How to address DC/DC noise, efficiency, and layout issues using integrated power modules
Quiet: There are two broad classes of noise that concern designers. First, the noise and ripple on the output of the DC/DC regulator must be low enough so that it does not adversely affect system performance. This is an increasing concern as rail voltages drop to low single digits in digital circuits, as well as for precision analog circuits where ripple of even a few millivolts can degrade performance. The other major concern is related to EMI. There are two types of EMI emissions: conducted and radiated. Conducted emissions ride on the wires and traces that connect to a product. Since the noise is localized 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 and filter design. Radiated emissions, however, are more complicated. Every conductor on a circuit board that carries current radiates an electromagnetic
increases real estate, makes thermal management and testing more difficult, and introduces additional assembly costs. Another technique is to slow down the switching edges of the regulator. However, this has the undesired effect of reducing the efficiency, increasing minimum on and off times as well as the required dead times, and compromising the current-control-loop speed. Still another approach is to adjust the regulator design to radiate less EMI by careful selection of the key design parameters. The task of balancing these regulator tradeoffs involves assessing the interaction of parameters such as switching frequency, footprint, efficiency, and resultant EMI. For example, a lower switching frequency generally reduces switch loss and EMI and improves efficiency, but requires larger components with associated increases in footprint. The quest for greater efficiency is accompanied by low minimum on and off times, resulting in higher harmonic content due to the faster switch transitions. In general, with every doubling of switching frequency, the EMI becomes 6 decibels (dB) worse, assuming all other parameters such as switch capacity and transition times, remain constant. The wideband EMI behaves like a first-order high-pass filter with 20 dB higher emissions
field: every board trace is an antenna, and every copper plane is a mirror. Anything other than a pure sine wave or DC voltage generates a wide signal spectrum. The difficulty is that even with careful design, a designer never really knows how bad the radiated emissions are going to be until the system gets tested, and radiated emissions testing cannot be formally performed until the design is essentially complete. Filters are used to reduce EMI by attenuating the levels at specific frequencies or over a range of frequencies using various techniques. Some of the energy radiating through space is attenuated by using sheet metal as a magnetic shield. The lower frequency part that rides on pc board traces (conducted) is controlled using ferrite beads and other filters. Shielding works but brings a new set of problems. It must be well-designed with good electromagnetic integrity (often surprisingly difficult). It adds cost,
passive components to complete the job. The task is made even easier as the datasheet for the regulator IC almost always shows a typical application circuit with a schematic, a board layout, and a BOM that may provide component vendor names and part numbers. The engineering dilemma is that a “good” level of performance may not be adequate with respect to some non-obvious regulator performance parameters. While the output DC rail may deliver enough current with adequate line/load regulation and transient response, those factors are only the beginning of the story for power rails. The reality is that in addition to those basic performance criteria, a regulator is also assessed by other factors, some of which are driven by external imperatives. The three critical issues which most regulators must address are not necessarily apparent, solely from the simplistic perspective of a functional block that accepts an unregulated DC input and provides a regulated DC output. They are ( Figure 2 ): ■ Cool: High efficiency and associated minimal thermal impact. ■ Quiet: Low ripple for error-free system performance, plus low EMI to meet radiated noise standards (non-acoustic).
■ Complete: An integrated solution that minimizes size, risk, BOM, time to market, and other “soft” concerns. Addressing these issues brings a set of challenges, and solving them can become a frustrating experience. This is in line with the “80/20 rule”, where 80% of the effort is devoted to getting the last 20% of the task done. Looking at the three factors in more detail: Cool: Every designer wants high efficiency, but exactly how high, and at what cost? The answer is the usual one: it depends on the project and its tradeoffs. Higher efficiency is important for three main reasons: ■ It translates into a cooler product that enhances reliability, may allow for operation at a higher temperature, may eliminate the need for forced air (fan) cooling, or may simplify setting up effective convection cooling if feasible. At the high end, it may be needed to keep specific components that run particularly hot below their maximum allowed temperature and within their safe operating area. ■ Even if these thermal factors are not a concern, efficiency translates to longer run time for battery-operated systems or a reduced burden on the upstream AC-DC converter.
Quiet
Success
Cool
Complete
Figure 2: A DC/DC regulator must do more than just deliver a stable power rail; it must also be cool and efficient, be EMI “quiet,” and be functionally complete. (Image source: Math.stackexchange.com; modified by author)
■ There are now many regulatory standards mandating specific efficiency levels for each class of end product. While these standards do not call out efficiency for individual rails in a product, the designer’s challenge is to ensure that the overall aggregate efficiency meets the mandate. This is easier when each contributing rail’s DC/DC regulator is more efficient, as that provides for headroom in the summation with the other rails and other sources of loss.
when the switching frequency increases by a factor of ten.
we get technical
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