Capacitor Selection for DC-DC Converters 09/12

CAPACITOR SELECTION FOR DC/DC
CONVERTERS:
WHAT YOU NEED TO KNOW TO PREVENT EARLY
FAILURES, AND REDUCE SWITCHING NOISE
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
The SIMPLE SWITCHER® Experience
Easy-to-Use ICs
Power Modules
WEBENCH® Power Designer
• Enables
designers at any
level to create a
power supply
easily and
quickly
• Reduces overall
design time with
proven solutions
Regulators
Reference Designs
Controllers
Design Tools
• Delivers faster
time to market
Table of Contents
Capacitors types for
DC/DC Conversion
Electrolytic
Typical
Characteristics
Ceramic
Advantages and
Disadvantages
Tantalum
Failure Modes
Polymer
Selection Process
Advanced Applications in DC/DC
Converters
Buck
RMS current
ratings by
topology
Measurement of
capacitor
parasitics
Boost
Estimating output
voltage ripple and
transient
response
Simple method to
reduce high
frequency noise
in SMPS
CAPACITOR TYPES
What is a Capacitor?
» Capacitance = The ability to store
charge in an electric field.
Capacitor Chemistry - Value and Voltage Rating
Electrolytic
100uF 10000u
F
Capacitance
Polymer
Tantalum
0.1uF100uF
X5R
X5R/X7R
COG
1pF –
0.1uF
Voltage
2V
4V
16V
25V
50V
100V
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 Mechanisms lead to – 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
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.
-
+
-
+
Aluminum Electrolytics - Advantages
» Low cost
» Mature technology with low cost materials
» Long history
» Industry started in the 1930s
» Many manufacturers to choose from
» High capacitance values available
» Only choice for SMPS that need high voltage
and high capacitance
Aluminum Electrolytics - Disadvantages
» Large swings in ESR vs temperature
» Cold temps have 4 - 8x higher ESR than room temps
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 the Aluminum oxide barrier layer to de-form.
– Capacitance drops
– ESR increases.
• Higher ESR causes more internal heat causing the electrolyte 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 most 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
Aluminum Electrolytics – 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
Ceramics - Overview
»
Lowest Cost devices
»
»
»
Primarily for decoupling and bypass
applications
» Multiple vendors, sizes
» Surface mountable
How is it made?
» Alternating layers of electrodes and ceramic
dielectric materials
Things to watch out for with 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
Ceramic Dielectric – 3 Character Codes
Class 1: (Best Performance)
Class 2: (Higher Capacitance)
»
»
Temperature Coefficient Decoder
Typical Values:
»
NP0, C0G, values up to
100,000 pF
Temperature & Capacitance
Tolerance Decoder
Typical Values:
»
X5R, X7R, values up to 150 uF
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)
» < 2nH
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 LDOs and parts using COT control
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 Bias Effect Including Case Size
X5R, 16V Rated Capacitors
Voltage de-rating
Class 1 Dielectrics
COG etc do not
require voltage
derating.
1uF 0603
1uF 1206
1uF 0805
Capacitance decreases more quickly with smaller case sizes
Class 2 Dielectrics
such as X7R and
X5R, lose significant
capacitance as you
approach the rated
voltage.
Ceramic Capacitance Change Due To
Temperature
» AVX Capacitor X5R dielectric – typical of any
brand
» You lose another 10% over temperature
Y5V Dielectric Characteristics
DO NOT USE Y5V and Z5U ceramic dielectrics for power supply
designs
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 MnÆMnO
» Epoxy encapsulated
2
Old technology
» Requires 50% Voltage de-rating
– PPM failure rates increase exponentially
above 50% voltage de-rating
» Can fail explosively
» High ESR compared to polymer types
» Fairly low cap roll off vs. frequency
Tantalum Model
Solid Tantalum Capacitors - Packaging
-
+
» Usually rectangular surface mount technology –
SMT machine mountable
» Capacitance ratings for 1 uF to 1,000 uF
+
-
+
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
Solid Tantalum Capacitors - Disadvantages
» Limited voltage range of 50V rating (max)
» Therefore, only reliable for operating voltages less
than 25 to 35VDC
» Fairly high in cost
» Historically tantalum has had supply shortages
» Limited in-rush surge current capability
» Do not use tantalum for hot pluggable input
capacitors!
Don’t use tantalum to hot plug!
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
Polymer - Overview
» Highest capacitance per unit volume technology
» Small package sizes available
» How is tantalum polymer 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 it still looks like a cap!
» Lower power dissipation
– Higher ripple current capability
– May need less capacitance
Polymer & Organic Capacitors Packaging
» SMT Block style similar to tantalum
» Round / Radial versions in SMT and throughhole
» Types: Tantalum polymer / Aluminum polymer /
Organic semiconductor
-
+
+
+
-
Kemet Tantalum Polymer
PosCap
OSCON
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
Polymer - Reliability
» Voltage de-rating is 10 - 20% depending on rated
voltage
» PPM failure rates significantly reduced
» Can withstand higher transient voltages
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
Capacitor Chemistry – General Parameters
DC/DC CONVERTER TOPOLOGIES
Capacitor Selection for DC/DC Converters
» Design factors that are known before
selecting capacitors:
» Switching frequency: Fsw ; from 50 KHz (high power)
to 6 MHz (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 ~30% of IOUT at the switching frequency
» Topology: chosen in architectural stage
Capacitor Selection for DC/DC Converters
» RMS current of a capacitor is one of the
most important specifications for capacitor reliability
» It also affects the converter’s 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
Common Topologies - BUCK
Buck
Converter
Switching Current exist
in the input side
Boost
Converter
Buck-Boost
Converter
Critical
path
Common Topologies - BUCK
Buck
Converter
Boost
Converter
Input Capacitor RMS Current
Output Capacitor RMS Current
Buck-Boost
Converter
Common Topologies - BOOST
Buck
Converter
Boost
Converter
Buck-Boost
Converter
Critical
path
Common Topologies - BOOST
Buck
Converter
Boost
Converter
Input Capacitor RMS Current
Output Capacitor RMS Current
Buck-Boost
Converter
Common Topologies – BUCK BOOST
Non-Inverting
Buck
Converter
Boost
Converter
Buck-Boost
Converter
Inverting
Critical path
Common Topologies – BUCK BOOST
Buck
Converter
Non-Inverting
Mode 1 (Buck)
Mode 2 (Boost)
Boost
Converter
Buck-Boost
Converter
Input Cap RMS Current
Input Cap RMS Current
Output Cap RMS Current
Output Cap RMS Current
Additional Topologies
SLUW001A
CAPACITOR PARASITICS
Ideal Capacitor Compared to Actual Capacitor
You get this
You buy this
Voltage and Temperature De-rated
Capacitance
(ESL)
Effective Series Inductance
22uF 4V X5R 0603 Ceramic
- Parasitic inductance term
(ESR)
Effective Series Resistance
- Parasitic resistance term
Get three parts for the price of one!
Important to Know Your Parasitics
»
»
Equipment to use to measure capacitor parasitic elements
RLC Analyzer
»
»
RF Network Analyzer
»
»
»
»
»
Some can apply DC bias
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) and several others
Æ Measure the parasitic terms and include them in the design Å
First Pass Parasitic Inductance for Ceramics
First Pass Trace Inductance for FR-4,
Microstrip
Typical Inductance for a
2500um (60mil) wide 1oz Trace
19.5 nH / inch, 19.5pH / mil , 767pH / mm
Typical Inductance for a
250um wide 1oz Trace
26.4 nH / inch, 26.4pH / mil , 1.039nH / mm
From ARRL Handbook
First Pass Trace Inductance for Via
From Dr. Howard Johnson - http://www.signalintegrity.com/Pubs/edn/ParasiticInductance.htm.
Comparison of Capacitor Types Using
Frequency Response Analyzer
(Shown in reactance coordinate system)
5 different types of 22uF capacitor
Comparative Performance of Different
Capacitor Types Using RF Network Analyzer
Same Five
22uF
Capacitors
1.0 Ohm
Le
a
de
d
0.316 Ohm
OS
-C
Ta
ON
n
t
Po
alu
s- C
m
ap
Aluminum Electrolytic
0.01 Ohm
45 Deg
0 deg
Ce
r am
ic
0.1 Ohm
0.0316 Ohm
90 deg
-45 deg
-90 deg
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
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
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 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
»
Capacitor impedance
» Does this capacitor chemistry look inductive at the frequency of
interest?
Capacitor - Selection Process Summary
» Transient and stability requirements
» Size bulk capacitance based upon voltage deviation requirements
» Check that the selected capacitor meets stability requirements for the part
» Most designs use a combinations of technologies
» Tantalums or Aluminum Electrolytics for bulk Capacitance
» Ceramics for decoupling and bypass
» Selection might also depend on mechanical challenges
» Vibration, Temperature,Cooling
» Lifetime comes into play
» Ceramics and polymer have improved lifetime over electrolytic and
tantalum
» Costs - Tradeoffs
» Component cost vs. total cost of ownership
PARALLELING CAPACITORS TO
REDUCE HIGH FREQUENCY OUTPUT
VOLTAGE RIPPLE
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)
High Frequency Ripple
Switch
waveform
(scope trigger)
Vout ripple w/ 20
MHz bandwidth (bw)
5 mV /div Æ
10 mv p-p
HF spikes ignored !
Use Zoom Function to Measure Ring
Frequency
Timebase Zoomed traces
20 MHz bw
Need to add
470 pF 0603
bypass SRF ~
300 MHz
10 mV/div
200 MHz bw
100 mV/div
2 GHz bw
100 mV/div
Å 300MHz
ring
Continue the Method
Timebase Zoomed traces
20 MHz bw
10 mV/div
200 MHz bw 100 mV/div
2 GHz bw
100 mV/div
Å 115
MHz ring
Measured after
adding a 470
pF 0603 but
before adding
2200pF 0603
Continue the Method
Timebase Zoomed traces
20 MHz bw
10 mV/div
200 MHz
bw
100 mV/div
2 GHz bw
100 mV/div
Å 60 MHz
ring
Measured after
adding a 470pF
0603 and a
2200pF 0603 but
before 4700pF
0805
Results After 3rd Added Small Capacitor
20 MHz bw
10 mV/div
200 MHz bw
100 mV/div
2 GHz bw
100 mV/div
ÅRing ~377MHz
Measured
after adding
a 470pF
0603,
2200pF
0603, and
4700pF 0805
Final Amplitude Improvement Results
20 MHz
bw
10
mV/div
200 MHz
bw
10 mV/div
2 GHz bw
10 mV/div
20 mV p-p @ 20MHz
bw
80 mV p-p @
200MHz bw
After 470pF
2200pF
4700pF
Starting Point for Comparison - 3 caps
Removed
20 MHz bw
10 mV/div
200 MHz bw 200 mV/div
2 GHz bw
200 mV/div
696mVp-p @ 200MHz bw
Final Schematic and BOM: 15 mins. Later
L1
3.3µH 6A
1.2VDC
OUT
@ 5A
+3.3VDC
OUT
COUT2
470 pF
NP0 (C0G)
GND
Specialty Polymer
800 kHz Buck Switcher
COUT1
220µF
6.3V
0.0075 ohm
GND
COUT3
470
pF
2200pF
NP0 (C0G)
GND
COUT4
4700 pF
X7R
GND
» Remember to reserve locations on the schematic and
PCB for these parts
» You won’t know the capacitor values until after you test
the running power supply for ringing noise
» Plan ahead
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
CH2 - 20MHz
RSOURCE
1.2VDC SOURCE
49.9
SOURCE
3 CHANNEL INPUT
CH3 - 200MHz
50 OHM COAX
CH4 - 2GHz
Example of 3 channel input adapters built for this tutorial (net 4x passive probe)
Use C0G (NP0) Dielectric for High
Frequency Shunt Filter Capacitors
Start with
manufacturer data
sheets, then measure
SRF on bench to
confirm
THANK YOU
Questions?