PC Board Layout of Switch Mode Power Supplies
Transcript of PC Board Layout of Switch Mode Power Supplies
• Basic circuit for a ‘Buck’ Converter –
• Output Voltage of this circuit is lower than Input.
Switch Mode Power Supplies
Cin
SW Lo
CoutVin
–
+
Rload
(Continuous)
Input Loop
Power Switch Loop
(SW - Turned On)
Output Loop
(Continuous)
D
Rectifier Loop
(SW - Turned Off)
– ++ –/ /
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• In circuit on previous page –
Switch Mode Power Supplies
− 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…...
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Switch Mode Power Supplies
− 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…..
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• Basic circuit for a ‘Boost’ Converter –
• Output Voltage of this circuit is Higher than Input.
Switch Mode Power Supplies
Cin
SW
Lo
Cout
D
Vin
–
+
Power Switch Loop
(SW - Turned On)
Rload
(Continuous)
Input Loop
Output Loop
(Continuous)
Rectifier Loop
(SW - Turned Off)
–
+ –
+
/
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• In circuit on previous page –
Switch Mode Power Supplies
− 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.
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Switch Mode Power Supplies
− 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’.
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• 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.
Switch Mode Power Supplies
as with this
TI controller, used
in many hand held
devices and low
cost systems.
• ‘SW’ Loop is through the IC.
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Switch Mode Power Supplies
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• 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 –
Switch Mode Power Supplies
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.
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… 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!
Switch Mode Power Supplies
• Of the 4 loops
shown at right,
all are High
Current loops...
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Switch Mode Power Supplies
• 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
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Switch Mode Power Supplies
• 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.
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Switch Mode Power Supplies
• 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.
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Switch Mode Power Supplies
• 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.
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Switch Mode Power Supplies
• 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.
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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.
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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
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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.
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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
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Switch Mode Power Supplies
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?
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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.
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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.
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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’.
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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!!!
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• PCB Footprint for Sense Resistor Feedback –
− Usual arrangement…
Switch Mode Power Supplies
Power
TracesSense Lines
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− 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.
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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.
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Switch Mode Power Supplies• Ground Distribution in Switcher Circuits –
− 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
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Switch Mode Power Supplies• Ground Distribution in Switcher Circuits –
− 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.
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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.
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Switch Mode Power Supplies• First, let us evaluate 1 & 2 layer boards with
several examples of Routes and NO Planes-
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Yield a 60 Ohm Impedance
Yield a 70 Ohm Impedance
Yield an 80 Ohm Impedance
Yield a 93 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?
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yields a 7.65 Ohm Impedancewith Ground Plane,
− Routes, approx 200 mils wide, 35 mils apart,S G
G
yields a 7.67 Ohm Impedancewith Ground Plane,
− A Single Route, approximately 200 mils wide,S
G
yields a 14.0 Ohm Impedancewith Ground Plane,
− 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,
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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,
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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.
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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.
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Switch Mode Power Supplies• Example Layout of Switcher Circuit –
Closer look at Feedback Lines.38
Switch Mode Power Supplies
• 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!!!
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Switch Mode Power Supplies
• 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
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Switch Mode Power Supplies
• 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
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