Serial Bus Debug

8/20/2019 Serial Bus Debug

http://slidepdf.com/reader/full/serial-bus-debug 1/64

Hands-on Class: Switched Mode

Power Supply Measurements





Agenda

In this workshop we’ll be learningı SMPS background and basics

ı Non-ideal behavior in switching components (switches, inductors, capacitors)

ı Key SMPS measurements

ı Measurement Accuracy (averaging, decimation and filtering)

ı

Hands On Example: averaging, decimation and filteringı Hands On Example: measuring switching voltage and inductor current

ı Hands On Example: input and output current and voltage ripple

3/3/2014 FAST: Advanced Triggering 3



Power Systems: Everywhere, in Every Size

ı MicroWatts to MegaWatts

03.03.2014 4

3 MW Wind Power Controller

2.56 mm2 500 mW LED Driver

Small Off-Line Power Supply



SMPS Applicationsı AC/DC Power Supplies for Industry – DC Drives/Welding/Motion Control

ı AC/AC Power Supplies for Industry – Synchronous Drives

ı

DC/DC Power Supplies Internal to Industrial Equipment

ı DC/AC for Automotive & Aircraft – AC drives / AC Power

ı DC/DC Low-voltage for Automotive & Aircraft – Numerous Systems Applications

ı DC/DC High-voltage for Automotive & Aircraft – Lighting, DC drives

ı AC/DC Power Supplies for Consumer Household Equipment

ı DC/DC Power Supplies Internal to Consumer Household Equipment

ı DC/AC Power Supplies Internal to Consumer Household Equipment

ı Power Tools

ı AC/DC Power Supplies for Consumer Electronics

ı DC/DC Supplies internal to Consumer Electronics

ı AC/DC Power supplies for Portable Electronics

ı DC/DC Power Supplies Internal to Portable Electronics

03.03.2014 5



Modern Power Supplies:

Inductors, Capacitors and Fast Switches

ı Use ‘Lossless’ Components, In ‘Switching’ Operation Inductors store energy, and can deliver the energy at arbitrary voltage

Capacitors store energy between ‘pumping’ operations of inductors

ı Replace Linear Series Pass And Shunt Regulators Linear regulators turn excess voltage into thermal energy

ı Effectively ‘Variable Transformer’ Operation Able To Provide Increase/Decrease, Or Both, In Voltage

Able To Operate Over Wide Ranges Of Input Voltage

03.03.2014 6



Modern Power Supplies:

Inductors, Capacitors and Fast Switches

ı The Conflicting Drivers For Efficient Power Conversion

Minimize Losses In Active And Passive Components

Minimize Cost

Minimize Size

Maximize Reliability

Minimize Adverse Effects On Other Systems

03.03.2014 7



SMPS | 3

Switched mode power supply basics

l Basic DC-DC converter

l Switches A and B alternately charge and discharge inductor through

load

l Switches are realized using power MOSFET, IGBT and d iodes

Vs(t)



SMPS | 4

Voltage regulation in SMPS

l Average voltage at the load is controlled by the duty cycle D

l Waveform assumes an ideal switch

DTs (1-D)Ts

Vs(t) Vg

0

Vs = DVg



SMPS | 6

Synchronous converter using FET switches

l Transistors Qa and Qb act as switches A and B

l Must operate in “ break before make” mode to prevent high current

through transistors

l Voltage at the load determined by the duty cycle



Agenda





ı Hands On Example: averaging, decimation and filtering

ı Hands On Example: measuring switching voltage and inductor current





The Conflicting Drivers For Efficient Power Conversion:Minimize Losses In Active And Passive Components

What Are These, Why Are They Important, And What Are The Trade-offs In SMPS

Technology For The Following Goals?

ı Switching Faster Transitions, Higher Frequencies

Lower Rds(on)

Lower Gate Drive Integrate

03.03.2014 12




Why Are These Important, And What Are The Trade-offs In SMPS Technology For

The Following Goals?

ı Inductors Lower DCR

Lower ACR

Reduced Parasitic Capacitance Reduced Size

Reduced Magnetic Leakage

03.03.2014 13




Why Are These Important, And What Are The Trade-offs In SMPS Technology For

The Following Goals?

ı Capacitors Smaller

Lower ESR

Lower Cost

03.03.2014 14



SMPS Passive Components Issues

ı Non-Ideal Behavior Parasitic R/C of Inductors

Inductor Saturation

Inductor Core Losses

Inductor Copper Losses

ESR and ESL of Capacitors

Dielectric and Aging in Electrolytic Capacitors

Heat sensitivity of Electrolytic Capacitors

Aging and Voltage Effects in Ceramic Capacitors

Piezoelectric Effects in Ceramic Capacitors

Impedance vs. Frequency in Ceramic Capacitors

03.03.2014 15



Agenda











Understanding What to Measureı Understanding Power Flow and Topology

ı The Basic SMPS - Buck Converter Topology

03.03.2014 17

A diode often replaces one switch




ı The Basic SMPS - Buck Converter Topology Measurement Points

03.03.2014 18

Vin

Vsw Vind

Iind

Vout




ı The Basic SMPS - Buck Converter Topology Why these points?

What will we see at each point?

03.03.2014 19

Vin

Vsw Vind

Iind

Vout




ı The Basic SMPS - Buck Converter Topology – Probing The Test PointsProbe Types for Each Point

03.03.2014 20

Vin

Vsw Vind

Iind

VoutPassive Voltage

Active SE Voltage(Voltage Range!)

ActiveDifferential

(CommonMode Range!)

Passive Voltage

Current Probe

IoutIin



Understanding Power Flow and Topology

The Basic SMPS - Buck Converter Topology

A more complete picture of the components and measurement points

03.03.2014 21



RT-ZF20 – Why deskewing?

ı

Skew between voltage and current probe leads to wrong power measurementresults

Feb. 2013 RT-ZF20 - Power Deskew Fixture 22

Deskewing with reference voltage and currentpulses essential for accurate power measurements

Positive voltage vs current pulse skew

Power measurement too low

Negative voltage vs current pulse skew

Power measurement too high

Positive voltage vs current pulse skew

Power measurement too lowDeskewed, accurate measurement



RT-ZF20 – How to deskew

1. Connect RT-ZF20 to USB

2. Connect current probe and voltage probe

to RT-ZF20

3. Overlay current and voltage pulse

Trigger condition rising + falling edge

Adjust vertical scale to same pulse height

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

Feb. 2013 RT-ZF20 - Power Deskew Fixture 23

Deskew

Voltage pulse

Current pulse

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




The Basic SMPS - Buck Converter Basic Waveforms

03.03.2014 24

Vswitch

Iinductor

Vout




The Basic SMPS - Buck Converter – Load Transient – Well-Damped Response, Little Overshoot

03.03.2014 25

ILoadVout




The Basic SMPS – 1.4 MHz Buck Converter – Vout Ripple Spectrum

03.03.2014 26

Iinductor

Vout

Spikes at multiples of Fswitch



Agenda











Maximizing measurement accuracy

ı Large dynamic range required for accurately measuring switching loss

On state is tens to hundreds (even thousands) of volts

Off state is often only several mV to a few volts

Typical 8-bit A/D provides only 6 to 7 effective bits (43 dB S/N)

This is equivalent to 39 - 78 mV out of 5 V

ı Three possibilities to improve signal to noise

Use waveform averaging

High resolution decimation (trade off sample rate and bandwidth for S/N)

Overdrive instrument front end



Waveform averaging

ı Increases resolution by averaging samples Effective in reducing thermal (random) noise

Will distort time varying waveforms

Can also reduce displayed rise time

Can not reduce deterministic noise sources such as interleaving artifacts



Noise reduction using averaging

1 mV on 5 Vscale (12.6 bits)50 averages

Zoom of thissegment



High Resolution Mode

ı

Combine consecutivesamples from A/D

converter with weighting

ı Preserves real time

sampling – no smearing

of dynamic signals

ı Reduces bandwidth

based on decimated

sampling rate

ı Compatible with

segmented memory so

that each cycle can beanalyzed

Combinesamples foreach point



High Resolution Decimation Mode

Decimate 10Gs/s to 1 Gs/s

~ 500 MHz BW

4.6 mV on 5 Vscale (10.4 bits)



Combining averaging and high resolution mode

Decimate 10Gs/s to 1 Gs/s50 averages

~ 500 MHz BW

500 uV on 5 Vscale (13.5 bits)



Viewing Multiple Waveforms



Resolution is Reduced by Half…

Full scale waveform

Half scale waveform



Using Multiple Grids



Agenda











What are we going to look at

ı The output voltage of the power supply


Measure

Vout

0.1Ω

5 Ω or20 Ω

2.2 µH

100 µF x3

Vsw

0.1Ω



Hands on Lab

ı Averaging, High Resolution and Filtering





VSW GND

W f A i



Waveform Averaging


ı Probe the Vsw point on the demo board

ı Set the demo board to mode "3." (20 Ω

load)

ı Press "Autosetup"

ı Measure the RMS value of the waveform to

the left of the center of the screen. Use

measurement gating to select the region of

the waveform to measureı Compare the value for the following settings

No averaging

50 averages

"High Res" mode with sampling rate set to

1 Gs/s and no averaging 50 averages and "High Res" mode with a

sampling rate of 1 Gs/s

W f A i dD i ti



Waveform Averaging and Decimation


High Res Average

H d L b



Hands on Lab

ı Hands On Example: Measure SMPS Output

and Input voltage and Current Ripple

Wh t i t l k t





Vout IoutGND

M t t lt ( ll l d)



Measure output voltage (small load)


ı Preset the oscilloscope

ı Connect passive probe to channel 1 of the scope

ı Probe the signal at the V_OUT pin on the demo boardı Set demo board to mode "3." note the period (20 Ω load)

ı Autoset scope

ı Measure p-p and mean

voltage

Ratio of P-P to mean = 2%

Mean voltage = 1.79 V

M t t lt (l l d)



Measure output voltage (large load)


ı Set demo board to "4." (5 Ω load)

ı Measure the p-p and mean voltage

Ratio of P-P to mean = 30%

Mean voltage drops to 1.77 V

M t t lt t



Measure output voltage spectrum

ı Touch FFT tool on tool bar and then

the output voltage waveform (CH1)

ı Set FFT start-stop frequency to 0 and

500 MHz, Res BW to 120 KHz


20 Ω load

5 Ω load

Note the increase in the spectrumin the region above 100 MHz withthe higher load

H d L b



Hands on Lab

ı Hands On Example: Measure Switch Node

Voltage and Inductor Voltage and Current

InductorCurrentWaveform



SMPS | 4

Inductor Current Waveform




SMPS | 4





SMPS | 4





SMPS | 4





SMPS | 4







VswIL GND

MeasureSwitchNodeVoltage



Measure Switch Node Voltage


ı Set the demo board to mode "4." (5 Ω load)

ı Move probe (still on channel 1) to switch voltage node on demo board

ı Adjust vertical scale to 1 V/div and horizontal to 100 ns/divı Set the decimation to "High Res" and Wfm Arithmetic to "Average" and the

average count to 50

ı Note large overshoot ( 4 V) and 500 mV pre-shoot

ı Rise time is 1.3 ns






ı Reduce the output load by switching to mode "3." (20 Ω load)

ı Note slower rise time and smaller overshoot ( 15.1 ns, 1 V)

MeasureSwitchNodeVoltageandCurrent



Measure Switch Node Voltage and Current


ı Set demo board to mode "4." ( 5 Ω load)

ı Connect current probe to channel 2

ı Set the decimation to "high res" and thewfm arithmetic to " average" on channel 2

with and averaging count of 50

ı Select pre-defined probe = "RT-ZC20" for

channel 2

ı Lock the jaw on the current probe andpress the "DEMAG" button on the probe

near the scope connection

ı Use the "ZERO ADJ" control to center the

trace at 0 A

ı

Adjust vertical scale of channel 2 to 100mA/div

ı Use smart grid to create a second grid for

the current waveform

Settingup theCurrentProbe



Setting up the Current Probe







ı Set demo board to mode "3." (20 Ω load)

Measure InductorVoltageandCurrent



Measure Inductor Voltage and Current


ı Connect passive probe to channel 3

ı Set scale to 1V/div

ı Set acquisition to "High Res" with Wfm Arithmetic = "Average" and an averagecount of 50 for Ch3

ı Define math waveform M1 = Ch1 – Ch3

Ts = 950 nsD = 0.35L = 2.2 µH

2*∆I = 3.2*950e-9*0.35(2.2e-6)

= 484 mA

Predicted current ripple:






ı Set demo board mode to "4." (5 Ω load)

ı Set the vertical scale for channels 1 and 3 to 2 V/div

ı Measure p-p current ( ~ 700 mA)ı Use inductor current equation to compute L based on p-p current

ı Note the onset of saturation in the inductor

Measured current ripple:

2*∆I = 720 mA Equivalent Inductance:

L = 950e-9*.35*3/0.721

= 1.39 µH

"Extra Credit"



Extra Credit


ı Create math waveform M2 = (integral(M1)/2.2e-6)[A]

ı Set the scale of M2 to 100 mA/div

ı Adjust the horizontal reference point so that the minimum of M2 and Ch2 areequal in the center of the screen

Measurementson M1

P-P current of Ch2

Note that ideal currentwaveform is muchsmaller than the actual

current (ch2). Thisindicates that theinductor is partiallysaturated

"Extra Credit 2"



Extra Credit 2


ı Set the board to mode "3."

ı Adjust the horizontal reference point so that the minimum of M2 and Ch2 are

equal in the center of the screen

Measurementson M1

P-P current of Ch2

The ideal currentwaveform amplitudematches the measured

current. Note that theactual current is notlinear indicating someresistance in the inductor

Conclusion



Conclusion

ı Increasing the load to 5 Ω results in reduced voltage (by approximately 200 mV)

and increased voltage ripple

Increased spectral power at 200 MHz

3% ripple voltage

ı Examining the switching node revealed that the inductor appears to be the root

cause

Non-linear IL with 5 Ω load Higher IL over predicted value for rated inductor value

Decreased rise time of Vsw with increased load

Very high inductor current ripple with increased load

ı Comparing measured to ideal waveforms using math

Measured values do not agree with expected values Results indicate that inductor is undersized ( core saturation, copper loss)

Documents

Serial Bus Debug