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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
DesignCon 2011, 13-TH2, February 3, 2011 3
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
DesignCon 2011, 13-TH2, February 3, 2011 4
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
DesignCon 2011, 13-TH2, February 3, 2011 5
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
DesignCon 2011, 13-TH2, February 3, 2011 6
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
4
DesignCon 2011, 13-TH2, February 3, 2011 7
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
DesignCon 2011, 13-TH2, February 3, 2011 8
X5R vs X7R
Vendor allocation for measuring X5R and X7R 1uF 0603 16V MLCC parts
5
DesignCon 2011, 13-TH2, February 3, 2011 9
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
DesignCon 2011, 13-TH2, February 3, 2011 10
X5R vs X7R at 10mV AC
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
Percentage capacitance, X7R parts [%]
-100
-80
-60
-40
-20
0
-20 -10 0 10 20DC bias [V]
D7
F(1)7
B7
1uF 0603 16V X7R parts only
6
DesignCon 2011, 13-TH2, February 3, 2011 11
X5R vs X7R at 10mV AC
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)
DesignCon 2011, 13-TH2, February 3, 2011 12
X5R vs X7R at 500mV AC
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
7
DesignCon 2011, 13-TH2, February 3, 2011 13
X5R vs X7R at 500mV AC
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
Percentage capacitance, X7R parts [%]
-100
-80
-60
-40
-20
0
-20 -10 0 10 20DC bias [V]
D7
F(1)7
B7
1uF 0603 16V X5R parts only
1uF 0603 16V X7R parts only
DesignCon 2011, 13-TH2, February 3, 2011 14
X5R vs X7R at 500mV AC
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
8
DesignCon 2011, 13-TH2, February 3, 2011 15
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
DesignCon 2011, 13-TH2, February 3, 2011 16
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
9
DesignCon 2011, 13-TH2, February 3, 2011 17
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
DesignCon 2011, 13-TH2, February 3, 2011 18
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
10
DesignCon 2011, 13-TH2, February 3, 2011 19
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.
DesignCon 2011, 13-TH2, February 3, 2011 20
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.
11
DesignCon 2011, 13-TH2, February 3, 2011 21
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.
DesignCon 2011, 13-TH2, February 3, 2011 22
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
Readings 100 sec after changing bias
12
DesignCon 2011, 13-TH2, February 3, 2011 23
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
Readings 10 sec after changing bias
Readings 100 sec after changing bias
DesignCon 2011, 13-TH2, February 3, 2011 24
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.
13
DesignCon 2011, 13-TH2, February 3, 2011 25
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
DesignCon 2011, 13-TH2, February 3, 2011 26
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
14
DesignCon 2011, 13-TH2, February 3, 2011 27
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
Voltage transfer function at 2V 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
Voltage transfer function at 4V 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
Voltage transfer function at 8V 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
DesignCon 2011, 13-TH2, February 3, 2011 28
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
15
DesignCon 2011, 13-TH2, February 3, 2011 29
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
DesignCon 2011, 13-TH2, February 3, 2011 30
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