DC-DC｜Design
Placement of Output Capacitors
2018.02.08
Points of this article
・An output capacitor should be placed as close as possible to an inductor.
・In order to reduce the propagation of high-frequency noise, the GND of CIN should be placed 1 to 2 cm distant from the GND of CO.
In succession to the previous section on “Placement of Inductors”, this time we explain “Placement of Output Capacitors”, which are an important component. In conjunction with this discussion, it will be good to review the role played by output capacitors and properties required of them. Refer to the DC-DC Design Edition, “Selection of Output Capacitors”.
Differences in Currents in Output and Input Capacitors
As a review, we begin now by recalling that there are differences in the currents flowing in an input capacitor CIN and in an output capacitor CO. In the waveform diagram, ICO is the current waveform for an output capacitor, and ICIN below it is that for an input capacitor.
In the input capacitor, a comparatively large current repeatedly flows suddenly, but in the output capacitor, smooth charging and discharging is repeated, linked with an output ripple voltage and centered on the output voltage. This is because an inductor is inserted in series into the output line; the inductor L and CO serve as an output filter.
Placement of the Output Capacitor
The GND connection of the capacitor CO should be at a position 1 to 2 cm distant from the GND connection of CIN, and as close to the inductor as possible.
As stated above, in the input capacitor, a sharply rising/falling current flows repeatedly, and so a high frequency in the range of several hundred MHz flow into the GND pattern connected to CIN. Of course the GND pattern to which CO is connected is the same GND pattern, and so if Co is placed close to where CIN is connected, there is the possibility that the high-frequency noise in the input may pass through CO and be conducted to the output. This is shown schematically in the diagram on the lower right.
The reason for separating the GND of CO from the GND of CIN by 1 to 2 cm is that the inductance and resistance components of the thin film wiring can serve as a filter to reduce high-frequency noise. In other words, this design makes clever use of parasitic components
For an example of overall positional relationships, refer to the PCB pattern diagram above.
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This hand book shows the basics and examples of board layout for DC-DC buck converters. It explains the concept and precautions of PCB layout, which also greatly affects the stability of switching type DC-DC converters.
DC-DC
Basic
- Operation During Shutdown of a Boost DC-DC Converter
- Linear Regulator Basics
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Switching Regulator Basics
- Types of Switching Regulators
- Advantages vs Disadvantages in Comparison with Linear Regulator
- Supplement－Current Paths during Synchronous Rectifying Step-Down Converter Operation
- Operating Principles of Buck Switching Regulator
- Differences between Synchronous and Nonsynchronous Rectifying DC-DC Conversion
- Control Methods (Voltage Mode, Current Mode, Hysteresis Control)
- Efficiency Improvements at Light Load for the Synchronous Rectifying Type
- Protective and Sequencing Functions
- Considerations on Switching Frequencies
- Behavior when Vin Falls Below Vout
- Supplement－Protective Function: Output Pre-bias Protection
- Seven Representative Power Supply Circuits: From Low-noise to Boost Specs
- Concluding Remarks
- What is a DC/DC Converter?
Design
- Overview of Selection of Inductors and Capacitors for DC-DC Converters
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Overview of DC-DC Converter PCB Layout
- Ringing at switching nodes
- Placement of input capacitors and output diodes
- Placement of Thermal Vias
- Placement of Inductors
- Placement of Output Capacitors
- Feedback Path Wiring
- Ground
- Resistance and Inductance of Copper Foil
- Noise countermeasures: corner wiring, conducted noise, radiated noise
- Noise countermeasures: snubber, bootstrap resistor, gate resistor
- Summary
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PCB Layout of a Step-Up DC-DC Converter – Introduction
- The Importance of PCB Layout Design
- Current Paths in Step-up DC-DC Converters
- PCB Layout Procedure
- Placement of Input Capacitors
- Placement of Output Capacitors and Freewheel Diodes
- Inductor Placement
- Placement of Thermal Vias
- Feedback Path Wiring
- Ground
- Layout for Synchronous Rectification Designs
- Resistance and Inductance of Copper Foil
- Relationship Between Corner Wiring and Noise
- Summary
Evaluation
- Overview of Characteristics and Evaluation Method of Switching Regulators
- How to Read Power Supply IC Datasheets: Cover, Block Diagram, Absolute Maximum Ratings and Recommended Operating Conditions
- Evaluating a Switching Regulator: Output Voltage
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Introduction
- Definitions and Heat Generation
- Losses in Synchronous Rectifying Step-Down Converters
- Conduction Losses in Synchronous Rectifying Step-Down Converters
- Switching Losses in Synchronous Rectifying Step-Down Converters
- Dead Time Losses in Synchronous Rectifying Step-Down Converters
- Controller IC Power Consumption Losses in a Synchronous Rectifying Step-Down Converter
- Gate Charge Losses in a Synchronous Rectifying Step-Down Converter
- Conduction Losses due to the Inductor DCR
- Example of Power Loss Calculation for a Power Supply IC
- Simplified Method of Loss Calculation
- Heat Calculation for Package Selection: Example 1
- Heat Calculation for Package Selection: Example 2
- Loss Factors
- Matters to Consider When Studying Miniaturization by Raising the Switching Frequency
- Important Matters when Studying High Input Voltage Applications
- Important Matters when Studying Large Output Currents Applications: Part 1
- Important Matters when Studying Large Output Currents Applications: Part 2
- Summary
Application
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Important Points in the Design of a Power Supply Using a Linear Regulator
- Typical Application Circuit Examples of Linear Regulator ICs
- Input/output capacitor design and ripple prevention for linear regulator ICs
- How to determine efficiency and Thermal design for linear regulator ICs
- Protection of Linear Regulator IC Terminals
- Soft Starting of a Linear Regulator IC
- Overcurrent Protection（OCP） and Thermal Shutdown（TSD） of Linear Regulator IC
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Important Points in the Design of a Power Supply Using a Floating Type Linear Regulator
- Example of Power Supply Circuit Based on a Floating Type Linear Regulator IC
- Input/output capacitor design and ripple prevention for linear regulator ICs
- How to determine efficiency and Thermal design for Floating Type Linear Regulator ICs
- Terminal protection for linear regulator ICs
- Startup characteristics for linear regulator ICs
- Failure to Start of a Power Supply Using a Linear Regulator, Case 1: Damage to the IC and Peripheral Components Due to Hand-Soldering
- About Parallel Connections of LDO Linear Regulators
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Introduction
- Power Supply Sequence Specification ①: Power Supply Sequence Specifications and Control Block Diagrams
- Power Supply Sequence Specification①: Sequence Operation at Power Turn-on
- Power Supply Sequence Specification①: Sequence Operation at Power Shutoff
- Power Supply Sequence Specification①： Example of Actual Circuit and Component Value Calculations
- Power Supply Sequence Specification①: Example of Actual Operations
- Power Supply Sequence Specification②:Power Supply Sequence Specifications and Control Block Diagrams
- Power Supply Sequence Specification②:Sequence Operation at Power Turn-on
- Power Supply Sequence Specification②: Sequence Operation at Power Shutoff
- Power Supply Sequence Specification②: Example of Actual Circuit and Component Value Calculations
- Power Supply Sequence Specification②: Example of Actual Operations
- Circuits to Implement Power Supply Sequences Using General-Purpose Power Supply ICs ーSummaryー
- Easy Stabilization/Optimization Methods for Linear Regulators – Introduction
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