Quality Standards and Environmental Certifications

测量电流超过50安培的秘密

测量电流超过50安培的秘密

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由Georges El Bacha，Evan Shorman和Harry Chandra，
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, will be used as examples.

It’s often best to measure current near the supply voltage of the load (the high-side) instead of near ground (the low-side). Measuring on the high-side brings immunity to ground bounces and allows for the detection of short circuits to ground. Depending on the supply voltage and the application, basic or reinforced isolation might be needed for sense-circuit connections. If a sense-resistor/op-amp are used to measure on the high-side, an op-amp with a high common-mode input range will be necessary, making the design more complex. To provide isolation, additional isolators (such as optocouplers) and isolated power supplies will be needed, increasing complexity and boosting costs.

On the other hand, Hall-effect current sensor ICs, such as those provided by Allegro, eliminate the need for a sense resistor. The current flows directly into the integrated conductor, generating a magnetic field that will be measured.

ACS780LR1

The ACS780 sits in a 6.4 × 6.4 mm surface mount LR package. Current flows into the integrated conductor and generates a magnetic field that on-die Hall elements then sense. Use of a flip-chip assembly technique brings the Hall elements close to the leadframe where the magnetic field is at its highest point. This packaging allows for superior signal-to-noise ratio.

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。

Figure 1: ACS770 in the CB Package (left) and ACS780 in the LR package (right)
Figure 1: ACS770 in the CB Package (left) and ACS780 in the LR package (right)

图2：在LR包装中构建ACS780
图2：在LR包装中构建ACS780

ACS770CB2

The ACS770 sits in a 14 × 21.9 mm through-hole CB package. 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.

The integrated conductor has 100 μΩ resistance, providing ultralow power loss. The ACS770 can measure 200 A continuously at an ambient temperature of 85°C and can be factory programmed to measure inrush currents up to 400 A.

Figure 3: Construction of the CB Package
Figure 3: Construction of the CB Package

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 4: ACS780 Evaluation Board
Figure 4: ACS780 Evaluation Board

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.
图5：LR包装保险丝和过温度时间作为应用的直流电流的功能
图5：LR包装保险丝和过温度时间作为应用的直流电流的功能

CB Package

所有测试的CB包使用急速地一个CS770 evaluation board. This is a two-layer board with four ounce (140 μm) copper and an FR4 substrate. Sixteen thermal vias of 0.5 mm diameter were placed next to each of the solder pads of the integrated current conductor (Figure 6).

Figure 6: CB Package Evaluation Board
Figure 6: CB Package Evaluation Board

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

Ambient Temperature
（°C）		 Maximum Current
(A)
Current is on for 10 s and off for 90 s, 100 pulses applied
25 350
85 350
150 260
Current is on for 3 s and off for 97 s, 100 pulses applied
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

DC Current Capability

Figure 7 shows the die temperature rises as continuous DC current is injected through the sensors and temperature reaches steady state. As expected, the CB package shows a smaller temperature increase because of its lower conductor resistance of 100 μΩ compared to 200 μΩ for the LR package.

Figure 7: Change in Die Temperature vs. DC Current
Figure 7: Change in Die Temperature vs. DC Current

Layout Guidelines for Thermal Performance

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.
Of the three methods, adding thermal vias has the least impact on PCB area and cost, and it is easy to implement. To understand the impact of vias and how many to use, a simulation was run on the ACS780LR evaluation board using a natural convection model. The model assumed an air enclosure of 300 × 300 × 300 mm with the enclosure wall set to 25°C. The injection of current caused the steady-state die temperature to reach 150°C.
将热通孔的数量减少50％（18代替每焊盘的36个通孔），使5.6°C的模具温度升高至156°C。去除所有热通孔，导致模33.5°C的模具温度上升至183.5°C。这些结果突出了热通孔的显着益处，同时表明通孔数量小（相对于Allegro评估板的减少远低于50％），应对热性能产生最小的影响。
Figure 8: Thermal Vias Pattern of the ACS780LR Evaluation Board
Figure 8: Thermal Vias Pattern of the ACS780LR Evaluation Board

Increasing the LR Package Current Sensing Capability

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
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（vPK.or DC) or 700 Vrms., while its reinforced isolation working voltage is 636 (VPK.或直流）或450 vrms..

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
Allegro Current Sensors when measuring >50 A
Item 感测电阻/ OP-AMP Allegro ACS780. 急速地一个CS770
BOM. Increased BOM list including sense
resistor		 Small surface-mount package with
150 A sensing range		 Through-hole package with 400 A
sensing range
PCB Area Larger BOM requires more PCB
area		 6.4 mm×6.4 mm 14 mm × 21.9 mm
Power Dissipation Higher resistance (2-4×) than
ACS780, generates more heat on
PCB.		 集成导体阻力
200 μΩ		 集成导体阻力
100μΩ
杂散磁场 Immune to stray magnetic fields 差分传感技术
rejects stray fields		 集成浓缩器核心拒绝
流浪领域
隔离 Requires external isolators and
more expensive isolated power
supply		 For <100 V applications. Ideal for
48 V systems		 UL 60950-1 2nd edition passes
4.8 kV，提供工作电压
up to 990 Vpk. Ideal for line
亚博尊贵会员
准确性和分辨率 Accuracy will depend on resistance
过温。难
measure small currents with low
sense resistor. Higher resistance
provides good resolution but more
power dissipation		 典型精度为±1％。
Measures small currents and can resolve down to 60 mA with proper
filtering
Noise High inductance switching creates
需要消隐的嘈杂事件
和settling times		 Allegro IC filtering and integrated shield layer couple noise to GND and
produces a cleaner output signal

Footnotes:

  1. /en/Products/Current-Sensor-ICs/Fifty-To-Two-Hundred-Amp-Integrated-Conductor-Sensor-ICs/ACS780.aspx
  2. /en/Products/Current-Sensor-ICs/Fifty-To-Two-Hundred-Amp-Integrated-Conductor-Sensor-ICs/ACS770.aspx

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