DC-DC｜Design
Placement of Output Capacitors and Freewheel Diodes
2021.02.24
Points of this article
・If the output current is small, only a comparatively small value is needed for the output capacitor. Hence a single ceramic capacitor can be used as both the output capacitor and as a high-frequency decoupling capacitor.
・The freewheel diode is positioned near the IC and the output capacitor on the same surface.
・If the wiring of the node connecting the diode and the switching MOSFET is long, high-frequency spike noise induced by the wiring inductance is superposed on the output.
・A snubber circuit can be used to deal with spike noise, but it should be recognized that losses occur in the snubber circuit.
Following the discussion of input capacitors of the previous article, we explain the placement of output capacitors and freewheel diodes, components which are nearly as important.
Placement of Output Capacitors and Freewheel Diodes
Following the input capacitor, the output capacitor and the freewheel diode are positioned.
This explanation of PCB layout assumes a step-up circuit the output current of which can be up to 1 A. For an output current of this magnitude, a comparatively small capacitance suffices for the output capacitor COUT, and so a single ceramic capacitor can also be used as a high-frequency decoupling capacitor COBYPASS. This is because the frequency characteristic of ceramic capacitors is better for smaller capacitances. However, frequency characteristics of ceramic capacitors differ depending on the capacitor type and the manufacturer, and so the frequency characteristic of the component that will actually be used must be checked.
The freewheel diode D2 is positioned close to the IC and COUT on the same surface. The node connecting the diode and the switching MOSFET Q2 is a switching node, and therefore is a source of high-frequency noise. If the wiring of this node is long, high-frequency spike noise induced by the wiring inductance is superposed on VOUT. Moreover, the loop from the switching MOSFET Q2 to the freewheel diode D2 and the high-frequency decoupling/output capacitor COUT should be made as small as possible, to hold radiation of high-frequency noise to a bare minimum. These components and wiring must always be placed on the same surface. If components are placed and wired on the back surface using vias, noise is worsened due to the effects of the via conductance; vias should never be used. A satisfactory layout example that takes these considerations into account is shown on the right. The relevant layout section is indicated by the darker color.
Example of Satisfactory Layout of Output Capacitor and Freewheel Diode
As indicated above, because wiring inductance increases if the switching node is made longer, high-frequency spike noise is increased, and in most cases this exerts adverse effects. One way to alleviate this high-frequency spike noise is to add an RC snubber circuit.
The snubber circuit must be placed close to the switching MOSFET Q2 and the GND pin of the IC, and so it is recommended that lands for the resistor RS and capacitor CS of the snubber circuit be prepared in advance.
As one point to keep in mind, because losses always occur in a snubber circuit during switching operation, it should be born in mind that they detract from efficiency. In some cases a balance will have to be struck between reducing the spike noise of the switching node and circuit efficiency.
Example of Satisfactory Layout of Snubber Circuit
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This is a hand book for understanding the basics of linear regulators, such as operating principles, classification, characteristics by circuit configuration, advantages and disadvantages. In addition, typical specifications of linear regulators, efficiency and thermal calculations are also explained.
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
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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
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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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