Method for measuring the saturation current of inductors and transformers - current probe

We often emphasize repeatedly: whether it is an ACDC converter or a DCDC converter, the saturation current of the transformer or inductor must be checked. Its saturation current must be greater than the OCP current set by the system and ensure sufficient margin.

The manufacturer's data sheet of the inductor usually gives the saturation current of its product, while the ACDC transformer, such as the flyback converter transformer, is basically designed by the engineers themselves. During the design process, the air gap size of the inductor core is adjusted based on the rounded primary turns or inductance, and the actual saturation current value of the transformer is rarely checked.

So, how can engineers measure the saturation current of an inductor or transformer by themselves in the laboratory?

Over the past few decades, I have communicated with many engineers and shared this little trick with everyone. Now, I will explain it in detail here.

1. Steps for measuring the saturation current of an inductor or transformer

The steps to measure the saturation current of an inductor or transformer are as follows:

Figure 1. Inductor saturation current measurement

1. Use wire L1 to connect the positive terminal of the DC power supply and pin A of the inductor. The end point of wire L1 and pin A of the inductor can be welded together.

2. Set the DC power supply output voltage to 10V and set the current limit of the DC power supply. You can first set a smaller value, such as 1A.

3. Connect the current probe to the interface of the oscilloscope. The bayonet end of the current probe is clamped on the wire L1. Pay attention to the direction of the current probe, which should be consistent with the direction of the measured current.

4. Connect one end of wire L2 to the negative terminal of the DC power supply.

5. Set the oscilloscope to trigger on the channel to which the current probe is connected, and set the trigger value to a lower value, such as 0.2A. At the same time, set the oscilloscope to single trigger.

6. Press the DC power output button to output DC voltage; hold the other end of wire L2 with your hand, let the wire head touch the B end of the inductor, and then quickly remove wire L2 to break contact between the wire head and the B end of the inductor.

7. Reduce the time base appropriately and observe the waveform on the oscilloscope. If the inductor current waveform becomes flat later, increase the current limit of the DC power supply and repeat steps 5 and 6 until the inductor current waveform shown in Figure 2 appears.

8. Measure the inflection point of the inductor current waveform, which is approximately the saturation current of the inductor. In Figure 2, the inflection point is approximately 13A, and the saturation current of the inductor is approximately 13A.

The time base (X-axis) and current scale (Y-axis) can be set to larger values ​​at the beginning, and then, based on the measured waveform, gradually narrow them to a suitable range to ensure that the complete current waveform can be seen.

Figure 2 Inductor current waveform, L=10uH, Vin=10V

The measurement process of the transformer primary saturation current is the same as above, except that the applied voltage can be higher, such as 20V or higher.

2. Measurement Principle

When a voltage is applied across the inductor, the inductor becomes magnetized and the inductor current increases linearly with time:

L*dt/dt=V

When the inductor is saturated, L suddenly decreases sharply, and the excitation voltage remains unchanged. Then, the rate of change of the inductor current di/dt will increase sharply:

dt/dt=V/L

The inflection point where the rate of change of the inductor current di/dt increases sharply is the saturation current of the inductor.

How to select a current probe

1. Current type: DC/AC or AC type current probe;

2. Current size: including maximum current and minimum current. The maximum current also needs to consider the frequency issue;

3. Bandwidth: Not only the frequency of the waveform but also the speed of the rising edge should be considered;

4. Accuracy: ≥1% when used with an oscilloscope, ≤1% when used with a power analyzer;

5. Diameter of the wire being measured: determines the size of the jaw aperture;

6. The size of the measured space: determines the shape and size of the jaws;

7. Jaw structure: open or closed;

8. Temperature range: Pay attention to the temperature range of the current probe and select a suitable probe.