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Do You Know How Much Capacitance Do You Really Get? I Presenter: Istvan Novak
• A Real-life Scenario
• The Problem
• The Reasons
• Does It Matter?
• Possible remedies
• Conclusions
INTRODUCTION
A Real-Life Scenario
• You purchase a 1uF +-20% ceramic capacitor.
• When it comes in, you measure it: it reads 0.6 uF.
• You send it back to the vendor for retesting.
• It comes back from the vendor: they say the part is in spec.
• You measure it again, you still get 0.6 uF.
• You check the measurement setups at both ends with precise reference pieces and conclude that measurements at both ends were correct.
How is this possible?
So What Is Going On?
To get a clue, we need to look at the way how the measurements are done.
• Vendors use standardized measuring conditions: impedance bridge, source voltage of 1Vrms or 0.5Vrms with 100 or 120Hz frequency. First hint: a typical capacitor at the measurement frequency is high impedance: most of the source voltage appears across the capacitor.
• You more likely use a small pcb as test fixture and a VNA with 0dBm source power at frequencies above 1 kHz. Second hint: in VNA measurements and in typical bypass applications the voltage across the capacitor is single or at most double digit mV.
MLCC Material and Construction
Layer count
N = H/th
th
VE
th
GLWNC r
=
−=
)2(0
Class II and higher ceramic materials are ferroelectric
Ferroelectric materials have saturating hysteretic D-E curves
and C are voltage dependent!
D
E
Effect of Temperature on MLCC Capacitance
EIA Class II and Class III ceramics
First character:Z: + 10CY: - 30CX: - 55C
Second character:2: + 45C4: + 65C5: + 85C6: + 105C7: + 125C8: + 150C9: + 200C
Third character:F: +- 7.5%P: +- 10%R: +- 15%S: +- 22%T: + 22 / - 33%U: + 22 / - 56%V: + 22 / - 82%
DC Bias Dependence (Old School Info)
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…
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 20
DC bias [V]
A5
B5
B7
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
A5
F(1)7
F(1)5B5
B7
1uF 0603 16V X5R and
X7R
Samples from Vendors A,B,C,D,F
Last digit refers to X5R or X7R
For more info see http://www.electrical-integrity.com/Paper_download_files/DCE11_200.pdf
AC Bias Dependence: The Problem
Testing and typical
usage apply very
different AC levels.
User application
Vendor testing
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]
1V
0.5V
0.2V
0.1V
50mV
20mV
10mV
5mV
2mV
1mV
AC bias [Vrms]
Percentage capacitance loss [%]
-100
-80
-60
-40
-20
0
-20 -15 -10 -5 0 5 10 15 20DC bias [V]
1m
2m
5m
10m
20m
30m
0.1
0.2
0.5
1
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]
0
2
4
6
8
10
12
14
16
18
20
DC bias [V]
High AC bias increases
capacitance at low DC bias.
At high DC bias the AC bias
sensitivity is lower.
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
OK, Less Capacitance, So What?
In SI (less of an issue)
If OK for the design, just oversize the capacitance (worst case: by 6x)
Design could become space limited
In differential DC block the two legs may become unbalanced at low frequencies
In PI (much more serious)
Results in more droop (in case of single capacitor bank)
Results in peaking (more noise)
May result in current amplification, increased dissipation
OK, Less Capacitance, So What?
1uF 0603-size 16V X7R part from Vendor-D and 47uF 1206-size 6.3V X5R part from Vendor-E
All three resonances
shift.
Impedance magnitude [dBOhm]
-55
-50
-45
-40
-35
-30
-25
1.E+5 1.E+6 1.E+7
Frequency [Hz]
10V8V
6V4V
2V
0V
For more info: http://www.electrical-integrity.com/Paper_download_files/DC16_Paper_ElectricalAndThermalConsequencesOf_Choi.pdf
Filter Response vs. DC Bias Voltage
No 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
Change Due to DC Bias Voltage and Current
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
For more info see http://www.electrical-integrity.com/Paper_download_files/DC19_Tutorial_SLIDES_HowToDesignGoodPDNFilter.pdf
What Else May Change the Capacitance
Percentage range [%] Relative multiplier
Initial tolerance +-10 0.9 … 1.1
Temperature effect +-15 0.85 … 1.15
DC bias effect +0 -70 0.3 … 1
AC bias effect +0 -30 0.7 … 1
Aging (over 3 years) +0 -7.5 0.925 … 1
When we multiply the worst-case contributors, we get 0.9*0.85*0.3*0.7*0.925 = 0.15, which means in worst case instead
of 1uF we have only 0.15uF capacitance.
Does This Happen In All Capacitors?
Luckily, NO.
This happens in high Dk MLCCs
It does not happen in
Low Dk MLCCs (COG/NPO), but we don’t get high density either
Electrolytics
Tantalum
Polymer
Film
How To Simulate
S-parameter models don’t easily scale
SPICE equivalent circuits scale easily, but in complex models, scaling is tricky due to multiple Cs describing a single device
Best option: dynamic models (available for most Murata MLCCs)
For more info on dynamic models: http://www.electrical-integrity.com/Quietpower_files/QuietPower-36.pdf
How To Mitigate
If the DC-AC bias effects have to be reduced, consider to
Create circuits, which are less sensitive to capacitance variations
Check carefully bias curves (if available) and select ‘better’ parts
Avoid using MLCC parts with the highest capacitance density
Avoid using MLCCs
Conclusions
DC and AC bias effect can reduce MLCC capacitance substantially
Total capacitance loss can be up to 80-90%
Capacitance loss can increase droop, resonance peaks and degrade filter
performance
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