PC Board Layout of Switch Mode Power Supplies

_______________________

[email protected]

PC Board Layout of

Switch Mode Power Supplies

• Basic circuit for a ‘Buck’ Converter –

• Output Voltage of this circuit is lower than Input.


Cin

SW Lo

CoutVin

–

+

Rload

(Continuous)

Input Loop

Power Switch Loop

(SW - Turned On)

Output Loop

(Continuous)

D

Rectifier Loop

(SW - Turned Off)

– ++ –/ /

2

• In circuit on previous page –


− Input Voltage Source (ie: +15V) will…..

− supplies current through ‘SW’ and Lo, (Power

Switch Loop) charging ‘Cout’. Voltage across

Cout rises, providing Output to Loads, until….

− output reaches predetermined voltage, slightly

higher than the expected level (ie- 3.3V)….

− at which time, energy in Feedback Loop (not

shown) triggers Controller IC (not shown) to

Turn SW to Off State.

− create continuous, mostly DC current, in Input

Loop, charging ‘Cin’, which…...

3


− Once SW in Off State, though input current is

removed, Inertia of Lo keeps current flowing,

in ‘Rectifier Loop’ …..

− causing Power Switch Loop to charge Cout

until output again reaches level just above

desired voltage (ie- 3.3V)….

− for brief period (ie: 1 mSec), until Output Volt-

age drops slightly below Desired Level (3.3V).

− At which time, Feedback Loop triggers Con-

troller IC to Turn SW back to On State……

− etc, etc, etc…..

4

• Basic circuit for a ‘Boost’ Converter –

• Output Voltage of this circuit is Higher than Input.


Cin

SW

Lo

Cout

D

Vin

–

+

Power Switch Loop

(SW - Turned On)

Rload

(Continuous)

Input Loop

Output Loop

(Continuous)

Rectifier Loop

(SW - Turned Off)

–

+ –

+

/

5

• In circuit on previous page –


− Input Voltage Source (ie: 3.3V) will…..

− supplies current through ‘SW’ and ‘Lo’, (Power

Switch Loop) storing energy in Lo in form of a

Magnetic Field. Polarity at top of Lo is +.

− This action makes ‘Lo’ a source of Current,

flowing through ‘D’, toward the Load, which…

− create continuous, mostly DC current, in Input

Loop, charging ‘Cin’, which…...

− When ‘SW’ opens (turns off), Magnetic Field in

‘Lo’ creates Back EMF, reversing ‘Lo’ polarity.

6


− places ‘Lo’ in series with the Input source,

boosting the voltage at ‘Cout’.

− Output voltage is controlled by Duty Cycle of

‘SW’, which is controlled by IC and Feedback

Loop, making layout of this loop very critical.

− Once voltage at ‘Cout’ reaches desired level,

Feedback Loop (not shown) triggers controller

IC (not shown) to turn on ‘SW.

− etc, etc, etc…..

…. allowing

voltage at ‘Cout’ to drop slightly, also restoring

energy (magnetic field) in ‘Lo’.

7

• Some Low Current SMPS circuits will have the

FET (‘SW’) inside Controller IC…

• TI wisely placed ‘GND’ pin between ‘SW’ and

Feedback (‘FB’) pins, of 5 pin package.


as with this

TI controller, used

in many hand held

devices and low

cost systems.

• ‘SW’ Loop is through the IC.

8


9

• Many of today’s Switch Mode Controllers

have the FET (SW) inside the IC package.

• This is a Great Idea…. IF –

− IC package is designed to have Low Inductance.

− This requires proper placement of IC Pins… and

− Proper design of the IC Substrate… and

− Proper attachment of the Die to the Substrate.

• FET and associated Parts have Very High

Current, at Very High Frequencies.

• Poorly designed ICs will cause EMI issues!!!

• Circuit for ‘Transformer Isolated’ Converter –

• With this circuit, output polarity can be + or –


Cin

SW

To CoutD

Vin

–

+

Power Switch Loop

(SW - Turned On)

Rectifier Loop

(SW - Turned Off)

Rload

(Continuous)

Input Loop

Output Loop

(Continuous)

• Output voltage can step up or step down.

• Grounds are isolated, Input to Output.

10

… making them all important, in terms of how

they are laid out on the PC Board.

• Most important of the 4 are the ‘Power Switch

Loop’ and the ‘Rectifier Loop’, because ….

• These 2 loops are also very High Frequency!


• Of the 4 loops

shown at right,

all are High

Current loops...

11


• AC Waveforms of ‘Power’ and ‘Rectifier’ Loops-

− Discontinuous Mode –

Current in ‘SW’ Loop

Current in ‘D’ Loop

Rising & Falling edges will

Never be ‘zero’ time, as shown

… but will be extremely fast!

− Continuous Mode –

Current in ‘SW’ Loop

Current in ‘D’ Loop

12


• Switch mode power supplies are typically

triggered at frequencies from 100’s of KHz

to several MHz.

• This is due to the high harmonics embedded

in the rapidly changing pulses.

• From an EMI perspective, the frequency of

greatest concern is not a function of the

trigger rate (clock), rather a function of the

rising and falling edge rate of the waveform

for the SW and D loops.

13


• Due to the low bit rate (long time period) of

the sawtooth waveform of a switcher AND

due to the extremely short rise time of the

fastest edge –

− Unlike a typical digital square wave (where

rise time is matched to the clock), yielding a

knee frequency at the 5th or 7th harmonic….

− Switch Mode Power Supplies typically have

‘Knee’ frequencies which extend to 100 times,

or more, of the switching frequency.

14


• This adds yet another complication during

PC board layout, in addition to the mere fact

of making the circuit function properly.

• This fact will often create an EMI signature

for a switcher circuit that extends well into

the hundreds of MHz… even with switching

rates from 100’s of KHz to Low MHz.

• Board layout is the most critical element in

the battle to make a switch mode supply

Work as Intended and to Control EMI.

15


• Layout of Switcher Circuits –

− Always start with High Current, High freq-

uency circuits…

− Next, focus on the Feedback Loop and the

Output Loop.

the Power Switch Loop and

the Rectifier Loop.

− Then make sure the Input Loop is tight.

− Finish layout with lower level control circuits.

− Due to Fast Rise Time Current Pulses, these

2 loops are extremely rich in high frequency

energy, needing Extraordinary attention.

16

Switch Mode Power Supplies• Layout of Switcher Circuits –

− Power Switch Loop and Rectifier Loop must

be laid out to Place components physically

close together, keeping Loops Tight.

− Make Traces as Fat as reasonably possible,

keeping Manufacturabilty in mind…

(Keeping Loops Tight and Traces Fat means

they will likely Radiate Less Energy!)

i.e.- make certain traces are not so Fat as to

make solderability very challenging!

− Make Traces as Short as reasonably possible.

17

Switch Mode Power Supplies• Fairchild Semiconductor- High Frequency Loops –

Source: Fairchild

Semiconductor

If a Ground

Plane is pre-

sent inside

the board

stack, does

this Wide

Ground

Route offer

any Value?

Are

These

Traces

Too

Wide?

What about

Feedback?

Fee

db

ack

to

IC

18

Switch Mode Power Supplies• Layout of Parallel Capacitors –

− Paralleling 2 identical capacitors, on the Input

or Output Loop, Doubles Capacitance and

Lowers Equiv Series Resistance (ESR) and

Equiv Series Inductance (ESL).

− Parallel Caps can ‘Source’ and ‘Sink’ higher

levels of Ripple with less overall heating.

− To function as intended, Parallel Caps must

equally ‘Share’ the high frequency energy.

− Proper Layout of the Caps, relative to source

of energy, plays a Major Role toward having

the Caps Share as intended.

19

Switch Mode Power Supplies• Layout of Parallel Capacitors –

− Proper layout dictates that the two leads of

each Parallel Cap be Positioned Symmetrically.

− Never like this –

− Or even this –

Source:

Fairchild Semiconductor

20


Source: Fairchild Semiconductor

• Layout of Parallel Capacitors –

− Rather like this – Balancing the Caps

at source and return

side allows them to

function equally

with high frequency

ripple current.

Is balance needed

if Caps are diff-

erent physical

sizes or different

values?

21

Switch Mode Power Supplies• Layout of Feedback Lines –

− There are fundamentally two types of closed

loop control (feedback) for Switchers.

− One is to feed a portion of the output voltage

back to the controller… as voltage goes too

high, IC turns Off SW…. as voltage drops

too low, IC turns SW back on.

− Second is to ‘Sense’ Output Current and use

that to control the ‘On/Off’ of SW.

− Both need Careful Routing of feedback lines.

22

Switch Mode Power Supplies• Layout of Feedback Lines –

‘Current Sense’

Resistor, to feed

small voltage,

representing

Output Current,

back to controller.

Voltage Divider,

to feed portion

of Output to IC.

Route as Diff pair, except

when lines are very short.

Route as Diff pair, if PCB

does Not have Ground Plane

on next layer AND Routes

are Very Long.

23

Switch Mode Power Supplies• Layout of Voltage Divider Feedback –

− The Voltage Divider Resistors are high value

to avoid loading the Output.

− In PCB w/ Gnd Plane, only Route as a Pseudo-

Diff Pair if All these conditions exist –

- The line to Controller IC is Very Long.

− Also Route as Diff Pair in board w/o planes.

− When conditions above exist, consider Cap-

acitor on lines, at Controller Pins.

- The line is routed on an Outer Layer.

- The Layer next to the line is Not ‘Ground’.

24

Switch Mode Power Supplies• Layout of Sense Resistor Feedback –

− Resistor is Low value to avoid loading Output.

− Sense Lines should Route as Differential Pair

if Long enough to route in Diff format.

− This is the case whether or not Board has

planes or where planes reside in the board.

− Diff Routing helps ensure that any noise

coupled to lines will couple equally to Both.

− If TOO short to route as Diff Pair…. Don’t!!!

− Length Matching is NOT required!!!

25

• PCB Footprint for Sense Resistor Feedback –

− Usual arrangement…


Power

TracesSense Lines

26

− Some Technical Papers Say- Resistance of solder

joints can cause errors in ‘sense’ reading.

− To resolve, some engineers use a ‘4 Terminal’

Resistor, designed for sensing applications. (Can

be Expensive)

− Same result- 2 Term Resistor w/ 4 Pad Footprint.

− Is this REALLY a problem???

Switch Mode Power Supplies• Position of Feedback Resistors –

− When possible, place Feedback Resistor(s)

close to BOTH the Output Capacitors and the

Feedback Input of Controller IC.

− Close proximity of Feedback Resistors also

helps to keep Noise close to Zero.

− Low Noise in Feedback Loop is essential for

proper operation of Switcher Circuit and

proper control of Output Voltage.

− This is an SI, Not an EMI issue.

27

Switch Mode Power Supplies• Ground Distribution in Switcher Circuits –

− On 1 and 2 Layer Boards, without Ground Plane –

− On Multi-Layer boards with Ground Plane(s) –

- Careful distribution of Ground Traces is needed,

to Control Paths of Return Currents.

- Concept is to prevent Common Impedance paths

in ‘Return’ Lines, helping to prevent energy

coupling that leads to ‘Interference’.

- Special distribution of Ground is NOT needed.

- IF components are placed as stated on previous

slides, splitting Ground planes offers NO benefit.

- Splitting Ground planes usually Cause Problems.

28


− On 1 and 2 Layer Boards, without Planes, Ground

distribution should follow this schematic.

− Concept is to direct the currents when Fields do

not have a Very Low Impedance Path, as they

would with Ground Plane on Next Layer.

Cin

SW Lo

CoutVin

–

+

Load

D

Controller

P A

GND

FeedbackGround

Input and SW Ground Output & Rectifier Ground

FB

29


− On 1 and 2 Layer Boards (as seen in schematic) –

- Structure Ground for ‘Switch’ and ‘Rectifier’

Loops, as High Current paths (Wide Traces),

Routed VERY close to Signal Paths.

- As much as possible, keep Input and Output

Ground separate from ‘SW’ and ‘D’ Ground.

- If Controller has ‘A Gnd’ pin, connect ‘SW’ and

‘D’ Ground to all Gnd Pins, EXCEPT ‘A Gnd’.

- Route Feedback as Diff Pair, having Signal in

Parallel with the Ground side of FB Resistor.

Connect Ground Line to ‘A Gnd’ at IC.

30

Switch Mode Power Supplies• Ground Distribution in Board without Plane(s) –

− How do things differ if board has Ground Plane(s)?

- Lower Line Impedance… Lowering Volume of Fields…

which Lowers EMI.

− Wide ‘Gnd’ Trace,

placed Close to for-

ward Traces will –

- Minimize Inter-

ference between

Return Currents in

Various Loops.

- Lower Voltage Drops in Loops… which Lowers EMI.

Gnd to IC

- Remember… have Separate Return Line for Feedback.

31

Switch Mode Power Supplies• First, let us evaluate 1 & 2 layer boards with

several examples of Routes and NO Planes-

32

Yield a 60 Ohm Impedance


Yield an 80 Ohm Impedance


Source: Polar Si9000 Field Solver

− Routes (previous slide), approx 200 mils wide,

35 mils apart-

S G

− Same Routes, approx 200 mils wide, 70 mils

apart-S G

− Routes, approx 100 mils wide, 35 mils apart-S G

− Routes, approx 100 mils wide, 70 mils apart-S G

Switch Mode Power Supplies• With Plane, 10 mils down, on Layer 2 of PCB,

does wide Ground Route offer any value?

33

yields a 7.65 Ohm Impedancewith Ground Plane,

− Routes, approx 200 mils wide, 35 mils apart,S G

G


− A Single Route, approximately 200 mils wide,S

G


− A Single Route, approximately 100 mils wide,S

G

Source: Polar Si9000 Field Solver

• As we know, with routes on layer 1 and Ground on

layer 2,

34

Noise, EMI & SI- Switch Mode Design

Layer 1(S)

Layer 2(G)

• Add a Component in Series, it becomes part of the

Transmission Line

Fields travel through the L1 to L2 Dielectric.

and part of the Path for the Fields.

• If we add Poured Copper under the Component, W/O

Ground Vias,

Layer 1(S)

Layer 2(G)Copper Pour

the path of the Fields is Disrupted!!!

• Be careful about Pouring Copper under Large Parts,

except for ICs (Always Use Return Vias).

Switch Mode Power Supplies• With a Ground Plane in PCB,

35

should we open

the plane under the Switch Node?

• Many IC company

App Notes state,

‘Plane should be Open

under the Switch Node’…

Why?

• Reasons given - “Don’t Place ‘Noisy’ Ground

under critical nodes”.

• With ‘Proper’ Transmission Line design, there are

NO ‘Noisy Grounds’. Do Not Open Ground!!!

Switch Mode Power Supplies• Of all the App Notes examined, found One (TI) where tests

were run to examine EMI vs Different Layout concepts.

36

Switch Mode Power Supplies• Example Layout of Switcher Circuit –

Ground Planes not shown for clarity.

− Where is ‘D’?

− What is Q9?

− Input Loop?

− Output Loop?

− Power Loops?

− Feedback?

− Notice how

components

wrap around

controller IC.

37

Switch Mode Power Supplies• Example Layout of Switcher Circuit –

Closer look at Feedback Lines.38


• Other Methods to Control EMI (*) –

SW1

D1

C1R1− Add Snubber

Network

across ‘SW1’

R4

R3

C2

− Add Snubber Network to ‘D1’

− Add Series Resistor

in Gate of ‘SW1’

R2

(*) These methods will certainly Lower the

EMI signature of the circuit… but will also

reduce efficiency of the Power Supply!!!

39


• Other Methods to Control EMI –

− Install Inductor (L) to place ‘Start Winding

Terminal’ at input side of circuit.

− Much Larger Magnetic Field is generated by

Inductor if mounted with ‘Start Winding’ at

Output side of circuit.

Source: TDK of America

40


• Other Methods to Control EMI –

− Use Lowest Height Inductor that will perform

as needed.

− Tall Inductors = Much Larger Magnetic Field!

- Inductors under 4.5mm Height are Best!.

− Worst Choice, by far, are

Stick Inductors

41

Documents

PC Board Layout of Switch Mode Power Supplies