how to avoid emc problems in pcb design
TRANSCRIPT
-
8/18/2019 How to avoid EMC problems in PCB design
1/16
How to Avoid EMC Problems in PCB Design?
-
8/18/2019 How to avoid EMC problems in PCB design
2/16
EMC Basics• EMC Defnition
– the ability of devices and systems to operate in theirelectromagnetic environment without impairing theirfunctions
Main elements to EMC – Emissions:
• !he EM" emissions refer to the generation of unwantedelectromagnetic energy#
• !hese need to be reduced below certain acceptablelimits to ensure they do not cause any disruption toother e$uipment#
– Susceptibility & immunity: • !he susceptibility of an item of electronics to EM" is the
way it reacts to unwanted electromagnetic energy#• !he aim of the design of the circuit is to ensure a
-
8/18/2019 How to avoid EMC problems in PCB design
3/16
Elements of Electromagneticinterference
• A generator is an E&E devicethat produces EM"
• A receptor is an E&E devicethat receives or couples inEM"
•
A coupling path allows EM"from the generator toproduce an undesiredresponse in the receptor
-
8/18/2019 How to avoid EMC problems in PCB design
4/16
!hree Main 'ays toMinimi(e EM"
• Reducing the noise rom the generator
– reducing the generator circuit area – using slower digital logic
– reducing the circuit current
– )ltering* and&or shielding#
• Altering the coupling path
– moving the receptor away from the generator
– or shielding the receptor or generator
• Improving the immunity o the receptor
– reducing the receptor circuit area
– using less susceptible electronic components
–
changing the impedance of the circuit
-
8/18/2019 How to avoid EMC problems in PCB design
5/16
+uidelines to address EMC issuesduring the PCB design
• !he following guidelines will describe how hardwareengineers can address EMC issues during the PCB design
,# +round planes
-# Component .egregation
/# Board 0ayers
1# Digital Circuits
2# Cloc3 !ermination
4# Analog Circuits
5# Decoupling Capacitors
6# Cables
7# Crosstal3
,8#.hielding
-
8/18/2019 How to avoid EMC problems in PCB design
6/16
,#+round Planes•
Ma9imi(ing the ground areaon a PCB reduces theinductance of ground in thesystem* which in turnreduces electromagnetic
emissions and crosstal3#
• A poor PCB design is onewhere components are
connected randomly toground points#
• A recommended designapproach is to have a full
ground plane because it
-
8/18/2019 How to avoid EMC problems in PCB design
7/16
-#Component .egregation
• Components on the PCB needto be grouped according totheir functionality* such asanalog* digital* power supplysections* low:speed circuits*
high:speed circuits* and so on#
• !he trac3s for each groupshould stay in their designatedarea
• ;or a signal to
-
8/18/2019 How to avoid EMC problems in PCB design
8/16
/#Board 0ayers• "f more than two layers are
used* then one completelayer should be used as aground plane#
• !he layer below the groundlayer should be used as apower plane#
•
"f a complete layer ofground is not possible* thenground grids should beused#
• "f a separate power plane isnot used then round
-
8/18/2019 How to avoid EMC problems in PCB design
9/16
1#Digital Circuits
• !races connecting cloc3 andhigh speed signals should be3ept as short as possible andbe ad=acent to the groundplane to 3eep radiation and
crosstal3 under control#
• !hese signals must also be3ept away from the powerplane since they can induce
noise on the power plane aswell#
• !he crystal should also be3ept close to the appropriatechips#
-
8/18/2019 How to avoid EMC problems in PCB design
10/16
2#Cloc3 !ermination
• !races carrying cloc3 signalsfrom a source to a devicemust have matchingterminations because
whenever there is an
impedance mismatch* a partof the signal gets reective termination*
including sourcetermination* endtermination* ac termination*etc
-
8/18/2019 How to avoid EMC problems in PCB design
11/16
4#Analog Circuits
• !races carrying analog signalsshould be 3ept away fromhigh:speed or switchingsignals and must always beguarded with a ground signal#
• A low:pass )lter shouldalways be used to get rid ofhigh:fre$uency noise coupledfrom surrounding analogtraces#
• !he ground plane of analogand digital subsystems should
not be shared#
-
8/18/2019 How to avoid EMC problems in PCB design
12/16
5#Decoupling Capacitor
• A decoupling capacitorprovides a low:impedancepath for high:fre$uencycurrent on the powerplane to ground#
• !his path should be 3eptto a minimum possiblelevel by placing a
decoupling capacitor veryclose to the "C
• Multiple capacitors withlow E.0 e$uivalent seriesinductance@ should be
-
8/18/2019 How to avoid EMC problems in PCB design
13/16
6#Cables•
Cables carrying digitalsignals act as e%cientantenna
• sing a twisted:pair cablehelps 3eep coupling to a
low level by cancelling anyinduced magnetic )elds
• 'hen a ribbon cable isused* multiple ground
return paths must beprovided
• ;or high:fre$uency signals*shielded cable must beused* where the shielding
is connected to ground
-
8/18/2019 How to avoid EMC problems in PCB design
14/16
7#Crosstal3
• Crosstal3 is a function ofmutual inductance mutual capacitance
•
Crosstal3 caused bymutual inductance istypically larger than thecrosstal3 caused bymutual capacitance#
• Mutual inductance canbe reduced byincreasing the spacingbetween the two tracesor by reducing the
-
8/18/2019 How to avoid EMC problems in PCB design
15/16
,8#.hielding
• A mechanical approach toreducing EMC
• .hield reduces the si(e of
loop antennas byabsorbing and re
-
8/18/2019 How to avoid EMC problems in PCB design
16/16
!han3 ou