Capacitors have a self-resonance frequency (SRF), newer SMPS have switching frequencies above the SRF. We show strategies, how to choose the right components and what impacts SRF on capacitors.

1. Above Resonance Frequency

2. Trends To High Switching Frequency
• Many manufacturers of power integrated circuits
(ICs) are aggressively marketing high-frequency DC/DC
converters that they claim reduce board space.
• decrease the size of the output capacitor and inductor
• A DC/DC converter operating at 1 or 2 MHz seems like a
great idea
• Upper Frequency is limited by min ON time and 𝐷𝐶 =
𝑈 𝑜𝑢𝑡
𝑈𝑖𝑛
http://www.ti.com/lit/an/slyt415/slyt415.pdf

3. TI Buck Regulator Datasheet

4. Schematic TPS54150 - 3,3V/1,5A
MPLC and MPC

5. Switching Frequency
We use 1.2MHz - what about the capacitor above SRF
C0805C476M8PAC = 0805 47µF 10V
SRF = 758kHz

6. SRF = Self Resonance Frequency
ESR Equivalent Series Resistance Frequency dependant ohmic
resistor.
Cause self heating P=I²*ESR
L = ESL Equivalent Series Inductance Frequency dependant inductance
Cause overshot and ringing

7. S21 - Parameter
• Magnitude minimum is at SRF
• Phase change approx. -90° to 90°

8. Real Capacitor
• Above resonance frequency

9. Calculated Values

10. How We Get ESL
assume ESR = 0

11. Compare Measure Vs Calc - Schematic

12. Compare Measure Vs Calc - Plot
measured
calculated

13. Model T520 A 47µF 4V
• RC - ladder drop capacitance over frequency

14. Polymer At 1.2MHz And 3.3V

15. MLCC Model

16. VCC
• VCC Voltage change capacitance aka DC-Bias effect
• Capacitance drops with applied DC voltage
• Reason is so called “locked dipoles”
• Dipoles locked by an electric field
• Thinner dielectric increase E-Field
• Downsizing typically accompanied by thinner dielectric
+
-
+
-

17. VCC - 0805/47µF/10V @3.3VDC
• Capacitance loss of -64%
Catalog Application
47µF 16,92µF

18. No Capacitance Vs Vbias For Polymer
Polymer
Ceramic

19. SRF 758kHz @ 0VDC Bias

20. SRF Shift 1.318MHz @3.3VDC
• SRF moves up as DC-Bias decrease capacitance!

21. Compare Different Case And Dielectric
• High capacitance values
• Dielectrics X5R, X7R and Polymer
• 0402, 0603, 0805, 1206, 1210 and A-Case (~1210)

22. Comparison
• netCap is at 1.2MHz@3.3V
• SRF* is with applied DC-Bias
Cap
/ µF
SRF* /
MHz
dC%
@3.3V
netCap/
µF
noOfPcs
0402 10u 4V X5R 10 5,754 -73,1 2,4 20
0603 10u 10V X5R 10 3,020 -63,2 5,0 10
0805 47u 10V X5R 47 1,318 -63,3 15,2 4
1206 47u 10V X7R 47 0,794 -13,6 36,2 2
1210 22u 25V X7R 22 1,660 -27 19,5 3
A 47u 4V Polymer 47 0,870 0 16 3

23. Downsizing Effect
• Downsizing (0603,0402,0201) requires thinner dielectric
• Higher permittivity more sensitive for VCC

24. Ripple Current
• 1.4A @ 1.2MHz

25. Self Heating At 1.2MHz
1.5Arms cause 3°C self heating in addition

26. Wrap Up
• Use K-SIM (ec.kemet.com/designtools)
• VCC cause significant capacitance drop for MLCC
• High switching frequency compensate at least partially
• VCC increase the SRF
• Small case sizes are stronger affected (dielectric thickness)
• Downsizing has some side effects
• Polymer has no VCC
• A - Case 3216-18 is drop in replacement for 1206 and 1210
• Ripple current can increased by moderate more self heating

27. Thank You!
Axel Schmidt
Field application Engineer
Cell +49 172 89 25 284
email: AxelSchmidt@kemet.com
Hermann-Köhl-Str. 2
86899 Landsberg am Lech
Germany
www.kemet.com