测量50安培以上电流的秘密
测量50安培以上电流的秘密
乔治·艾尔·巴查、埃文·肖曼和哈里·钱德拉,
Allegro MicroSystems, LLC
介绍
感应电流超过50A可能是一个挑战,因为这项任务通常涉及热管理,必须在有限的PCB区域内进行,并且在某些情况下,需要一个电压隔离装置。两种广泛使用的高电流感测方法是感测电阻/运算放大器法和霍尔电流感测法。比较这两种技术是有用的。最近开发的Allegro MicroSystems集成电流传感器,亚博棋牌游戏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件来检测和拒绝任何外部杂散磁场。集成导体具有200μΩ的低电阻以降低功耗,允许在120 kHz带宽下进行100 a以上的连续电流测量。热性能在很大程度上取决于PCB设计和布局。
ACS770CB2
ACS770坐落在14×21.9毫米的通孔CB封装中。随着电流在其集成导体中流动,集成的低滞后芯将磁场集中在霍尔元件的典型精度为±1%和120 kHz带宽的典型精度。核心也充当磁屏蔽,拒绝外部流浪场。
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.
热性能
为了确定合适的传感器,了解高电平瞬态电流和恒定DC/RMS电流下的热性能是非常重要的。对于以下示例,所有测量均在25°C环境温度下进行,可用于在不同工作温度下降低传感器的额定值。
大电流脉冲测试
LR包
使用Allegro ACS780评估板对LR封装进行了高电流脉冲测试。这是一个八层板,带有两盎司(70μm)铜和FR4基板。36个直径为0.2毫米的热通孔被放置在集成电流导体的每个焊盘旁边。
然后,包装经历了设定幅度的电流脉冲,并且测量了两个条件的时间:模具温度超过最大结温的时间超过165°C,以及熔断电流导体的时间。
CB包装
All testing of the CB package took place using the Allegro ACS770 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).
当进行大电流脉冲测试时,CB封装在执行此测量的实验室设备的最大电流容量为1.2 kA时没有熔断。下表显示了最大电流脉冲持续时间和占空比,可用于保持在不超过165°C模具温度的安全操作区内。
表1:CB封装过热时间作为应用直流电流的功能
Ambient Temperature (摄氏度) |
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 |
电流开启1秒,关闭99秒,施加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.
Layout Guidelines for Thermal Performance
Increasing the LR Package Current Sensing Capability
模拟说明了这种方法的热能力。假设电路板用电流比为6.7:1(即通过迹线:电流通过传感器的电流)以及以下规格:六个铜层(顶部和底层厚度为二盎司(70μm),内层三盎司(105μm)),FR4基板,36个直径为0.2mm的热通孔,以及5mm直径通孔,用于电流在PCB上注射。铝制散热器为94×70毫米,在PCB下连接。
在PCB中注入250A,模拟假设具有300×300×300mm的空气机箱容积的自然对流,外壳壁设定为25°C。最高观察到的温度在顶部金属上为74℃(相对于环境温度〜50°C),而模芯温度达到71℃。
隔离
结论
表2:比较感测电阻/ OP-AMP和
Allegro Current Sensors when measuring >50 A
物品 | 感测电阻器/运算放大器 | 快板ACS780 | 快板ACS770 |
物料清单 | Increased BOM list including sense resistor |
小型表面贴装包装 150 A sensing range |
Through-hole package with 400 A sensing range |
PCB Area | Larger BOM requires more PCB area |
6.4毫米×6.4毫米 | 14毫米×21.9毫米 |
Power Dissipation | 更高的阻力(2 - 4×)than ACS780, generates more heat on 印刷电路板 |
集成导体电阻 200 μΩ |
集成导体电阻 100 μΩ |
杂散磁场 | 免于杂散磁场 | 差分传感技术 rejects stray fields |
集成集中器核心拒绝 杂散场 |
隔离 | Requires external isolators and 更昂贵的孤立权力 供应 |
For <100 V applications. Ideal for 48 V系统 |
UL 60950-1第2版通过 4.8 kV,提供工作电压 up to 990 Vpk. Ideal for line 亚博尊贵会员 |
精度和分辨率 | 准确性将取决于阻力 温度过高。难以 measure small currents with low 感测电阻。更高的抵抗力 提供良好的分辨率但更多 power dissipation |
典型精度为±1%。 测量小型电流,可以正确地解析为60 mA filtering |
|
Noise | 高电感切换创造 需要消隐的嘈杂事件 和建立时代 |
Allegro IC filtering and integrated shield layer couple noise to GND and 产生更清洁的输出信号 |
脚注:
- /en/Products/Current-Sensor-ICs/Fifty-To-Two-Hundred-Amp-Integrated-Conductor-Sensor-ICs/ACS780.aspx
- /en/products/current-sensor-ics/fifty-to-two-hundred-amp-隔膜 - 通讯程序 - sensenor-ics/acs770.aspx.
文章发表在电力电子手册2017年3月。转载许可。