FEAT RE FEA ATURE
COVER
COVER STORY
MINIMISING EMI IN INDUSTR LAI ENVIRONMENTS
MINIMISING EMI IN INDUSTRIAL ENVIRONMENTS
manager, power management products at Linear Technology del vering the desired eff
Tony Arm trong, director of productm rketing, pow r products and Christian Kück, strategic marketing owerm nagement products at Linear Technology Corporation explore how a new regulator is ed boxwithout using one to solve com n EM sues
Tony Armstrong, director of product marketing, pow nager, pow
ffects of a shi ower products an ristian ck, strategicm rketin gy Corporation explore how owa newregul tor i delivering the desired effects of a shielded box without usi g one to solve common EMI issues
ADDRESSING SW CHING REGULAT NOISE EMISSIONS
ADDRESSING SWIITCHING REGULA O R NOISE EMISSIONS
ATOR
In an industrial environment, switching regulators usually replace linear regulators in areas where low heat dissipation and efficiency are valued. Moreover, the switching regulator is typically the first active component on the input power bus line, and therefore has a significant impact on the EMI performance of the complete converter circuit.
There are two types of EMI emissions : conducted and radiated. Conducted emissions ride on the wires and traces that connect up to a product. Since the noise is localised to a specific terminal or connector in the design, compliance with conducted emissions requirements can often be assured relatively early in the development process with a good layout or filter design as already stated. Radiated emissions, however, are another story. Everything on the board that carries current radiates a n
C
omponent selection and circuit board layout play a significant role in
determining the success or failure of virtually all power supplies. These aspects set their functional, electromagnetic interference (EMI) and thermal behaviour. For the uninitiated, switching power supply layoutmay seemlike a “black” art, but it is infact a basic aspect of a design, which is often overlooked in the early stages of the process.
Since functional EMI requirements always have to bemet, what is good for functional stability of the power supply is also usually good for its EMI emissions too. Furthermore, good layout fromthe beginning does not add any cost to the design, but it can actually provide cost savings by eliminating the need for EMI filters,mechanical shielding, EMI test time and numerous boar d revisions. Moreover, the potential problemfor interference and noise can be exasperated whenmultiple DC/DC switchmode regulators are paralleled for current sharing and higher output power. If all are operating (switching) at a similar frequency, the combined energy
12 OC OB 12 OCTOBER 2016 2016 || ELEC RO ELECTRONICS CS
generated bymultiple regulators in a circuit is then concentrated at one frequency. Presence of this energy can become a concern especially if the rest of ICs on the printed circuit boards (PCBs), as well as other systemboards are close to each other and susceptible to this radiated energy.
This can be particularly troubling in industrial automation systems which are densely populated and are often in close proximity to electric noise generating sources, such asmechanically switched inductive loads, PWMdrive power outputs,microprocessor clocks and contact switching.
Figure 1:
The LT8614 Silent Switcher minimises C emissions
Figure 1: he LT
LT8614 Silent
Switcher minimises EMI/EMC emissions while delivering high
while delivering high efficiency at frequencies
efficiency at frequencies up to 3MHzup to 3MHz
Figure 1:
electromagnetic field. Every trace on the board is an antenna and every copper plane is a resonator. Anything, other than a pure sine wave or DC voltage, generates noise all over the signal spectrum. Even with careful design, a power supply designer never really knows how bad the radiated emissions are going to be until the systemgets tested. And radiated emissions testing cannot be formally performed until the design is essentially complete .
Filters are often used to reduce EMI by attenuating the strength at a certain frequency or over a range of frequencies. A portion of this energy that travels through space (radiated) is attenuated by addingmetallic andmagnetic shields. The part that rides on PCB traces (conducted) is tamed by adding ferrite beads and other filters. EMI cannot be eliminated but can be attenuated to a level that is acceptable by other communication and digital components Moreover severa l regulatory bodies enforce standards to ensure compliance.
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Modern input filter components in surfacemount technology have better performance than through-hole parts. However, this improvement is outpaced by the increase in operating switching
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