Ferrite Magnetic Core Testing

Ferrite core inspection mainly conducts scientific testing and analysis on the initial permeability, saturation flux density, power loss, Curie temperature, amplitude permeability and hysteresis loop parameters, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Initial permeability: The initial permeability is the characterization of the magnetization capability of the ferrite cores under the weak magnetic fields, defined as the ratio of the magnetic flux density to the magnetic field strength at the starting point of the magnetization curves. This parameter directly affects the inductance design of the inductors, and is the core indicator of evaluating the performance of the soft magnetic materials under the low fields. The testing is usually carried out at the specific frequencies and temperatures, using the impedance analysers or LCR meters to measure the inductance values of the coils with the cores under test, and then calculated through the formulas.

Saturation flux density: The saturation flux density refers to the maximum magnetic flux density that the ferrite core materials can reach under the given magnetic field strengths. It determines the capability of the cores to withstand the DC bias or large currents without saturation in the power applications, and is the key boundary parameter of the switching power supply transformer and choke designs. The testing method is usually to wind the coils on the toroidal cores, apply the gradually increasing DC or low-frequency AC magnetic fields, and determine it by measuring the flux changes until they no longer significantly increase.

Power loss: The power loss mainly refers to the energy loss generated by the cores in the alternating magnetic fields due to the hysteresis, eddy currents and residual losses, usually expressed as the power values per unit volume or unit mass. This parameter is directly related to the temperature rises and overall energy efficiencies of the cores during the operation, and is particularly crucial in the high-frequency switching power supply applications. The testing is often carried out using the power analysers or dedicated core loss testers under the specified sinusoidal flux density, frequency and temperature conditions.

Curie temperature: The Curie temperature is the critical temperature point at which the ferrite materials transform from the ferromagnetism to paramagnetism. Above this temperature, the magnetism of the material will basically disappear. This parameter determines the maximum safe operating temperatures of the cores, and has the decisive effects on the application stability in the high-temperature environments. The testing is usually determined by measuring the inflection point at which the permeability or saturation magnetization sharply decreases with the temperature rise.

Amplitude permeability: The amplitude permeability refers to the permeability measured under the specific alternating magnetic field strength amplitudes, reflecting the magnetic properties of the material under the larger alternating magnetic fields. Unlike the initial permeability, it is more suitable for evaluating the performance of the cores under the non-weak-field conditions such as the power conversion. The testing needs to be obtained by measuring the ratio of the magnetic flux density to the magnetic field strength under the specified magnetic field strength amplitudes and frequencies.

Hysteresis loop parameters: The hysteresis loop is the closed curve describing the relationship between the magnetic flux density and magnetic field strength of the ferrite cores during the alternating magnetization processes. From it, the multiple key parameters such as the remanence, coercivity and hysteresis loss can be extracted. These parameters comprehensively reflect the magnetization and demagnetization difficulty degrees and energy loss characteristics of the material. The testing usually uses the B-H analysers or observes the loop patterns output by the integration circuits through the oscilloscopes.

Testing Scope

Toroidal cores: The toroidal cores are the most common testing objects. Their magnetic circuits are closed without the air gaps, with the extremely small leakage flux, very suitable for the precise measurement of the intrinsic magnetic properties of the material, such as the initial permeability, saturation flux density and power loss. The preparation and winding of the toroidal samples are relatively standardized, with the strong comparability of the testing results. It is the preferred shape of the material development and incoming inspection.

E-type, U-type and other special-shaped cores: The cores such as the E-type, U-type and RM-type usually have the air gaps or are the combined structures, widely applied to the transformers and inductors. For the testing of this type of cores, the structural factors such as the effective magnetic path lengths and equivalent air gaps need to be considered, and the effective permeability, DC superposition characteristics and the overall performance after the assembly are emphatically tested, which is closer to the actual application scenarios.

Wide temperature range performance testing: The properties of the ferrite cores change significantly with the temperature. The testing scope needs to cover the wide temperature ranges from the low temperatures (such as -40°C or -55°C) to the high temperatures (such as +100°C to +150°C and above). The main examinations are the permeability temperature coefficients, power loss temperature characteristics and Curie temperatures, to ensure the reliability of the devices under the extreme environments.

Wide frequency band performance testing: The operating frequencies of the modern electronic equipment range from tens of kHz to several MHz or even higher. The testing needs to cover the wide frequency band range from the low frequencies (such as 10kHz) to the high frequencies (such as several MHz), emphatically evaluating the frequency response characteristics of the cores, including the variation laws of the permeability with the frequencies and the power loss densities at the different frequencies (such as the Pcv curves).

DC bias characteristic testing: In the actual circuits, the cores often simultaneously withstand the AC excitation and DC bias magnetic fields. The testing needs to include the performance under the DC bias conditions, that is, measuring the inductance drop curves (L-I curves) or permeability changes of the cores when the different DC currents are superimposed. This is the key of evaluating the anti-saturation capabilities of the cores for the power inductors.

Mechanical and physical property testing: In addition to the electromagnetic properties, the testing scope should also include the physical characteristics of the cores, such as the dimension tolerances, apparent densities, flexural strengths, hardnesses and thermal expansion coefficients. These properties affect the mechanical strengths, processability, assembly consistency and structural stability of the cores in the temperature cycles.

Testing Methods

AC bridge method: The AC bridge method is a classic precision measurement method, mainly used to measure the initial permeability and loss angles of the cores under the weak fields. Through the construction of the balanced bridge circuits, the equivalent series inductances and resistances of the coils with the cores under test are measured. This method has the high precision, suitable for the material property research under the low flux densities, but the testing frequency ranges are limited by the bridges.

Volt-ampere method: The volt-ampere method is the direct measurement method based on the electromagnetic induction law. The sine wave voltage excitation is applied to the primary windings of the toroidal cores, the magnetic flux densities B are obtained through the integration circuits (or digital integration), and at the same time, the magnetic field strengths H are sampled through the series resistors, thereby drawing the B-H curves and calculating the various parameters. This method has the intuitive principles and is the common method of measuring the dynamic hysteresis loops and power losses.

Impedance analyser method: Using the impedance analysers or precision LCR meters, the complex impedances of the core coils can be automatically scanned and measured in the wide frequency ranges. Through the analysis of the real and imaginary parts of the impedances, the complex permeabilities of the cores (the real part μ' and imaginary part μ'') can be calculated, thereby separating the magnetic loss components. This method has the high automation degree and fast data acquisition, suitable for the frequency characteristic analysis.

Power meter method: The power meter method is the practical method of directly measuring the total losses of the cores. The test coils with the cores are connected as the loads into the output loops of the power amplifiers, and the wide-band power analysers are used to directly measure the active power input to the cores. The results directly reflect the total losses under the specific excitation conditions, often used for the product acceptance and comparative testing.

Pulse testing method: The pulse testing method is used to evaluate the saturation characteristics of the cores under the large currents or DC bias. Through the application of a rapidly rising current pulse to the core windings, the oscilloscopes are used to observe the voltage waveform changes at the two ends of the windings. When the cores approach the saturation, the inflection points will appear in the voltage waveforms, from which the saturation flux densities and saturation ampere-turns can be calculated. This method is closer to the actual operating states of the switching power supplies.

Thermal analysis method for the Curie temperature measurement: In addition to the magnetic methods, the thermal analysis methods such as the differential scanning calorimetry (DSC) or thermogravimetric analysis (TGA) can also be used to measure the Curie temperature. During the temperature rise process, the ferrites will have the endothermic or specific heat capacity changes related to the magnetic phase transitions at the Curie points. The starting point of the endothermic peaks on the DSC curves corresponds to the Curie temperature. This method can be used as the supplement and verification of the magnetic measurement methods.

Testing Standards

Domestic Standards:

1. GB/T 9632, Measuring methods of inductors and transformers for telecommunications — Magnetic cores

International Standards:

1. IEC 62044, Cores made of soft magnetic materials — Measuring methods

2. IEC 60424, Ferrite cores — Guide on the limits of surface irregularities

3. IEC 62317, Ferrite cores — Dimensions

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Impedance analyser/precision LCR meter: The impedance analysers and high-end LCR meters are the core equipment of measuring the complex impedances and inductance parameters of the cores. They can provide the high-precision measurements in the wide frequency ranges (from tens of Hz to over a hundred MHz), and have the automatic scanning and equivalent circuit analysis functions. They are the necessary instruments of obtaining the permeability frequency response curves and quality factors (Q values).

B-H analyser: The B-H analyser is the integrated instrument specially used to measure the dynamic hysteresis loops and losses of the soft magnetic materials. It usually has the built-in high-precision power amplifiers, integrators and data acquisition systems, and can automatically carry out the testing under the set flux densities, frequencies and waveform conditions, directly outputting the parameters such as the B-H loops, loss values and permeabilities. The operation is convenient with the good repeatability.

Power analyser and power amplifier: The high-precision wide-band power analysers combined with the linear power amplifiers constitute the standard testing system of directly measuring the core power losses. The power analysers can accurately measure the voltages, currents and active powers under the non-sinusoidal waves, especially suitable for the loss evaluation under the high flux densities and arbitrary waveform excitation conditions.

Oscilloscope and current probe: The high-performance digital oscilloscopes combined with the high-bandwidth current probes and differential voltage probes are the key tools of carrying out the pulse testing, observing the transient waveforms and performing the manual volt-ampere method measurements. Their powerful waveform capture and mathematical operation functions can be used to analyse the voltage and current phases and calculate the flux changes, etc.

High and low temperature chamber: The programmable high and low temperature test chambers are used to provide the stable temperature environments, for the temperature characteristic testing of the core properties. The measuring devices with the test fixtures and cores are placed in the chambers, and through the lead wires connected to the external instruments, the automatic temperature cycling testing from the low temperatures to the high temperatures can be realized, evaluating the temperature stability of the core parameters.

Standard test fixtures and winding devices: The standardized test fixtures (such as the solenoids and fixture coils) and precision winding machines are the basis of ensuring the testing consistency and repeatability. The winding of the toroidal cores needs to be uniform and tight with the accurate numbers of turns; for the special-shaped cores, the dedicated fixtures can ensure that the cores are tightly closed, reducing the effects of the contact air gaps on the measurement results.