Abstract:
An oscilloscope performs an in-circuit measurement of voltage across, and current through, a core winding of an inductor, and derives the actual B and H values with n number of turns, receives data indicative of the magnetic length of the circuit, and plots the B_H curve. The oscilloscope then derives the value of Saturation flux density (Bsat), Remnant flux density (Br), Permeability (μ), and Coercivity (Hc) from this B-H plot. Characterizing the operating region of the magnetic component while it operates in a Switch Mode Power Supply (SMPS) under test, provides information concerning the stability of the power supply that was heretofore unavailable.

Description:
CLAIM OF PRIORITY  
       [0001]     The subject application claims priority from U.S. Provisional Patent Application No. 60/504,159 entitled, Measuring Saturation Flux Density Bsat, Coercivity Hc, Permiability of the Magnetic Components in an In-circuit Operation Using DSO (Ramesh, et al.), filed 19 Sep. 2003. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The subject invention generally concerns measurement of magnetic components and specifically concerns in-circuit measurements of magnetic components using a DSO.  
       BACKGROUND OF THE INVENTION  
       [0003]     Switching power supplies are aimed to operate at the maximum efficiency by operating at the higher switching frequency. This makes the design deviations and operating tolerance of the switching frequency stringent The operating switching frequency and its stability depends upon the magnetic components used as the part of Switching (Oscillating) element in the switch mode power supply.  
         [0004]     Switching Power Supplies are expected to have very high reliability at different operating conditions, such as, Power-On state, Steady state, Load change-over and settling-down, Based design methodologies such as topologies, and Type of converter such as AC/DC and DC/DC.  
         [0005]     During these modes of operation the current and voltage characteristics change. It is expected that magnetic components such as inductors, coupled inductors and transformers operate in stable condition such that power supply is not driven to an unstable condition.  
         [0006]     For optimum performance, designers generally design magnetic components such as transformers and Inductors using the magnetic characteristics curve, popularly known as a hysteresis curve of the core material, that is supplied by magnetic component manufacturers. Each magnetic component is designed based on the operating voltage, current, topology, and type of converter, In an attempt to ensure that it will operate in the linear region of its hysteresis curve. Unfortunately, there is no mechanism available to verify the design under actual operating conditions.  
         [0007]     The operating region of the magnetic components determines the stability of the switching power supply. The power supply operating voltage and current characteristics also depend on Power-on state, Steady state, Load-change state, Type of topology, and Type of converter.  
         [0008]     It is extremely difficult, in view of the different scenarios of signal characteristics, to design a magnetic component and ensure that it will operate in a linear region.  
         [0009]     What is needed is an apparatus and method to observe and measure the B-H characteristics of the magnetic component in an in-circuit operation.  
       SUMMARY OF THE INVENTION  
       [0010]     An oscilloscope according to the subject invention performs an in-circuit measurement of voltage across, and current through, a core winding of an inductor, and derives the actual B and H values with n number of turns, receives data indicative of the magnetic length of the circuit, and plots the B_H curve. The oscilloscope then derives the value of Saturation flux density (Bsat), Remnant flux density (Br), Permeability (μ), and Coercivity (Hc) from this B-H plot.  
         [0011]     It is herein recognized that characterizing the operating region of the magnetic component while it operates in a Switch Mode Power Supply (SMPS) under test, provides information concerning the stability of the power supply that was heretofore unavailable.  
         [0012]     In a first aspect of the invention, the operating point of a magnetic component may be viewed by plotting variation of the Flux density against magnetic field strength (Magnatomotive force).  
         [0013]     In a second aspect of the invention, B-H characteristics may be quantified by measuring Peak flux density, remnant flux density, permeability, and coercivity.  
         [0014]     In a third aspect of the invention the cycle related to maximum flux density can be selected and used to calculate permeability, for analyzing inductance variation in in-circuit operation.  
         [0015]     A fourth aspect of the invention is the ability to calculate the operating DC flux density and positioning the curve at the operating point and plot B-H characteristics during CCM, DCM and while switching between CCM to DCM and DCM to CCM in an in-circuit operation.  
         [0016]     A fifth aspect of the invention is the ability to characterize complex magnetic components having multiple windings such as, transformers or coupled inductors.  
         [0017]     A sixth aspect of the invention is the ability to verify the actual behavior of a magnetic component, under in-circuit operating conditions. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0018]      FIG. 1  is a plot of a hysteresis curve for a typical soft magnetic material, as know from the prior art.  
         [0019]      FIG. 2  is an illustration of a test setup for measuring B and H, as known from the prior art.  
         [0020]      FIG. 3  is a flowchart of a method of measuring a BH characteristic of a magnetic component operating In-circuit, in accordance with the subject invention.  
         [0021]      FIG. 4  is a chart of the magnetic properties of a particular material.  
         [0022]      FIGS. 5, 6 , and  7  are screenshots useful for understanding the invention. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0023]     Some definitions that may be useful in understanding the invention are: Magnetic Field Strength (H): Magnetic field employed to induce magnetic flux in the material under test. Units are Amperes/meter  
         [0024]     Saturation Flux Density (Bs): Maximum magnetic flux density that can be induced in the material regardless of the magnitude of the externally applied field H.  
         [0025]     Remnant Flux Density (Br): Induced magnetic flux density that remains in the material after the externally applied magnetic filed (H) is returned to zero during the generation of the Hysteresis loop.  
         [0026]     Coercive Force (Hc): Value of H found at the intercept of the H-axis with the Hysteresis loop. This represents the external field required to cause the induced flux density (B) to reach zero during the measurement cycle of a Hysteresis loop. HC is symmetrical with positive and negative axis  
         [0027]     Initial Permeability (u I ): The ratio of induced magnetic flux density (B) to an applied field (H) as H approaches zero. It is ratio of B to H at any point on the Hysteresis loop.  
         [0028]     Maximum Amplitude Permeability: Maximum ratio of B to H on the first quadrant positive cycle of the Hysteresis loop. It is the slope drawn from the Origin.  
         [0029]     It is herein recognized that characterizing the magnetic properties of materials by measuring Saturation flux density (Bsat), Coercivity (Hc), Permeability for magnetic components in an In-circuit operation, enables engineers, such as Switch Mode Power Supply (SMPS) designers, to better select appropriate magnetic components for their designs.  
         [0030]     Use of the subject invention allows a magnetic component designer to view the operating point of the magnetic component by plotting the variation of the Flux density against the magnetic field strength (Magnetomotive force). It also allows the designer to quantify the B-H characteristics of the component by measuring peak flux density, remnant flux density, permeability, and coercivity, all of which helps in analyzing the stability of a power supply under test  
         [0031]     Use of the subject invention allows a magnetic component designer to select a cycle related to maximum flux density and calculate permeability, which helps to check inductance variation in in-circuit operation. It also allows the designer to calculate the operating DC flux density, to position the curve at the operating point, and to plot B-H characteristics during CCM, DCM and while switching between CCM to DCM and DCM to CCM in an in-circuit operation. One skilled in the art will know that CCM means Continuous Current Mode operation wherein a current through the inductor never becomes zero; it means carry DC current. One skilled in the art will also know that DCM mean Discontinuous Current Mode operation wherein a current through the inductor does become zero. This term indicates that the current through the inductor can change from DCM to CCM as the operating load varies on the power supply. DC flux density Bdc also varies with operating load on the power supply.  
         [0032]     Use of the subject invention allows a magnetic component designer to characterize complex magnetic components having multiple windings (e.g., transformers, coupled inductors). It also allows the designer to verify the actual behavior of the magnetic component, during in-circuit operating condition, thus enabling the designer to optimize the design.  
         [0033]     Consider a coil is wound on a closed ferromagnetic core, or PERM Alloy ring, or amorphous core. Depending upon the excitation levels applied, its cross sectional area, and the number of turns, the core can be driven into saturation by an applied current. It is herein recognized that one can measure the voltage across the core winding and the current through it, and derive the actual B, H values with n number of turns, and the length of the magnetic circuit, then plot a B-H curve. It is then possible to derive the value of Saturation flux density (Bsat), Remnant flux density (Br), Permeability (μ), and Coercivity (Hc) from this plot.  
         [0034]     Referring to  FIG. 1 , a B-H curve is displayed showing Saturation Flux Density (Bs), Remnant Flux Density (Br), Coercive Force (Hc), Initial Permeabilty (μ i ), and Maximum Amplitude Permeability (μ a(Max) ). It should be noted that the data waveform (B-H curve) of  FIG. 1  starts from the maximum value of H (i.e., at point M) and decreases, then increases again along a path designated M-N-O-P. As noted above,  FIG. 1  is a typical B-H curve (i.e., hysteresis plot) for a soft magnetic material.  
         [0035]     When testing a single winding inductor (not shown), an engineer would connect a differential voltage probe across the inductor and connect a current probe to measure the current through the inductor.  
         [0036]     A measurement setup arrangement  200  for testing a transformer is shown in  FIG. 2 . Referring to  FIG. 2 , a four-channel oscilloscope  210 , such as a Tektronix TDS7054B or TDS5104 Digital Phosphor Oscilloscope (DPO), manufactured by Tektronix, Inc., Beaverton, Oreg., is coupled to a transformer  260  for measurement of the magnetic characteristics of transformer  260 . It is to be understood that transformer  260  is part of a user&#39;s circuit (not shown) and receives energizing current from the remainder of the user&#39;s circuit, as indicated by the arrows associated with transformer  260 .  
         [0037]     A differential probe  220 , such as a Tektronix P5205 differential probe, has its probes  220   a  and  220   b  coupled to primary winding  260   a  of transformer  260  for providing a non-ground-referenced measurement of the voltage developed across primary winding  260   a  to channel  1  of oscilloscope  210 , via a single-ended output of differential probe  220 . In addition, a first current probe  230  is coupled to channel  2  of oscilloscope  210  for measuring the current through primary winding  260   a,  a second current probe  240  is coupled to channel  3  of oscilloscope  210  for measuring the current through secondary winding  260   b,  and a third current probe  250  is coupled to channel  4  of oscilloscope  210  for measuring the current through secondary winding  260   c.    
         [0038]     The measured current at each secondary winding  260   b,    260   c  is added with the measured current through primary winding  260   a  in accordance with it&#39;s respective turns ratio. That is: 
        if Ip equals the current through primary winding  260   a,  having N turns;     if I1 equals the current through secondary winding  260   b;  having N 1  turns; and     if I2 equals the current through secondary winding  260   c;  having N 2  turns; then: the Magnetizing Current Iz is defined by the following equation:  
       Iz   =     Ip   +         N   ⁢           ⁢   1     N     *   I   ⁢           ⁢   1     +         N   ⁢           ⁢   2     N     *   I   ⁢           ⁢   2           
       
 
         [0042]     In operation, an engineer characterizes magnetic materials of either an inductor or a transformer in the following manner. He first winds a magnetic core with a known number of turns of wire, and excites the core winding with an external voltage-controlled and current-controlled source. The engineer then measures the magnetizing voltage across the core winding and the current through the windings. The engineer then processes the acquired data with software according to the subject invention as described in the flow diagram of  FIG. 3  to derive and plot the BH characteristic of the core.  
         [0043]      FIG. 3  shows a flowchart  300  of a program for performing an in-circuit measurement of the B-H characteristic of a magnetic component in accordance with the subject invention. The program is entered at step  305  with measurement data of voltage measurements, current measurements, waveform edge measurements, and user-entered data indicative of the number of turns and path length, and then advances to step  310  wherein waveform edges are identified using the edge waveform data. At step  315 , the number of waveform edges is determined. If the number is zero, then an error message is generated at step  320 . A non-zero number causes the program to advance to step  325  wherein a complete number of cycles is identified, and passed to step  330  wherein a check is performed to see if all cycles in the waveform have been processed.  
         [0044]     If cycles remain to be processed, the YES path is taken to step  335  wherein a check is made to see if the present duty cycle has varied from earlier cycles. If not, the NO path is taken to step  340  wherein the mean value of voltage on each cycle is subtracted, and the program advances to step  345 . If, at decision step  335 , the duty cycle had varied, then the program would have advanced directly to step  345 . At step  345 , the voltage waveform is integrated, and the mean value of the integrated voltage waveform is subtracted from the result. If, at step  350 , the edge source is voltage data, then the YES path is taken to step  360  wherein the value of the inductance L is calculated for the complete cycle. Otherwise, the NO path is taken to step  355  wherein the inductance L is calculated at mid-level by taking ±5% of the level of the current. After performing either step  355  or step  360 , the program advances to step  365  for calculation of Bdc.  
         [0045]     The program then decrements the number of cycles and loops back to step  330  to see if it is done (i.e., to see if any cycles remain to be processed). If, at step  330 , no cycles remain, then the program advances to step  370 , calculates B and H for each cycle then proceeds to step  380 . In step  380 , the following procedures are performed: Identification of Bpeak cycle and calculation of Br, Hc, and Hpeak. The BH values are converted to Plot data values, and the BH curves are plotted on the display screen of the oscilloscope. Finally, the Permiability is computed for the data between the cursors on screen.  
         [0046]     In the above flowchart steps, calculation of the B Value from the measured voltage data is accomplished by identifying each cycle using the edge source data. Assume, for purposes of this explanation, that there are “m” number of complete cycles on the acquired waveform Further assume that each cycle has “q” number of data points.  
         [0047]     If the duty cycle is varied (see step  335 ), the mean average value of the voltage is not zero for each cycle, whereas it had been zero for over a set of cycles. In this case, the voltage waveform is not subtracted from its mean value before doing the integration.  
         [0048]     Assume that the mean voltage for the k th  cycle is  
         Vmean   k     =       1   q     ⁢       ∑     i   =   1       i   =   q       ⁢           ⁢     V   ⁡     (   i   )               
 
 Where i varies from 1 to q data points, Where Vk(t) is the normalized voltage waveform data 
 
 Vk ( t )= V ( i )− V mean k  
 
 of Kth cycle 
 
         [0049]     Continuos Integration of the Voltage waveform is performed for the k th  cycle where k th cycle 
 
φ( t ) k   =∫Vk ( t ) dt    Eq(1) 
 
 has 1 to q points and is done for all cycle from 1 to m cycles. 
 
         [0050]     B k (t) is the Flux density of the k th  cycle. 
 
 B   k ( t )=φ k   (t)   /N*S    Eq(2) 
 
         [0051]     To derive only the changes in the flux density, the calculated flux density is normalized. The normalized flux density for the k th  cycle is B meank   
         Bmean   k     =       1   q     ⁢       ∑     i   =   0       i   =   q       ⁢           ⁢       B   k     ⁡     (   i   )               
 
         [0052]     Now the normalized flux density of kth cycle is 
 
 Bn   k ( t )= B   k(t)   −B   meank    Eq(3) 
 
         [0053]     The DC flux density that determines the operating point of the inductor or transformer in an in-circuit operation is calculated for this cycle as B dck    
         [0054]     The B dck  flux density for k th  cycle (step  370 ) is given by  
               Bdc   k     =         L   k     *     Iavge   k       S             Eq   ⁢           ⁢     (   4   )               
 
 Wherein, L k  is the Inductance derived for that cycle. The value of inductance is calculated with the integral voltage data and current data using the method stated in U.S. patent application Ser. No. 10/628,022, entitled, MEASUREMENT OF INDUCTANCE USING A DIGITAL STORAGE OSCILLOSCOPE UNDER REAL TIME OPERATING ENVIRONMENTS, (Ramesh, et al.), filed 25 Jul. 2003, assigned to the same assignee as the subject invention, and herein incorporated by reference in its entirety. 
 
         [0055]     lavge k  is the average value of the current for the k th  cycle and S is the Core cross sectional area of the magnetic material  
         [0056]     Bdata of the k th  cycle is 
 
 B ( t )k =B   dck   +B   nk ( t ) 
 
         [0057]     H data for the k th  cycle is  
               Hk   ⁡     (   t   )       =       I       k   ⁡     (   t   )       *   N       l             Eq   ⁢           ⁢     (   6   )               
 
 Where N is the number of turns, I is the length of the magnetic path. Values for N and I are entered by the user. 
 
         [0058]     B and H values are calculated (see step  370 ) for all the complete number (m) of cycles in the entire acquisition record using equations Eq(1) to Eq(6).  
         [0059]     The cycle that has maximum value of B(t) is identified and this cycle is known as the peak cycle “p”. (Step  380 ). The value of Hc, Br is calculated on the data of the Peak Cycle form Bp(t) and Hp(t) data. (Step  380 ). All the cycles are plotted on the plot and B p(t) data for the p cycle is preferably colored differently to show the customer that the maximum B has occurred on this cycle. (Step  380 )  
         [0000]     Permeability Calculation:  
         [0060]     In the plot on the screen of the oscilloscope, there is a provision to select the data point between the cursor  1  and a cursor in an anti-clockwise direction of the plot.  
         [0061]     The slope of the data selected between the cursors on the B and H peak cycle is calculated as follows. Assume that there are “y” data points between the Cursors on the peak cycle. 
 
Find  H   av =( H   1   +H   2   + . . . H   y )/ Y  
 
 B   av =( B   1   +B   2   + . . . B   y )/ Y  
 
 H   normi   =H   i   −H   av   , i= 1 . . .  Y  
 
 B   normi   =B   i   −B   av   , i= 1 . . .  Y  
 
 B/H=SUM ( H   norm1   *B   norm1   +H   norm2   *B   norm2   + . . . +H   normY   *B   normY )/ SUM ( H   norm1   *H   norm1   +H   norm2   *H   norm2   + . . . H   normY   *H   normY ) 
 
 Permeability μ=B/H 
 
         [0062]     The results of validation of the BH property Measurements include Bpeak, Hmax, Br, Hc, Permeability and Ripple Current. Results of validations are divided into two sections, one with Stimulus, and one using prototype kits. The validations are for 
        a) Single Winding     b) Multiple Winding (Simultaneous and non-simultaneous transformers)        
 
         [0065]     Section 1 Stimulus Verification and Validation 
        Material: NC-7     Manufacturer: Nippon Ceramics Co Ltd        
 
         [0068]     This material is wire-wound and the physical properties are  
                                                       Area   13.58 usqmtr           Length    0.0265 mtr           Number of Turns   50                      
 
         [0069]     A data sheet for this material is shown in  FIG. 4 .  
         [0070]     The A F G (Arbitrary Function Generator) Setup is as follows: 
        A stimulus signal having the following characteristics is applied to the inductor.     Amplitude=6 Volts     Frequency=1 KHz     Shape=Triangular Wave        
 
         [0075]     The results, as measured by the inventive apparatus and method, are shown in the screenshot of  FIG. 5 .  
         [0076]     A second example is as follows:  
                                                       Material: 3F3 type               Manufacturer: SIEMENS (Feroxcube)           Units: SI           Area   37e-6           Length   29.2e-3           Number of Turns   18                      
 
         [0077]     The results for the 3F3 material, as measured by the inventive apparatus and method, are shown in the screenshot of  FIG. 6 .  
         [0078]     Results Verification for Multiple-Winding Inductors  
         [0079]     The Bill of Materials (BOM) of manufacturer provides the value of inductance. Then, the value of delta, Relative permeability are theoretically calculated and verified with calculated results.  
         [0080]     Here, for the given value of Inductance, the delta B flux is calculated, then the permeability is calculated theoretically and is compared with measured results.  
                                             IRSMPS Transformer Proto type Power supply                                    Material Type Part Number   EFD25-1S-10P           Material Type           Cross Section Area Ae   58e-6 sqmtr           Magnetic Length   57e-3 mtr           Number of primary Turns   58           Number of Secondary turns    6                      
 
         [0081]     A DSO screen shot of the results, as provided by an apparatus and method of the subject invention, is shown in  FIG. 7 .  
         [0082]     Calculations: 
 
Inductance=1 mH 
 
ΔI=Imax−Imin=1.31 
 
       L   =       Δ   ⁢           ⁢   B   *   N   *   Ae       Δ   ⁢           ⁢   I           
  L=   1  mH,  N=   58,    Ae=   58.1   e−   6 sqmtr 
 
 AB =(1 e− 3*1.31)/(58*58 e− 6) 
 
ΔB=389 mT 
 
 Note, from  FIG. 7 , that the measured value of ΔB is 353 mT, and compares favorably with the theoretical calculation of ΔB of 389 mT. 
 
         [0083]     Permeability can be derived from the measured value of inductance  
       L   =         μ   0     *   μ   ⁢           ⁢     rN   2     ⁢   A     l         
 
 μo*μr=(1e−3*57e−3/58*58*58e−6)=232. This compares favorably with the DSO&#39;s calculated permeability for the given range of about 245, as shown in the screenshot of  FIG. 7 . 
 
         [0084]     What has been described is an apparatus and method for in-circuit characterization of the magnetic properties of magnetic materials. The scope of the invention is defined by the following claims.