camsemi - pcb layout guidelines

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PCB Layout Guidelines

Applicat ion Note AN-3185

AN-3185-0906© Cambridge Semiconductor Ltd 2008 30-Jun-2009

INTRODUCTION

This application note presents guidelines for creatingsuccessful PCB layouts for SMPS applications usingCamSemi controller ICs. It is relevant to both RDFC(Resonant Discontinuous Forward Converter) andPSS (Primary Side Sensing) products. Topicsaddressed include:

General good and bad design practices;

Achieving low EMI;

Thermal optimisation

Design for manufacture

Further information on CamSemi products isavailable from www.camsemi.com.

RELEVANT STANDARDS

IPC-2221A Generic Standard on PCB Design

IPC-7351A Generic Requirement for Surface MountDesign and Land Pattern Standard

IEC60617 Graphical Symbols for Diagrams

IEC60950-1 IT Equipment Safety Requirements

IEC61000 Electro Magnetic Compatibility (EMC)

Emissions and Immunity

GOOD & BAD DESIGN PRACTICES

PCB layout is usually a compromise betweenconflicting requirements for size, shape, cost andlow EMI. However there some simple, good practiceguidelines that should be kept in mind:

Good Routing

Make track widths appropriate to currentcarried

Space tracks according to voltage differencebetween them

Keep tracks as short as possible. Prioritisecritical ones: highest first (high current, highfrequency, high voltage)

Keep thin tracks away from board edge

Route tracks to centre of a connecting pad

De-Coupling

CamSemi controller IC’s are mixed signal andshould be treated as analogue IC’s during PCBlayout. Power supply decoupling capacitors shouldbe placed close to the pin they are connected to,and the routing distance should be as short aspossible.

Figure 1: Critical Current Loops in a Typical Primary Side Sensing SMPS Application Circuit

OUTPUT

SWITCH

VDD and AUXINPUT RECTIFIER AND FILTER

+ +

VDD

CS

RC

FB

ED

GND

Qsw

+

-

+

Cin1 Cin2

Rht1

Rht2

Dbase

Lfilt

CssnubRssnub

Dout

CoutRout

0V

Dbridge

Rin

Rosc

Rcs

Cdd Cosc

Rbase

Cfb

Q1

Daux

Rfb1

Rfb2

Caux

Rdd

Csw

T1

CURRENT

SENSE

CONTROLLER FEEDBACK

TRANSFORMER

IC1



Appl ication Note AN-3185

AN-3185-0906© Cambridge Semiconductor Ltd 2008 30-Jun-2009

Figure 11: Transformer to Output Diode Path

Output Snubber Components

The output snubber components (Figure 12)should be as close to the output diode as possibleto help eliminate EMI from the secondary circuit.NB These components are not always required.

Figure 12: Output Snubber Components

Magnetic CouplingMagnetic coupling occurs when a wound inductivecomponent is close enough to the transformer topick up EMI from the magnetic flux of thetransformer.

There are two key places where this is a problem:

Input inductor (Lfilt) - part of the Pi filter

Any output inductor (if fitted)

This is difficult to fix because most designs aresmall, so there is limited space available to keep

an inductor away from the transformer.Using components such as electrolytic capacitorsas a shield does not help, because any shieldused needs to be made from a ferrous material toblock the magnetic flux from the transformer.

EMI Control on RDFC Applications

Although PSS and RDFC applications workdifferently, many of the circuit elements aresimilar, so the ways of eliminating EMI are similar.The layout guidelines that apply to RDFC are:

Primary Current Path - Figure 2

Auxiliary Decoupling - Figure 6

Output Current Path - Figure 7

Input Rectifier Loops - Figure 8

VDD Decoupling - Figure 9

Q1 Collector Path - Figure 4

TX to Output Diode Path - Figure 11

Current Sense Resistors - Figure 3

Output Snubber Components - Figure 12

Also, bear in mind that magnetic coupling willapply as well.

THERMAL CONSIDERATIONS

Thermal management in enclosed power suppliescan be very challenging, particularly in low-costplug-top adapters. PCBs are generally singlesided and do not have much copper to conductheat away from hot spots. In a sealed plasticenclosure, there is no airflow so poor aircirculation. The limited options for dispersing heatfrom particularly hot components are:

Use available PCB copper

Keep components that get hot away fromother components

Three components that can get particularly hot arecovered in the following sections.

Thermal Control of Switching Transistor

The switching transistor (highlighted red in Figure13) switches through the primary winding of thetransformer. To help keep its temperature down:

Make pads as large as possible

Make connections to pads as wide aspossible to conduct heat away

These methods have little or no cost penalties. Another way to increase the copper at thetransistor is by increasing the copper thickness,but this will carry a cost penalty.

The switching transistor is close to the transformerbecause of EMC and space requirements, but it is

also a source of heat. This means a balance isrequired between too close for thermal reasonsand too far for EMC reasons; Figure 13 shows alayout that strikes a balance between the two.

It is also good practice to have some distancebetween the switching transistor and the nearestelectrolytic bulk capacitor(s).

NOTE: Using a TO92 transistor as the switchingdevice is acceptable for applications up to about4 W, above this:

Use a larger transistor package or

Use a heatsink.

Q1C2C1

D2

D3

D4 TX1

D1

D6

F L 1 a

F L 2 a

NEGPOS

L1

R1CamSemi

R13

LK1C102

D1 0 1

C103

C102

Q1C2C1

D2

D3

D4 TX1

D1

D6

F L 1 a

F L 2 a

NEGPOS

L1

R1CamSemi

R13

LK1

D1 0 1

C103

camsemi - pcb layout guidelines

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