1. Switching Power Supply Component Selection
7.1a Capacitor Selection – Overview

2. Capacitor Selection for DC/DC Converters
Design factors that are known before selecting capacitors:
Factor
Description
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 ~30-40% of IOUT at the switching frequency
Topology
Chosen in architectural stage
These factors dictate what capacitors can be used and how to derive the best application scenario based on all factors combined

3. Selection Process Summary Electrical Specifications
Electrical Performance
Transient Requirements
Capacitor Impedance
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
Size bulk capacitance based upon voltage deviation requirements
Check that the selected capacitor meets stability requirements
Does this capacitor chemistry look inductive at the frequency of interest?

4. 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. Large ceramics can crack due to vibration.
Costs - Tradeoffs
Component cost vs. Total cost of ownership

5. 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

6. Selection Process Summary
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

7. Capacitor RMS Current
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
A Capacitor is two conducting parallel plates with a dielectric compound (insulator) between them. The type of dielectric distinguishes the various types of capacitors. Electrolytic capacitors are polar; their terminals are not reversible. Aluminum and Tantalum capacitors are a type of electrolytic capacitors. Ceramic capacitors are not electrolytic; their terminals are reversible. Electrolytic capacitors are more stable over voltage and temperature than the Ceramic capacitors but they have worse ESR.

8. 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. 8 8 8

9. Common Topologies: BUCK
Buck Converter
Boost Converter
Input Capacitor RMS Current
Buck-Boost Converter
Output Capacitor RMS Current 9 9 9

10. 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. 10 10 10

11. Common Topologies: BOOST
Buck Converter
Boost Converter
Input Capacitor RMS Current
Output Capacitor RMS Current
Buck-Boost Converter 11 11 11

12. 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. 12 12 12

13. Common Topologies 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 13 13 13

14. Additional Topologies
SLUW001A

15. Thank you!