Agilent Wide Range DC Current Biased Inductance Measurement Application Note 369-8 Agilent E4980A Precision LCR Meter Agilent 4284A Precision LCR Meter Agilent 42841A Bias Current Source Introduction A large number of switching power supply inductors with extended high frequency characteristics have recently been developed. The reason for this is the increase in the switching frequency to reduce size of switching power supplies which are being built using electronic components which are more compact than are conventional components. However, if components which are not suitable for high frequency are used, the increase in the frequency lowers the efficiency of the switching power supply and creates electrical noise. Consequently, lower noise components and circuits for use at higher frequencies must be developed for future switching power supply designs. Inductors are one of the easiest components to reduce in size by raising the frequency and will require the development of low-loss, low leakage cores. The development and production of such inductors requires DC current biased inductance measurements to evaluate the inductance characteristics under actual operating conditions. This application note describes DC current biased inductance measurements that are more accurate and made over a wider frequency range than was previously possible. Table 1. Measurement instruments Problems concerning DC current biased Instruments Max. bias current Max. bias current inductance measurements 20 A 40 A LCR meters E4980A E4980A DC current biased inductance measurements involve the (with Option E4980A-002) (with Option E4980A-002) following problems. 4284A 4284A (with Option 4284A-002) (with Option 4284A-002) Bias current source 42841A Two 42841A units • Measurement preparations and procedures are 1 Bias current test fixture 42842A 42842B time-consuming • An external bias circuit is required Bias current cable Not required 42843A - Setting and confirming current values are Test leads 16048A 16048A troublesome - Automation of measurement procedures is difficult - Safety problems • Frequency range is insufficient • Not enough bias current can be generated • Measurement accuracy is not guaranteed Solutions offered by the Agilent E4980A or 4284A and Agilent 42841A The E4980A or 4284A precision LCR meter (with Option E4980A-002/4284A-002 current bias interface) in combination with the 42841A bias current source ensures simple and safe DC current biased inductance Figure 1. 42842A bias current test fixture measurements. The E4980A and 4284A allow for DC current biased inductance measurements with the following advantages. • Wide 20 Hz to 2 MHz (E4980A), 1 MHz (4284A) frequency range measurements • DC current biased inductance measurements up to 40 A using two the 42841As, • Basic accuracy of 1% • List sweep function for bias sweep measurements of up to 10 points • The bias current is easily set using the 4284A's front panel keys or by using an external controller via GPIB. • The 42842A/B bias current test fixtures which Figure 2. 42843A bias current cable protect the operator and instrument are provided. • Built-in memory function and removable memory (USB memory for E4980A, memory card for 4284A) for storing instrument setups Measurement Preparation Accessories required When DC current biased inductance measurements are made using an E4980A or 4284A, the accessories required depend on the maximum bias current to be used. Table 1 is a list of what accessories are required. Figures 1, 2, and 3 show the external appearance of the 42842A bias current test fixture, the E4980A or 42843A Figure 3. 16048A test leads bias current cable and the 16048A test leads. 1. 42842B can be used for 20 A DC current biased measurements 2 Connections Measurement safety The table shows which accessories are to be connected Large DC current biased measurements have to be for maximum bias currents of 20 A and 40 A. The 42841A conducted with utmost care. The spike voltages caused is connected to the E4980A or 4284A by plugging in the by accidental removal of the device under test from provided interface cable. The E4980A and 4284A use the measurement terminals while a DC biased current the 16048A test leads to connect to the 42842A/B. Two is applied are particularly hazardous. If current 42841A units have to be connected parallel when making exceeding the rating is run through a device under bias current measurement up to 40 A. (See Figure 4) test (DUT), the heat generated may cause a fire or smoke. Following precautions should be taken when The 42842A/B are equipped with a voltage monitor terminal DC current biased measurements are being made. for connecting a digital voltmeter (DVM) to monitor the bias voltage applied to the device under test directly. Only • The bias current must be switched off before the a DVM with an input impedance of 10 MΩ or more should DUT is disconnected. be connected to the voltage monitor terminal, since the • Make sure that the test leads between the DUT and output monitor has 10 kΩ resistance. The DC resistance the LCR meter are securely connected to prevent (DCR) of the device under test can be derived from this bias accidental disconnections. voltage measurement according to the following formula. • Check at all times that not too much current is put through the DUT to prevent abnormally high V MON temperatures. (Check for heat or smoke.) -3 - [ ] DCR = 3x10 Ω I BIAS • The bias current must be turned off after a bias sweep operation is made with the list sweep function. (If the V is the bias voltage measurement value (unit is V), bias current is not turned off, the last bias current MON I is the bias current (unit is A) setup value and the sweep value will continue to flow through the DUT.) BIAS -3 3 x 10 [Ω] in the formula is the residual DCR of the The 42842A is provided with the following safety features. fixture. Refer to ‘Appendix A’ for information on the accuracy of DCR measurements using this method. • Components are automatically discharged when the protective cover is opened, to ensure the safety of the E4980A/4284A operator while disconnecting a DUT. Precision LCR meter (with Option E4980A-002/4284A-002) • Transparent protective covers are used to facilitate monitoring the DUT during a measurement. Interface cable • Protective circuits are built in to prevent damage to (Furnished with 42841A) 16048A the LCR meter from voltage spikes. Test leads • The bias current is automatically cut off if the 42842A temperature in the fixture becomes abnormally 42841A Bias current Bias current high (i.e. 200 °C in the DUT and 70 °C at the test fixture source measuring terminal.) DVM Compensation (a) 20 A Since the residual impedance caused by the 42841A E4980A/4284A Precision LCR meter is negligible, no compensation is required for normal Interface cable (with Option E4980A-002/4284A-002) inductance measurements. However, when measuring (Furnished with 42841A) 16048A devices with an inductance lower than 10 µH use the Test leads E4980A or 4284A's short compensation function to Interface cable reduce errors. (Furnished with 42841A) 42842A Bias current test fixture 42843A Bias current DVM cable 42841A Bias current source (2 units) (b) 40 A Figure 4. Measurement configuration 3 Measurement Results The purpose of measuring the DC current biased The result shown in Figure 6 shows that there are inductance of inductors is to derive the current differences in the L-IDC characteristics depending on rating from the measured inductance versus DC the frequency used. The program (running on an current biased (L-IDC) characteristics. The current HP 9000 series 300 computer) used to conduct these rating is defined as the value of the bias current when measurements is described in ‘Appendix B’. the inductance is decreased by 10% (or 30% to 50%). Measurements up to 40 A The E4980A and 4284A can measure L-IDC characteristics and the measurements can be easily automated by using DC current biased inductance measurements up to an GPIB interface and the bias sweep function (list 40 A require the use of two 42841A units. Figure 7 sweep) are used. Actual measurement examples and the shows the measured L-IDC characteristics when DC information required for such measurements are given current bias up to 40 A is used. in the following paragraphs. L-IDC characteristics measured with the list SYS MENU sweep function The list sweep function of the E4980A and 4284A can MODE : SEQ be used to sweep up to 201 bias (E4980A) or 10 bias BIAS [ A ] Ls [ H ] Rs [ ] CMP (4284A) current points. Figure 5 shows the rough L-IDC 100 . 00m 544 . 933u 0 . 11931 characteristics and the rated current. The E4980A and 200 . 00m 545 .282u 0 . 11863 4284A automatically waits until the bias current has settled 500 . 00m 544 . 529u 0 . 11723 (settling time) at the specified current value before starting 1 . 000 538 . 915u 0 . 11503 a measurement. Since the meter wait for the optimum 2 . 000 522 . 914u 0 . 11138 moment to start ordinary measurements or list sweep 5 . 000 444 . 466u 0 . 09126 measurements, the settling time need not be considered 10 . 000 330 . 656u 0 . 06747 when the bias current is changed. Consequently, 12 . 000 296 . 950u 0 . 06206 measurements are always made after the bias current 15 . 000 258 . 190u 0 . 05593 has settled. 20 . 000 213 . 129u 0 . 04150 However, temporary discrepancies in the measured values Figure 5. Measurement result using the list sweep function result after bias current changes during measurement of the device that are slow to respond to changes in the bias current. This occurs when transient response of the Inductance [uH] device is longer than the settling time of E4980A or 500 4284A. A suitable delay time should be set with the E4980A or 4284A to compensate for this. Always make sure to turn off the bias current to ensure that no current is flowing through the DUT after a bias 10 sweep operation. 20 Bias [A] 100 1000 10000 Measurements of L-IDC characteristics using 100000 an external controller 1.E+6 Since bias current values can be controlled by an external Freq . [Hz] GPIB controller when the 42841A bias current source is used together with the E4980A or 4284A, it is possible Figure 6. Frequency characteristics of L-IDC to perform L-IDC measurements automatically. Furthermore, the wide measurement frequency range of E4980A or 4284A make it possible to check the L-IDC characteristics per frequency as shown in Figure 6. 4 Conclusion The E4980A and 4284A equipped with the Option E4980A-002/4284A-002 and the 42841A bias current 300 source will permit highly accurate and efficient DC Freq. = 1 kHz current biased inductance measurements up to the 1 MHz frequency range. All of these combine to promote 200 the development and production of high frequency switching power supply inductors. 100 0 0 10 20 30 40 Bias [A] Figure 7. Measurement results up to 40 A Appendix A. Accuracy of DCR Measurements (Typical Values) Accuracy of DCR measurements are as follows. Here I is the bias current set value. BIAS When I ≤ 1 A BIAS 0.5 5 ±{(1.2+ )% + mΩ} I I BIAS BIAS When 1 A < I ≤ 5A BIAS 0.5 ±{2.2% + mΩ} I BIAS When I > 5 A BIAS 5 ±{3.2% + mΩ} I BIAS Note that the input impedance of the DVM must be more than 10 MΩ. 5 Inductance [uH] Appendix B. Sample Program List 1000 DIM XP ( 100, 20 ), Yp (100, 20 ) ! 1010 DIM Work$ [100] ! 1020 DIM Bias (200), Freq (20) ,A(200, 20) ,B (200, 20) ! 1030 DIM Xyz (3 ) ! 1040 DIM Axis (3, 3) ,Axis$ (3) [10] ! 1050 ! 1060 Agt4284a=7l7 ! Address of 4284A 1070 ASSIGN @Work TO "WORK" ! Assign I/O path to store data 1080 Min_bias=0 ! Min. bias value is OA 1090 Max_bias=20 ! Max. bias value is 20A 1100 Step_bias=1 ! Step of bias sweep 1110 READ Nfreq ! read number of frequency 1120 FOR Ifreq=1 TO Nfreq ! 1130 READ Freq(Ifreq) ! read meas. frequency 1140 NEXT Ifreq ! 1150 Nbias=(Max_bias-Min_bias)/Step_bias+1 ! calc. number of bias points 1160 IF Nbias>200 THEN STOP ! check number of bias points 1170 FOR Ibias=1 TO Nbias ! 1180 Bias(Ibias)=Min_bias+Step_bias*(Ibias-1) ! set bias value 1190 NEXT Ibias ! 1200 ! << 4284A initialization>> 1210 OUTPUT Agt4284a;"TRIG:SOUR BUS" ! Trigger mode is Bus trigger 1220 OUTPUT Agt4284a;"FUNC:IMP LSRS" ! Meas function is Ls-Rs 1230 OUTPUT Agt4284a;"INIT:CONT ON" ! 1240 OUTPUT Agt4284a;"DISP:PAGE MEAS" ! Display page is Meas. page 1250 OUTPUT Agt4284a;"INIT" ! Initialize 1260 OUTPUT Agt4284a;"BIAS:STAT ON" ! Bias ON 1270 ! <> 1280 FOR Ifreq=1 TO Nfreq ! Freq. sweep loop < + 1290 OUTPUT Agt4284a;"FREQ "&VAL$(Freq(Ifreq)) ! 1300 FOR Ibias=1 TO Nbias ! Top of bias. sweep loop < + 1310 OUTPUT Agt4284a; "BIAS : CURR " &VAL$ (Bias (Ibias) ) ! Set bias 1320 OUTPUT Agt4284a;"*TRG" ! Triggering 1330 ENTER Agt4284a;Work$ ! Enter Meas. data 1340 A(Ibias,Ifreq)=VAL(work$[1,12]) ! 1350 NEXT Ibias ! Bottom of bias loop < + 1360 NEXT Ifreq ! Bottom of freq. loop < + 1370 OUTPUT Agt4284a;"BIAS:STAT OFF" ! Bias OFF 1380 OUTPUT @work;Nfreq,Nbias ! Store meas. condition 1390 FOR Ifreq=1 TO Nfreq ! 1400 FOR Ibias=1 TO Nbias ! 1410 OUTPUT @work;A(Ibias,Ifreq) ! Store meas. data 1420 NEXT Ibias ! 1430 NEXT Ifreq ! 1440 ! <> 1450 CLEAR SCREEN ! Clear screen 1460 GOSUB Trans_init ! Initialize Trans subroutine 1470 WINDOW -2,2,-2,2 ! Set graphic window 1480 GOSUB Axis ! Draw axes 1490 Amax=MAX(A(*)) ! Find max. value of meas. data 1500 FOR Ifreq=1 TO Nfreq ! <> 1510 FOR Ibias=1 TO Nbias ! 1520 Xyz(1)=LOG(Freq(Ifreq))/LOG(Freq(Nfreq)) ! 1530 Xyz(2)=Bias(Ibias)/Bias(Nbias) ! 1540 Xyz(3)=A(Ibias,Ifreq)/Amax ! 1550 GOSUB Trans ! Make graphic data of 3D 1560 Xp (Ibias,Ifreq)=Xyz(1) ! 1570 Yp (Ibias,Ifreq)=Xyz(2) ! ! 1580 NEXT Ibias ! 1590 NEXT Ifreq ! 6 1600 MOVE Xp(1,1),Yp(1,1) ! <> 1610 FOR Ifreq=1 TO Nfreq ! Top of freq. loop < + 1620 FOR Ibias=1 TO Nbias ! Top of bias loop < + 1630 DRAW Xp(Ibias,Ifreq),Yp(Ibias,Ifreq) ! Draw graph 1640 NEXT Ibias ! bottom of bias loop + 1650 MOVE Xp(1,Ifreq+1),Yp(1,Ifreq+1) ! 1660 NEXT Ifreq ! bottom of freq. loop + 1670 MOVE Xp(1,1),Yp(1,1) ! 1680 FOR Ibias=1 TO Nbias ! 1690 FOR Ifreq=1 TO Nfreq ! 1700 DRAW Xp(Ibias,Ifreq),Yp(Ibias,Ifreq) ! Draw grid 1710 NEXT Ifreq ! 1720 MOVE Xp(Ibias+1,1),Yp(Ibias+1,1) ! 1730 NEXT Ibias ! 1740 STOP ! 1750 1760 Trans_init:! ! <> 1770 Xd=.5 ! 1780 Yd_1 ! 1790 RETURN ! 1800 ! 1810 Trans: ! <> 1820 Xxx=Xyz (1) ! 1830 Xyz (1)=Xyz (2) - Xxx*Xd ! 1840 Xyz (2)=Xyz (3) - Xxx*Yd ! 1850 RETURN ! 1860 ! 1870 Axis: ! <> 1880 Axis$(1)="FREQ." ! Label of Y axis 1890 Axis$(2)="BIAS" ! Label of X axis 1900 Axis$(3)="INDUCTANCE" ! Label of Z axis 1910 MAT Axis= (0) ! Init. axes data 1920 FOR Iax+1 TO 3 ! 1930 Axis(Iax,Iax)=l.2 ! 1940 NEXT Iax ! 1950 MAT Xyz= (0) ! 1960 GOSUB Trans ! Make 3D graph data of zero 1970 Xzero=Xyz(1) ! 1980 Yzero=Xyz(2) ! 1990 FOR Iax=1 TO 3 ! 2000 MAT Xyz= Axis(Iax,*) ! 2010 GOSUB Trans ! Make 3D graph data of axes 2020 MOVE Xzero,Yzero ! 2030 DRAW Xyz(1),Xyz(2) ! Draw axis 2040 LABEL Axis$(Iax) ! plot label 2050 NEXT Iax ! 2060 RETURN ! 2070 ! <> 2080 DATA 17 ! Number of data 2090 DATA 20,50,100,200,500,1E3,2E3,5E3,1E4,2E4,5E4,1E5,2E5,3E5,4E5,5E5,7E5 2100 END 7 Agilent Email Updates Remove all doubt www.agilent.com/find/emailupdates Get the latest information on the products and applications you select. Our repair and calibration services will get your equipment back to you, performing like new, when promised. Agilent Direct You will get full value out of your Agilent equipment throughout its lifetime. 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