BYE-BYE TO EMI
Switchers say bye-bye to EMI EMI can emanate from high di/dt loops found in some switch-mode supply topologies. New controllers
TONY ARMSTRONG | POWER BY LINEAR ANALOG DEVICES INC.
overcome these difficulties by integrating key components inside the chip package.
EMI generation in buck converters
IT
goes without saying that PCB layout sets functional, electromagnetic interference (EMI) and thermal behavior of every power supply design. Switching power supply layout is not black magic, but it is often overlooked until late in the design process. Many switchmode power supply designers are familiar with the design complexities and nuances of switch mode operation. But a lot of these old hands are literally retiring and leaving the industry! Consequently, more and more digital designers are being asked to take on switch-mode supply designs for no other reason than too few analog power supply designers to get the job done. Most digital designers know how to design with a simple linear regulator; it is less clear they are equipped to handle more complex designs such as step-up mode (boost) or even a buckboost topology (buck and boost modes combined). This leaves many electronic systems manufacturers wondering how their switch-mode supply circuits will get done. Companies that make ICs for switch-mode supplies are aware of this brain drain. So they are devising chips that help remove some of the complexity involved in the design of switch-mode circuitry. To understand these developments, consider the example of the basic buck regulator as diagramed in the nearby schematic. High di/ dt and parasitic inductance in the switcher “hot” loop causes electromagnetic noise and switch ringing. EMI emanates from the high di/dt loops. The supply wire as well as the load wire should not have high ac current content. Accordingly, the input capacitor C2 should source all the relevant ac currents to the output capacitor where any ac currents end.
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ADI — Power Electronics HB 02-19.indd 11
A simplified synchronous buck regulator and the high di/dt loops its topology creates.
During the on cycle with M1 closed and M2 open, the ac current follows the solid blue loop. During the off cycle, with M1 open and M2 closed, the ac current follows the green dotted loop. Most people have difficulty grasping that the loop producing the highest EMI is not the solid blue nor the dotted green. Only in the dotted red loop flows a fully switched ac current, switched from the zero to I peak and back to zero. The dotted red loop is commonly referred to as a hot loop because it has the highest ac and EMI energy. It is the high di/dt and parasitic inductance in the switcher hot loop that causes electromagnetic noise and switch ringing. To reduce EMI and improve performance, one must minimize the radiating effect of the dotted red loop. If we could reduce the PCB area of the dotted red loop to zero and buy an ideal capacitor with zero impedance, the problem would be solved. However, in the real world, it is the design engineer who must find an optimal compromise.
2 • 2019
DESIGN WORLD — EE NETWORK
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2/19/19 3:27 PM
