Switch Mode Power Supply Measurements - Application Note
Keysight TechnologiesSwitch Mode Power Supply MeasurementsApplication Note
IntroductionUsing a Keysight InfiniiVision 3000T/4000 X-Series OscilloscopeWith the Power Measurements OptionKeysight Technologies, Inc. 3000T and 4000 X-Series oscilloscopes with the Power Measurementsoption provide a quick and easy way to analyze the reliability and efficiency of switching powersupplies. This application note provides step-by-step instructions on how to perform a broad range ofpower analysis measurements on your switch mode power supply (SMPS), as well as detailedinstructions on how to perform these same measurements when using Keysight’s SMPSMeasurements Training Kit as the device under test (DUT). The following power analysismeasurements are enabled when a Keysight InfiniiVision 3000T or 4000 X-Series oscilloscope hasbeen licensed with the Power Measurements option (DSOX3PWR or DSOX4PWR):Input analysis–– Power Quality–– Current Harmonics–– Inrush CurrentSwitching/modulation analysis–– Switching Loss–– Slew Rate–– ModulationOutput analysis–– Output Ripple–– Turn on/Turn off–– Transient Response–– Power Supply Rejection Ratio (PSRR)–– Efficiency
03 Keysight Switch Mode Power Supply Measurements - Application NoteTable of ContentsIntroduction. 2Probing Tips. 4Deskewing the Probes. 6Power Quality Analysis. 7Current Harmonics Analysis. 8Inrush Current Analysis . 9Switching Loss Analysis . 10Slew Rate Analysis . 14Modulation Analysis. 15Output Ripple Analysis. 16Turn-on/Turn-off Analysis. 17Transient Response Analysis. 19PSRR Analysis. 21Efficiency Analysis. 22Related Literature. 23Figure 1: Keysight’s Switch Mode Power Supply Measurements Training KitRequired Equipment–– Keysight 3000T or 4000 X-Series Oscilloscope with the Power Measurements option(DSOX3PWR or DSOX4PWR)–– U1880A Deskew Fixture with USB cable for power–– N2790A High-voltage Differential Active Probe, or equivalent–– 1147B 15A Current Probe, or equivalent–– 10:1 Passive Voltage Probe–– Your Switch Mode Power Supply (SMPS) or Keysight’s SMPS Measurements TrainingKit (Figure 1)
04 Keysight Switch Mode Power Supply Measurements - Application NoteProbing tipsIf using Keysight’s N2790A high-voltage differential active probe(Figure 2) for your power measurements, this probe can bemanually set for either 50:1 attenuation for up to 140 V (DC pluspeak AC) measurements, or 500:1 attenuation for up to 1400 Vmeasurements. If you are using Keysight’s SMPS training kit asyour DUT, the 50:1 setting is appropriate. But, if you areperforming measurements on your own switching power supplythat may have higher input and switching voltages, then you mayneed to use the 500:1 setting.When you initially connect the N2790A probe to one of the inputchannels of the Keysight 3000T/4000/6000 X-Seriesoscilloscope, the scope will automatically detect and set its ownprobe attenuation factor for that channel to 50:1, but if you havethe probe attenuation manually set on the probe to 500:1, thenyou will need to manually enter a 500:1 attenuation factor in thescope’s probe menu for that particular channel.Figure 2: Keysight’s N2790A 100-MHz high-voltage differential active probeThe 1147B or N2893A current probe (Figure 3) is a 0.1 V/A probe(10:1 attenuation). When this probe is connected to any inputchannel of a Keysight 3000T/4000/6000 X-Series oscilloscope,the scope will automatically detect that a current probe isconnected to provide measurements in units of Amps. It will alsoautomatically detect and compensate for the 10:1 probeattenuation. No user settings are required.When you connect the current probe to a current loop in yourdevice under test, make sure to fully close AND lock the probeclamp. To lock it in place, slide the locking mechanism fullyforward until you feel and hear it click in place.Figure 3: Keysight’s 1147B 50-MHz, 15A AC/DC current probe
05 Keysight Switch Mode Power Supply Measurements - Application NoteProbing tips (continued)When using the 1147B or N2893A current probe to make currentmeasurements on Keysight’s SMPS training kit, it is very easy toconnect to various designed-in PC board current loops. But, ifyou are making measurements on your own switching powersupply, rarely are designed-in current loops available. This meansyou will probably need to create temporary current loops in yourprototype power supply’s circuitry in order to measure currentand power. Figure 4 shows an example of a wire-loop in serieswith the Source terminal on a FET switching device in order tomeasure drain-to-source current. To measure input AC current,engineers will often just strip back the insulation on the powercord to access the line current wire. Note that although this maybe quick and easy, this is NOT a Keysight-recommendedtechnique.After extended use, a magnetic field can build up in a currentprobe. When you are making power supply measurements usinga current probe, you should occasionally “degauss” (de-magnify)the probe. To do this, simply disconnect the current probe fromthe device under test, close and lock the current probe clamp,and then press the DEMAG button near the base of the probewhere it connects to the scope. Note that you can also calibratethe offset of the probe (and scope) at the same time by rotatingthe ZERO ADJ thumb-wheel until the baseline current waveformaligns with the ground indicator on the scope’s display.Figure 4: Creating a special current loop in your power supply to measure current
06 Keysight Switch Mode Power Supply Measurements - Application NoteDeskewing the probesWhen using dissimilar type probes, such as a voltage probe andcurrent probe, it is important to perform a probe deskewcalibration. This automatic procedure using the recommendedU1880A deskew fixture (Figure 5) will null-out the difference inpropagation delay between your probes to provide moreaccurate power measurements. This is especially important forthe Switching Loss measurement where just a few nanosecondscan sometimes make a big difference in power and energy lossmeasurements during the turn-on and turn-off phases oftransistor switching.1.2.3.4.5.6.7.8.9.Connect the U1880A’s USB cable between the deskewfixture and the USB port on the back of the oscilloscope.Press the [Default Setup] front panel key.Press the [Analyze] front panel key; then press the Featuressoftkey and select the Power Application.Press the Analysis softkey; then select Power Quality.Press the Signals softkey; then press the Deskew softkey.Note that the automatic deskew calibration is also availablewithin the Current Harmonics and Switching Lossmeasurement menus.Set the S1 switch on the deskew fixture to the “Small Loop”setting as shown in the on-screen diagram in Figure 6.Connect the N2790A high-voltage differential active probebetween the scope’s channel-1 input to J6 (red lead tored test point) and J7 (black lead to black test point) onthe U1880A deskew fixture as illustrated in the on-screendiagram.Connect the current probe from the scope’s channel-2input to the “Small Loop” in the direction indicated on thedeskew fixture (current flow towards the top of the board).Press the Auto Deskew softkey on the scope.Figure 5: Keysight’s U1880A deskew fixtureFigure 6: Deskew Connections diagramWhen the automatic deskew calibration completes, your scope’sdisplay should look similar to Figure 7. The deskew calibrationfactor will remain in the channel-2 probe settings menu and isnon-volatile. But you can manually change the deskew factor, orperform a “factory default setup” within the scope’s [Save/Recall]menu to reset it back to zero. Performing a standard front panel[Default Setup] will NOT reset the deskew calibration factor.Figure 7: Deskew display after completing automatic deskew between voltage andcurrent probes
07 Keysight Switch Mode Power Supply Measurements - Application NotePower Quality AnalysisPower Quality analysis measures the quality of the input AC linesignal that supplies power to your switching power supply whileoperating. This measurement provides the following input signalquality parameters:–– Real Power (P VInstantaneous x I Instantaneous Averagedover N cycles)–– Apparent Power (S VRMS x IRMS over N cycles)–– Reactive Power (Q Apparent x SIN())–– Power Factor (PF Real/Apparent)–– Voltage Crest Factor (CF V VPeak /VRMS)–– Current Crest Factor (CF I IPeak / IRMS)–– Phase Angle (φ ACOS(PF))1.2.3.4.5.6.7.8.9.If using Keysight’s SMPS training kit, set the S2 load switchto the ON position (high-load, maximum current).Press the [Default Setup] front panel key.Press the [Analyze] front panel key; then select the PowerApplication under the Features softkey.Press the Analysis softkey; then select the Power Qualitymeasurement.Press the Signals softkey.Ensure that Voltage is set to 1 (channel-1), and Current isset to 2 (channel-2).Connect the N2790A high-voltage differential active probefrom the scope’s channel-1 input to Line (red lead) andNeutral (back lead) as illustrated in the connectionsdiagram shown in Figure 8. If using Keysight’s SMPStraining kit, connect the probe to TP2 (red lead) and TP1(black lead).Connect the current probe from the scope’s channel-2input to a wire-loop of the input line signal. If usingKeysight’s SMPS training kit, connect the current probe tothe J1 loop in the direction indicated.Press the AutoSetup softkey; then press Apply.Real PowerP (W)ΦApparentPowerS (VA)Figure 7: Apparent, Real, and Reactive powerFigure 8: Connections diagram for the Power Quality measurementWhen AutoSetup is pressed, the scope optimally scales thevoltage (yellow trace) and current (green trace) waveforms, turnson the waveform math power waveform (purple trace), and thensets the timebase to display two cycles (default setting) acrossthe screen. Note that if you observe the current waveform (greentrace) out of phase relative to the voltage waveform (yellow trace),which will result in negative power waveform pulses (purpletrace), then your current probe is probably connected backwards.When Apply is pressed, the 4000 or 6000 X-Series scope willautomatically measure all of the power quality parameters asshown in Figure 9. The 3000T X-Series scope will measure justPower Factor, Real Power, Apparent Power, and Reactive Power.To measure the Crest Factor of the voltage and currentwaveforms, make sure Type is set to Crest; then press Apply. Tomeasure the phase between the voltage and current waveforms,press the Type softkey, select Phase Angle, and then press theApply softkey again.ReactivePowerQ (VAR)Figure 9: Input Power Quality measurements
08 Keysight Switch Mode Power Supply Measurements - Application NoteCurrent harmonics analysisCurrent harmonics analysis measures the amplitude of frequencycomponents that can be injected back into the AC lines.End-products must often meet specific standards of compliancein order not to disturb other equipment connected to the ACsupply grid. This measurement will perform an FFT measurementon the current waveform, compare the results of the amplitudesof odd and even harmonics against a user-selected IEC standard,and also provides color-coded pass/fail indicators for each testedfrequency up to the 40th harmonic.1.If using Keysight’s SMPS training kit, set the S2 load switchto the ON position (high-load, maximum current).2. Press the [Analyze] front panel key; then select the PowerApplication under the Features softkey.3. Press the Analysis softkey; then select the CurrentHarmonics measurement.4. Press the Signals softkey.5. Connect the N2790A voltage probe from the scope’schannel-1 input to Line (red lead) and Neutral (back lead)as illustrated in the connections diagram shown In Figure10. If using Keysight’s SMPS training kit, connect the probeto TP2 (red lead) and TP1 (black lead).6. Connect the current probe between the scope’s channel-2input to a wire-loop of the input line signal. If usingKeysight’s SMPS training kit, connect the current probe tothe J1 loop in the direction indicated.7. Ensure that Voltage is set to 1 (channel-1), and Current setto 2 (channel-2).8. Press AutoSetup. If you use the default settings, the scopewill display 20 cycles of the input line voltage (yellow trace)and current (green trace) waveforms on the scope’s display.9. Press the Settings softkey; then press the Line Freq softkeyand set the frequency to either 50 Hz, 60 Hz, or 400 Hzdepending upon the line frequency in your part of the worldor application. Note that you can also select theappropriate IEC standard to test against in this menu aswell.10. Press the [Back] softkey to return to the previous menu;then press the Apply softkey to begin the CurrentHarmonics measurement.Figure 10: Connections diagram for the Current Harmonics measurementFigure 11: Current Harmonics measurement in a tabular formatWhen Apply is pressed, the scope performs an FFT waveformmath operation (purple trace) on the current waveform with theresults shown in a tabular format in the upper half of the scope’sdisplay as shown in Figure 11. The scope measures up to the 40thharmonic and compares against the selected IEC standard. Toview the higher harmonics, press the Scroll Harmonics softkeyand then rotate the knob.11. To view the results in a bar chart format, press theSettings softkey, then press the Display softkey andchange from the Table setting to the Bar Chart setting asshown in Figure 12.Figure 12: Current Harmonics measurement in a Bar Chart format
09 Keysight Switch Mode Power Supply Measurements - Application NoteInrush current analysisInrush Current Analysis measures the peak input current (positiveor negative) when power is initially turned on. Since thismeasurement is based on the acquisition of a single-shot event,there is not an AutoSetup selection for this measurement(AutoSetup requires a repetitive input signal). For this reason,you must enter the “expected” peak current surge, as well as thesteady-state peak-to-peak line voltage so that the scope canestablish initial vertical scaling. The scope then providesstep-by-step instructions on how to perform this single-shotmeasurement.1.2.3.4.5.6.7.If using Keysight’s Power Measurements training kit, setthe S2 load switch to the ON position (high-load, maximumcurrent).Press the [Analyze] front panel key; then select the PowerApplication under the Features softkey.Press the Analysis softkey; then select the Inrush Currentmeasurement.Press the Signals softkey; then connect the N2790Avoltage probe between the scope’s channel-1 input to Line(red lead) and Neutral (back lead) as illustrated in theconnections diagram shown in Figure 13. If using Keysight’sSMPS training kit, connect to TP2 (red lead) and TP1 (blacklead) on the demo board.Connect the current probe between the scope’schannel-2 input to a wire-loop of the input Line signal. Ifusing Keysight’s training kit, connect the current probe tothe J1 loop in the direction indicated.Ensure that Voltage is set to 1 (channel-1), and Current isset to 2 (channel-2).Set Max Vin and the approximate Expected surge currentfor your device under test. If using Keysight’s SMPS trainingkit, then the default settings should be appropriate for thismeasurement.Note that the default Expected surge current and Max Vinsettings have been optimized (default setup) for Keysight’s SMPStraining kit. Although the steady-state peak current of the demoboard is approximately 1 A (with the S2 load switch set ON), thepeak inrush current is much higher and can sometimes exceed 30 A. Determining the “expected” peak current of your deviceunder test will be an iterative/trial & error process. The value thatyou enter for Expected should be higher than the actual peakcurrent that will be captured and measured.8.After accepting or changing the settings in the Signalsmenu, press the [Back] front panel key (left side of scope)to return to the previous menu.9. Press the Apply softkey; then follow the step- by-step onscreen instructions, which are repeated below.10. Turn OFF power; then press Next.11. Turn ON power; then press Next.12. Repeat several times in order to obtain the worst-casepeak inrush current as shown in Figure 14.Figure 13: Setting “expected” inrush current and peak-to-peak line voltage for asingle-shot inrush current measurementFigure 14: Inrush current measurementIf the Peak Current measurement indicates either “ ” or “ ”, this isan indication that the current waveform is “clipped” and that youshould then increase the Expected current setting in the Signalsmenu for a more accurate measurement. The absolute peakinrush current will typically occur if the power switch is turned onat the same instance in time that the input line voltage is eitherat its absolute positive or negative peak value. This means thatyou should repeat this measurement several times in order tomeasure the worst-case inrush current.
10 Keysight Switch Mode Power Supply Measurements - Application NoteSwitching loss analysisSwitching loss analysis measures power and energy losses ofyour switching device (typically an FET). In a Switch Mode PowerSupply (SMPS), most power and energy losses occur during theswitching phases of the transistor when it turns on and turns off.During these phases the switching transistor goes in and out ofsaturation and momentarily operates in its linear region. Powerand energy losses also occur during the conduction phase of theswitching transistor. This is when voltage is at the transistor’ssaturated minimum and current flows. Losses during thenon-conduction phase are typically insignificant, and shouldtheoretically be zero. The Switching Loss measurementautomatically measures losses over one switching cycle. But youcan then optimize the scope’s horizontal and vertical settings toperform more accurate power and energy loss measurementsduring particular switching phases.1.2.3.4.5.6.7.8.If using Keysight’s SMPS training kit, set the S2 load switchto the ON position (high-load, maximum current).Press the [Analyze] front panel key; then select the PowerApplication under the Features softkey.Press the Analysis softkey; then select the Switching Lossmeasurement.Press the Signals softkey.Connect the N2790A high-voltage differential probebetween the scope’s channel-1 input to the drain/ collector(red lead) and source/emitter (black lead) of yourswi
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