Summary: The Hall effect can be used to measure the magnetic field generated by the integrated current-carrying circuit. This technique has many advantages. If a magnetic core is not used, there will be some problems, that is, the sensor IC is susceptible to stray magnetic fields, and the Hall plate will have stray fields generated by high-current carrier fluid or solenoids, which may cause errors in current measurement. The fundamental solution to this problem is integrated differential current sensing technology. Integrated differential current sensing can reduce the error generated by stray magnetic fields by one to two orders of magnitude. In this way, users of such sensor ICs no longer have to worry about the measurement of stray field interference currents, and the PCB layout can be simplified.
Key words: Hall effect; current sensor; differential current sensing; common mode field suppression; stray magnetic field
1 Technical background
The Allegro current sensor IC uses the Hall effect to measure the magnetic field generated by the integrated current-carrying circuit, and can convert the magnetic field into a voltage proportional to the current. This technology has many advantages, including galvanic isolation, low power loss, and high accuracy at different temperatures. This technology does not use a magnetic core to concentrate the magnetic field, so its hysteresis is almost zero. However, there is a disadvantage of not using a magnetic core, that is, the sensor IC is susceptible to stray magnetic fields. When using a magnetic core, stray magnetic fields can be shunted around the sensor IC because the magnetic core provides a low magnetic resistance path around the sensor IC. When the filter element is not used, stray fields generated by high-current carrier fluid or solenoids will appear on the Hall plate, which may cause errors when measuring current. The correct circuit board and system design can eliminate these sources of error when measuring current; but the optimized trace layout may also limit the design of the PCB and system. The solution to this problem is integrated differential current sensing technology. ACS724 integrated current sensor IC is shown in Figure 1.
Figure 1 ACS724 integrated current sensor IC
2 The principle of differential current sensing
The basic principle of differential current sensing is that the magnetic fields generated on both sides of the current-carrying conductor have opposite polarities. That is to say, when the current-carrying lead frame shown in Figure 2 is used, the Hall plate 1 (H1) will have a magnetic field outside the range generated by the current shown, and the Hall plate 2 (H2) will have the current generated by the shown current. Magnetic field within range. When there is a common mode field on the current sensor IC, the same magnetic field appears on the two Hall plates. By subtracting the output of the two Hall plates, we can suppress these externally generated magnetic fields. The output of the differential current sensor IC is shown in formula (1):
Among them, B1 represents the magnetic field of H1, B2 represents the magnetic field of H2, and G represents the gain of the sensor IC (unit: mV/G). If there is current flowing through the lead frame (I) and there is a common mode field on the sensor IC (BC), the output of the differential sensor IC is:
Among them, C1 represents the coupling factor of H1 (unit: G/A), and C2 represents the coupling factor of H2 (unit: G/A). After simplifying the equation, we can get:
The common mode field (BC) cancels out, and the output signal is only proportional to the current through the sensor IC. Similarly, since the Hall plate can only measure magnetic fields of one size, the sensor IC ignores external magnetic fields in other planes.
Figure 2 Integrated type with differential Hall plate configuration
Current sensor IC lead frame
3 Limiting factors of differential current sensing
There are two main limitations to the suppression capability of differential current sensing:
(1) Hall plate matching: Under the action of the common mode field, the mismatch of the two Hall plates will cause some changes in the output of the differential sensor IC. The Allegro current sensor IC is a monolithic device, so both Hall plates are on the same chip, which can produce a high degree of match between nominal and over-temperature conditions. The Hall plate matching on a single chip is usually higher than 1%.
(2) Field gradient: If the external interference magnetic field passing through the two Hall plates is not uniform, the difference in the interference magnetic field will propagate to the output of the sensor IC. To deal with this limitation, place the two Hall plates as close as possible while making them on the other side of the conductor.
4 Common mode rejection of uniform external magnetic field
The matching of the Hall plate on the chip is usually about 1%, which limits the suppression of the common mode field to about 40 dB. Under the action of this uniform external magnetic field (BC), the output error of the sensor IC (unit: A) is:
Where CFRepresents the coupling factor (unit: G/A) of the current flowing through the sensor IC to the Hall plate, which is equal to the sum of C1+C2 above. The coupling factor of most Allegro integrated current sensor ICs is about 10 to 15 G/A, which will produce the proportional relationship between the output error (unit: A) shown in Figure 3 and the external magnetic field. In order to understand how to generate this kind of magnetic field, we can generate a 10 G magnetic field on the sensor IC by applying a current of 50 A in a wire that is only 10 mm away from the sensor IC. When the matching of the Hall plate is 1%, due to the presence of the magnetic field, the output of the sensor IC will only produce an error of about 10 mA. In contrast, when common mode field suppression is not used, an error of 1 A will occur.
Figure 3 When the two Hall plates 1% do not match
Comparison of error (unit: A) and common mode field CF=10 G/A
5 Common mode suppression of the magnetic field generated by adjacent current-carrying conductors
In current sensor IC applications, one of the most common interference magnetic fields is adjacent to current-carrying conductors. These may be other phases or ground loops. The magnetic field generated by the current-carrying conductor may produce uneven fields on the two Hall plates, depending on the direction of the current. The worst case is that the current direction is perpendicular to the two Hall plates, as shown in Figure 4.
Figure 4 External current perpendicular to the two Hall plates
In this case, the magnetic fields of H1 and H2 are:
用这些磁场除以耦合因数CF(~10 to 15 G/A)，可将这些干扰磁场转换为误差（单位：A）。图5显示了只使用一个霍尔板时的误差与距离的关系。
图6显示了使用差分配置时的误差。图7显示了单独霍尔配置与差分霍尔配置之间的抑制比（单位：dB）。值得注意的关键点是在10X抑制时，抑制比为-20 dB，30X抑制时，抑制比为-30 dB。这些点取决于D和d的比率，如图8所示。图8中的所有D和d值保持不变，也就是说，减少霍尔板之间的距离，并增加霍尔板到外部载流导线的距离，会减少测量值的误差量。大多数Allegro集成式电流传感器IC的霍尔间距(d)约为0.6～1 mm。
(2)与一个霍尔板的耦合是11 G/A，与另一个霍尔板的耦合是2.8 G/A，所以总耦合因数(CF)是13.8 G/A。
总之，集成式差分电流传感使杂散磁场产生的误差降低了一到两个数量级。这样，此类传感器IC的用户就不必再担心杂散场干扰电流的测量，而且能简化PCB布局，并使用外形更精巧的系统。对于采用载流线迹或磁场发生器件(如螺线管)的高度压缩系统，可采用本文的分析，以快速估算这些杂散场产生的误差量。这样设计人员就能预见和改正可能在系统内引入过大误差的系统配置或 PCB 布局，从而显著减少设计迭代的次数。
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