How to measure the inductor current in a switching power supply

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How to measure the inductor current in a switching power supply [Copy link]

Switching power supplies often use inductors to temporarily store energy. When evaluating these power supplies, measuring the inductor current often helps to understand the complete voltage conversion circuit. But what is the best way to measure inductor current?

Figure 1 shows the recommended setup for this type of measurement using a typical step-down converter (buck topology) as an example. A small auxiliary cable is connected in series with the inductor. This is used to connect a current probe and display the inductor current on an oscilloscope. It is recommended to measure on the side of the inductor that has a stable voltage. Most switching regulator topologies use the inductor in such a way that the voltage on one side switches between two extremes while the voltage on the other side remains relatively stable. For the step-down converter shown in Figure 1, the voltage on the switching node (i.e., the left side of the inductor L) switches between the input voltage and ground at the rate of the switching edge. On the right side 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. The inductor is lifted and one of the two terminals is soldered diagonally to the board. The other terminal is connected to the board via an auxiliary wire. This transition can be easily accomplished. Hot air flow desoldering is a well-established method for removing inductors. Many SMD rework stations offer hot air flow processing with adjustable temperature.

Figure 2. Practical setup for inductor current measurement.

Current probes are provided by oscilloscope manufacturers. Unfortunately, they are usually very expensive, so a question is constantly raised whether the inductor current can also be measured via a shunt resistor. In principle, this is possible. However, a disadvantage of this measurement method is that the switching noise generated in a switching power supply can easily couple into the voltage measurement via the shunt resistor. Therefore, especially at the point of interest, when the inductor current changes direction, the measurement result does not truly represent the behavior of the inductor current.

Figure 3. The inductor current measurement is shown in blue, with the behavior of the saturated inductor shown in additional purple.

Figure 3 shows the measurement of the inductor current (blue) of a switching power supply as detected by a current probe compatible with the oscilloscope used. In addition to the measurement shown in blue, a purple marker is added, which indicates the current condition flowing through the inductor as it begins to approach the peak current and enters excessive saturation. This occurs when the selected inductor does not provide enough rated current for a given application. One of the main reasons for making inductor current measurements in switching power supplies is that it can help identify whether the inductor has been selected correctly or if inductor saturation will occur during operation or during fault conditions.

Using a shunt resistor instead of a current clamp for measurement will result in strong coupled noise, especially at the peak current, which makes it very difficult to detect inductor saturation. (From Analog Dialogue on the ADI website)