Realization of large DC current measurement

Although there are many instruments that can measure small direct currents (3A), few instruments can measure (better than 1%) direct currents above 50A. Such a large current range is a typical load value for electric vehicles (EV), grid energy storage, and photovoltaic (photovoltaic) renewable energy devices. In addition, these systems need to predict the state of charge (SOC) of the relevant energy storage battery. The estimation of the charge state can be realized based on the current and charge (coulomb count) measurement, and the measured data is a necessary condition for the charge state estimation.

Generally speaking, any system for current or charge measurement is designed to include built-in data acquisition components, such as appropriate amplifiers, filters, analog-to-digital converters (ADC), etc. The current sensor is used to detect current. The output of the current sensor needs to be converted into a usable form (ie voltage) by a circuit. The signal is then filtered to reduce electromagnetic and radio frequency interference. Then zoom in and digitize. Then multiply each current data sample by the appropriate time interval, and (by digital calculation) accumulate the charge value.

On the other hand, if digitization is performed at a constant frequency, the current samples accumulated first, and then when the accumulated charge value is read out or used in some way, is it multiplied by the appropriate time interval. At the same time, it is necessary to consider selecting a suitable Nyquist sampling rate and using a sufficiently narrow anti-aliasing filter before the analog-to-digital converter.

Figure 1: The signal chain in a typical modern current measurement system.

Practical sensor technology for large current measurement

In the technology used to measure large currents, there are two common sensor technologies. One technique is to detect the magnetic field around a conductor carrying current. The second technique is to measure the voltage drop across the resistor (often called a shunt) that carries the current (and charge) to be measured. This voltage drop follows Ohm's law (V = I × R).

The device used for large current measurement is usually called Hall effect current sensor. This sensor has a built-in current-carrying element. When current and an external magnetic field are applied to the element, a pressure difference perpendicular to the direction of the current and perpendicular to the direction of the external magnetic field will appear on both sides of the element. The Hall effect pressure difference in ordinary metals is very small. It is worth noting that not all DC current sensors that measure the magnetic field around current-carrying conductors are based on the Hall effect. The differences between them will be briefly introduced below.

High current Hall effect sensor

In order to make a current sensor with a Hall effect device, a magnetic core is needed to concentrate the magnetic field around the conductor current, and a slot is required in this magnetic core to accommodate the actual Hall element. A slot with a relatively small size (relative to the entire magnetic circuit length) will form a nearly uniform magnetic field perpendicular to the plane of the Hall element. When the Hall element gains current energy, it will generate a voltage proportional to the excitation current and the magnetic core magnetic field. This Hall voltage is amplified and output from the output terminal of the current sensor.

Figure 2: Schematic diagram of the magnetic field around the conductor, the linear open-loop Hall effect sensor and the closed-loop sensor.

Since there is no electrical connection between the current-carrying conductor and the magnetic core (only the magnetic field is coupled), the sensor is actually isolated from the circuit under test. The current-carrying conductor may have a very high voltage, and the output of the Hall-effect current sensor can be safely connected to the ground circuit, or connected to a circuit of any potential relative to the current-carrying conductor, thus providing clearance and creepage values that meet strict safety standards It is also relatively easy.

However, this linear sensor also has some disadvantages. The unimportant disadvantage may be the fact that the Hall-effect sensor requires a constant excitation current. In addition, the amplification and adjustment circuits that process the signal from the Hall-effect sensor usually consume significant energy. Of course, this energy consumption may not be so significant, depending on the specific application. Nevertheless, the energy consumption of Hall sensors for continuous current measurement cannot be as small as milliwatts.

Hall effect sensor: large drift, small available operating temperature range

Because the output of a typical linear sensor is measured proportionally (not only depends on the measured magnetic field strength, but also depends on the value of the excitation current), the stability of the excitation current will greatly affect the magnitude of the current to be measured and the zero when no current flows. Offset. Generally speaking, the latter two depend on the stability of the supply voltage and temperature changes (because the Hall sensor element resistance that affects the excitation current and the Hall voltage itself depends on the operating temperature).

A sensor variant that measures the excitation current and takes this factor into account in the output is possible. But it requires sophisticated external components and larger processing circuits. Moreover, the Hall voltage is a non-linear function of the magnetic field to be measured, which further increases the error of the sensor.

Because different errors will occur under different conditions, most manufacturers of linear Hall-effect devices will decompose the total error into many individual components. Sometimes it is difficult to calculate the total composite error.

Closed loop current sensor

In order to solve the nonlinear problem of Hall sensor elements, the industry has developed another technology. This technique relies on detecting the presence or sign of the magnetic field in the sensing core, rather than measuring the strength of this magnetic field. In addition, it can avoid measurement errors caused by the unstable excitation current in the Hall element.

This technique is to add a winding to the magnetic core to generate a magnetic field with the opposite sign, but the intensity is exactly the same as the magnetic field generated by the current to be measured. Now the Hall sensor element is only used to detect the magnetic field sign and not the magnetic field strength. This winding is connected to a circuit with an op amp. The circuit maintains the current in this compensation winding and makes the magnetic field sensed by the Hall sensor zero. The current in the compensation winding is many times smaller than the current in the conductor under test (perhaps more than 1000 times). This function can be realized by only winding a few more turns on the magnetic core when making the winding, and the number of turns can be controlled.

In view of the role of the compensation winding in the feedback loop of the op amp, this type of current sensor is often referred to as a "closed loop" sensor. In contrast, the aforementioned simple linear Hall-effect sensors are often considered "open loop" sensors in order to emphasize that there is no feedback mechanism during their operation.

In the Hall-effect device, the (offset) error in detecting the zero magnetic field cannot be reduced to an arbitrarily small value. This is due to various drifts, and most of them are due to temperature-dependent drift. This is also the reason why some higher-performance current sensors do not rely on the Hall effect technology. However, these sensors are still generally referred to as Hall-effect sensors, simply because they are very similar in appearance to Hall-effect devices.

Other magnetic field detector

In non-Hall devices, some sensors based on various physical phenomena can be used to perform the function of a magnetic field detector. One of these technologies is based on the magnetoresistive effect, which means that when a magnetic field is applied to the sensor, the resistance of the sensor changes.

Another technique for magnetic field detectors uses the non-linear properties of ferrite between the magnetic field strength (represented by H), the magnetic flux density (represented by B), and a special phenomenon called saturation. When the H field increases, the magnetic flux density B will eventually reach a point where it no longer increases significantly-this point is called the saturation point. Some specially formulated materials have very low saturation points, and they are widely used in devices called fluxgates.

In fact, a fluxgate-based sensor can convert a constant magnetic field into a "gated" or "cut" magnetic field that alternates between full scale and almost zero. This magnetic field change can be easily picked up by a winding on the magnetic core, and then amplified by an AC amplifier. Use the so-called synchronous detection (because the circuit itself controls the cutting action) technology to restore the value proportional to the constant magnetic field to be measured.

It is worth noting that the complexity of the mechanical structure and related circuits of this sensor is much higher than that of a closed-loop sensor. In addition, their work is very difficult-when the sensor does not receive energy, or the compensation winding circuit is open due to the loose connection with the external detection resistor, the current measurement often leads to irrecoverable offset and gain indicators. Since the compensation winding cannot cancel the magnetic field from the current to be measured, the magnetic element in this sensor will be magnetized.

Need precision resistance

The output signal of the closed-loop sensor is the current in the compensation winding (its value is many times smaller than the current to be measured). This current is usually converted into a voltage value, and then further processed and digitized. At this time, just use ordinary resistors.

However, the accuracy and stability of this resistance will directly affect the accuracy and stability of the closed-loop current sensor. If a detection resistor with 1% accuracy is used, a closed-loop sensor with a basic accuracy of 0.0.01% will soon be reduced to 1% accuracy.

But it is difficult to buy a certain commercial quantity of resistors with an accuracy higher than 0.01%, even if they only work in a narrow temperature range.

high current shunt

As mentioned earlier, the second current measurement technique uses a voltage drop across a resistor. When determining the current according to Ohm's law, a unique set of factors need to be considered, which are related to the current size. For relatively small currents, the voltage drop on the shunt resistor can be made quite large to overcome any error caused by the heat dissipation of the detection connection and the shunt resistor or the temperature difference caused by the working environment. However, when the current exceeds 50A, heat dissipation and thermoelectric errors are important. Similarly, since the shunt resistor is always heated by the current flowing through it, and may work in an environment with unstable temperature, the stability of the shunt resistor's resistance with respect to temperature is particularly important.

The physical composition of the shunt

At first glance, the shunt device is a simple resistor. Some conductive materials with appropriate properties in terms of volume resistivity, (temperature and time) stability, and appropriate mechanical shape can be used as shunt resistors. The low-precision shunt resistor can be a length of wire or a rectangular shape constructed with a suitable alloy, and simply welded in series with the current-carrying conductor (or in some kind of electrical connection). However, it is almost impossible to insert such a shunt element into the measurement circuit without affecting its resistance (due to changes in the amount of solder at the connection point, or changes in the mechanical details of the connection).

In addition, for stability reasons, it is very beneficial to arrange the shunt resistors in such a way that the current density in any given cross-section of the shunt resistors is mostly uniform. This prevents the formation of so-called hot spots-defined as the area inside the shunt resistor that has a higher temperature than the rest of the material. In addition to simple resistance changes, the elevated high temperature at the hot spot may bring the resistive material to the annealing point temperature, at which the resistance value of the material (achieved by careful control of chemical composition and processing) may begin to change.

Even if the actual presence of hot spots does not affect the accuracy, it is impossible to ensure that they are formed in exactly the same place when calibrating the shunt resistor. Therefore, the shunt resistor design includes the method of evenly distributing the current between the cross section of the resistive material, or between a single parallel resistive part and the inside of each part.

This is the reason why most high-precision shunt resistors consist of three different parts: the two areas are terminals for access to the circuit (almost always made of thick, high-conductivity materials, such as copper), and One zone or multiple parallel zones make up most of the shunt resistance. The two terminal areas are connected with a resistance section or a section using welding or metallurgical processes, and have a very uniform seam.

The resistive part (also called the effective part) material of the precision shunt resistor must have resistance characteristics that are low in temperature dependence. Because of its suitable resistance and low temperature resistivity (TCR), one of the common alloys used for precision shunt resistors is the manganese copper developed in 1892 by Edward Weston (famous for the development of the electrochemical battery-Weston battery).

Heat dissipation in shunt resistor

The heat dissipated by a resistor is proportional to the square of the current and the resistance (W = I2 × R). For example, a 1mΩ shunt resistor has a power consumption of 2.5W when a current of 50A flows through it. This power consumption is a controllable value with a moderate radiator and still air. On the contrary, when the current is 1kA, the same shunt resistor will dissipate 1kW of heat, which requires a large physical size and may be forced to air-cooled (or liquid-cooled) devices.

Figure 3: The relationship between the heat dissipated in the shunt resistor and the resistance and current.

Figure 4: The relationship between the heat dissipated in the shunt resistor and the full-scale output voltage and current.

It should be clear from the above figure that the way to reduce the heat dissipated in the shunt resistor under a given current condition is to reduce its resistance. However, this will also reduce the voltage value measured on the shunt resistor, and the signal will become more sensitive to errors caused by the shunt resistor and the detection circuit, resulting in a degradation of accuracy in the case of small currents.

The source of error in the shunt measurement method

The high operating temperature and the temperature difference in the shunt resistor will negatively affect the gain and offset errors. For a shunt-based measurement system, not only the ambient temperature plays a role, but the measured current itself also plays a role, because large currents heat the shunt resistor.

Although the resistance (effective) part of the shunt element is made of low TCR materials, the high operating temperature will inevitably promote the resistance value to deviate from the calibration value, no matter how small the change is. This will produce sensitivity (gain) errors.

Since different materials are used in the shunt resistor structure (that is, the materials of the connecting terminal and the detection wire are generally different from the material of the resistance value of the shunt resistor), there is a so-called thermoelectric error (such as the Seebeck effect), which will affect the bias. Shift error (there is a current reading when the actual current is zero). Since the heat dissipation effect of shunt resistors can be measured and expressed in a predictable manner, some systems based on shunt resistors can compensate for the thermal effects of shunt resistors that cause offset and gain errors. In any case, when designing a current measurement system based on shunt resistance as shown in Figure 1 (the signal chain of a typical modern current measurement system), it is necessary to carefully select components that can provide errors and drift.

Choose the right measurement method

For measuring large DC currents, the basic problems are measurement accuracy and cost. Other important considerations include: working environment (especially temperature range), power consumption, size and durability (considering possible overload, transients and unstimulated operation). In order to judge the measurement accuracy of any given method, it is important to consider all possible sources of error under all relevant extreme operating conditions.

Table 1: Comparison of current dividers.