dc and ac bias dependence of...

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DC and AC Bias Dependence of CapacitorsIstvan Novak, Kendrick Barry Williams, Jason R. Miller, Gustavo Blando,

Nathaniel Shannon

DesignCon 2011 13-TH2, February 3, 2011

DesignCon 2011, 13-TH2, February 3, 2011 2

Outline

Introduction and backgroundScope of workInstrumentation setupMeasurement results

Unit-to-unit variationsComparing X5R and X7R partsESR and ESL variationsBeware of detailsAC bias dependenceDependence of timing and sweep type

How all this may impact our designParalleled capacitorsLC filters

Conclusions

2


Introduction and Background

Layer count

N = H/thth

VE

th

GLWNC r

)2(0

Class II and higher ceramic materials are ferroelectric

Ferroelectric materials have saturated hysteretic D-E curves

D

E


Introduction and Background Percentage loss of capacitance [%]

-80

-60

-40

-20

0

20

1.E-01 1.E+00 1.E+01 1.E+02DC bias percentage of rated voltage [%]

Y5V

X5R

X7R

For some time, it was a common assumption that X7R MLCCs had less DC bias sensitivity than X5R parts.

But lately…

3


Scope of Work

Class II X5R and X7R parts

0402, 0603, 0805, 1206 and 1210 body sizes

4-16VDC nominal voltage rating

Six different MLCC vendors

25 different part numbers

Multiple pieces of each part number

Room temperature testing only


Instrumentation Setup

Constant AC across DUT

Calibration before each measurement

Impedance sweep 100Hz-10MHz range

Script steps DC bias in 1% increments and repeats sweep

User-defined dwell time

One sweep takes 100 seconds

Full sweep of DC bias takes 6-10 hours

RT LF OUTPort-R & T:Hi-Z inputs

Rc = 50 ohm

Open Short Load

50 ohm resistor

DUT (MLCC)Power splitter

Test board

Calibration standards:

E5061B-3L5 network analyzer

AC +DC

Rc = 50 ohmT-conn.

Alternative solution

For details, see: Novak-Mori-Resso, “Accuracy Improvements of PDN Impedance Measurements in the Low to Middle Frequency Range,”Proceedings of DesignCon 2010, Santa Clara, CA, February 1-4, 2010

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Unit-to-Unit Variations

1uF 0603 16V X5R

Capacitance [F]

0.E+0

2.E-7

4.E-7

6.E-7

8.E-7

1.E-6

-20 -15 -10 -5 0 5 10 15 20DC bias [V]

Percentage capacitance loss [%]

-100

-80

-60

-40

-20

0

-20 -15 -10 -5 0 5 10 15 20DC bias [V]

Ten samples from same vendor

Capacitance measured at 100 Hz


X5R vs X7R

Vendor allocation for measuring X5R and X7R 1uF 0603 16V MLCC parts

5


X5R vs X7R at 10mV AC

Capacitance [F]

0.E+0

2.E-7

4.E-7

6.E-7

8.E-7

1.E-6

-20 -10 0 10 20DC bias [V]

A5

B5B7

C5

D7

F(1)5

F(1)7

F(2)5

Percentage capacitance [%]

-100

-80

-60

-40

-20

0

-20 -10 0 10 20DC bias [V]

F(2)5 D7

C5

A5F(1)7

F(1)5B5

B7

1uF 0603 16V X5R and X7R

Samples from Vendors A,B,C,D,F



1uF 0603 16V X5R parts only

Percentage capacitance, X5R parts [%]

-100

-80

-60

-40

-20

0

-20 -10 0 10 20DC bias [V]

F(2)5

C5

A5

F(1)5B5


-100

-80

-60

-40

-20

0

-20 -10 0 10 20DC bias [V]

D7

F(1)7

B7


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X5R vs X7R from the same vendor

Percentage capacitance,Vendor-B [%]

-100

-80

-60

-40

-20

0

-20 -10 0 10 20DC bias [V]

X7R X5R

Percentage capacitance, Vendor-F [%]

-100

-80

-60

-40

-20

0

-20 -10 0 10 20DC bias [V]

X7R X5R(1)

X5R(2)



Capacitance [F]

0.0E+0

2.0E-7

4.0E-7

6.0E-7

8.0E-7

1.0E-6

1.2E-6

-20 -10 0 10 20DC bias [V]

A5

B5B7

C5D7

F(1)5

F(1)7

F(2)5

Percentage capacitance [%]

-100

-80

-60

-40

-20

0

-20 -10 0 10 20DC bias [V]

F(2)5 D7

C5

A5F(1)7

F(1)5B5

B7

1uF 0603 16V X5R and X7R

Samples from Vendors A,B,C,D,F

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-100

-80

-60

-40

-20

0

-20 -10 0 10 20DC bias [V]

F(2)5

C5

A5

F(1)5B5


-100

-80

-60

-40

-20

0

-20 -10 0 10 20DC bias [V]

D7

F(1)7

B7





Percentage capacitance,Vendor-B [%]

-100

-80

-60

-40

-20

0

-20 -10 0 10 20DC bias [V]

X7R X5R

Percentage capacitance, Vendor-F [%]

-100

-80

-60

-40

-20

0

-20 -10 0 10 20DC bias [V]

X7R X5R(1)

X5R(2)

X5R vs X7R from the same vendor

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X5R Correlation at 500mV AC Percentage capacitance,Vendor-A, X5R [%]

-100

-80

-60

-40

-20

0

-20 -10 0 10 20DC bias [V]

Vendor data

Measured

Percentage capacitance,Vendor-B, X5R [%]

-100

-80

-60

-40

-20

0

-20 -10 0 10 20DC bias [V]

Vendor data

Measured

Percentage capacitance,Vendor-C, X5R [%]

-100

-80

-60

-40

-20

0

-20 -10 0 10 20DC bias [V]

Vendor data

Measured

Percentage capacitance,Vendor-F, X5R(1) [%]

-100

-80

-60

-40

-20

0

-20 -10 0 10 20DC bias [V]

Vendor data

Measured


X7R Correlation at 500mV AC Percentage capacitance,Vendor-B, X7R [%]

-100

-80

-60

-40

-20

0

-20 -10 0 10 20DC bias [V]

Vendor data

Measured

Percentage capacitance,Vendor-D, X7R [%]

-100

-80

-60

-40

-20

0

-20 -10 0 10 20DC bias [V]

Vendor data

Measured

Percentage capacitance,Vendor-F, X7R [%]

-100

-80

-60

-40

-20

0

-20 -10 0 10 20DC bias [V]

Vendor data

Measured

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ESR and ESL vs. Bias

• ESR does not change above SRF• ESR increases below SRF as C drops• Piezo effect shows up with increasing bias• ESL shows no measurable difference

ESR [Ohm]

0.01

0.1

1

1.E+5 1.E+6 1.E+7

Frequency [Hz]

0V

20V


Beware of Details

Sensitivity vs. body height

Data from vendor

Lower body height comes with higher

sensitivity

Percentage capacitance, Vendor-C [%]

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

10

0 2 4 6 8 10 12 14 16DC bias [V]

X5R 0.5mmX5R 0.8mm

X7R 0.8mm

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AC bias dependence

4.7uF 0805-size 16V X5R parts from Vendor F

Capacitance [F]

0.E+0

1.E-6

2.E-6

3.E-6

4.E-6

5.E-6

-20 -15 -10 -5 0 5 10 15 20DC bias [V]

1V0.5V0.2V0.1V

50mV20mV10mV5mV2mV1mV

AC bias [Vrms] Percentage capacitance loss [%]

-100

-80

-60

-40

-20

0

-20 -15 -10 -5 0 5 10 15 20DC bias [V]

1m2m5m

10m20m30m0.10.20.51

AC bias [Vrms]

Capacitance [F]

0.E+0

1.E-6

2.E-6

3.E-6

4.E-6

5.E-6

1 10 100 1000

AC bias voltage [mVrms]

02468

101214161820

DC bias [V]

High AC bias increases capacitance at low DC bias.

High AC bias lowers capacitance at high DC bias.


Relaxation of Part 1

Part 1: Vendor-A, 47uF 1206-size 6.3V X5R part.

100Hz, 10mVrms AC bias.

Capacitance, Part 1 [uF]

0

5

10

15

20

25

30

0 20 40 60 80 100Time [s]

2V

4V

6V

8V10V

Capacitance change, Part 1 [%]

y = -2.7793x - 1.5553

-8

-6

-4

-2

0

2

1.2 1.4 1.6 1.8 2Log time [-]

2V

4V

6V

8V

10V

Absolute capacitance change. Relative capacitance change.

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Relaxation of Part 2

Capacitance, Part 2 [uF]

0

0.5

1

1.5

2

2.5

3

3.5

0 50 100 150 200Time [s]

4V

8V

12V

16V20V

Capacitance change, Part 2 [%]

y = -13.786x + 3.9297

-30

-25

-20

-15

-10

-5

0

1 1.5 2 2.5Log time [s]

4V

8V

12V

16V

20V

Part 2: Vendor-D, 4.7uF 0805-size 16V X7R part.

100Hz, 10mVrms AC bias.

Absolute capacitance change. Relative capacitance change.


Quick Relaxation Part

Readings 10 sec after changing bias

‐20‐16

‐12‐8

‐40

48 12

1620

0.00E+00

2.00E‐07

4.00E‐07

6.00E‐07

8.00E‐07

1.00E‐06

1.E+2

1.E+3

1.E+4

1.E+5

1.E+6

DC bias [V]

Capacitance [F]

Frequency [Hz]

Vendor‐A 1uF 0603 16V X5R at 10mVrms AC

‐20‐16

‐12‐8

‐40

48 12

1620

0.00E+00

2.00E‐07

4.00E‐07

6.00E‐07

8.00E‐07

1.00E‐06

1.E+2

1.E+3

1.E+4

1.E+5

1.E+6

DC bias [V]

Capacitance [F]

Frequency [Hz]

Vendor‐A 1uF 0603 16V X5R at 10mVrms AC


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Slow Relaxation Part

‐20‐16

‐12‐8

‐40

48 12

1620

0.00E+00

2.00E‐07

4.00E‐07

6.00E‐07

8.00E‐07

1.00E‐06

1.E+2

1.E+3

1.E+4

1.E+51.E+6

DC bias [V]

Capacitance [F]

Frequency [Hz]

Vendor‐D 1uF 0603 16V X7R at 10mVrms AC bias

‐20‐16

‐12‐8

‐40

48 12

1620

0.00E+00

2.00E‐07

4.00E‐07

6.00E‐07

8.00E‐07

1.00E‐06

1.E+2

1.E+3

1.E+4

1.E+5

1.E+6

DC bias [V]

Capacitance [F]

Frequency [Hz]

Vendor‐D 1uF 0603 16V X7R at 10mVrms AC bias




Paralleled Capacitors

1uF 0603-size 16V X7R part from Vendor-D and 47uF 1206-size 6.3V X5R part from Vendor-E

Impedance magnitude [dBOhm]

-55

-50

-45

-40

-35

-30

-25

1.E+5 1.E+6 1.E+7

Frequency [Hz]

10V8V6V

4V2V

0V

All three resonances shift.

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Capacitors in Filters, Test Setup

C1: 390uF 16V OSCON, DUT: 47uF 6.3V X5R 1206-size MLCC from Vendor-E, L1: 2A ferrite bead

No DC current through L1 With DC current through L1


Filter Response vs. DC Bias VoltageNo DC current bias through L1

No change below 10 kHz and above 1 MHz

No change in peaking

Peak frequency and cut-off frequency increases with increasing bias

Voltage transfer function [dB]

‐100

‐80

‐60

‐40

‐20

0

20

1.E+2 1.E+3 1.E+4 1.E+5 1.E+6 1.E+7

Frequency [Hz]

Voltage transfer function [dB]

‐40

‐30

‐20

‐10

0

10

1.E+4 1.E+5Frequency [Hz]

0 10VDC bias

Zoom

24 kHz 55 kHz

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Filter Response vs. DC Bias Voltage and Current

Parameter: DC current through

L1

Voltage transfer function at 0V bias [dB]

‐90

‐80

‐70

‐60

‐50

‐40

‐30

‐20

‐10

0

10

1.E+3 1.E+4 1.E+5 1.E+6 1.E+7

Frequency [Hz]

1.5A

1.0A

0.5A

0.0A


‐90

‐80

‐70

‐60

‐50

‐40

‐30

‐20

‐10

0

10

1.E+3 1.E+4 1.E+5 1.E+6 1.E+7

Frequency [Hz]

1.5A

1.0A

0.5A

0.0A


‐90

‐80

‐70

‐60

‐50

‐40

‐30

‐20

‐10

0

10

1.E+3 1.E+4 1.E+5 1.E+6 1.E+7

Frequency [Hz]

1.5A

1.0A

0.5A

0.0A


‐90

‐80

‐70

‐60

‐50

‐40

‐30

‐20

‐10

0

10

1.E+3 1.E+4 1.E+5 1.E+6 1.E+7

Frequency [Hz]

1.5A

1.0A

0.5A

0.0A


Filter Response vs. DC Bias Voltage and Current

Voltage transfer function at 0A bias [dB]

‐90

‐80

‐70

‐60

‐50

‐40

‐30

‐20

‐10

0

10

1.E+3 1.E+4 1.E+5 1.E+6 1.E+7

Frequency [Hz]

16V

8V

4V

2V

0V

Voltage transfer function at 0.5A bias [dB]

‐90

‐80

‐70

‐60

‐50

‐40

‐30

‐20

‐10

0

10

1.E+3 1.E+4 1.E+5 1.E+6 1.E+7

Frequency [Hz]

16V

8V

4V

2V

0V

Voltage transfer function at 1A bias [dB]

‐90

‐80

‐70

‐60

‐50

‐40

‐30

‐20

‐10

0

10

1.E+3 1.E+4 1.E+5 1.E+6 1.E+7

Frequency [Hz]

16V

8V

4V

2V

0V

Voltage transfer function at 1.5A bias [dB]

‐90

‐80

‐70

‐60

‐50

‐40

‐30

‐20

‐10

0

10

1.E+3 1.E+4 1.E+5 1.E+6 1.E+7

Frequency [Hz]

16V

8V

4V

2V

0V

Parameter: DC voltage across

C2

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Overall Change in Filter Response

Current bias through L1 eliminates peaking

Voltage transfer function, 0‐4V, 0‐1A [dB]

‐90

‐80

‐70

‐60

‐50

‐40

‐30

‐20

‐10

0

10

1.E+3 1.E+4 1.E+5 1.E+6 1.E+7

Frequency [Hz]

50 dB

3.2 : 1

Voltage transfer function, 0‐16V, 0‐1.5A [dB]

‐90

‐80

‐70

‐60

‐50

‐40

‐30

‐20

‐10

0

10

1.E+3 1.E+4 1.E+5 1.E+6 1.E+7

Frequency [Hz]

68 dB

7.2 : 1


Conclusions

High volumetric density creates big capacitance drop in Class II MLCCs with DC bias

X7R parts are not necessarily less sensitive to DC bias than X5R parts

Slow relaxation may result in an additional 20-30% capacitance drop over time (not on the spec sheet!)

Major vendors provide models for DC and AC bias sensitivity DC and AC bias sensitivity is very different across vendors>> DC and AC bias sensitivity must be taken into account in

alternate source selection

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THANK YOUIstvan Novak,

[email protected]

dc and ac bias dependence of...

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