Closed-loop Hall scheme and fluxgate scheme---current sensor

The principle of the open-loop Hall current sensor is introduced above. Today we introduce two other solutions: the closed-loop Hall solution and the fluxgate solution .

Closed Loop Hall Current Sensor

The picture below is the structural diagram of the open-loop Hall current sensor introduced earlier (picture from LEM official website). After the current Ip to be measured establishes the magnetic field in the core, the current is measured through the Hall voltage induced by the Hall element. value; what it outputs externally is a voltage analog quantity.

The closed-loop Hall current sensor is based on the open-loop principle (the picture below is from the Internet), and then introduces a compensation circuit; there are two parts of current passing through the magnetic core: the primary side current to be measured and the secondary side compensation current; the primary side current to be measured It refers to the large current flowing through the busbar copper bar from the battery, and the secondary compensation current is generated internally by the closed-loop Hall current sensor and flows through the secondary coil on the magnetic core.

Specifically (the picture below is from the LEM official website), the Hall voltage induced by the Hall element is not directly used for measurement. Instead, the Hall voltage generates a secondary current after passing through the amplification circuit. This secondary current flows through the winding coil on the magnetic core and then flows to ground through the sampling resistor Rm. In this case, the secondary current will also generate a magnetic field in the magnetic core, and the design is to make this magnetic field have the opposite direction and the same intensity as the magnetic field generated by the primary current to be measured, then the total magnetic flux is 0, that is, the Hall element is at 0 magnetic flux. Environment .

Next, when the magnetic flux in the Hall element is 0, the following formula will be obtained. By measuring Is, Ip can be obtained; Ns is generally 1000~5000, and Is is generally around 25mA~300mA.

Fluxgate current sensor (Fluxgate)

Fluxgate current sensors are products we often encounter, such as LEM's CAB series.

Fluxgate current sensors can be divided into several types, as shown in the figure below (picture source: LEM official website): such as standard type, C-type, IT-type, low-frequency type , etc. Here we introduce the basic principle of standard fluxgate. .

The structure of the standard fluxgate current sensor is very similar to the closed-loop Hall structure, as shown in the figure below (the picture is from the Internet), except that instead of a Hall element placed in the air gap of the magnetic core, a fluxgate sensor is placed. , that is, the inductor can be saturated .

Specifically, the structure also has the primary current to be measured Ip (current in the bus) and the secondary feedback current Is (in the secondary coil). Similarly, as long as the total magnetic flux in the air gap is 0, according to the following formula Calculate the IP:

The principle block diagram of this solution is as follows (from the LEM official website). Previously we knew the method of calculating the current Ip, that is, adjusting the secondary current Is so that the total magnetic flux at the air gap is 0, then we can get Ip; then, how do we Detect the magnetic flux at the air gap in real time and then adjust the magnetic flux to 0?

Here, a saturable inductor is used as a probe at the air gap to identify the magnetic flux at the air gap (the picture below is from the LEM official website). It is an inductance probe composed of a magnetic core and a coil.

Furthermore, the magnetic flux at the air gap will affect the inductance of the probe (the inductance of the probe changes due to the influence of the external magnetic field). We only need to distinguish the inductance when the magnetic flux is 0 and the inductance when the magnetic flux is not 0. Just size.

So how to identify the inductor size under different magnetic fluxes?

One solution is to pass a current Isi through the coil of the inductance probe (a voltage source u(t) is applied to both ends). The magnetic flux it generates is caused by the total magnetic flux of the external air gap (including both Ip\Is). The magnetic flux) is superimposed and acts on the magnetic core of the inductance probe. This accumulated magnetic flux will affect the inductance of the inductance probe, and the inductance is related to the size of the current, so as long as you know how the current of Isi is affected by the magnetic flux at the air gap It's OK if it passes the influence.

Omitting some theoretical analysis processes in the middle, when the total magnetic flux at the air gap is 0, the current in the inductance probe is as shown below, where the dotted line represents the square wave voltage u(t) applied to both ends of the probe, and the solid line is the current value. i(t).

When the total magnetic flux at the air gap is not 0, the current waveform in the inductance probe is as follows: So by detecting this current value, we can equivalently determine whether the total magnetic flux at the air gap is 0, and then adjust the secondary coil The current Is in the air gap makes the total magnetic flux at the air gap 0.

Summarize:

The derivation of some processes may have been omitted during the analysis, but the logic of the entire expression is rigorous. You can look for a more in-depth explanation by yourself; all the above are for reference only.