Designing second stage output filters for switching power supplies
These days switching power supplies are nearly ubiquitous and used throughout every electronic device. They are valued for their small size, low cost, and efficiency. However, they have a major drawback in that their outputs can be noisy due to the high switching transients. This has kept them out of high performance analog circuits where linear regulators have ruled the roost.
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. 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. Therefore, there is a need to be able to design optimized, damped multistage filters to clean up the output from switching power converters. In addition, it is important to realize how the filter design will affect the compensation of the switching power converter.
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. Shown in Figure 1 are the basic waveforms in a boost converter in constant-current mode (CCM).
Figure 1 Basic voltage and current waveforms for a boost converter
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. This tends to excite parasitic inductances in the switch, the layout, and the output capacitors. 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.
Figure 2 Typical measured waveforms of a boost converter in DCM
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. Generally, this output noise is in the 10 MHz to 100+ MHz range, well beyond the self-resonant frequency of most ceramic output capacitors. Therefore adding additional capacitors will do little to attenuate the noise.
There are a couple of reasonable choices for different types of filters to filter this output. This article will illustrate each type of filter and give a step-by-step process to a design. The equations are not rigorous and some reasonable assumptions are made to simplify them somewhat. There is still some iteration required since each component will affect the values of the others. 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. However, for a first pass at a design, this level of complexity is not necessary. 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.
Before designing the filter, consider what is achievable with a single stage filter RC or LC filter. 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. A buck converter can be made somewhat quieter since the power inductor provides significant filtering. These limitations are because once the ripple is down in the μV the component parasitics, and noise coupling between filter stages starts to become the limiting factors. If even quieter supplies are required, then a third stage filter can be added. However, switching power supplies do not generally have the quietest references and also suffer from jitter noise. These both result in low frequency noise (1 Hz to 100 kHz) that cannot be easily filtered out. Therefore, for extremely low noise supplies it may be better to use a single second stage filter and then add an LDO to the output.
Before diving into a more detailed design process for each type of filter, some values that will get used in the design process for each of the types of filters are defined as follows:
The simplest type of filter is just an RC filter as shown attached to the output of a low current ADP161x-based boost design shown in Figure 3. This filter has the advantage of low cost and will not need to be damped. However, due to power dissipation it is only useful for very low output current converters. For this article, ceramic capacitors with small ESR are assumed.
Figure 3 ADP161x low output current boost converter design with an added RC filter on the output