A buck converter can be made somewhat quieter since the power inductor provides significant filtering. Typically with a second stage filter it is reasonable to get the ripple down to a few hundred μV p-p and the switching noise down below 1 mV p-p. With the provided calculations and possibly using a SIMPLIS simulator like the free ADIsimPE ™, or some bench time in the lab, a satisfactory design can be found with a minimal amount of effort.īefore designing the filter, consider what is achievable with a single stage filter RC or LC filter. However, for a first pass at a design, this level of complexity is not necessary. The ADIsimPower design tools get around this problem by using linearized equations for component values, like cost or size, to do an optimization before actual components are selected, and then optimize the outputs once real components are chosen from the database of thousands of parts. There is still some iteration required since each component will affect the values of the others. The equations are not rigorous and some reasonable assumptions are made to simplify them somewhat. This article will illustrate each type of filter and give a step-by-step process to a design. There are a couple of reasonable choices for different types of filters to filter this output. Therefore adding additional capacitors will do little to attenuate the noise. Generally, this output noise is in the 10 MHz to 100+ MHz range, well beyond the self-resonant frequency of most ceramic output capacitors. The switching ripple (at the switching frequency) caused by the change in charge of the capacitor is very small compared to the undampened ringing of the output switch, which we will refer to as output noise. Typical measured wave forms of a boost converter in DCM. & amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp lt img src='/en/_/media/analog/en/landing-pages/technical-articles/designing-second-stage-output-filters-for-switching-power-supplies/figure2.png?w=435 ' alt='Figure 2'& amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp gt įigure 2. The result is that in the real world the output waveforms look much more like Figure 2 rather than Figure 1, even with a good layout and ceramic output capacitors. This tends to excite parasitic inductances in the switch, the layout, and the output capacitors. The issue that makes an output filter so important for a boost or any of the other topologies with discontinuous current mode is the fast rise and fall in the current time in Switch B. Basic voltage and current waveforms for a boost converter. & amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp lt img src='/en/_/media/analog/en/landing-pages/technical-articles/designing-second-stage-output-filters-for-switching-power-supplies/figure1.png?w=435 ' alt='Figure 1'& amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp amp gt įigure 1. Shown in Figure 1 are the basic wave forms in a boost converter in constant-current mode (CCM). In this article, boost circuits will be used for the example circuits, but the results will be directly applicable to any dc-to-dc converter. In addition, it is important to realize how the filter design will affect the compensation of the switching power converter. Therefore, there is a need to be able to design optimized, damped multistage filters to clean up the output from switching power converters. Even in those demanding applications where an extremely low noise supply is required, there is probably a switching circuit somewhere upstream in the power tree. It has been shown that in many applications an appropriately filtered switching converter can replace a linear regulator for production of a low noise supply. This has kept them out of high performance analog circuits where linear regulators have ruled the roost. However, they have the major drawback in that their outputs can be noisy due to the high switching transients. They are valued for their small size, low cost, and efficiency. These days switching power supplies are nearly ubiquitous and used throughout every electronic device.
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