Tips for Electronic Printed circuit Board Design

EMI/EMC problems may be approached at the component, PC board or enclosure levels.  However, it is much more efficient to deal with these problems as close to the source or susceptible victim as possible.  Therefore, it is important to consider these tips as guidelines for PCB design and layout so that problems may be identified and prevented prior to actual fabrication of the equipment.

(1) EMI controls should be applied at the circuit and box levels prior to addressing EMI at the interconnected and system levels.

(2) Digital circuits are more likely to be the source of emissions due to the handling of periodic waveforms and the fast clock/switching rates.  Analog circuits are more likely to be the susceptible victims due to higher gain functions.

(3) The source or susceptible victim of most EMI problems is typically an electronic component.  Although active components are usually the sources of EMI, passive components often contribute to it, depending on the signal frequencies and component's characteristics.   For example, an inductor can become predominantly capacitive due to the high frequency parasitic coupling between windings.  A capacitor can develop parasitic series inductance due to its internal inductance and external lead inductance at high fundamental and harmonic frequencies. 

(4) EMI problems involving an active component can be the result of the device's output transferring the emissions or its input providing the path for susceptibility.   However, at high frequencies the active component may become a direct radiator or receptor of EMI.  Also, the component’s power and ground connections can provide paths for both emissions and susceptibility.

(5) Although common mode currents are usually small compared to differential mode currents, they can be the main cause of radiated emissions.

(6) Emissions and susceptibility that are typical in single layer, free wired (using power and ground traces instead of planes) PC Board design, can be greatly improved by using multi-layer PC boards with power planes.  High capacitance between a forward signal and its return path (ground plane) provides containment of the electric field. Low inductance of the paths provides for magnetic flux cancellation.  Although not always realistic in a PCB stack-up design, a trace should be spaced one dielectric layer away from its associated return path and the voltage and ground planes should be as closely spaced as possible.

(7) PCB stack-up design is important in containing the electromagnetic fields, while providing for additional bypassing and decoupling of the power bus and minimizing bus voltage transients.  Some of the benefits of multi-layer PC board design with power planes are: 

a.      The power planes, if properly designed, will provide an image plane effect.  Since the return currents in the power planes are equal and opposite polarity to the associated signal currents, their electromagnetic fields will tend to cancel.  Power planes can also reduce the loop areas of signal and power traces, resulting in a decrease of EMI emissions and susceptibility. 

b.     A ground plane can lower the overall ground impedance, thus reducing high frequency ground bounce.  Also, the impedance between the ground and voltage planes is lowered at the high frequencies and this reduces power bus ringing. 

(8) Clocked IC’s with rapid output transitions can be very demanding on voltage and current distribution components such as the power supply, power bus, and power planes.  The inductance of the power bus can prevent the rapid energy transfer needed to meet the quick output transitions and fast rise times.  This can be improved with the placement of decoupling capacitors at the IC’s power pins.  The capacitors must be properly selected in their frequency response to deliver the energy needed at the IC’s output frequency spectrum.  However, as the number of decoupling paths increase, so do the number of voltage drops across them and this can result in power bus transients along with the associated common mode emissions.   This problem can be minimized with proper power plane design in the area of the IC’s.  The power plane acts as an effective high frequency capacitor, and consequently, as an additional energy source needed for cleaner IC outputs. 

Happy Reading

Ruby