# Inductor Current Measurement in Switched Power Supplies

### QUESTION:

How do you measure inductor current?

Switched-mode power supplies commonly use inductors for temporarily storing energy. In the evaluation of these power supplies, it is often useful to measure the inductor current to gain a complete picture of the voltage conversion circuit. But what is the best way to measure the inductor currents?

Figure 1 shows a suggested setup for such a measurement using the example of a typical step-down converter (buck topology). A small auxiliary cable is inserted in series with the inductor. It is used to attach a current probe and display the inductor current with an oscilloscope. It is recommended that measurements be made on the side of the inductor where the voltage is stable. Most switching regulator topologies use the inductor in such a way that the voltage on one side is switched between two extreme values but remains relatively stable on the other side. With the buck converter shown in Figure 1, the voltage at the switching node—that is, the left side of inductor L—switches between the input voltage and the ground voltage at the speed of the switching edges. On the right side of the inductor is the output voltage, which is usually relatively steady. To reduce the 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 illustrating measurement of the inductor current in a switched mode power supply.

Figure 2 shows the practical setup for this measurement. The inductor is lifted and obliquely soldered back with one of the two terminals on the board. The alternate terminal is connected to the board with the auxiliary wire. This conversion can be accomplished quite easily. Desoldering with hot air is a proven method for detaching the inductor. Many SMD rework stations offer hot air with an adjustable temperature.