测量电流超过50安培的秘密

由Georges El Bacha，Evan Shorman和Harry Chandra，
亚博棋牌游戏Allegro Microsystems，LLC

介绍

感测电流超过50 A可能具有挑战性，因为任务往往涉及热管理，必须在有限的PCB区域进行，并且在某些情况下，需要电压隔离装置。用于传感高电流的两种广泛使用的方法是一种感觉电阻/ OP-AMP方法，以及基于霍尔的电流感测。比较这两种技术是有用的。最近开发了Allegro Micros亚博棋牌游戏ystems集成电流传感器，ACS780LR.和ACS770CB.，将用作示例。

通常最好测量负载（高端）的电源电压附近的电流而不是近地（低端）。在高端测量为接地反弹带来抗扰度，并允许检测到地面的短路。根据电源电压和应用，可能需要对感测电路连接需要基本或增强隔离。如果使用感测电阻/运算放大器测量高侧，则需要具有高共模输入范围的OP-AMP，使设计更加复杂。为了提供隔离，需要额外的隔离器（例如光耦合器）和隔离电源，提高复杂性和升高成本。

另一方面，霍尔效应电流传感器IC，如Allegro提供的那些，消除了对感测电阻的需求。电流直接流入集成导体，产生将测量的磁场。

ACS780LR.¹

ACS780位于6.4×6.4 mm的表面安装LR封装中。电流流入集成导体，并产生磁场，然后导通霍尔元素感知。使用倒装芯片组件技术使霍尔元素接近磁场处于最高点的引线框架。该包装允许出色的信噪比。

该设备使用两个霍尔元素来检测和拒绝任何外部杂散磁场。集成导体具有低200μΩ电阻以降低功耗，允许具有120 kHz带宽的连续电流测量超过100μ。热性能高度依赖于PCB设计和布局。

ACS770CB.²

ACS770坐在14×21.9毫米金属化孔CBpackage. As current flows in its integrated conductor, an integrated low-hysteresis core concentrates the magnetic field which is then sensed by the Hall element with a typical accuracy of ±1% and 120 kHz bandwidth. The core also acts as a magnetic shield, rejecting external stray fields.

集成导体具有100μΩ电阻，提供超级功率损耗。ACS770可以在85°C的环境温度下连续测量200A，可以工厂编程，以测量浪涌电流，最高可达400 A.

热性能

为了确定应用的适当传感器，重要的是要在高级瞬态电流和恒定的DC / RMS电流下理解热性能。对于如下实施例，所有测量均在25°C环境下进行，并且可用于在不同的操作温度下缩小传感器。

高电流脉冲测试

LR包装

使用Allegro ACS780评估板进行LR包的高电流脉冲测试。这是一个带有二盎司（70μm）铜和FR4基板的八层板。在整个电流导体的每个焊盘旁边放置0.2mm直径的三十六个热通孔。

The package then experienced a current pulse of a set magnitude and the time was measured for two conditions: the time for the die temperature to exceed the maximum junction temperature of 165°C, and the time to fuse the current conductor open.

Figure 5 shows in green the area of safe operation where the die temperature remains below 165°C. The orange area shows the conditions under which the maximum junction temperature is exceeded but the current conductor is not fused.

CB Package

CB包的所有测试使用Allegro ACS770评估板进行。这是一个双层板，具有四盎司（140μm）铜和FR4基板。在集成电流导体的每个焊盘旁边放置0.5mm直径的16个热通孔（图6）。

当经过高电流脉冲测试时，CB封装在1.2 ka的CB封装 - 执行该测量的实验室设备的最大电流能力。附近的表格显示了最大电流脉冲持续时间和占空比，可以应用于保留在安全操作区域内，其中不超过165°C的芯片温度。

Table 1: CB Package Overtemperature Time as Function of Applied DC Current

环境温度（°C）	最大电流（一种）
电流为10 S及OFF，施加了100个脉冲
25.	350.
85	350.
150.	260.
电流为3 S及OFF，施加了100个脉冲
25.	450.
85	425.
150.	375.
电流为1 S及OFF，施加了100个脉冲
25.	1200.
85	900
150.	600

直流电流能力

图7显示了芯片温度上升，因为通过传感器注入连续的DC电流，温度达到稳定状态。正如预期的那样，Cb封装显示出较小的温度升高，因为它为LR封装的200μΩ相比其导体电阻为100μΩ。

热性能布局指南

The system thermal performance depends greatly on the PCB layout and can be improved in several ways: by incorporating multiple layers of metal to better dissipate the heat under the IC, by adding a heat sink as close as possible to the IC, or by adding thermal vias (that connect all metal layers) surrounding the Allegro IC integrated conductor solder pads.

在三种方法中，添加热通孔对PCB区域和成本最小，并且易于实现。要了解VIVE的影响以及使用多少，使用自然对流模型在ACS780LR评估板上运行模拟。该模型假设空气罩300×300×300毫米，外壳壁设定为25°C。喷射电流导致稳态模具温度达到150℃。

Reducing the number of thermal vias by 50% (18 instead of 36 vias per solder pad), brought a 5.6°C rise in die temperature to 156°C. Removing all thermal vias caused a 33.5°C rise in die temperature to 183.5°C. These results highlight the significant benefits of having thermal vias, while showing that a small reduction in the number of vias (much less than a 50% reduction relative to the Allegro evaluation board) should have a minimal impact on the thermal performance.

增加LR封装电流传感能力

The small footprint of the ACS780 in the LR package and its ease of surface-mount assembly brings advantages for measuring currents exceeding 100 A. The approach is to reroute a portion of the current to be sensed through a trace on the PCB. Thus, a portion of the current to be sensed does not pass through the Allegro IC. Here, the current ratio of the splitter is critical. It must be set so the maximum possible current flows through the sensor (while the sensor remains in the thermal safe operating zone) to get the best accuracy (Figure 9).

**Figure 9: Current Splitting using the ACS780LR**

A simulation illustrates the thermal capability of this approach. Suppose a board is used with a current ratio of 6.7:1 (that is, current through trace: current through sensor) and the following specifications: six copper layers (top and bottom layer thickness of two-ounce (70 μm), inner layers of three-ounce (105 μm)), an FR4 substrate, 36 thermal vias of 0.2 mm diameter around each pad, and 5 mm diameter through-holes for current injection on the PCB. An aluminum heat spreader of 94 × 70 mm connects under the PCB.

AN296141图10. — **Figure 10: Current Splitting Board used in Thermal Simulation of the ACS780LR Package**

With 250 A injected in the PCB, a simulation assumed natural convection with an air enclosure volume of 300 × 300 × 300 mm with the enclosure wall set to 25°C. The highest observed temperature was 74°C on the top metal (~50°C rise relative to the ambient temperature), while the die temperature reached 71°C.

隔离

Allegro current sensors are galvanically isolated, offering an efficient way to measure on the high-side. The ACS780LR targets applications where the supply voltage is less than 100 V. Its construction provides inherent isolation, because the active circuitry on the die is not electrically connected to the current conductor.

ACS770认证到UL 60950-1第2版，通过4.8 kV 60秒。其基本隔离工作电压为990（v_PK.或直流）或700 v_rms.，而其增强隔离工作电压为636（v_PK.或直流）或450 v_rms.。

结论

总而言之，包装和电路设计的进步已经简化了使用霍尔电流传感器IC来测量PCB上超过50 A的电流的任务。通过使用小型表面安装ACS780或通孔ACS770，可以经济地进行准确和电流隔离的感测。

Table 2: Comparing Sense-Resistor/Op-Amp and
测量时的Allegro电流传感器> 50 a

Item	感测电阻/ OP-AMP	Allegro ACS780.	Allegro ACS770.
BOM.	增加了BOM列表，包括感觉电阻器	Small surface-mount package with 150传感范围	带400a的通孔包装传感范围
PCB区域	更大的BOM需要更多的PCB 区	6.4 mm×6.4 mm	14 mm × 21.9 mm
功耗	较高的电阻（2-4×）比 ACS780，产生更多的热量 PCB.	集成导体阻力 200μΩ.	集成导体阻力 100μΩ
杂散磁场	Immune to stray magnetic fields	差分传感技术拒绝流浪领域	集成浓缩器核心拒绝流浪领域
隔离	需要外部隔离器和 more expensive isolated power 供应	对于<100 V应用程亚博尊贵会员序。理想的 48 V systems	UL 60950-1 2nd edition passes 4.8 kV，提供工作电压高达990 vpk。适合线路亚博尊贵会员
准确性和分辨率	Accuracy will depend on resistance 过温。难测量小电流低 sense resistor. Higher resistance provides good resolution but more 功耗	典型精度为±1％。 Measures small currents and can resolve down to 60 mA with proper 过滤
噪声	High inductance switching creates noisy events that require blanking 和settling times	Allegro IC滤波和集成屏蔽层耦合噪声到GND和 produces a cleaner output signal

Footnotes:

Article published in Power Electronics Handbook, March 2017. Reprinted with permission.

测量电流超过50安培的秘密