TWHB-03e-001 ROHM罗姆公司 DC-DC降压转换器PCB布局基础手册

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TWHB-03e-001

DC/DC Buck Converter

PCB Layout Basics

Powere

Hand Book

TWHB-03e-001

DC/DC Buck Converter

PCB Layout Basics

Hand Book

－ Table of Contents －

Introduction ....................................................................................................................................... 1

1. What You Need to Know First ....................................................................................................... 1

1.1 Main Principles of PCB Layout .................................................................................... 1

1.2 Current Path of DC/DC Buck Converter in Operation ................................................. 1

1.3 Ringing at Switching Nodes ......................................................................................... 2

1.4 PCB Structure and Characteristics .............................................................................. 4

1.5 Resistance of Copper Foil ........................................................................................... 4

1.6 Inductance of Copper Foil ............................................................................................ 5

2. PCB Layout Method and Examples of DC/DC Buck Converters .................................................. 6

2.1 Input Capacitors and Diodes ....................................................................................... 6

2.2 Thermal Vias ................................................................................................................ 9

2.2.1 About Thermal Vias ..................................................................................... 9

2.2.2 Placement of Thermal Vias .......................................................................... 9

2.3 Inductors .................................................................................................................... 10

2.3.1 Inductor Characteristics Related to PCB Layout ....................................... 10

2.3.2 Placement of Inductors .............................................................................. 10

2.4 Output Capacitors ...................................................................................................... 12

2.4.1 Difference Between Output and Input Capacitor Current .......................... 12

2.4.2 Placement of the Output Capacitors .......................................................... 12

2.5 Feedback Path ........................................................................................................... 12

2.6 Ground ....................................................................................................................... 14

2.6.1 Analog Small-Signal Ground and Power Ground ...................................... 14

2.6.2 Ground Plane ............................................................................................. 15

3. Noise Countermeasures in PCB Layout ..................................................................................... 15

3.1 Corner Wiring ............................................................................................................. 15

3.2 Noise Terminal Voltage (conducted emissions) ......................................................... 16

3.3 Noise Electric Field Intensity (radiated noise) ........................................................... 16

3.4 Addition of a Snubber Circuit ..................................................................................... 16

3.5 Noise Suppression for Bootstrap Circuits .................................................................. 17

3.6 Noise Suppression of High-side MOSFET ................................................................ 17

4. Conclusion .................................................................................................................................. 18

Revision History .............................................................................................................................. 19

Notes ............................................................................................................................................... 20

- 1 -

TWHB-03e-001

DC/DC Buck Converter

PCB Layout Basics

Hand Book

Introduction

It goes without saying that circuit configuration and

component selection are important in the design of a

DC/DC converter, but the layout of the PCB (Printed

Circuit Board) is equally or even more important. Even if

the circuit drawing and component constants are correct,

if the PCB layout is not appropriate, not only will

performance not be achieved, but it will not even work. In

many cases, the PCB layout is the cause of problems

such as I made a prototype, but it doesn't work properly.

In the case of a switching power supply that has a lot of

noise but is still working, the power supply may be able to

handle the situation, but the noise it emits may cause the

S/N of the system to drop, and the system may not meet

the specifications.

Common problems caused by PCB layout include noise

at the output (including spikes and oscillations), poor

regulation, and unstable operation. These can be solved

by optimizing the PCB layout, but redoing the PCB design,

even for prototyping, is a major loss of time, money, and

effort. For a quick and reliable design, it is important to

have a high-quality PCB design from the beginning, with

the key points of PCB layout.

This document basically describes the PCB layout of a

DC/DC buck converter (switching power supply) with

diode rectification or synchronous rectification. Although

there are parts that can be shared with other methods and

topologies, the contents are specific to these buck

converter types.

1. What You Need to Know First

The key to designing a board is the layout of the

components and wiring, and there are reasons why it is

better to do so. Some of the reasons include those

specific to switching circuits, electrical characteristics of

components, and those related to the materials and

structure of the PCB. The following is an overview of the

minimum things you should know when designing a PCB.

1.1 Main Principles of PCB Layout

A list of the main principles of PCB layout is given from

the beginning. The following explanations are based on

these points. I hope you will read this book with these

points in mind.

Main principles of buck converter PCB layout

▶ Input capacitors and output diodes are placed on the

same surface as IC pins, and as close to ICs as

possible.

▶ Thermal vias are provided as necessary.

▶ Inductors minimize radiation noise from switching

nodes, and so although while not as important as

input capacitors, are arranged close to ICs.

▶ The copper layer pattern area should not be made

larger than necessary.

▶ Output capacitors are placed close to inductors.

▶ Feedback paths are wired so as to be kept far from

noise sources such as an inductors and diodes.

▶ Corner wiring is rounded.

1.2 Current Path of DC/DC Buck Converter in

Operation

A switching regulator is an analog circuit, but in contrast

with circuits that operate mainly in linear modes, current

and voltage are switched, that is, turned on and off.

Therefore, it is necessary to determine the optimal

component placement and current path by considering

which node and line will have what nature of voltage and

what kind of current will flow.

This means that the size and nature of the voltages and

currents in each part of the circuit have a great deal to do

with the PCB layout. Therefore, it is important to know the

current paths and properties of the buck converter during

operation in order to understand what needs to be

considered in the PCB layout.

Figure 1 shows a schematic diagram of a diode rectified

(asynchronous rectification) DC/DC buck converter

circuit: the capacitor on the BOOT pin is for bootstrapping

to drive the built-in Nch-MOSOFET, and the resistor and

capacitor connected to the COMP pin are for phase

compensation. These pins may not be present in some

ICs. The other pins and components are basic pins and

necessary external components.

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摘要：

本手册针对ROHM罗姆公司DC-DC降压转换器，系统梳理了PCB布局中的关键要点与基础规范。内容涵盖功率回路与信号回路的区分、输入/输出电容的放置原则、电感与反馈走线的优化策略，以及热管理与接地设计的实操建议。通过遵循本手册中的布局基线，可有效降低开关噪声、提升转换效率并确保系统稳定性。适合电源设计工程师、硬件开发人员及PCB设计初学者作为参考，帮助快速掌握工业级降压转换器的布局要求，规避常见陷阱，缩短产品开发周期。

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