Advances in Soft Ferrite Measurement Technology and Standards

Published: 2021-05-29 Views: 次

Development of soft magnetic ferrite measurement technology and standardization

I. Introduction
      The performance parameters of soft magnetic ferrite materials and cores are not only indicators of their own quality, but also the basis for designing various soft ferrite devices such as electronic transformers and inductors. Therefore, accurate measurement of these parameters is essential. Soft ferrite measurement technology encompasses:

(1) Principles and methods for measuring soft magnetic performance parameters,

(2) Measurement setups (systems) and instruments for soft magnetic performance parameters,

(3) Test conditions and test procedures,

(4) Error analysis and data processing of measurement results.
       Soft magnetic measurement technology evolves with advances in electronics and computer technology, as well as with the development and application needs of soft magnetic material products. Standards for soft magnetic parameter measurement methods are continuously being revised, and new standards are being drafted and issued.

      This paper discusses measurement methods for soft magnetic performance parameters, along with the development of measurement setups (systems) and instruments, and progress in domestic standards for measurement methods and adoption of IEC standards. Test conditions and measurement procedures play a critical role in the reliability and repeatability of parameter measurements; these are specified in detail in the measurement method standards and are not covered here. Error analysis and data processing of measurement results are closely tied to advances in error theory and instrument manufacturing technology, and are likewise not discussed in detail.

II. Advances in Magnetic Measurement Technology at Low Excitation Levels
      Soft magnetic ferrite materials are used in weak AC magnetic fields (<5mT), and their main characteristic parameter is complex permeability = μ′-jμ″. μ′ has the same physical meaning as initial permeability μi, while μ″ represents the energy loss of the material. However, the characteristic parameters commonly used in technical standards for materials and magnetic products include initial permeability μi, inductance L, inductance factor AL, loss tangent tgδ, Q factor, tgδ/μi, and hysteresis constant ηB, among others.
      Typically, μi, μ′, tgδ, and AL are calculated from measured core inductance. Thus, ultimately it comes down to measuring the inductance L of the core (sample). The main methods for inductance measurement are the bridge method and the complex impedance method. Both can also measure tgδ, Q factor, ηB, etc. of the core.
      As is well known, in the mid-20th century, many instruments based on the bridge method were produced, both manual and semi-automatic. Examples include the Re13R217 bridge (West Germany), Re13R218 bridge (West Germany), CD-5 (Nantong, China), CD-24 (Nantong, China), and CD-16 magnetic parameter measuring instrument (Nantong, China). Using operational amplifiers to form an active voltage regulation network in the bridge was an evolution of the classic bridge; employing a phase-sensitive detector (PSD) in the bridge circuit allows the unbalanced voltage to be decomposed into in-phase and 90° quadrature components, making bridge balancing extremely convenient. Most early digital automatic balancing bridges adopted this approach. Domestic models such as CDBA, CD-7, CDG-A, and the U.S. GR1608-A, the UK's Wayne Kerr B221, and Marconi TF2700 all belong to this type—essentially second-generation bridge products. Traditional bridge-method instruments, while the most mature in theory and measurement method, have many drawbacks: narrow range, narrow frequency band, limited functions, slow test speed, and inconvenient operation. Consequently, since the 1970s, test instruments have been continuously updated in measurement principles and display methods. Since the 1980s, the complex vector impedance method, combined with microprocessor technology, has enabled multi-frequency measurement, digital display, automatic parameter conversion, auto-calibration, programmable output, and IEEE-488 bus compatibility, producing various high-precision LCR meters and impedance analyzers. Examples include the HP4274A, HP4192A, GR1688, and GR1689. Domestic models include the YY2815 and ZL-5. The basic measurement error of these instruments is 0.02%–0.1%. The HP4291A and Agilent 4291B cover 1 MHz–1.8 GHz, with impedance parameters including |Z|, |Y|, θ, R, X, G, B, CP, CS, LP, LS, RP, RS, D, Q; conversion parameters; and material parameters |ε|, θ, ε′, ε″, |μ|, μ′, μ″. These instruments offer wide ranges—for example, the HP4274A measures inductance from 100 nH to 1000 H and resistance from 100 mΩ to 10 MΩ. They are easy to operate, can automatically display and print data and curves, and offer fast measurement speed. Recently, Agilent (Japan) introduced the Agilent E4991A RF impedance/material analyzer with a test frequency of 1 MHz–3 GHz.
      For tgδ and Q measurement, GB9632 (IEC367-1) recommends the transmission method and the damped oscillation method. The Southwest Institute of Applied Magnetics developed a test system based on the transmission method. Various Q meters based on the resonance method have been produced, with test frequencies all below 300 MHz—for example, the TF1245 Q meter (Marconi, UK) covers 40 kHz–300 MHz, and the HP4342A Q meter covers 22 kHz–70 MHz. The HP4285A and HP42851A Q adapters measure Q from 75 kHz to 30 MHz. Domestically, the AS-2851 Q meter (Shanghai Radio Instrument Factory) covers 50 kHz–50 MHz, and the HL2801 digital meter (Heilongjiang Institute of Electronic Technology) covers 50 kHz–30 MHz.
      The Magnetic Core Product Quality Supervision and Inspection Center of the Ministry of Information Industry possesses the Agilent 4294A precision impedance analyzer and the 4291B impedance/material analyzer—the best instruments for measuring soft magnetic materials and cores under weak magnetic fields, covering 40 Hz–1.8 GHz. In particular, μ′μ″–f curves can be measured from 40 Hz to 1.8 GHz using a coaxial fixture.
      Domestically, although several manufacturers (companies) such as Tianjin No. 6 Radio Factory produce various models of LCR bridges and impedance analyzers, their test frequencies are mostly within 200 kHz; instruments above 1 MHz are rare, and the oscillator level cannot be adjusted to the level required for measuring initial permeability. Consequently, high-precision LCR meters and impedance analyzers above 1 MHz are almost all imported from the U.S., Japan, and other countries. At present, domestically produced magnetic parameter test instruments for low excitation levels lag far behind the level of the U.S., Japan, and other countries.
III. Advances in Measurement Technology for Soft Magnetic Characteristic Parameters at High Excitation Levels
      The power loss P and amplitude permeability μa of power ferrite materials should be measured at high excitation levels.
1. Before 1990, China mainly used the resonance method to measure power loss and amplitude permeability; the setup circuit is shown in Figure 1.

    In Figure 1, N1=1, N2=60, R=60Ω, CN is a standard variable capacitor, and the oscilloscope monitors the parallel resonance state of CN and LX. Adjust CN until the Lissajous figure on the screen becomes a straight line, indicating the phase difference between UL and I (UR) is 0°. The core power loss is then:

(1)

    The parallel amplitude permeability is:

(2)

    In equation (2), C1 is the core constant of the sample, in units of (m-1). In China's early research and production of power materials, the resonance method for measuring P and μa played an important role.
    2. In 1982, IEC367-1, "Measuring methods for cores for inductors and transformers for telecommunications," was published. The standard specifies three methods: the multiplier voltmeter method, the bridge method, and the oscilloscope method. The second edition of IEC367-1 (1992) added the RMS method for power loss measurement. The 2002 GB9632 standard equivalently adopted the RMS method. Since the publication of IEC367-1 in 1982, more than 20 years have passed, and it has made enormous contributions to the production and application of power ferrite materials and cores worldwide.
    3. In December 2000, the International Electrotechnical Commission published IEC62044-3, "Measuring methods for cores made of soft magnetic materials—Part 3: Magnetic properties at high excitation levels." Content and features of this standard: The standard has 8 clauses and Annexes A, B, C, D, and E. Clauses 1–3 cover scope, normative references, terms, definitions, and symbols. Clause 4 covers general principles for testing at high excitation levels. Clause 5 covers specimens. Clause 6 covers test procedures. Clause 7 covers statement of specified information. Clause 8 covers test report. Annexes A, B, C, D, and E provide recommended circuits for amplitude permeability and power loss measurement, respectively.
      In the general testing principles, the standard discusses that the magnetic parameters of the core equivalent circuit differ from material properties. It particularly emphasizes that specimens should be restored to a magnetically neutral state before testing. It explains the principles for using single-winding and double-winding test coils. When the test frequency exceeds 200 kHz, a two-winding test coil is recommended. Performance requirements for test instruments are specified.
      In Clause 6, test procedures, the standard describes in detail the measurement principles for amplitude permeability and power loss P, and recommends various measurement circuits consistent with these principles—see Annexes A, B, C, D, and E of IEC62044-3. Test procedures for these methods are also specified.
      The test circuits for measuring μa and P in IEC62044-3 differ significantly from those in IEC367-1 (1982). In the two μa measurement circuits, current sampling uses not only non-inductive resistors but also current probes (transformers). The current sampling resistor is connected in series with the excitation coil, with the other end grounded. In IEC367-1 (1982), the current sampling resistor is connected in series with the excitation coil, with the other end connected to the high side of the power signal source. The power loss measurement methods specified in IEC62044-3 include: the RMS method, the product method, the reflection method, and the calorimeter method. The product method is further divided into the V-A-W meter method, the impedance analyzer method, the analog-digital method, the vector spectrum method, and the quadrature power method. The CH258 V-A-W meter, CH2330 V-A-W meter, and CH2335 V-A-W meter from Clarke-Hess (U.S.) employ the product method principle, using digital sampling and computer control technology to automatically display current, voltage, and power measurement results digitally. The power loss measurement uncertainty is no greater than ±1.2%.
       Domestically, Sichuan Mianyang Jinpeng Company, Tianjin No. 6 Radio Factory, Guangdong Tongxin Technology Company, and Shandong Yangguang Company have produced and sold V-A-W meters. Features of the IEC62044-3 standard:
    a. Scientific rigor
      IEC62044-3 represents the long-accumulated expertise of many international electromagnetic measurement experts and scholars in scientific research, production, measurement technology research, and use of IEC367-1:1982. The standard is therefore theoretically mature and reliable, methodologically reasonable and feasible, and operationally safe and straightforward.
    b. Uniformity
      The publication of IEC62044-3 further unified measurement methods for magnetic properties at high excitation levels. It provides a uniform test method for quality control of soft magnetic power materials and cores worldwide, helping eliminate trade barriers between countries in soft magnetic power material and core products.
    c. Systematic structure
      The content and structure of the standard are systematic. For each parameter, detailed provisions are made covering test purpose, specimen selection, measurement principle, test method, test instruments and equipment, test circuits, test conditions, coil winding and selection, test procedures, and more.
    d. Flexibility
      The power loss and amplitude permeability of material and core specimens are nonlinear, requiring different excitation levels. Test frequencies also vary among specimens. A single test method or circuit can hardly meet all requirements. Therefore, multiple test methods are specified and multiple test circuits recommended. In material and core specifications, the appropriate test method can be selected according to applicable conditions.
4. Power signal sources
      The power signal source is the key equipment for measurements at high flux density and a headache for many testers, because power sources meeting the measurement conditions are scarce. Measurement conditions require the power source to supply the specified voltage and current with a sinusoidal waveform, total harmonic content below 1%, and low output impedance (generally below 8Ω).
      Voltage output amplitude and frequency must be stable with minimal drift. In most cases, the power signal source consists of a signal generator and a power amplifier.
      Before 1985, China Great Wall Radio Factory developed the TDS-4 power amplifier with 50 W output and frequencies up to 50 kHz. In 1988, the Central Iron and Steel Research Institute developed a 400 Hz–500 kHz, 200 W power amplifier. In 1990, the Southwest Institute of Applied Magnetics developed a 10 kHz–1 MHz, 100 W power amplifier; by 1996, a 10 kHz–1 MHz, 100 W locked power source was available for sale. In 1997, Mianyang Saimao Power Supply Company developed and sold a 10 kHz–100 kHz, 200 W power source. Electronic fabrication technology has continuously improved, with ongoing design and manufacturing refinements. Since 1997, Mianyang Saimao Power Supply Company's power signal sources have been reliable in quality and excellent in performance.
      In 1984, Japan developed a 50 Hz–2 MHz, 50 W power amplifier. In 1988, Japan's Ryowa Co., Ltd. introduced power amplifiers covering 50 Hz–1 MHz at 70 V/6 A and 1–15 MHz at 50 V/4 A. In recent years, the SY-8216 B-H analyzer from Iwasaki Communication Equipment Co., Ltd. is equipped with a 50 Hz–1 MHz, 200 W power source, and the SY-8232 B-H analyzer with a 10 Hz–10 MHz, 800 W power source.
      To meet the needs of power loss measurement for high-frequency, high-power soft ferrite materials, China must not only develop high-frequency power loss measurement technology but also strive to develop high-frequency, high-power sources that satisfy the test conditions of various power ferrite materials—especially digital power signal sources with keypad output.
IV. Advances in Measurement Technology for Hysteresis Loops and Bs, Br, Hc
      Measurement of the saturation hysteresis loop mainly falls into static and dynamic categories. Static measurement methods include the ballistic galvanometer method and the electronic integration method. The ballistic method has been in use for 50 years and remains as an accurate method of metrological value. IEC401, the U.S. MMPA standard, and the national standards of Japan, Germany, and Russia still adopt this method. The former Soviet Union's early Y541 magnetic parameter tester was the first complete ballistic-method system used in China; its domestically produced counterpart, the CC4 ballistic measurement setup, was once essential equipment for many magnetic core material factories. The CD4 magnetic material measuring instrument, priced at less than one-tenth of the CC4, was also widely used.
      There are many types of static hysteresis loop testers based on the electronic integration method. In the mid-to-late 20th century, representative models included the CL-6 from Shanghai No. 4 Meter Factory, the 3257 from Yokogawa (Japan), and the LDJ5500 from the U.S. In 2001, the Magnetic Product Quality Supervision and Inspection Center of the Ministry of Information Industry imported the C-750 hysteresis loop tester from Magnet-Physik (Cologne, Germany), featuring microcomputer control, automatic display and printing of hysteresis loops and Bs, Br, Hc, good stability and repeatability, and 0.8% error.
      AC hysteresis loop testers use the electronic integration method. At 50–60 Hz, the AC and DC hysteresis loops of magnetic materials are basically similar; especially for soft ferrite materials with very high resistivity, no difference between the two can be observed. At low frequencies, high-precision electronic measurement setups can also achieve measurement errors below 3%. The Department of Solid Electronics at Huazhong University of Science and Technology developed the M-1A and M-1B oscilloscope-method hysteresis loop measuring instruments, operating at 50–60 Hz.
      The second edition of IEC401 (1993) changed the frequency test condition for Bs, Br, Hc to f ≤ 10 kHz. The U.S. MMPA standard (SFG-89) uses the oscilloscope method at 10 kHz to measure Bs, Br, Hc, and the Southwest Institute of Applied Magnetics has also established an oscilloscope-method setup for measuring Bs, Br, Hc at 10 kHz. A typical oscilloscope-method setup is shown in Figure 2. The oscilloscope method is simple and reliable; at 10 kHz, it can trace the saturation hysteresis loop and measure small specimens such as Φ16 without amplifiers in the B and H channels—sensitivity is sufficient. The oscilloscope method measures Bs quickly and is suitable for batch sample sorting and comparative measurement on production lines; changes in Bs can be continuously observed as environmental conditions vary. Therefore, in industrial measurement, Bs, Br, and Hc of soft ferrites are best measured at 10 kHz using the oscilloscope method.

       With the development of computer technology, Japan has used waveform memory methods to continuously raise the test frequency of hysteresis loop measurement. The AC-100 hysteresis loop tester developed by Riken Denshi Co., Ltd. (Japan) operates at 30 Hz–1000 kHz. The SY-8232 AC B-H loop tester from Iwasaki Communication Equipment Co., Ltd. (Japan) operates at 10 Hz–10 MHz. These instruments are all computer-controlled, automatically displaying and printing hysteresis loops and Bs, Br, Hc, with B and H measurement errors of 2%.
V. Standardization of Soft Ferrite Methods
      Test method standards are the foundation for judging and evaluating core quality. China places great importance on research and formulation of soft ferrite test method standards. As early as 1974, the standard SJ859-74, "Ferrite short-wave (f≤12MHz) antenna cores," specified antenna rod test methods. Subsequently, with the development and widespread application of soft ferrite production, standards were successively issued: SJ1511-79, "General specification for ferrite antenna cores"; SJ/Z1766-81, "Soft ferrite material series and test methods"; SJ/Z2110-82, "Test method for disaccommodation coefficient of soft magnetic materials"; SJ/Z2090-82, "Test methods for power loss and amplitude permeability of soft ferrites"; SJ/Z2007-82, "General specification for soft ferrite pot cores"; and SJ1174-81, "General specification for ferrite U-cores for television receivers." In 1999, SJ/T11206, "Measuring methods for properties of magnetic oxide double-aperture and multi-aperture cores," was published.
      Magnetic measurement technology is mainly reflected in the IEC standards issued by the International Electrotechnical Commission, which are recognized worldwide as authoritative technical standards. Its two subordinate bodies, TC51 and TC68, have extensively collected opinions and suggestions from various countries and formulated a series of recommended standards for measuring magnetic materials and magnetic components—for example, IEC367-1, "Measuring methods for cores for inductors and transformers for telecommunications," and IEC62044-3, "Measuring methods for magnetic properties at high excitation levels."
      With scientific and technological innovation and progress, new measurement method standards continue to emerge, and existing methods are continuously revised and supplemented. The IEC367-1 standard formulated in the 1970s was supplemented and revised in 1982, 1984, and 1992. Thus, IEC magnetic measurement technology standards have long maintained their advanced nature and practicality, are widely accepted and applied worldwide, and demonstrate strong vitality. Therefore, since 1988, China has focused on adopting IEC soft magnetic measurement method standards.
      In 1988, GB9632-88, "Measuring methods for cores for inductors and transformers for telecommunications," was published, equivalently adopting IEC367-1 (1982), Part 1: Measuring methods. GB9635-88, "Measuring methods for antenna rods," equivalently adopted IEC492. SJ3175-88, "Measuring methods for cylindrical cores, tubular cores, and threaded cores made of magnetic oxides," equivalently adopted IEC732.
      In 1997, SJ/T9072.3-1997, "Guidance on ferrite material data in manufacturers' catalogs for transformer and inductor cores," was published, equivalently adopting IEC401.
      In January 1992, IEC367-1 (1982) was supplemented with the RMS method for power loss measurement. The GB9632.1-2002 standard added the RMS method for power loss measurement.
      In December 2000, the International Electrotechnical Commission published:

u        IEC62044-1 Measuring methods for cores made of soft magnetic materials—Part 1: General specification (draft for comment) 

u       IEC62044-2 Measuring methods for cores made of soft magnetic materials—Part 2: Magnetic properties at low excitation levels

u       IEC62044-3 Measuring methods for cores made of soft magnetic materials—Part 3: Magnetic properties at high excitation levels

u       IEC62044-4 Non-magnetic properties (draft for comment)

u       The magnetic industry should actively promote the IEC62044 standard and adopt IEC62044-3 as soon as possible.
      New soft ferrite materials and applications are moving toward higher frequencies, demanding ever-higher test frequencies that some current standard methods can hardly handle. Future research on standard methods should advance in the following areas:
       (1) Standard test methods for Bs, Br, Hc at test frequencies ≤10 kHz
       (2) Standard method for magnetic spectrum (μ′μ″) measurement

  (3) Standard method for B-H loop measurement
       (4) Standard method for power loss measurement at MHz frequencies
       (5) Standard method for AC/DC resistivity measurement of ferrites
       (6) Standard method for material density measurement
       (7) Standard method for Curie temperature measurement
       (8) Standard method for measuring the performance factor of power materials
       (9) Standard test method for compressive strength of cores
       (10) Standard test method for mechanical strength of cores
VI. Committing to Technological Innovation and the Path of Domestic Production
      Over the past half century, science and technology workers in China's applied magnetics community and magnetic industry have tracked advanced international magnetic measurement technology while, on the basis of introduction, digestion, and absorption, pursued independent innovation and proposed many novel and unique magnetic measurement methods. From 1975 to 1998, the Chinese National Committee of IEC/TC51 submitted multiple IEC standard proposals—for example, the amendment to Annex N of IEC367-1, "Measuring circuit for (effective) amplitude permeability," and the standard proposal "Measuring methods for properties of magnetic oxide double-aperture and multi-aperture cores," submitted by the Magnetic Product Quality Supervision and Inspection Center of the Ministry of Information Industry. To align with international practices as quickly as possible, China's magnetic industry has actively adopted, equivalently or identically, the measurement methods recommended in IEC and ISO standards, and has also widely absorbed measurement methods from standards of advanced industrialized countries and internationally renowned corporations, such as the U.S. MMPA, Japanese JIS, German DIN, and Russian ROCT standards. Practice has proven that China's research on soft magnetic measurement theory and methods is approaching international levels.
      Internationally, the development and production of dedicated soft magnetic measurement instruments and equipment are concentrated in a few advanced industrialized countries—the U.S., Germany, and Japan. Examples include the HP4284A LCR meter (U.S.), Agilent 4294A precision impedance analyzer (U.S., Japan), Agilent 4291B RF impedance/material analyzer (U.S., Japan), CH2330 sampling digital V-A-W meter (U.S.), CH2335 sampling digital V-A-W meter (U.S.), C-750 B-H tester (Germany), and SY-8232 B-H analyzer (Japan). These instruments represent the current state of the art in soft magnetic test instruments worldwide. Domestically, because there are many manufacturers of magnetic measurement instruments, each small in scale with dispersed technical resources and working independently, the instruments produced lag far behind foreign instruments in frequency range, reliability, multifunctionality, automation, and appearance quality. Therefore, domestic instrument R&D should restructure the industry, concentrate resources on the path of domestic production, and produce more and newer instruments that comply with soft magnetic measurement method standards, striving