Knowledge

Current Measurement in the Drive Unit of Electric Vehicles – Inverters that are More Compact Require Shunts that are More Precise

08/31/2022 Knowledge

As for many other devices and applications, smaller and smaller components are required for electric cars. In the case of inverters for the control unit of the electric motor, miniaturization entails higher switching frequencies. This increases the influence of parasitic inductance. Low-inductive shunts offer the highest precision for measuring the drive current.

Battery-powered electric vehicles are currently regarded as the basis for future mobility. The concept is as easy, as it is old: A battery stores the electric energy that is used to run an electric drive, which sets the car in motion. The objective of modern technology is a drive unit as efficient as possible, which in the end will help to increase the driving range. At the same time, the charging time should be kept as low as possible.

Mainly brushless three-phase motors are used in current e-cars, because they are maintenance-free, nearly free from wear, and highly effective. Power electronics are needed to control the motor, as the battery provides direct current. Figure 1 shows the schematic description of such a drive unit.

The shown power electronic consists of an inverter module, which transforms the direct current into three phases. So far, this was mainly done with IGBTs, which are silicon-based insulated-gate bipolar transistors. As an output filter an inductor, which is adapted to the inverter, is used.

Approaches to miniaturization

In the course of increasing the driving range, as much weight as possible should be cut down. With higher battery voltages, for example it is possible to use a smaller wire cross section. To shrink the completely electric system, silicon carbide (SiC) technology offers the possibilities of higher switching frequencies and lower switching losses together with a smaller architecture. The higher switching frequencies also offer the advantage that smaller passive components can be used, including a smaller coil in the output filter.

In order controlling the drive current properly, the motor control unit needs to have the actively flowing current as an input parameter. Thus, the current needs to be measured continuously. The most accurate results are obtained with a current-measuring resistor (shunt). So-called metal plate shunts in the milliohm to microohm range are most frequently used here. They normally consist of an alloy and are suitable for particularly high currents. The principle of measurement is quite simple: If a current is flowing through a resistor, there will be a measurable voltage drop. As the ohmic value of the resistor is known, the flowing current can be measured with these two values by using Ohm’s law.

In practice, many factors can influence the measurement and lead to a false measuring result if not taken care of. For high frequency currents, the parasitic inductance has the biggest influence. Having a change in the amount of flowing current, it induces an electromotive force (EMF). The stronger the current change in a certain period or the shorter the time period of a certain current change is, the bigger the EMF will be. Figure 2 shows a typical current and voltage across a resistor with high parasitic inductance.

As the voltage is meant to be measured with the shunt, the induced EMF needs to be kept as low as possible to reduce the error in measurement. The higher the switching frequency will be, the higher the influence of the parasitic inductance will turn out. That is why, especially for SiC inverters, shunts with particularly low parasitic inductance should be used to guarantee a precise measurement of the current.

Another possibility is to choose a higher ohmic value, so that the influence of the parasitic inductance will be smaller compared to the voltage drop, as the measured signal will be much higher. On the other hand, this will lead to higher electric losses due to the higher energy consumption. Therefore, it is more advantageous to look out for a shunt with especially low parasitic inductance from the beginning. First, very flat SMD (surface-mount device) metal plate shunts offer a very low parasitic inductance, due to their structural shape and material.

As an initial tolerance for current sensing resistors, a value of ±1% was established. To reach this value during production, the resistor needs to be trimmed. Special trimming processes that work without cutting into the resistive element offer the best solution here. With the TLR and PS series from KOA Europe, Rutronik offers two particularly low-induction shunt series.

For the TLR series, KOA has implemented a trimming process completely without cutting into the material, which leads to parasitic inductance values in the range of 0.1 nH. The PS series is designed for much higher currents, up to 244 A. These values are a bit higher, but significantly below 1 nH. Therefore, these parts are perfectly suited for current measurement in the drive unit of electric vehicles, especially when SiC-based inverters are used.

 


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Schematic description of a drive unit for a battery electric vehicle (picture source: KOA Europe)

Voltage and current across a shunt with 50 mΩ and 10 nH (picture source: KOA Europe)