Some techniques for measuring inductor current

Switching power supplies can be seen everywhere, so how to measure the current of switching power supplies? Switching power supplies often use inductors to temporarily store energy. When evaluating these power supplies, measuring the inductor current often helps 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 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 on an oscilloscope. It is recommended to measure on the side of the inductor with a stable voltage. Most switching regulator topologies use an inductor in such a way that the voltage on one side switches between two extreme values, while the voltage on the other side remains relatively constant. For the buck converter shown in Figure 1, the voltage at the switching node (ie, to the left of the inductor L) switches between the input voltage and ground at the rate of the switching edge. 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.

Figure 2 shows the actual setup used for this measurement. Lift the inductor up and bias-solder one of the two terminals to the circuit board. The other terminal is connected to the circuit board via 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 flow processing.

Current probes are provided by the oscilloscope manufacturer. Unfortunately, they are usually very expensive, so a question that is constantly raised is whether the inductor current can also be measured 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 couple into the voltage measurement through the shunt resistor. So, especially at the point of concern, the measurements don't really represent the behavior of the inductor current when it changes direction.

Figure 3 shows a 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 measurements shown in blue, a purple marker has been added, which indicates the current conditions flowing through the inductor as it begins to approach oversaturation with peak current. This occurs when the inductor selected does not provide sufficient current rating 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 chosen correctly or if inductor saturation is occurring during operation or under fault conditions.

Measurements using shunt resistors instead of current clamps will exhibit strong coupling noise, especially at peak currents, making detection of inductor saturation very difficult. Detection of coil current is very useful in power supply evaluation and can be easily achieved with suitable equipment. The above is the best method for measuring inductor current, and I believe it will be of certain reference significance to designers.