Bode 100 - Application Note - All
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1. OMICRON LAB Bode 100 Application Note DC DC Converter Stability Measurement 1976IFE q TP3 vouT 3 3V 1A C6 Optional Strongly supported by LT MUR By Stephan Synkule Lukas Heinzle amp Florian Hammerle 2014 by OMICRON Lab V2 1 Visit www omicron lab com for more information Contact support omicron lab com for technical support Smart Measurement Solutions Bode 100 Application Note DC DC Converter Stability Measurement Page 2 of 17 Table of Contents 1 INTRODUC IN cases a secs seca ce ce vecceedececeiscenccesatesdanicevuncucecanuctseccinnecenecenesendcateseuccsectiessncesiecaumesancsaseteosseestecs 3 2 MEASUREMENT SETUP seen te a en ee ne a eer ae 4 2 1 THE CIRCUIT UNDER TEST eee ee me an eee meen eee vere ae 4 2 2 SELECTING THE INJECTION POINT easiest easter y cols peau coccteaceda nea alanine danentace snacanseas annostpscanbecinieasnededs ocsadejaaseacesiocssdee 5 2 3 CONNECTING THE BODE 100 sccescsccviccaconecnctedcoueaesacacaetedewkecaanneuneteheowedecedaanoteiendsiinciavncsdauetanddoe suotsldeewinensivhcinvnsdanenaddoesnns 6 2 4 PHASE MARGIN AND GAIN MARGIN cscccscccsccsccesecucecucccucccuccauccesecusccasececueccsecesecsecesecauccuuecasecuseseseusceasceuscenseens 8 3 DEVICE CONFIGURATION seoseoseoseosecosscscssssesseeseosecsecsecsecsscsssecsscsscsscsssecsecsecsecsecsscsecssosssessecsecsecsecsecsscssessoe 9 4 MEASUREMENT amp RESULTS cccccscccscccscccscc2. of a switched mode power supply such as a step down DC DC converter To guarantee a stable output voltage of a power supply and to reduce the influence of supply voltage variations and load changes on the output voltage of a power supply a compensating controller is necessary The quality of the design of this control circuit determines the stability and dynamic response of the entire DC DC converter system The following pages show you how you can measure the loop response of such control systems using the Bode 100 vector network analyzer in combination with the B WIT 100 wideband injection transformer For the characterization of the loop we measure the open loop gain by using the voltage injection method This method is commonly used to analyze the control loop stability of voltage regulators such as switched mode power supplies In this document we will discuss the following points in detail e How to choose the correct injection point to measure the loop gain e Determining gain margin and phase margin from the frequency response e How do supply voltage and load current influence the systems dynamics e Using the shaped level feature of the Bode 100 to improve the measurement results OMICRON E LAB Smart Measurement Solutions 7 Bode 100 Application Note DC DC Converter Stability Measurement Page 4 of 17 2 Measurement Setup 2 1 The Circuit under Test The demo board 481A is a step down buck converter featuring the LT1976
3. In this case we use 10 1 probes to pick up the signals but any standard oscilloscope probe can be used for this measurement Attention lf hazardous voltages are present make sure that suitable probes are used to protect operator and device from any dangerous voltage gt 33 V OMICRON E LAB Smart Measurement Solutions 7 Bode 100 Application Note DC DC Converter Stability Measurement Page 7 of 17 To ensure good measurement results it is strongly recommended to place the injection resistor the injection transformer and the probes close to the circuit to keep leads short Furthermore it is very important to avoid mechanical stress at soldering pads to prevent damage to the test object The following figures show how we have realized the modification on the demo board and how the probes and the injection transformer are connected to the circuit TECHNOLOGY 408 432 1900 www linear com orre o DEMO CIRCUIT 481A Figure 4 Demo board prepared for connecting the measurem i J p Do With lures Mode ym nana OGY 6 F COM BEN Tig pon n e a I _ ie 2 at tar hm Figure 5 The probes and the injection transformer connected to the circuit AA OMICRON OT Smart Measurement Solutions Bode 100 Application Note DC DC Converter Stability Measurement Page 8 of 17 Figure 6 Measurement setup with power supply resistive load Ampere meter and Bode 100 2 4 Phase Marg
4. Receiver Bandwidth 30 Hz ma We set the reference level to 20 dBm and increase the output level from 100 Hz to 500 Hz from 20 dBm to 0 dBm by entering a delta level of 20 dB File Help 4 OK M Cancel i Print E Print Preview Output Level Reference Level Preview Output Level Measurement Range Full Frequency Range OMICRON Smart Measurement Solutions Bode 100 Application Note DC DC Converter Stability Measurement Page 14 of 17 Restarting the measurement leads to the following gain phase curve L Cursor 1 20 496k 0 000 83 007 60 ___Lursor2 92 060k 9 499 0 000 2 C1 71 563k E 2 A TH ry ie 404 ee SA Py i ON 0 7 oe X E E N _ Ee es Uae rene rT ETT 207 rr r fr rt 71 Inve eenee an 40 Az TRIB TRZ fp M maan T 9 499 83 007 102 103 104 ro f Hz m R1 Mag Gain _ TR2 Phase Gain Figure 10 loop gain measurement 12 V Input voltage and 1 A load current By using the cursors we can read the Gain Margin and Phase Margin of the system The measurement indicates a Phase Margin of PM 83 and a Gain Margin of GM 9 5 dB 4 3 Injection Level You may have noticed that we use a very low output level of 20 dBm for this measurement The reason is that we want to analyze the small signal behavior of
5. be amplified or attenuated and shifted in phase The Bode 100 output will provide the disturbance signal whereas the inputs will measure the transfer function of the loop To ensure that the measured loop gain equals the real loop gain we need to choose a suitable point First we need to find a point where the loop is restricted to one single path to make sure that there are no parallel signal flows Then we need to make sure that at this point the impedance looking in the direction of the loop is much bigger than the impedance looking backwards The following figure shows the feedback loop of the circuit and indicates the suitable injection point The impedance looking backwards equals the output impedance of the converter which is very low in the range of several mQ The impedance looking in direction of the loop is formed by the compensator and the voltage divider and is in the range of several KQ C4 010F Point TP3 a VOUT ul LT1976IFE q 33uH CDRH8D28 330NC c5 100uF D 6 3V Optional 1 0MQO60N TP6 GND Av Figure 2 Feedback loop and injection point More details on the selection of the injection point and the theory of the voltage injection method can be found in the article Loop Gain Measurement which is available for download at http www omicron lab com bode 100 application notes know how articles use cases htmI 3 OMICRON E LAB Smart Measurement Solutions 7 Bode 100 Applicatio
6. nen nnn na eee nn ne anne named 40 Vz TRIB Cursor 1 10 521k 0 000 60 C Cursor 2 81 025k 25 385 _ C2 C1 70 504k 25 385 36 i 102 103 104 10 f Hz m R1 Mag Gain m TR2 Phase Gain Figure 12 loop gain measurement 5 V Input voltage and 1 A load current The phase margin did decrease to PM 37 whereas the gain margin did increase to GM 25 4 dB 4 5 Load Current Influence By varying the load current and keeping the supply voltage of the regulator constant we can check the sensitivity of the system to different load currents The following graph shows the loop gain measurement at different load currents All measurements were performed with a supply voltage of 12 V 80 ee ee 200 ao 7 00 C 50 N 50 0 20 40 Hl 150 60 200 102 108 104 10 f Hz 100MA Mag Gain 500mA Mag Gain 1000mA Mag Gain 100mA Phase Gain 5Q0O0mMA Phase Gain 1000mA Phase Gain Figure 13 loop gain depending on load current 4 4 OMICRON E LAB Smart Measurement Solutions 7 Bode 100 Application Note DC DC Converter Stability Measurement Page 16 of 17 5 Conclusion The Bode 100 in combination wit
7. the regulator Some regulators are very sensitive to the injected level and show nonlinearities or big signal effects if the injected level is too high If we i e set the load to 80 mA and use an output level of 18 dB for the measurement the result will be erroneous as shown below L oS E TT sn 100 ee 50 o g oc al ee J o 50 404 Lo I Poof ff yf 100 go 150 102 103 408 105 f Hz m TR1 Mag Gain TR2 Phase Gain Figure 11 big signal effects nonlinearities due to excessive injection signal Such erroneous measurements can be avoided by reducing the injection signal level The shaped level feature provides the possibility to reduce the output level exactly at the frequencies where it is necessary OMICRON E LAB Smart Measurement Solutions 7 Bode 100 Application Note DC DC Converter Stability Measurement Page 15 of 17 4 4 Supply Voltage Influence With our next measurement we will check how supply voltage changes influence the characteristic of the LT1976 control circuit To do so we change the supply voltage to 5 V Restarting the sweep and placing the cursors again at the 0 dB and 0 points leads to the following graph 60 tao 150 40 100 20 ee a eee eee COE ea eae el Mee eee a EN ES Stee aH Dement TR1 dB 20 Ren enn enn nn nn en eee nen nnn
8. IMP OFF Xj Trace Functions TRI AVG OFF TR2 AVG OFF X E Start Frequency 100 000 Hz Stop Frequency 200 000 kHz Center Frequency 100 050kHz Span 199 900 kHz Sweep Mode Loganthmic Number of Points 401 Configuration Shaped Level 20 00 dBm Attenuator CH1 ods Attenuator CH2 0 dB X Receiver Bandwidth 30 Hz Y Measurement Reference Resistance 50 00 Q 10 f Hz TR1 Mag Gain TR2 Phase Gain TR1 AVG Off TR2 AVG Off Gain inp ERT 7 of rc 2013 07 10 EM Figure 9 Screenshot of the Bode Analyzer Suite showing the loop gain curve The red line shows the gain magnitude and the blue curve the gain phase Above 1 kHz the result does not show much noise whereas in the lower frequencies the curve is very noisy The reason is the very small injection level and the high gain of 60 dB In order to reduce the noise in the low frequency range we use the shaped level feature of the Bode 100 OMICRON eee LAB Smart Measurement Solutions 7 Bode 100 Application Note DC DC Converter Stability Measurement Page 13 of 17 On the left hand side in the Bode Analyzer Suite click on the arrow right to the Level and select Shaped Level A Shaped Level button will appear By clicking this button the shaped level can be entered in the Shaped Level window Attenuator CH1 7 dB Attenuator CH ode
9. The output is optimized for 3 3 V at a load current of 1 A The following figure shows the schematics of the demo board 481A Detailed information on the demo circuit can be found at htip Awww linear com OTR 408 432 1900 www linear com LTI976IFE DEMO CIRCUIT 481A 1 5A 200kHz Step Down Converter with Burst Mode LINEAR TECHNOLOGY CORPORATION LTC CONFIDENTIAL FOR CUSTOMER USE ONLY sSEE QUICK START GUIDE VIN TPI 4v 60v SEE QUICK START GUIDE TP3 VOUT 3 3V 1A c6 Optional TP6 GND Optional R12 Optional PCB Layout This circuit i i to Linear Technology and supplied dy ise aia TaT ale Notice Linear Technology has made a best effort to 1630 McCarthy Bivd Milpitas CA Customer howaver itremaie te cuscmersreepenabity vey pr 95035 7487 Jeemeni Aiie pke Lineer Technology Corp Phone 408 432 1 900 Fax 408 434 0507 with Burst Mode Document Number Demo Board 481 ka esday Dec 02 2003 Sheet 1 of i o Z Figure 1 LT 481A demo board schematics OMICRON EE R Smart Measurement Solutions Bode 100 Application Note DC DC Converter Stability Measurement Page 5 of 17 2 2 Selecting the Injection Point In order to measure the loop gain of a voltage feedback loop we need to break the loop at a suitable point and inject a disturbance signal at this point The disturbance signal will be distributed around the loop and depending on the loop gain the signal will
10. asurement shows a flat line at OdB and 0 which indicates that both probes have the same frequency response and calibration is not necessary in this case 60 150 40 100 20 50 L a 4 oO E No i a j 20 7 50 40 1 100 60 450 102 103 104 105 f Hz m R1 Mag Gain TR2 Phase Gain Figure 8 THRU Measurement result Note If this measurement shows a gain or phase curve deviating from 0 dB and 0 calibration is necessary Phase and amplitude difference can be compensated by performing a THRU calibration in the Bode Analyzer Suite Details on how to perform a THRU calibration can be found in the Bode 100 User Manual OMICRON Smart Measurement Solutions Bode 100 Application Note DC DC Converter Stability Measurement Page 12 of 17 4 2 Shaped Level We perform the first stability measurement with a supply voltage of 12 V and a load current of 1 A Please do not use electronic loads for frequency response measurements as the control circuit of the electronic load could interfere with the circuit under test Starting a frequency sweep leads to the following bode plot mm OMICRON Lab Bode Analyzer Suite C temp loopgain Bode File Measurement Configuration Calibration Trace Tools Help FHURAN AmO O RAR s amp User Calibration GAIN OFF IMP OFF ZX Probe Calibration GAIN OFF
11. csccesccesccsccesccescsccescccsccesccessccscocsconccescccsccescccsseuss 10 A T UTE TON areca ec seen A E E E EEEE wats pene eats 10 A 2 OHAPED LEVEL orenera tence tt eee ee ee tenet erie tesa et reticent eet octets ate aero wool esto ants doe au ae ae de acs AAT 12 A US Gea GN EVE Deane Sn ne ee re ne en ee eer 14 4 4 SUPPLY VOLTAGE INFLUENCE cccccceccceccccscccesccncccceccccceceucccseccuucecseccusccaucecsuceuececeuccusecuucecsecuusecuuceusceceucenececuacs 15 4 5 LOAD GURRENT UI INS corocisirurindr Erorien ire rieri RNEER ENRE E sets sae baci antes oats 15 5 CONCLUSION oee cece acettece tecencee eeuctteca cece ence vapercceissnecsavesastucuseenucescesaeesteccs santeucessnteccecaceecceeeeeeeceseceauceneess 16 Note Basic procedures such as setting up adjusting and calibrating the Bode 100 are described In the Bode 100 user manual You can download the Bode 100 user manual at www omicron lab com bode 100 downloads 3 Note All measurements in this application note have been performed with the Bode Analyzer Suite V2 43 Use this version or a higher version to perform the measurements shown in this document You can download the latest version at www omicron lab com bode 100 downloads OMICRON E LAB naa Smart Measurement Solutions Bode 100 Application Note DC DC Converter Stability Measurement Page 3 of 17 1 Introduction In this application note we show you how to analyze the stability respectively the control loop behavior
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13. h the B WIT 100 wideband injection transformer offers a perfect toolkit for the quick and easy stability analysis of control systems It enables to measure the gain margin and phase margin of control systems such as switched mode power supplies or linear regulators Gain margin and phase margin are widely accepted indicators for the stability of a control loop Furthermore the Bode Analyzer Suite provides great functionality to display the system response on changing operating conditions such as supply voltage changes or load current changes To ensure Stability of a power supply in the field the combination of all acceptable load and environmental conditions have to be tested This provides detailed information on the dynamic behavior of a DC DC converter in various operating conditions OMICRON E LAB Smart Measurement Solutions 7 Bode 100 Application Note DC DC Converter Stability Measurement Page 17 of 17 IF DE METT i f ah 1 5 OMICRON Lab is a division of OMICRON electronics specialized in providing Smart Measurement Solutions to professionals such as scientists engineers and teachers engaged in the field of electronics It simplifies measurement tasks and provides its customers with more time to focus on their real business OMICRON Lab was established in 2006 and is meanwhile serving customers in more than 40 countries Offices in America Europe East Asia and an international network of distributors enabl
14. in and Gain Margin According to Nyquist the stability of a feedback system can be verified by checking two critical points These are the Gain crossover point where the Phase Margin is measured and the Phase crossover point where the Gain Margin is determined Note When analyzing the open loop gain for stability as it is done in text books positive feedback occurs at 180 phase Therefore the phase margin is measured by determining the phase difference to 180 In this measurement we measure the open loop gain in a closed loop system The phase margin must therefore be measured relatively to the 0 line This is Somehow confusing but gets clearer if you imagine a signal that is injected at the feedback input and appears at the output without any phase shift Such a signal that passed the loop with 0 phase will again be injected at the feedback and sum up with the previous one This is exactly the point where positive feedback and therefore instability will occur in a negative feedback system OMICRON 44 Smart Measurement Solutions 7 Bode 100 Application Note DC DC Converter Stability Measurement Page 9 of 17 3 Device Configuration In order to measure the transfer function of the loop we need to set up the Bode 100 correctly The measurement of the loop gain is performed in the Frequency Sweep mode of the Bode Analyzer Suite uite ga F Sl Open the device configuration window and apply the following settings The e
15. n Note DC DC Converter Stability Measurement Page 6 of 17 2 3 Connecting the Bode 100 We have selected the injection point and now need to break the loop at this point To ensure that the measurement does not change our system behavior we place a small resistor at the injection point that does not significantly change the feedback divider In this case we use a 10 Q resistor The disturbance voltage is applied in parallel to the injection resistor using the B WIT 100 injection transformer The transformer is necessary to isolate the output of the Bode 100 from the DC operating point of the feedback loop The following figure shows how the Bode 100 is connected to the circuit 1976IFE 4 i TP3 VOUT 45 z AA Swie Sumida 3 3VELA 14 CDRH8D28 330NC C6 16 7 PGFB D1 R10 R4 1 OMQO60N 13 0 Ohm 24k 9 1 C10 4TpF a 1 0uF cc VC ee eee BEOR eee nanan VEEE Penn EEEE Lew ere men ayer Figure 3 Connecting the Bode 100 to measure the loop response of the regulator The inputs of the Bode 100 are connected to either side of the injection transformer CH1 measures the disturbance signal that is applied to the feedback divider and CH2 measures the signal that appears at the output of the converter By dividing the voltage at CH2 by the voltage at CH1 we get the transfer function from the feedback input to the output of the power supply This transfer function we Call the loop gain T jw T j 2 VcH1
16. xternal reference is switched on and both inputs are set to high impedance Configuration RECBIVER 2 OUTPUT The following settings are applied Start Frequency 100 Hz Stop Frequency 200 kHz Sweep Mode Logarithmic Number of Points 201 or more Level 20 dBm Attenuator CH1 amp CH2 0 dB Receiver Bandwidth 30 Hz OMICRON E LAB Smart Measurement Solutions 7 Bode 100 Application Note DC DC Converter Stability Measurement Page 10 of 17 Trace 1 amp 2 are set up as shown below to display a Bode plot W Trace 1 TR1 4 Measurement amp Results 4 1 Calibration We first check if a calibration is necessary for this measurement Calibration would be necessary if the two probes used to connect the Bode 100 to the circuit have a different frequency response This would introduce phase shift and attenuation errors in the measurements In order to check if our probes have the same frequency response we connect both probes to the same side of the injection resistor as shown in the picture below T1976IP i DEMO CIRCUIT 200k Step Down wit Burst Mode t oe CONFIDENTIAL FQ sSEE QUICK START Cut Figure 7 Connection during calibration OMICRON EE a Smart Measurement Solutions Bode 100 Application Note DC DC Converter Stability Measurement Page 11 of 17 After connecting the probes we start a measurement by pressing the single sweep putton He The me
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