How to use a digital oscilloscope to produce the dynamic B-H Curve of magnetic components?
Keywords: Magnetic components, B-H Curve, Magnetic saturation
Preface:
This article will elaborate how to use a digital oscilloscope to produce magnetic flux density (B) and magnetic field strength (H), and discuss the differences between static (Sine Wave) B-H curve measurement and dynamic excitation source measurement.
Inductors and transformers are common magnetic components in power circuits and play the role of energy storage and energy conversion. Magnetic components are composed of windings and magnetic cores. The magnetic permeability (μ), temperature characteristics, frequency characteristics and structure of the magnetic material determine the specifications of the magnetic components.
How to use a digital oscilloscope to input voltage and current into two formulae to calculate magnetic flux density (B) and magnetic field strength (H)
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Figure 1: Structure of toroidal inductor |
H:Magnetic field strength B: Magnetic flux density n: Windings
AC: Cross Section Area The digital oscilloscope has the function of advanced calculations. By setting the formula and inputting variables such as n,, AC, voltage and current and using calculations such as multiplication, division and integration, the magnetic field intensity H can be obtained via Formula 1 and the magnetic flux density B can be calculated via Formula 2.The magnetic field strength (H) is proportional to the current; the magnetic flux density (B) is proportional to the voltage integral. |
Figure 2: Setting screens for variables such as n、
and AC in GDS-3000A power supply analysis
Finally, the B-H curve can be obtained by displaying the converted B and H in X-Y mode.
Understand the terminology used in B-H curve
Magnetic hysteresis is a ferromagnetic material. Under the action of an external magnetic field (H), its magnetic dipole (M) intensity will increase along the direction of the external magnetic field. When the intensity of the external magnetic field is greater, the magnetization of the magnetic material is stronger, the magnetic flux density (magnetic dipole strength) will also be greater, and it will eventually reach magnetic saturation.
Magnetic saturation (Ms): Even if the external magnetic field is increased, the magnetism of the ferromagnetic material itself cannot be increased. Magnetic remanence (Mr) or remanence (Retentivity): When the intensity of the external magnetic field decreases, the strength of the magnetic dipole (M) will also weaken. When the external magnetic field drops to zero, the magnetic dipole of the magnetic material will not be zero and still retains some magnetization.
When the external magnetic field in the opposite direction increases, the remaining magnetization intensity (M) will decrease again. Only when the external magnetic field in the opposite direction reaches a certain intensity can the residual magnetic dipole disappear (that is, the magnetization intensity becomes zero). The strength of the magnetic field applied in the opposite direction is called coercivity, Hc.
When the external magnetic field in the opposite direction continues to increase, the magnetic material will produce non-zero magnetization intensity (M). As the strength and direction of the external magnetic field changes, the magnetic dipole of the magnetic material will have specific changes, which is called the hysteresis curve or B-H Curve. Note 1~3
Figure 3：B-H Curve
The difference between static (Sine Wave) B-H curve measurement and dynamic excitation source measurement
Static measurement uses a fixed-frequency as the excitation source. The design goal of magnetic components is to operate normally under the maximum peak current, that is, magnetic saturation cannot occur. The actual maximum excitation current should be measured on the actual dynamic current waveform to obtain the true working state. This true working state includes the actual temperature conditions. Magnetic substances have a so-called Curie temperature. The Curie temperature of each material is different. When a magnetic substance exceeds its Curie temperature, the magnetic dipoles inside the substance will gain enough energy to break away from their alignment, changing the orderly alignment direction to disorderly, and the magnetism disappears Note 4.
Displays below show the measurement results of multiple switching cycles of the MOSFET. The B-H curve measurement results of GDS-3000A are close to the test results of Lecroy WaveRunner 8108HD.
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GDS-3000A B-H curve |
Lercory WaveRunner 8108HD |
Figure 4: Important specifications of GDS-3000A
Other than the B-H Curve measurement function, GDS-3000A provides a total of 13 power supply measurement functions (as shown in Figure 5) that completely cover AC input, DC output, Switching component analysis, Magnetics analysis and frequency response analysis to accelerate power supply verification.
Figure 5: GDS-3000A power supply analysis screen
References:
Note 1: Magnetic hysteresis from Wikipedia https://zh.wikipedia.org/zh-tw/%E7%A3%81%E6%BB%9E%E7%8E%B0%E8%B1%A1
Note 2: National Chiao Tung University Physics Experiment Manual B-H Curve (Experiment 19)
Note 3: National Tsing Hua University Department of Physics website magnetic hysteresis
Note 4: National Taiwan University Science Online https://highscope.ch.ntu.edu.tw/wordpress/?p=29468
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