This filter must be sufficiently large to carry the input current of the
supply, so it is a considerable task to expect it to offer 49dB of attenuation at 1MHz as well. This is just one example of how a switching voltage or current causes conducted EMI, how it is measured and the levels involved. Consequently all power supplies must be designed to attenuate or suppress these emissions to acceptable levels.
Avoiding EMI problems? All EMI generally resonates from a changing voltage or current. For EMI to be a an issue there must be three things present in a system
1. A source or transmitter, 2. A receiver, and 3. A medium to radiate or conductive medium.
A source can be considered as a component where there are changing
currents (di/dt’s) or voltages (dv/dt’s) such as switching FETs and rectifiers. The receiver or is usually the input mains while the transmission media is capacitive or inductive coupling. To avoid EMI problems we need only eliminate one of these.
In short we need only consider how dV/dT and dI/dT couple their energy into other circuits. If we can eliminate this, then we eliminate the emissions. Only one third of the components affecting EMI are on the schematic and bill of materials. Another third are parasitic components within the components you build or buy. The PCB track routing, component mounting, placement and even orientation create the final third. Some EMI sources are often overlooked as trivial but every pF and nH can cause a substantial EMI problem.
Of course the most obvious solution to the problem of conducted EMI is not to generate any in the first place. Therefore it is best to consider EMI in the design phase. Some useful methods for reducing EMI by design are: Use of ground planes - Large copper areas of the PCB may be used as
ground planes. For a monopole magnetic source the intensity of the magnetic field is proportional to the distance from the source squared. A ground plane has the effect of producing an image current resulting in a dipole magnetic field whose intensity is proportional to the distance cubed. As an aside additional ground planes also have advantage of thermal heat spreading and PCB stiffening. Copper areas to heatsink diodes - With axial diodes, often the only way to apply heat sinking is to use large copper areas of the PCB. Di/dt’s in diodes as they turn on and off can have an impact on emissions. If you are using this approach ensure that the heat sinking copper is on the DC or ‘quiet’ side of the diode. Ground choke cores - For first order filters the attenuation would roll
off at 40dB per decade starting at a frequency of (2πLC)-1. For the ideal filter this attenuation is unchanged. An actual filter has many parasitic components, which reduce the overall effectiveness of the filter. The effect of the ESR of Cf can be reduced by using several smaller values capacitors in parallel ensuring that the legs are kept as short as possible. To reduce the effect of the end to end capacitance of the inductor Lf the core should be connected to the filter common or AC quiet rail. Faraday shields - When using Faraday shields, always connect the shield to the primary or secondary circuits - do not connect the shield to the chassis or earth. Also the tap from the shield should be made at the center and not at either end. Shield resonance can be damped using a resistor RD or a ferrite bead. The shield should form full coverage around the winding with a non-shorting turn and be made from relatively thin (0.003”) material.
Points to note for a systems designer
In the same way that the power supply designer needs to ensure that the sources for radiation are kept to a minimum, the system designer also has a role to play in ensuring that the overall system has the optimum performance. Compliance of the power supply in a test house to various limits does not necessarily guarantee the user that it will pass when installed into a system.
Cabling arrangements and PCB tracking layouts are the greatest contributing factor to system EMC performance. It is important that PCB tracks and power cables are arranged to minimize current carrying loops that can radiate, and to minimize loops that could have noise currents induced into them. All cables and PCB tracks should be treated as radiation sources and antennae and every effort should be made to minimize their interaction.
Many consider Electro Magnetic Interference a black art, but the truth of the matter is that with sufficient knowledge, due diligence and attention to detail you can design your power supply to have the optimum performance. Trying to address this issue at the back end of a design process can be costly, time consuming, and end up having to use relatively large filters. Power supplies by their nature are noisy environments, but taking every aspect of the design into consideration a market leading performance is achievable. Reduce your common mode impedances, know your components, and most of all layout your PCB correctly at the beginning of your design cycle. This will minimize your troubleshooting EMI problems later. Above all remember that EMI design is the sum of many small items, and only when all of these are addressed will you see improvements in the emissions.
Excelsys Technologies |
www.excelsys.com
Shane Callanan is Director of Applications Engineering at Excelsys Technologies
April 2012 CIE Power Supplement 11
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