How to address DC/DC noise, efficiency, and layout issues using integrated power modules
To overcome this, experienced pc board designers will make the regulator’s current loops (“hot loops”) small, and use shielding ground layers as close to the active layer as possible. Nevertheless, pinout, package construction, thermal design requirements, and package sizes needed for adequate energy storage in decoupling components dictate a certain minimum hot-loop size. To make the layout problem even more challenging, the typical planar pc board has magnetic or transformer-style coupling between traces above 30 megahertz (MHz). This coupling will attenuate the filtering efforts since the higher the harmonic frequencies, the more effective unwanted magnetic coupling becomes.
These standards are complex and define the test procedures, probes, instrumentation, data analysis, and more. Among the many limits defined by the standard, the Class B radiated emission limit is often of most interest to designers. Complete: Even when the design situation is fairly well understood, selecting and employing the needed support components in just the right way is a challenge. Slight differences in component placement and specifications, pc board grounds and traces, and other factors can adversely affect performance. Modeling and simulation are necessary and can help, but it’s very difficult to characterize the parasitics associated with these components, especially if their values shift. Further, a change in vendors (or unannounced change by the preferred vendor) may induce a subtle shift in second- or third-tier parameter values (such as inductor dc resistance (DCR)),
which could have significant and unanticipated consequences. Further, even slight repositioning of the passive components or adding “just one more”, can change the EMI scenario and result in emissions exceeding allowable limits. SilentSwitcher µModules resolve the issues Anticipating and managing risk is a normal part of a designer’s job. Reducing the number and intensity of these risks is a standard end- product strategy. A solution is to use a functionally complete DC/ DC regulator that, through good design and implementation, is cool, quiet, and complete. Using a known device reduces uncertainty while addressing size, cost, EMI, BOM, and assembly risks. Doing so also accelerates time to market and reduces regulatory compliance angst.
By looking at a complete family of such regulators, such as the Silent Switcher µModules from Analog Devices, designers can choose a DC/DC regulator matched to the needed voltage and current rating, while being assured that EMI mandates will be met, size and cost will be known, and there will be no surprises. These regulators incorporate much more than innovative schematics and topologies. Among the techniques they use are: ■ Technique #1: The switching of the regulator acts as an RF oscillator/source and combines with the bond wires, which act as antennas. This turns the assembly into an RF transmitter with undesired energy that may exceed allowed limits ( Figures 3, 4, and 5 ). ■ Technique #2: The use of symmetrical input capacitors bounds EMI by creating balanced, opposing currents ( Figure 6 ). ■ Technique #3. Finally, the use of opposite current loops to cancel magnetic fields ( Figure 7 ).
Figure 4: The Silent Switcher assembly begins by replacing the wire bonds with flipchip technology, thus eliminating the energy-radiating wires. (Image source: Analog Devices)
Which standards are relevant?
Figure 5: The flipchip approach effectively eliminates the antennas and minimizes radiated energy. (Image source: Analog Devices)
There is no single guiding standard in the EMI world, as it is largely determined by the application and relevant governing mandates. Among the most cited ones are EN55022, CISPR 22, and CISPR 25. EN 55022 is a modified derivative of CISPR 22 and applies to information technology equipment. The standard is produced by CENELEC, the European Committee for Electrotechnical Standardization, and is responsible for standardization in the electrotechnical engineering field.
Figure 6: Dual, mirrored input capacitors are also added to constrain EMI. (Image source: Analog Devices)
Figure 3: The bond wires from the IC die to the package function as miniature antennas and radiate undesired RF energy. (Image source: Analog Devices)
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