Killing the Bode Plot Final - Picotest · Title: Microsoft PowerPoint - Killing the Bode Plot...

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Killing the Bode Plot

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Killing the Bode PlotSteve Sandler (Picotest) ImageSteve Sandler, (Picotest)

Copyright © 2016 Picotest.com, All Rights Reserved

Killing the Bode PlotKilling the Bode PlotSteve Sandler, PicotestSteve Sandler, Picotest


SPEAKERSSteve Sandler

Managing Director, Picotest@ i | i | @ [email protected] | picotest.com | @stevenmsandler

Steve Sandler has more than 35 years’ experience in the design, analysis, and troubleshooting of power conversion and system level equipment for commercial military and space applicationsequipment for commercial, military, and space applications.

Steve is the CEO of Picotest.com, a company that designs and distributes test equipment accessories designed for test and troubleshooting power systemstroubleshooting power systems.

Steve is also the founder and chief engineer of AEi Systems, a leading analysis and modeling company specializing in worst case analysis of high reliability systemshigh reliability systems.

Steve has authored several books related to power electronics including most recently the book entitled "Power Integrity - Measuring, Optimizing and Troubleshooting Power Systems "and Troubleshooting Power Systems.


The Bode Plot’s Days are Numbered …..

R.I.P.phase

gainThe worlds most popular stability assessment test

Rest in Peace

1947‐201?


So What’s Wrong with Bode Plots

Wh ’ The first problem is that the Bode plot is a POORindicator of RELATIVE stability – this is well known and well published It means that Bode

What’s RIGHT known and well published. It means that Bode

Plot results can be inaccurate and misleading.

A th bl i th t th t

RIGHTmight be Another problem is that the measurement

requires us to inject a signal into the loop. Many devices simply don’t provide access to the loop.

might be a better

We’ll show some examples…questionCopyright © 2016 Picotest.com, All Rights Reserved

Stability STILL Matters – A LOT

Closed Loop Performance is directly related to the Stability Performance.

That is why you DO need to assess Stability, maybe even more than ever – it’s essential to optimum, low noise, performance.

bili i ’ i l hStability assessment isn’t going away, only the Bode plot will, fortunately, better methods are currently availablecurrently available……


Why are Bode Plots Becoming IrrelevantIn other cases we might not have physical1. Poor or No Control Loop Access

3.3V/1.5A LDOIn other cases, we might not have physical space to inject into the loop or their might be a large number of power supplies

p

Fixed regulators/references don’t

6 output power supply

Fixed regulators/references don t allow access to the control loop 8.5mm

Or we might not want to gain access because it i i i

8.5mm More power supplies

requires cutting a trace or a wire


High Frequency Injection is Impractical

500MHz Bandwidth eGaNLinear Regulator


Why are Bode Plots Becoming Irrelevant

2. Many new devices have multiple internal loops

All need to be accounted forAll need to be accounted for–Digital Control Loops


Why are Bode Plots Becoming Irrelevant

3. You can have both good phase margin and good gain margin and yet have poor stabilitypoor stability– There may be multiple crossovers– Control loops can be high order

SM = Stability MarginPM = Phase MarginGM = Gain Margin

p g– Multiple or non‐linear loops

SM provides the most accurate measure of stability

ONLY this one point is UNSTABLEONLY this one point is UNSTABLE


What is RELATIVE Stability & Why Bode Plots Can Mislead

A STABLEt lcontrol

loop is This is the gain/phase vector

Unity gain negative feedback results in a denominator ofpone that doesn’t

Unity gain negative feedback results in a denominator of zero = unstable

The minimum magnitude of the denominator provides the doesn t oscillate

g pRELATIVEmeasure – also known as the ‘Stability Margin’

Nyquist is a better assessment tool for Stability than Bode


A DC-DC Converter Example What’s Interesting

• Bode & Nyquist ‐ same data

1 210 100

Bode & Nyquist same data

• Bode didn’t indicate stability

• Scope Owners: load step Bode

Nyquist

0

5

R1/

dB

0

50

TR2

ringing may or may not indicate a stability issue

• Must use a Closed Loop Test Nyquist Impedance etc

Bode

-5

TR

-50

2/° Nyquist, Impedance, etc.

Min. Distance from (1,0) Load Step

103 104-10 -100

f/HzTR1: Mag(Gain) TR2: Phase(Gain)

78 degrees phase margin6.7dB gain margin

0.38

But it Rings Severely


Output Impedance is as Good as Nyquist12 gain 15 phase PM = 50 Deg, 658 Hz

I d

20.0

40.0

volts

)

90.0

180

gree

s

frequency = 851 hertzphase = 21.3 degrees

BW = 658 hertz, PM = 50.2 degrees Impedance Peak, 860Hz

-20.0

0

gain

in d

B(v

-90.0

0

phas

e in

deg

Plot

2

15frequency = 851 hertzgain = -3.85 dB(volts)

x = 1.09k hertz, GM = 8.70 dB(volts) 500m

1.00

st

10 20 50 100 200 500 1k 2k 5k 10kfrequency in hertz

-40.0-180

12

, ( )

-2.00 -1.00 0 1.00 2.00real

-1.00

-500m

0

imag

Nyq

uis

16 frequency = 851 hertzreal = 598mimag = 232m

Ringing at 860Hz

Distance from (1,0)

200m

600m

1.00

1.40

1.80

dist

ance

Dis

t117

y (min) = 464.318m x = 851.138 hertzFlyback

Converter Flyback Converter Nyquist

851Hz100 200 500 1k 2k 5k 10k

frequency in hertz


So How Can We Easily and Accurately Assess Stability?

Non‐Invasive Stability Measurement – ‘NISM’

No Control Loop – No ProblempStability Margin can be determined from a single output impedance measurement!


The NISM Concept is Simple

Yes. You can measure phase

From 1 Impedance Measurement ‐ 3 data points– Impedance Magnitude, Phase, and Group Delay

margin from a single measurement

The NISM software extracts data from output impedance and the ‘Q’ via group delay and allows the ZS and ZL to be mathematically determined so they can be converted tomathematically determined so they can be converted to Stability Margin

Phase margin is determined by setting |Tm| = 1 and solving for phase


What is NISM and Why It’s a Critical TechnologyNyquist solves our problem but is not always simple to computeNyquist solves our problem, but is not always simple to compute

NISM accomplishes the same thing more simply thru closed loop output impedance

NISM determines control loop margins without needing access to the feedback loop

NISM is based on proven Minor Loop Gain Theory

– Similar to assessing Load Step ‘Q’

The NISM technology is licensed and promoted by Picotest.com

The measurement capability can be found on various VNAs

– OMICRON Lab, Keysight, Copper Mountain


NISM is Based on Fundamentals

D R D Middl b k l i d th t i fMi L G i T ithDr. R.D. Middlebrook popularized the topic of Minor Loop Gain, Tm with his introduction of the extra element theorem which allowed us to assess the stability of power supplies and input filters Minor loop gain, based on Nyquist criteria, is now one of the most researched electronics topicsMany articles can be found with an internet search of “forbidden regionMany articles can be found with an internet search of forbidden region stability criteria”


How Do I Make a NISM Measurement

Step 1: Measure ImpedanceStep 2: Set Cursors

Practically speaking, output impedance is measured with a suitable probe in a 1 or 2 port configurationprobe in a 1 or 2 port configuration

The software converts the impedance to group delay and Q

The user positions waveform cursors on the impedance and Q waveforms and the conversion to phase margin is

d h ’read out on the instrument’s screen


NISM on the OMICRON Lab Bode 100


NISM on the Keysight E5061B


NISM – Just as Accurate as a Bode Plot

Accurate to within 1 degreeAccurate even when Bode Plots are notAccurate up to 65 degrees

Lab comparisons of 7 regulators/capacitorsBode plot and the NISM methods were within +0.9 degrees

Noise free simulation results were within +0.3 degrees, attributable to cursor resolution


LT1086 Linear RegulatorVout

Min = 25mANom = 500mA

X2RH1086

1 80V

R1249

ADJ

C10.1uF

C20.1uF

Vin5

Vout

CoutCinT409G226M015

+/-0.5V

I125m

Vin

3.28%45.24% 45.24%

Max = 1AIN OUT

ADJUST I2AC = 1

f 1 Vf req1

1.80V

557mV

5.00V

NISM Bode Plot PM

Measured 59 deg 56 deg

R2110

Cadj68n

3.28% 5.87%B1Currentmag(f req)/(2*3.14159265)

R31

f req1 Vf req10V

12010

10-1

100

TR1 TR

2

1

0

50

100

0

100

200

R1/

dB

TR2/

f/Hz TR1 TR2 PM: Cursor 1 52.580k 325.128m 504.129m 58.904 °Cursor 2 40.777k 302.517m 597.409m

C2-C1 -11.803k -22.611m 93.280m

-110

10-3

10-2

102 103 104 105 106 107

T 2

f/H

f/Hz TR1/dB TR2/°Cursor 1 52.743k 0.000 55.467

-100

-50

-200

-100

102 103 104 105 106 107TR

/°

f/Hzf/HzTR1: |Mag(Gain)| TR2: |QTg(Gain)|

f/HzTR1: Mag(Gain) TR2: Phase(Gain)


LP2998 DDR Termination Regulator

29.43 deg

28.9 deg


TPS7A4501 Linear Regulator 5mA

NISM Bode Plot

Measured 51 deg 53 deg

A low‐noise, fast‐transient‐response 1.5‐A low‐dropout (LDO) voltage regulator

htt // ti /lit/ / l 259 / l 259 df

100 40

60

150

http://www.ti.com/lit/ug/slvu259a/slvu259a.pdf

010

10-2

10-1

100

TR1 TR

2

40

-20

0

20

50

100

TR1/

dB

TR2/°

f/Hz TR1 TR2 PM: Cursor 1 6.165k 1.264 811.220m 51.214 °Cursor 2 5.782k 1.259 835.934m

C2-C1 -382.160 -5.214m 24.714m-110

10-3

10

102 103 104 105 106

f/Hz5mA : |Mag(Gain)| 5mA : |QTg(Gain)|

f/Hz TR1/dB TR2/°Cursor 1 7.153k 45.204m 53.384-60

-40

0102 103 104 105 106

f/Hz5mA (2.37k) : Mag(Gain) 5mA (2.37k) : Phase(Gain)


VRG8666 (RH3080) Linear RegulatorX1VRG8666

NISM Bode Plot Vin

Vout

SET

VcBodeInj

2

43

1

VRG8666

V15

C2330u C3

330uI125m

R219

1 2001210

Measured 20 deg 21 deg R1250k

C11u

1

20

40

60

80

0

50

100

150

R1/

dB

TR2/°-30

-20

-10

0

10-1

100

TR1/

dB TR2

f/Hz TR1/dB TR2/°Cursor 1 14.330k 0.000 21.222-40

-20

0

-200

-150

-100

-50

102 103 104 105 106 107

T

°

f/Hz

f/Hz TR1/dB TR2 PM: Cursor 1 14.818k -21.472 2.546 20.497 °Cursor 2 15.304k -21.510 2.696

C2-C1 485.650 -38.053m 150.007m-60

-50

-40

10-3

10-2

102 103 104 105 106 107

T

f/Hz f/HzTR1: Mag(Gain) TR2: Phase(Gain)

f/HzTR1: Mag(Gain) TR2: |QTg(Gain)|


High Speed OpampUsing the Keysight E5061B VNA with the NISM softwareUsing the Keysight E5061B VNA, with the NISM software, we were able to test the stability of this 245 MHz op-ampThe big WOW is that we obtained the (very poor) phase margin from the impedance measurement using NISM (just about 2 degrees)g )This is a great capability; to be able to accurately assess stability at 100's of MHz or higher without lifting any wires (which would interfere with the measurement)

T tTest Point

NISM Bode Plot

Measured 2 deg N/A


Applications for NISM

• Op-amps• Voltage Regulators – including LDOs, POLs, VRMs…. g g g , ,• Voltage References• Audio Amplifiers – including switching typesp g g yp• Input Filter Stability• System Level Box-to-Box Stability• Current Regulator Stability (electronic load, LED, etc.)• Almost any control loop


A New Path To Stability Testing is Here

Technology, Integration, Shrinking Form Factors Have Forced the IssueReduced the practical application of Bode Plot measurement

The load (impedance and current) are critical to an accurate stability

assessment

What’s needed is an in-circuit measurement

NISM – Non-Invasive Stability Measurement– The final nail in the Bode Plot coffin is the availability of an alternative assessment

– Available on popular VNAs

I l d d ith th I t t il bl ft dd– Included with the Instrument or available as a software add-on


Thank you!---

QUESTIONS?


Killing the Bode Plot Final - Picotest · Title: Microsoft PowerPoint - Killing the Bode Plot...

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