Types And Characteristics OF Current Sensors

Currently, current sensors have been widely used in a variety of applications, starting with industrial equipment. In industrial equipment, current detection usually requires insulation measures, which are shunt resistor + insulated op amp/insulated ADC mode, cored current sensor mode and coreless current sensor mode (current sensor IC). This paper will introduce the summary of various current detection methods and their advantages and disadvantages.

Current Detection Method

Current detection methods can be divided into three types: shunt resistor + isolated op amp/isolated ADC mode, cored current sensor mode and coreless current sensor mode (current sensor IC).

The following table summarizes the characteristics of the various methods. The advantages and disadvantages of each approach are explained in the following sections.

How to choose a current detection method

The shunt resistor + insulated op amp/insulated ADC method is suitable for high-precision detection of small currents that do not cause heating problems (below 10A) in environments with large interfering magnetic fields.

Fluxgate is suitable for applications that require the highest accuracy in current current detection technology.

General coreless current sensors are suitable for applications that require small size in small devices and where cost is more important than detection accuracy.

The performance of AKM's coreless current sensor ics is not only comparable to that of general coreless current sensors, but it is comparable to coreless current sensors (open loop, closed loop) in all projects.

Shunt Resistor + Insulated Op Amp/insulated ADC Method

The shunt resistor + insulated op amp/insulated ADC method is a method in which the current to be measured flows through a known resistor and the current value is calculated based on the drop voltage value of the resistor. 

The insulation measure needs to be implemented through the back-end using a edge amplifier or ADC.

This method is the most important current detection metho

Magnetic current sensor is a method to detect the current flow by using a magnetic sensor to detect the magnetic field generated around the current line.

Unlike the shunt resistance method, the magnetocurrent sensor has an internal insulation construction, so there is no need to install an insulating amplifier or isolation ADC in the rear segment.

In addition, because the resistance value does not need to be changed according to the current flow, it has attracted attention as a solution that can solve the shortcomings of the shunt resistance mode that can only detect the current with a resistor with a lower resistance value.

3-1. Current sensor with core

In magnetic current sensors, a sensor that uses a magnetic core to collect and detect the magnetic field around the current line is called a cored current sensor.

Current sensors with core can be divided into three categories: 1. Open-loop type, 2. Closed-loop type, 3. Fluxgate type.

Comparison of 3 categories

Price from low to high: Open loop < Closed loop < fluxgate

Accuracy from low to high: open loop < closed loop < fluxgate

Common advantage

The resistance value of the primary conductor is small, and the heat is small.

The number of parts is small (no other parts are required).

Common disadvantage

Because of the magnetic core material, it is difficult to reduce the height, so the installation location is limited.

Since the magnetic core in principle has hysteresis, the sensor output (bias voltage) at zero current will change before and after applying a large magnetic field, which will cause errors.

In the fluxgate method, there is a probe coil between the magnetic cores that is driven by a high frequency alternating current to be used as a sensor.

Therefore, the unique advantage of the fluxgate approach is that, in principle, no shift will occur regardless of the temperature.

However, due to the configuration of the probe coil, the structure is complex and expensive, and the need for feedback current, so the current consumption is large.

The way to solve the shortcomings of the above cored current sensor is to use the "coreless current sensor".

d, which is suitable for detecting small currents without worrying about heating. However, in the application of tens of A or more large current flow, the following disadvantages will be more significant.

advantage

It is the most mainstream detection method at present, and various companies have accumulated a certain level of technology.

shortcoming

Since there are many components on the primary side (high voltage side), such as an insulated power supply and a bipolar power supply, the design takes time and effort.

Because the heat is proportional to the resistance value, the heat is several times to dozens of times that of other methods, resulting in difficult heat dissipation design.

Due to the large number of parts, it is difficult to wire, and the installation area increases.

One way to address the shortcomings of the shunt resistor + insulated amplifier/insulated ADC method described above is called the magnetic "current sensor".

Magnetic Current Sensor

3-2. Coreless current sensor

The so-called coreless current sensor is an extremely simple current sensor that does not use a magnetic core to detect the magnetic field generated by the primary conductor current, but directly detects through the Hall element and amplifies and corrects its output voltage through the IC.

It was developed as a sensor that can solve problems caused by magnetic cores while maintaining the advantages of current core sensors, and has been used since the first half of 2010 in applications that emphasize dimensional miniaturization.

3-2-1. Common coreless current sensor

Silicon Hall elements are less sensitive, so to compensate for this must take some measures, such as making the primary conductor thinner to increase the magnetic field strength, or increasing the gain of the correction IC.

As a result, there are the following advantages and disadvantages.

advantage

Since there is no magnetic core, the height can be reduced. Simple internal structure, easy to reduce costs.

Since there is no magnetic core, there is no hysteresis error.

Low current consumption.

shortcoming

Because the thinning of the primary conductor leads to a higher resistance value, it is easy to generate heat. High currents cannot be measured.

The sensitivity of Hall elements is low, and the frequency band is designed to be narrow in order to obtain sufficient resolution. As a result, the response is slower.

Increasing the gain of the correction IC also amplifies the offset of the Hall element, making it difficult to improve the accuracy.