How to solve the noise of switching power supply
Power off, small size, low cost, and high efficiency make it a great value.
However, its biggest drawback is high output noise due to high switching transients. It is this shortcoming that prevents them from being used in high-performance analog circuits mainly powered by linear regulators.
However, it has been proven that in many applications a properly filtered switching converter can replace a linear regulator to generate a low noise power supply.
Therefore, it is necessary to design an optimized and damped multi-stage filter to eliminate the output noise of the switching power converter.
The example circuit in this article will use a boost converter, but the results can be directly applied to any DC-DC converter. Figure 1 shows the basic waveforms of a boost converter in constant current mode (CCM).
Figure 1. Basic Voltage and Current Waveforms of a Boost Converter
The output filter is important for a boost topology, or any other topology with discontinuous current mode, because of the fast rise and fall times of the current in switch B. This results in parasitic inductance in the excitation switches, layout, and output capacitors. The result is that in actual use, the output waveform looks more like Figure 2 than Figure 1, even with good layout and ceramic output capacitors.
2. Typical measured waveforms of a boost converter in DCM
The switching ripple (switching frequency) due to changes in capacitor charge is very small compared to the undamped ringing of the output switch, hereafter referred to as output noise. Typically, this output noise ranges from 10 MHz to over 100 MHz, well beyond the self-resonant frequency of most ceramic output capacitors. Therefore, adding extra capacitance does not do much for noise attenuation.
There are also many types of filters suitable for filtering this output. We will explain each filter and give a design step by step.
The formulas in this paper are not rigorous, and some reasonable assumptions are made to simplify these formulas to a certain extent. Some iterations are still required, as each component affects the values of the other components.
The ADIsimPower design tool avoids this problem by using a linearization formula for component values (such as cost or size) to optimize before actually selecting components, and then optimize the output after selecting actual components from a database of thousands of devices . But this level of complexity is unnecessary when starting out with a design. Using the provided calculations, using a SIMPLIS simulator—such as the free ADIsimPE™—or spending some time at the lab bench, you can arrive at a satisfactory design with minimal effort.
