图2所示，虽然一次电流的路径_第，与霍尔元件在同一平面上，电流通过U形回路产生的磁通矢量垂直于霍尔元件平面。与霍尔元件相交的元件在霍尔元件上产生电压，然后将其放大并用于产生输出电压。

图2。U形回路和霍尔元件。流过U形回路的电流在与其路径正交的平面上产生磁通线。垂直于霍尔元件平面的磁通能产生霍尔电压。

如果可能，设备附近的大电流导体应垂直于安装设备包的板上的平面。如图3所示。通过这种排列，磁通量将在霍尔元件的平面内循环，而不是穿过霍尔元件，并且对霍尔集成电路的输出几乎没有影响。

图3. Adjacent Conduction Perpendicular to Hall Element Plane. Flux lines generated perpendicular to current flow are parallel to Hall plane and do not generate Hall voltage.

间距和布局指南

当铺设的PCB ACS71x设备be mounted, it is good practice to keep as much space as possible between the device and other traces carrying significant amounts of current. Figure 4 shows the measured effect of an adjacent current trace on the same side of the PCB as the device. Although this is a consideration for optimizing designs, the effect of the neighboring trace is small and much less than the signal generated by the current being measured within the device.

Figure 4. Adjacent Conduction Traces Carrying 50 A, Effect on an ACS71x device. The minimal influence of magnetic effects from an adjacent trace diminish rapidly as the distance, D, is increased.

屏蔽

Figure 4 shows the effect of the magnetic flux generated by an adjacent current-carrying trace on the Hall element when the current path is oriented in the same plane as the Hall element, and so its magnetic flux lines are perpendicular to the plane of the Hall element.

如果需要更大的外部磁场保护，建议使用表面贴装磁性合金屏蔽层覆盖设备包。如图5所示的屏蔽将外部磁通量从SOIC8封装分流出去，并且不会对封装内部围绕一次电流路径产生的磁场产生显著影响。典型的磁场结果如图6所示。

图5。简单的防护罩。这种设计可以防止垂直磁通线从SOIC8封装的Allegro电流传感器IC上方撞击。如有必要，可以在PCB的另一侧放置第二个屏蔽。

图6. Magnetic Field Map of Shield. A simple ferrous alloy shield deflects virtually all perpendicular flux lines, effectively shielding the Hall element.

屏蔽层可通过环氧树脂固定在PCB上，并且不强制要求其具有接地的导电路径。由于霍尔元件不易受到平行于其平面的磁通线的影响，因此无需封闭屏蔽的侧面。事实上，出于漏电和间隙的考虑，最好在IC引线附近保持屏蔽打开。

In cases where a high level of shielding is required, a second shield of a similar type can be attached to the underside of the PCB, protecting the Hall element from the perpendicular flux passing through the PCB from below.

实验结果

An experiment was performed with ±240 G applied perpendicularly to the Hall element of an 8-pin device package, through the use of an air core field source. The attenuation levels achieved by shields constructed of various ferrous alloys and material thicknesses were recorded. The results are provided below. Two types of ferrous alloys were used, silicon steel (SiFe), and HyMu alloy, which is a generic class of alloys that have a high magnetic permeability level, µ. It should be noted that in most applications the Hall element will not be exposed to field levels as great as 60 G. For example, up to 60 G of field would pass through the Hall transducer only if the ACS71x were placed within 6 mm of an adjacent busbar carrying 500 A.

Summary

当应用ACS71x电流传感器ic时，本说明中描述的对准、间隔和屏蔽技术可被用作减轻外部磁场影响的越来越激进的步骤。

如果您在处理外场效应方面需要进一步的帮助，请联系您的当地Allegro销售办事处to be put in touch with one of our applications engineers.