Different types of switching regulator topologies
You need to understand which terminal has the highest noise.
You can plan your filters in advance and secure the space you need.


There are many different switching regulator topologies. The most widely used is the traditional step-down converter, also known as the buck converter. There are also lesser-known switch-mode DC/DC converters, such as the zeta topology.

They can be divided into basic topologies and extended topologies. The basic topology is a non-isolated switching regulator that uses only two switches, one inductor, and two capacitors. In other words, it is a switching regulator that is not electrically isolated. Buck converters, boost converters, and inverting buck-boost topologies belong here.

All other topologies require additional components. For example, the SEPIC converter requires a coupling capacitor and a secondary inductor. Unlike non-isolated switching regulators, there are also several regulators that use a transformer to provide galvanic isolation.

Circuit design engineers sometimes refer to power supplies as black boxes or 4-pole devices, each with two input lines and two output lines. [Figure 1] shows a block diagram of a DC/DC converter. The one above is a DC/DC converter without galvanic isolation, and the one below is a converter with galvanic isolation.

No noise information is available regarding the terminals here. The behavior of the two-port network terminals varies depending on the switching regulator topology. [Figure 2] shows the ADP2441 from Analog Devices (ADI) as a general-purpose step-down converter for industrial applications.

This converter converts 24 V input to 3.3 V output. This topology is very noisy because it generates pulsed current at the input. When the high-side switch of ADP2441 is on, current flows to terminal A.

When this switch is off, no current flows through node A. However, there is almost no noise on the output side C. This is because the inductor in the output path prevents pulse-like current from being generated at the output terminal.

[Table 1] is intended to help system design engineers understand the basic operation of a switching regulator. This table summarizes the major switching regulator topologies in common use.

The first row shows whether the noise level at the input is high or low. That is, it indicates the noise level at terminals A and B of the two-port network.

The second row indicates whether the noise level at the output is high or low, that is, the noise level at terminals C and D of the two-port network.

Conductive noise in switching regulator circuits can be significantly reduced by additional filtering, for example, a separate LC filter. This method can overcome the weaknesses shown in Table 1.

However, system design engineers need to understand which DC/DC converters have particularly high noise at which terminals, so that they can design appropriate filters in advance during the design phase and reserve the necessary space for them.



This article is a summary of the article titled “Noise from Different Switching Regulator Topologies” by Frederik Dostal, Power Management FAE at Analog Devices.