1. Capacitor Selection for DC/DC Convertors: From Basic Application to Advanced Topics
TI – Silicon Valley Analog in Santa Clara California USA -
This course written by: Simple Switcher Applications Team Members:
Alan Martin
Marc Davis-Marsh
Giuseppe Pinto
Ismail Jorio
Additional material provided by Chuck Tinsley of capacitor manufacturer KEMET 1

2. Table of Contents Capacitors types for DC/DC Conversion
Electrolytic
Ceramic
Tantalum
Polymer
Typical Characteristics
Advantages and Disadvantages
Failure Modes
Selection Process
Advanced Applications in DC/DC Converters
Buck
Boost
RMS current ratings by topology
Measurement of capacitor parasitics
Estimating output voltage ripple and transient response
Simple method to reduce high frequency noise in SMPS 2

3. Capacitor Types 3

4. Capacitor Chemistry - Value and Voltage rating
100uF uF
Electrolytic
Polymer
0.1uF-100uF
Tantalum
Capacitance
X5R
X5R/X7R
1pF – 0.1uF
COG
Voltage 2V 4V
16V
25V
50V
100V

5. Aluminum Electrolytics - Overview
Least Expensive Capacitors for Bulk Capacitance
Multiple vendors
Small size, surface mountable
How is it made?
Etched foil with liquid Electrolyte
Placed in a can with a seal/vent
Highest ESR
Low Frequency Cap roll off due to higher ESR
Wear Out Mechanism – Limited lifetime
Liquid electrolyte – with a vent
Cap changes over time with Voltage and Temp
ESR changes over time
Mounting
High shock and vibration can cause failure 5

6. Aluminum Electrolytics - Packaging
Through hole versions, usually in a round can.
Large ones have screw terminals or solder lugs
Radial or axial leads
Non SMT may have higher inductance because of long leads
Surface mountable versions, are modified from radial leaded versions.
SMT versions usually have the capacitor value visibly printed on can.
SMT versions may use letter codes instead of numeric rating.
- + 6

7. Aluminum Electrolytics - Advantages
Low Cost
Mature technology with low cost materials
Long History
(Industry started in the 1930’s )
Many Manufacturers to choose from.
High capacitance values available.
Only choice for SMPS that need high voltage and high capacitance. 7

8. Aluminum Electrolytics - Disadvantages
Large swings in ESR vs temperature
Cold temps have 4-8x higher ESR than at room temperature 8

9. Aluminum Electrolytics - Disadvantages
Large Parasitics
High ESR (Effective Series Resistance)
High ESL – (Effective Series Inductance).
Electrolytic capacitors eventually degrade over the life of the product.
The electrolyte eventually dries out.
Long term storage may cause Aluminum oxide barrier layer to de-form.
Capacitance drops
ESR increases.
Higher ESR causes more internal heat causing it to dry out even faster.
This effect is worse at high temperatures.
(Lesson: Don’t use “old stock” aluminum capacitors in your product.)
Needs a ceramic in parallel for switch mode applications.
High esr and esl can cause SMPS malfunction.
Have measurable dc leakage current.
Probably not an issue in power circuits;
Leakage current can be a problem in timing circuits. 9

10. Aluminum Electrolytic - Venting failure
Fails open or shorted.
Catastrophic explosive venting
From Over-Voltage of the capacitor.
From exceeding the Ripple Current Rating of a capacitor
May have the same effect as overvoltage; but it takes longer for the capacitor to overheat and vent.
450V rated capacitors after accidental application of 600V 10

11. Ceramics - Overview Lowest Cost devices How is it made?
Primarily for decoupling and bypass applications
Multiple vendors, sizes
Surface mountable
How is it made?
Alternating layers of electrodes and ceramic dielectric materials
Significant effects for Class 2 Dielectrics i.e. X5R, X7R.
Voltage bias effect
Temperature effects
Ageing
2%/decade hour for X7R,
5%/decade hour for X5R
Starts decay after soldering
High Q
Frequency selective 11

12. Ceramic Dielectric – 3 Character Codes
Class 1: (Best Performance)
Temperature Coefficient Decoder
Class 2: (Higher Capacitance)
Temperature and Capacitance Tolerance Decoder
Typical Values:
NP0,C0G, values up to 100,000pF
Typical Values:
X5R,X7R, values up to 150uF

13. Ceramic Capacitors - Advantages
Low Cost
Mature technology with low cost materials
Many Manufacturers to choose from.
Reliable and rugged
Extremely tolerant of over voltage surges
Best Choice for local bypassing
Not Polarized
Lowest effective series resistance (Low ESR)
several milliohms
Leads to high RMS current rating
Low effective series inductance (Low ESL)
< 3nH 13

14. Ceramic Capacitors - Disadvantages
Capacitance limited to around 150 uF / 6.3V
Large body sizes prone to cracking with PCB flexing. Several small units in parallel may be a better choice.
Have both a voltage and temperature coefficient that reduces capacitance value.
Some large package size units exhibit piezo-electric audible “singing”.
Difficult to control. (Ceramic speaker effect.)
More noticeable with Class 2 dielectrics
Incomplete data sheets!!
ESR, ESL, SRF and Ripple Current rating often missing from data sheets
Contact the manufacturer for ripple current
Capacitance value not printed on SMT device package.
Impossible to visually inspect for value once mounted on the PCB.
Some power supply circuits are not stable with ceramic output capacitors.
Usually LDO’s and parts using COT control 14

15. Ceramics - Cracking Flex cracking – Number 1 Failure Mode!
Cracks formed after mounting to PCB
Mechanically stressed after assembly
Larger parts, generate cracks more easily
Usually Fails shorted
Voltage de-rating
Not required for COG, X7R and X5R…but…
Voltage bias or capacitance loss increases as you approach rated voltage on X5R/X7R devices 15

16. Voltage Bias Effect on X5R - example
X5R, 10uF, 6.3 volt Ceramic capacitor at 0, 3.15 and 6.3 volt bias
10uF becomes 2.9uF under 6.3V bias condition
Doesn’t include, temperature, ageing and initial tolerance!!!!!!!!!
10uF
0V Bias
3.15V Bias
2.9uF
6.3V Bias 16

17. Voltage Bias Effect Including Case Size
X5R, 16V Rated Capacitors
1uF 0603
1uF 0805
1uF 1206
Capacitance decreases more quickly with smaller Case sizes 17

18. Ceramic capacitance change due to temperature
AVX Capacitor X5R dielectric - typical of any brand
You lose another 10% over temperature 18

19. Y5V dielectric characteristics
DO NOT USE! Y5V and Z5U ceramic dielectrics for power supply designs 19

20. Tantalum – Overview (MnO2 based)
High Capacitance per unit Volume Technology
Small package sizes available
Thin devices are available
How is it made?
Tantalum Anode pressed around a tantalum wire
Oxide grown on surface
Cathode formed by dipping and heat conversion MnMnO
Epoxy encapsulated
Old technology
Requires 50% Voltage derating
PPM failure rates increase exponentially above 50% voltage derating
Can fail explosively
High ESR compared to Polymer types
Fairly low cap roll off vs. frequency 2
Tantalum Model 20

21. Solid Tantalum Capacitors - Packaging
Usually rectangular Surface Mount Technology – SMT machine mountable
Capacitance ratings for 1uF to 1000uF + - + - 21

22. Solid Tantalum Capacitors - Advantages
Lots of capacitance in a small package.
1uF to 1000uF max
Medium-high effective series resistance (Low ESR)
10 to 500 milliohms
Medium level of RMS current
Low effective series inductance (Low ESL)
< 3nH
Numerous manufacturers
Good datasheet vs. electrolytic 22

23. Solid Tantalum Capacitors - Disadvantages
Limited voltage range 50V rating max.
Therefore for circuit voltages less than 25 or 35VDC.
Fairly high in cost
Historically Tantalum has had supply shortages
Limited inrush surge current capability.
Do not use tantalum for hot pluggable input capacitors!
Don’t Hot plug tantalum! 23

24. Solid Tantalum Capacitors – Application Safety
ALWAYS observe voltage polarity
DO NOT exceed voltage rating.
DO NOT exceed inrush surge rating
Can fail catastrophically if misapplied.
Can fail open or short 24

25. Polymer - Overview Highest Capacitance per unit Volume Technology
Small package sizes available
How is it made?
Tantalum Anode pressed around a tantalum wire
Oxide grown on surface
Cathode formed by dipping into Monomer and cured at room temperature
Epoxy encapsulated
Lower ESR vs MnO2 based Tantalums
Higher frequency operation – over a Mhz…still looks like a cap!
Lower power dissipation
Higher ripple current capability
May need less capacitance 25

26. Polymer & Organic Capacitors - Packaging
SMT Block style similar to tantalum.
Round / Radial versions in SMT and through-hole.
Types
Tantalum polymer / Aluminum polymer / Organic semiconductor
PosCap
Kemet Tantalum Polymer
OSCON 26

27. Polymer & Organic Capacitors - Advantages
Low ESR
but not as low as equivalent ceramic.
Low ESL
depending on construction method
New technology
Designed for SMPS.
Can be very low profile.
High capacitance per unit volume.
Much better performance than aluminum electrolytic and much smaller in size.
No voltage coefficient.
Viable alternative to solid tantalum. 27

28. Polymer - Continued
Voltage de-rating is 10/20% depending on rated voltage
PPM failure rates significantly reduced
Can withstand higher transient voltages 28

29. Polymer & Organic Capacitors - Disadvantages
High cost
Voltage surges capability depends on chemistry.
Oscon very intolerant of voltage surges
Tend to be from a single supplier.
May have availability issues.
Polymer & Organic Capacitors – Failure Mode
Tantalum Polymer
Less prone to catastrophic failure than solid tantalum but will still vent and emit smoke.
Organic (OSCON)
Emits noxious smoke. 29

30. Capacitor Chemistry - Quick Comparison
Numerical Rankings from 1 (Best) to 5 (Worst) 30

31. Capacitor Chemistry – General Parameters31

32. DC/DC Converter Topologies32

33. Capacitor Selection for DC/DC converters
Design factors that are known before selecting capacitors :
Switching frequency Fsw : From 50KHz (High power) to 6MHz (Low power)
Input voltage range Vin
Output voltage Vout
Switch duty factor Duty Cycle (D) ~ Vout/Vin (for Buck/Step Down)
Output current Iout
Inductance L is usually designed such that the ripple current is ~ % of Iout at the switching frequency
Topology Chosen in architectural stage 33

34. Capacitor Selection for DC/DC converters
RMS current of a capacitor is one of the most important specifications for capacitor reliability
It also effects the converters performance, and varies by topology
Self-Heating Proportional to RMS Current and Internal Losses
Voltage Ripple Higher RMS Current leads to larger voltage ripple
Let’s calculate RMS current for different topologies 34

35. Common Topologies - BUCK
Buck Converter
Switching Current exist in the input side
Critical path
Boost Converter
Buck-Boost Converter
In a Buck converter, it is obvious that the power switches are going to be part of the high di/dt loop, but which ground connection is more critical than the others? We find out is by identifying the current path of the two sub-sections in switching period of this converter.
When the high-set FET is on, the current is going to go through the blue trace, and when the low-set FET is on, the current is going to through the pink trace, shown in this slide. And if a branch contains both of the two colored lines, that means 100% of the time of the switching period, there is current going through this branch, and we consider that a DC current path. If a branch only contains one of the colored lines, that means only a part of the switching period has a current going through this branch, and the current is going to be discontinuous.
So, we consider those to be high di/dt paths and the critical paths of the PCB layout. So, in Buck converter, this is shown as the shaded loop, which contains the high-side switch, the low-side switch, and the input capacitor. 35 35 35

36. Common Topologies - BUCK
Buck Converter
Boost Converter
Input Capacitor RMS Current
Buck-Boost Converter
Output Capacitor RMS Current 36 36 36

37. Common Topologies - BOOST
Buck Converter
Critical path
Boost Converter
Buck-Boost Converter
We can do the same thing in the Boost converter, and the critical path is contained by the two switching components and the output capacitor. 37 37 37

38. Common Topologies - BOOST
Buck Converter
Boost Converter
Input Capacitor RMS Current
Output Capacitor RMS Current
Buck-Boost Converter 38 38 38

39. Common Topologies – BUCK BOOST
Non-Inverting
Buck Converter
Critical path
Boost Converter
Inverting
Buck-Boost Converter
The inverting Buck boost is shown here, and the Buck boost. In both, the input and output current are discontinuous. 39 39 39

40. Common Topologies – BUCK BOOST
Buck Converter
Non-Inverting
Mode 1 (Buck)
Mode 2 (Boost)
Boost Converter
Input Cap RMS Current
Input Cap RMS Current
Buck-Boost Converter
Output Cap RMS Current
Output Cap RMS Current 40 40 40

41. Additional Topologies
SLUW001A

42. Capacitor Parasitics 42

43. Ideal capacitor compared to actual capacitor
You buy this
You get this
Voltage and Temperature De-rated Capacitance
(ESL)
Effective Series Inductance
- Parasitic inductance term -
22uF 4V X5R 0603 Ceramic
(ESR)
Effective Series Resistance
- Parasitic resistance term -
Buy one part – Get three for the price of one! 43

44. Know your parasitics! They are very important!
Equipment to use to measure capacitor parasitic elements.
RLC Analyzer
Some can apply DC bias
RF Network Analyzer
DC bias can easily damage analyzer source and receiver inputs
AC performance measurement very accurate.
Agilent (aka Hewlett Packard) i.e. HP3755A goes to 200MHz
Many other brands
Frequency Response Analyzer
Allows DC bias so voltage coefficient can be measured. RLC results are less accurate. Frequency range is lower than network analyzer
30 MHz max; Usually just 1 or 2 MHz range. May allow plotting on reactance paper with line of constant capacitance and constant inductance. FRA is also used for loop stability analysis.
Brands – Venable Industries – Ridley (A/P) several others.
 Measure the parasitic terms and include them in the design  44

45. 5 different types of 22uF capacitor
Comparison of capacitor types using Frequency Response Analyzer - Shown in reactance coordinate system -
5 different types of 22uF capacitor 45

46. Comparative performance of different capacitor types using RF Network Analyzer46

47. Reducing Ripple and Transient Response47

48. Output Caps Selection
The output capacitor must be designed to fulfill mainly two requirements:
To keep the steady-state peak-to-peak output voltage ripple below the maximum allowed value ∆Vopp
To keep the output voltage waveform within the required regulation window [Vo ± ∆Vo_reg] during the overshoot and undershoot caused by output current transients
The output capacitor design of a buck converter will be discussed in this presentation 48

49. Output Caps Selection – output ripple
Boundary conditions formulated separately for ESR and C of the output capacitor may lead to selection of oversized commercial components. Please note that there is a phase shift between the contribute of C and the contribute of ESR.
In this presentation an approach based on Acceptability Boundary Curves which jointly considers the effect of ESR and C will be shown. 49

50. Output Caps Selection – stray inductances in output ripple analysis
Depending on the constant ESR x C, the output voltage waveform can change from quasi-triangular to quasi-sinusoidal.
The effect of stray inductances can be easily separated from the effect of the principal parameters ESR and C of the capacitor. In fact, stray inductances produce additive step-up and step-down effects on the output voltage, whose amplitude is given by:
Dominant ESR
Tantalum
Electrolytic
Dominant C
Ceramic
Bad layout
NO ESL
Includes ESL 50

51. Output Voltage Ripple by Chemistry
Inductor Current
Ceramic
Tantalum Polymer
OSCON
Electrolytic
This plot shows a comparison of the output voltage ripple of a buck converter using 4 different capacitor chemistries.
All caps = 47uF. Scale = 20mV/div 51

52. Output Caps Selection – stray inductances in output ripple analysis
In order to take in account the effect determined by the equivalent series inductance (ESL) of the output capacitor and by the inductance of the printed circuit board trace LPCB it is sufficient to replace the maximum allowed peak-to-peak output ripple voltage amplitude ΔVOpp with the net effective peak-to-peak ripple voltage ΔVOppeff allowed to ESR and capacitance C, given by:
If ESL and LPCB are unknown, the previous formula can also be used to determine the maximum allowed ESL compatible with the ESR and capacitance C of a capacitor selected with the algorithm not including the LPCB : 52

53. Output Caps Selection – output ripple analysis
ON TIME
Current flowing into the output capacitor is given by :
OFF TIME
The remaining part of the peak to peak output ripple voltage can be determined by analyzing the circuit in the time intervals between switching instants and can be derived by integrating the current flowing into the output capacitor:
Initial voltage charge
ESR contribute
capacitance contribute
ON TIME
OFF TIME
Where: 53

54. Output Caps Selection – output ripple analysis
In order to determine analytical expression of output voltage ripple, maximum and minimum values of output voltage waveform must be computed.
Instant when minimum occurs is given by:
vO(tmax)
DOMINANT ESR case
Instant when maximum occurs is given by:
vO(tmin)
The analytical output ripple is given by:
∆VOpp = vO(tmax) - vO(tmin) 54

55. Output Caps Selection – output ripple analysis
Two different Domain Boundary Curves (DBC) can be defined as for D < 0.5 and D > 0.5
Three different Acceptability Boundary Curves (ABC) can be defined for HIGH, MID and LOW ESR case
HIGH ESR
MID ESR
LOW ESR
For D < 0.5 :
ESR ≥ Rs-  HIGH ESR
Ri- < ESR < Rs-  MID ESR
ESR ≤ Ri-  LOW ESR
Please note that boundaries are duty cycle and switching frequency dependent
ESR > Rs+ +
HIGH ESR
Rs+< ESR < Ri+
MID ESR
ESR < Ri+
LOW ESR 55

56. Output Caps Selection – output ripple analysis
The following figure shows Domain Boundary Curves (DBC) and the Acceptability Boundary Curves (ABC) in ESR-C plane for D = 33% , ∆iLpp = 0.8A, ∆Vopp_eff = 55mV, fS = 500kHz. In applications where D > 0.5 the DBC are inverted.
ABCs allow to quickly figure out real feasible capacitors representing possible design solutions when load transient constraints are not needed. A real capacitor whose values of ESR and C correspond to a point located below ABCs (green area) is suitable to meet ripple constraints requirements.
HIGH ESR
MID ESR
HIGH ESR
LOW ESR
MID ESR
LOW ESR 56

57. Output Caps Selection – output ripple analysis – simplified formula
A simplified equation can be derived by calculating the fundamental component of the output ripple voltage as:
There is an overestimation of the needed output cap nearby the MID ESR area 57

58. Output Caps Selection – load transient
The overshoot (or undershoot) occurs because of the surplus (or deficit) of charge in the output capacitor.
Let’s define the allowed variation for the output voltage as ∆Voreg
Because of slope constraints due to the inductor: when D < 0.5 the overshoot is greater than the undershoot and vice versa when D > 0.5
OVERSHOOT
UNDERSHOOT 58

59. Output Caps Selection – load transient analysis
The instant when the peak of the overshoot occurs depends on the type of the output capacitor.
The exact point where peak overshoot is reached, and its magnitude, must be calculated by taking into account joined influence of C and ESR. 59

60. Output Caps Selection – load transient
Three different cases can be considered when load transient occurs:
stepwise load transient: load transient duration TLT is much smaller than closed loop system’s response time given by the crossover frequency fC, TLT << 1/(4fC)
fast load transient: load transient duration TLT is smaller than closed loop system’s response time TLT < 1/(4fC)
slow load transient: load transient duration TLT is larger that closed loop system’s response time TLT > 1/(4fC)
In 1st and 2nd case the control network is not able to compensate output voltage variations suddenly after a load variation. Hence, the output filter must be designed to keep the output voltage within the maximum allowed range in early time after the load transient until the loop has the chance to respond.
The impact of load transient constraint on the output capacitor size is lower in 3rd case. For this reason worst case stepwise load transient is treated. The output current slew rate will be considered as infinite (TLT = 0). This is a reasonable assumption in application such FPGA supplies where load-current slew rate may range up to 100A/µs. 60

61. Output Caps Selection – load transient analysis
Is it possible to analyze a buck converter during load transient by means of the following circuit.
tLT is the instant when the load transient occurs, it can occur during ON time or during OFF time.
ON TIME
OFF TIME
Worst case condition occurs when:
tLT = DTS for overshoot
tLT = TS for undershoot
Let’s assume a high cross over frequency controller. Minimum duty cycle is assumed to be 0 and maximum duty cycle is assumed to be 1. 61

62. Output Caps Selection – load transient analysis – overshoot
Assuming that the step down load transient occurs at tLT = DT (overshoot worst case), the instant of peak variation of the output voltage and its maximum overshoot peak value are given by:
It tOS = DTS the magnitude of VOS is determined by ESR only (high ESR case ESRH)
If tOS > DTS the magnitude of VOS jointly depends on C and ESR values (low ESR case ESRL) 62

63. Output Caps Selection – load transient analysis – overshoot
The three curves cross at the boundary value of capacitance CB given by:
A real capacitor whose values of ESR and C correspond to a point located below this curve (green area) can be considered suitable to maintain the output voltage within the given regulation window in presence of a charging load transient.
Negative slope CB
Positive slope
Zero slope 63

64. Output Caps Selection – load transient analysis – overshoot
It makes sense to consider the effect of stray inductances LESL = ESL + LPCB only it the real slew-rate SR of the load current transition is known.
After the instant tr, the output voltage evolves in the time as if there was no ESL.
Load transient constraints are met if C, ESR and ESL are such that both conditions are fulfilled:
∆VO(tr) < ∆VOreg
∆VO(tos) < ∆VOreg
The effect of LESL can be accounted by including the following constraints for the ESR
∆VOreg 64

65. Output Caps Selection – load transient analysis – overshoot
LESL = 10nH
SR = 3 A/µs
∆VOreg 65

66. Output Caps Selection – load transient analysis – overshoot – simplified equation
It is possible to derive simplified boundaries for C and ESR.
Let’s assume that the minimum output capacitance required is CB previously defined as:
It is possible to calculate the maximum allowed value of the ESR by replacing CB value into the ESRcrit formula previously defined:
Any capacitor whose value of capacitance and ESR is higher than Cmin and lower the ESRmax is suitable to meet load transient requirements for a buck converter. 66

67. Capacitor - Selection Process Summary
Electrical specifications:
Electrical performance
RMS Current in the capacitor
Look for RMS current equation in the chosen DC/DC topology
Applied voltage at the capacitor
De-rate the capacitor based on the chemistry
Transient requirements
Size bulk capacitance based upon voltage deviation requirements
Check that the selected capacitor meets stability requirements
Capacitor impedance
Does this capacitor chemistry look inductive at the frequency of interest? 67

68. Capacitor - Selection Process Summary
Most designs use a combinations of technologies
Tantalums or Aluminum Electrolytics for bulk Capacitance
Ceramics for decoupling and bypass
Depends on Mechanical Challenges
Vibration
Temperature
Cooling
Lifetime comes into play
For longer life, improve the quality of the components
Ceramics and polymer have improved lifetime over electrolytic and tantalum
Costs - Tradeoffs
Component cost vs Total cost of ownership 68

69. Capacitors – Selection Process Summary
Use Equations for selected topology
Calculate RMS Currents, Peak voltages, Minimum capacitance, Maximum esr
Select Chemistry based upon the designs needs
Remember to de-rate voltage by at least 20% for all chemistries
50% for tantalum to improve reliability
50% for class 2 ceramics to decrease capacitance lost to DC biasing
Note: Capacitor data sheet MUST include 100kHz data if the capacitor is to be applied in a switch mode power supply (SMPS). 120 Hz only versions are not suitable for SMPS
Consider NP0 (C0G), X7R, X5R and X7S ceramic dielectrics* - in this order.
DO NOT USE Y5V
Place additional units in parallel if one is not enough
Combine chemistries to benefit from their various advantages
Use polymer, electrolytic and tantalum for bulk
Use Ceramics as your primary decoupling capacitor 69

70. Output Caps Selection – References
A. De Nardo, N. Femia, G. Petrone, G. Spagnuolo, “A unified method for optimal buck converter output capacitor design”, Proc. Of ISIE2008, Cambridge, UK, pp
S. Maniktala, Switching Power Supply Design & Optimization, Mc Graw Hill, New York, 2004
G. Spagnuolo, N. Femia, A. De Nardo, “Optimal Buck Converter Output Filter Design for Point of Load applications”, IEEE Trans. Ind. Electron., DOI /TIE , in press
R.W. Erickson, D. Maksimovic, “Fundamentals of Power Electronics”, 2001 Kluwer Academic Publishers 70

71. Paralleling capacitors to reduce high frequency output voltage ripple71 71

72. A technique for reducing High Frequency output noise
If the output capacitor(s) is not ceramic; then adding a small ceramic(s) in parallel with the output will reduce high frequency ripple.
Choose a ceramic capacitor that has an impedance null (self resonance) that is the same as the frequency to be attenuated.
One, two or three small ceramics can give 10X improvement (-20 dB.)

73. High frequency ripple Switch waveform (scope trigger)
Vout ripple w/ 20 MHz bandwidth (bw)
5 mV /div  10 mv p-p
HF spikes ignored !

74. Use Zoom function to measure ring frequency
Timebase Zoomed traces
20 MHz bw
10 mV/div
 300MHz ring
200 MHz bw
100 mV/div
2 GHz bw
100 mV/div
Need to add 470 pF 0603 bypass SRF ~ 300 MHz

75. Continue the method Timebase Zoomed traces 20 MHz bw 10 mV/div
 115 MHz ring
2 GHz bw
100 mV/div
Measured after adding a 470 pF 0603 but before adding 2200pF 0603

76. Continue the method Timebase Zoomed traces 20 MHz bw 10 mV/div
 60 MHz ring
200 MHz bw
100 mV/div
2 GHz bw
100 mV/div
Measured after adding a 470pF 0603 and a 2200pF 0603 but before 4700pF 0805

77. Results after 3rd added small capacitor
20 MHz bw
10 mV/div
Ring ~377MHz
200 MHz bw
100 mV/div
2 GHz bw
100 mV/div
Measured after adding a 470pF 0603, 2200pF 0603, and 4700pF 0805

78. Final amplitude improvement results
20 MHz bw
10 mV/div
20 mV 20MHz bw
200 MHz bw
10 mV/div
80 mV 200MHz bw
2 GHz bw
10 mV/div
After 470pF 2200pF 4700pF

79. Starting point for comparison - 3 caps removed
20 MHz bw
10 mV/div
200 MHz bw
200 mV/div
200MHz bw
2 GHz bw
200 mV/div

80. Final schematic and bill of materials: 15 minutes later Cost Approx < $0.03 USD
1.2VDC 5A
2200pF
Remember to reserve locations on the schematic and PCB for these parts.
You will not know the capacitor values until after you test the running power supply for ringing noise.
 Plan ahead 

81. Bench requirements
2GHz bw / 20Gsps Digital oscilloscope with zoom feature and adjustable channel bandwidth.
Selection of small capacitors pre-characterized by Self Resonant Frequency.
High quality interconnections with controlled impedances.
Example of 3 channel input adapter built for this tutorial (net 4x passive probe)

82. Use C0G (NP0) dielectric for high frequency shunt filter capacitors – lower ESR – more stable over temp
Start with manufacturer data sheets, then measure SRF on bench to confirm 82

83. Thank You
Questions?