question:
How to measure inductor current?
Answer:
Switching power supplies typically use inductors to temporarily store energy. When evaluating these power supplies, measuring the inductor current is often helpful in understanding the complete voltage conversion circuit. But what's the best way to measure inductor current?

Figure 1 shows the recommended setup for this type of measurement using a typical buck converter (buck topology) as an example. Connect a small auxiliary cable in series with the inductor. Use it to connect a current probe and display the inductor current through an oscilloscope. It is recommended to measure on the side of the inductor that has a stable voltage. The way most switching regulator topologies use inductors is that the voltage on one side switches between two extremes, while the voltage on the other side remains relatively constant. For the buck converter shown in Figure 1, the voltage on the switching node (ie, to the left of inductor L) switches between the input voltage and ground at the rate of switching edges. To the right of the inductor is the output voltage, which is usually relatively stable. To reduce interference due to capacitive coupling (electric field coupling), the current measurement loop should be placed on the quiet side of the inductor, as shown in Figure 1.

Figure 1. Schematic diagram of inductor current measurement in a switching power supply.

Figure 2 shows the actual setup used for this measurement. Lift the inductor up and diagonally solder one of the two terminals to the board. The other terminal is connected to the board with an auxiliary wire. This conversion can be done easily. Hot air desoldering is a proven method for removing inductors. Many SMD rework stations offer temperature-adjustable hot-air processing.

Figure 2. Actual setup for inductor current measurement.

Current probes are provided by the oscilloscope manufacturer. Unfortunately, they are usually very expensive, so the question is constantly being raised whether it is also possible to measure the inductor current through a shunt resistor. In principle this is possible. However, the disadvantage of this measurement method is that the switching noise generated in the switching power supply can easily be coupled into the voltage measurement through the shunt resistor. So, especially at the point of interest, the measurement doesn't really represent the behavior of the inductor current when it changes direction.
Figure 3. Inductor current measurements are shown in blue, with saturated inductor behavior shown in additional purple.

Figure 3 shows the measurement of the inductor current (blue) of the switching power supply detected by a current probe compatible with the oscilloscope used. In addition to the measurement shown in blue, a purple marker has been added, which indicates the current flow through the inductor as it begins to approach peak current and goes into oversaturation. This happens when an inductor is chosen that does not provide enough current rating for a given application. One of the main reasons for taking inductor current measurements in switching power supplies is that it can help identify if the inductor is selected correctly, or if inductor saturation will occur during operation or under fault conditions.
Measuring with a shunt resistor instead of a current clamp will introduce strongly coupled noise, especially at peak currents, making detection of inductor saturation very difficult.
The detection of coil current is very useful in power supply evaluation and can be easily implemented with suitable equipment.