Troubleshooting Switched Mode Power Supplies With A Digital Oscilloscope

Switched Mode Power Supply Measurements

FAST: Advanced Triggering

AgendaIn this workshop we’ll be learningı SMPS background and basics ı Measurement setupı Oscilloscope measurements

Averaging, filtering, gridsProbing and bandwidthCurrent measurements and deskew

ı Measurement Example: startup waveformsoutput voltage rippletransient behaviorswitch node voltage and current

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SMPS | 3

Switched mode power supply basics

l Basic DC-DC converterl Switches A and B alternately charge and discharge inductor

through loadl Switches are realized using power MOSFET, IGBT and diodes

Vs(t)

SMPS | 4

Voltage regulation in SMPS

l Average voltage at the load is controlled by the duty cycle Dl Waveform assumes an ideal switch

DTs (1-D)Ts

Vs(t) Vg

0

Vs = DVg

Understanding Power Flow and Topology

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SMPS | 4

Inductor Current Waveform


Measurement Setup

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SMPS

Passive voltage probes

Single ended and differential active probes

De-skew fixture

Current probes

Programmable power supply variable current and voltage remote sensing

Oscilloscope: 500 MHz or more

Maximizing measurement accuracy

ı Large dynamic range required for accurately measuring switching lossOn state is tens to hundreds (even thousands) of voltsOff state is often only several mV to a few voltsTypical A/D converters provide only 6 to 8 effective bits (50 dB S/N)This is equivalent to 20 mV out of 5 V

ı Maximizing signal to noiseWaveform averagingHigh resolution and filtering filtering (trade off sample rate and bandwidth for

S/N)Multiple grids

ı Probing and bandwidthSMPS contain high slew rate signals and high frequency contentProbing is critical for accurate measurements – bandwidth and connectionOscilloscope bandwidth and sample rate must be high enough to measure

fast edges and high frequency interference

Waveform averaging

ı Increases resolution by averaging samplesEffective in reducing thermal (random) noiseWill distort time varying waveformsCan also reduce displayed rise timeCan not reduce deterministic noise sources such as interleaving artifacts

High Resolution Mode or Digital Filter

ı Combine consecutive samples from A/D converter

ı Preserves real time sampling – no smearing of dynamic signals

ı Reduces bandwidth based on decimated sampling rate

ı Should be combined with filtering to reduce interpolation error

Combine samples for each point

Viewing Multiple Waveforms

Using Multiple Grids

Resolution is Reduced by Half…

Full scale waveform

Half scale waveform

Passive Probes – Ground Lead Length

Long ground lead Short ground lead

Active Probes

Slew Rate and Vertical Resolution

ı Both vertical and horizontal resolution are criticalHigh slew ratesMeasuring short, high

amplitude peaks that could damage active components

ı 4.4 V/ns = 880 mV per sample @ 5 Gs/s

ı 4.4 V/ns = 4.4 V sample @ 1 Gs/s

ı Compare to digitizer range39 mV @ 8 bits2.4 mV @ 12 bits

ı Measurement is limited by the sampling rate

Slew Rate and Vertical Resolution

ı Use high bandwidth probeShortest lead lengthsActive probes if possible

ı Maximize sampling rate and bandwidthSampling rate 5 to 10 times the scope bandwidthOscilloscope rise time 10x faster than switch time

ı Use averaging whenever possibleHigh resolution mode reduces rise time, bandwidth and sampling rateAveraging preserves sampling rate and rise time


Measuring Currentı Clamp-on current probes

Both DC and AC current measurementMust be “de-magnetized” Requires a loop in the circuitLimited bandwidth

ı Shunt resistorMeasure the voltage drop across a small

resistor – usually 0.1 ohmResistor must have stable value over

temperature and currentHighest bandwidth

ı Indirect method using near field probeOnly AC current proportional to -d(i(t))/dtLimited sensitivityMeasurement on very small geometry

and without disturbing the circuit

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Using a Near Field Probe to Measure Current

ı Probe voltage proportional to the derivative of the current

ı Small form factor probes can reach tight spots

ı Integrate signal to measure current Integral reduces noise on small signal

Current flow

H field

Vo

RT-ZF20 - Power Deskew Fixture

Probe De-skew

ı Skew between voltage and current probe leads to wrong power measurement results

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Deskewing with reference voltage and current pulses essential for accurate power measurements

Positive voltage vs current pulse skewPower measurement too low

Negative voltage vs current pulse skewPower measurement too high

Positive voltage vs current pulse skewPower measurement too low

Deskewed, accurate measurement

RT-ZF20 - Power Deskew Fixture

RT-ZF20 – How to deskew1. Connect RT-ZF20 to USB

2. Connect current probe and voltage probe

to RT-ZF20

3. Overlay current and voltage pulseTrigger condition rising + falling edgeAdjust vertical scale to same pulse height

4. Adjust „Deskew“ parameter of scope for current probe

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Deskew

Voltage pulse

Current pulse

Different propagation delay between current and voltage pulse Current and voltage pulse aligned

Start-up Behavior


Startup Waveforms

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No load

20 W load

5 W load

Output Voltage Ripple and Spectrum


Measure output voltage

ı Measured using passive probe with long ground lead

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ı Measured using passive probe with short ground lead

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ı Measured using passive probe with an active probe

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Measure output voltage spectrum

ı Spectrum measured out to 30 MHzı Spurs look very similar in both cases

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20 Ω load

5 Ω load


Measure output voltage spectrum

ı Spectrum measurement up to 500 MHz

ı Increased noise between 100 and 300 MHz with 5 ohm load

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20 Ω load

5 Ω load

Transient Response


Measure The Voltage Transient Response

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ıOutput voltage during load transient

No load

20 Ω load5 Ω load 4 Ω load


Measure The Voltage Transient Response

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ı Examining voltage after filteringı Stability is determined by analyzing overshoot and any ringingı Can be measured in-circuit

Measuring Switch Node Voltage and Inductor Current


Measure Switch Node Voltage and Current

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ı 20 ohm loadı Current measured using near field probeı Math waveform computes integral of near field probe voltageı Averaging and high resolution mode applied to signals


Measure Switch Node Voltage and Current

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ı 5 ohm loadı Current measured using near field probeı Math waveform computes integral of near field probe voltageı Averaging and high resolution mode applied to signals ı Slope of voltage increased compared to 20 ohm case and inductor current is non-linear


Conclusion

ı Increasing the load to 5 Ω results in reduced voltage (by approximately 200 mV) and increased voltage ripple Increased spectral power above 100 MHz3% ripple voltage

ı Examining the switching node revealed that the inductor appears to be the root causeNon-linear IL with 5 Ω loadDecreased rise time of Vsw with increased loadHigher slope on Vsw at higher load

ı The problem is traced to an undersized inductor

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Engineering

Troubleshooting Switched Mode Power Supplies With A Digital Oscilloscope