As an electrical equipment manufacturer, you must consider many factors when looking at the EMC aspects of a design, and it’s easy to overlook an important point. For that reason, follow the below 10- point basic checklist to assess your product design for EMC compliance.
Product design for EMC compliance checklist
#1. Design your product for EMC compliance from the beginning by knowing what performance requirements the product must fulfil. To do so, check the relevant EMC standards.
#2. Divide the system into critical and non-critical sections to:
- determine the circuits which will be noisy or susceptible and those which will not
- lay the circuits out in separate areas as far as possible
- select internal and external interface locations to allow optimum common-mode current control.
#3. Select electrical components with EMC in mind:
- use slow and/or high-immunity logic
- use good power decoupling techniques
- maximise the dynamic range of analogue signal paths
- minimise analogue signal bandwidths
- use series R buffering on all high-speed clock and data lines
- reduce fan-out on clock circuits by liberal use of buffers
- use series ferrite chips in the supplies in order to create power segments
- check stability in wideband amplifiers
- include resistive, capacitive or ferrite filtering at all sensitive analogue inputs
- don’t leave unused IC input pins floating
- if possible, avoid edge-triggered digital inputs
- incorporate a watchdog circuit on every microprocessor.
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#4. Cables
- isolate or avoid parallel runs of signal and power cables
- choose RF-screened cables if the signal cannot be adequately filtered
- use a twisted pair both outside and within an enclosure to ensure balanced or high DI/DT lines
- use properly designed ribbon, looms or flexi for internal wiring
- apply ferrite suppressors to damp resonances and control common mode currents
- place cables away from apertures in the shielding
- terminate lines that carry high-frequency signals by using the correct transmission line impedance.
#5. PCB layout (before routing begins)
- identify and label the high DI/DT circuits and sensitive circuits
- identify 0V plane(s), power plane segments, power plane layer(s), 0V plane layers, interface ground plane, constant impedance layer(s)
- decide on layer stack-up
- identify points for bonding the chassis to the ground plane(s)
- ensure that ground pins are distributed along multi-way connectors close to sensitive or high-speed signals.
#6. PCB layout (during routing)
- ensure no tracks cross any breaks in the 0V plane
- identify and control any common impedance current paths for sensitive wideband circuits and power switching circuits
- flag any breaks or gaps in a 0V plane
- ensure that critical and constant-impedance tracks don’t swap layers
- minimise surface areas of nodes with high DV/DT
- ensure balanced differential signal track paths by confirming that adequate balance is maintained along the entire run
- ensure that the decoupling capacitors are adequately placed.
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#7. Grounding:
- design the ground system at the product definition stage and consider it as a return current path
- ensure metal-to-metal bonding of filters, screens, connectors and enclosure panels
- keep earth straps short
- ensure that bonding methods won’t deteriorate in adverse environments
- apply a conductive finish to contact surfaces
- provide an interface ground area for filtering and decoupling
- avoid common ground impedances for different circuits.
#8. Shielding:
- design all metallic structures
- if necessary, design plastic enclosures to allow internal conductive coating
- avoid large apertures in a shield
- if the large apertures are unavoidable, take measures to mitigate them
- consider segregated enclosures
- avoid dipole-like structures in a metallic enclosure
- implement DC or RF tie points between the shield and circuit 0V
- use multiple internal tie-points in order to minimise box resonances
- use conductive gaskets to ensure that separate panels are well bonded along their seams.
#9. Filters:
- design the supply filter for the application
- filter all I/O lines, using common mode chokes and three-terminal capacitors to interface ground
- ensure a specified ground return for each filter
- locate all filter components and associated tracks and wiring adjacent to the interface being filtered
- apply filtering to interference sources directly at their terminals.
#10. Test and evaluate for EMC compliance continuously as the design progresses.
