SMPS SWITCHING POWER SUPPLY DESIGN BASICS

If you want to know what's SMPS and how to design it, you came to the right place. The industry push toward smaller, lighter and more efficient electronics has led to the development of switching-mode power conversion technology some four decades ago. By definition, switching power supplies (SMPS) are devices which use in their operation power handling electronic components which are continuously commutating on and off with a relatively high frequency. These electronic switches effectively connect and disconnect energy storage inductor(s) and capacitor(s) to and from the input source or the output. Output filters are then "averaging" energy transfer rate and provide continuous current flow into the load. By varying duty cycle, frequency or phase shift of these commutations, a desired output parameter (such as voltage) is controlled. The high operating frequency ("F") results in the smaller size of switch-mode power supplies since generally, the size of power transformers, inductors and filter capacitors is inversely proportional to the frequency. Switch mode operation also reduces energy losses and increases efficiency- when a switch is "off", its current is near zero; when it is "on", the voltage across it is low.

Of course, you can't learn SMPS design from a single webpage-- this guide is intended to provide just a starting point.

DESIGN PROCEDURE

Before you start an SMPS design or selection, you should make a list of both technical and regulatory requirements. Nowadays, electronics manufacturers rarely design their own power supplies. In-house designs are usually done for military and airborne applications, or for peculiar requirements. Most commercial and PC PSU are made by specialized off-shore ODMs. By searching PSU manufacturers websites you can usually find a standard off-the-shelf unit that meets your needs and that already have the required safety agency certifications. If you still need to design one, the first step would be to select the best cost-effective topology for your application. There are about a dozen basic topologies practically used in the industry. The best configuration for a given application obviously is selected based on the specific requirements for the PSU (including cost and time factors). Since a great deal of overlap exists in the topology usages, in practice this selection is usually influenced by a personal experience of the designer- the engineers like to do what they are comfortable with. If this is your first project, you may start with this guide to topology selection. Then read application notes, such as TI seminar manuals, which contain detailed reference designs with practical circuits and calculation procedures. If you are already familiar with power electronics and need quick reference and equations, get my SMPS handbook for all essential design information. If you are looking for digitally controlled power, see Microchip's intelligent power solutions.

Having selected a topology you need to come up with operation frequency. The typical frequency range of an offline SMPS circuit is from 50 kHz to 500 kHz. DC-DC converter modules for low-voltage (<100V) input can operate up to several MHz. The downside of SMPS is the switch-mode operation introduces the switching losses due to overlap of current and voltage during each transition. It also introduces additional electrical noise (EMI). The EMI acceptable limits depend on the application. The EMI limits in most industry standards start at 150 kHz. Therefore, the engineers sometimes select F<150 kHz in order to place the fundamental harmonic below the requirement range. In general, the switcher's optimization is based primarily on size, efficiency, cost and EMI considerations. Realizing the switcher's advantages also requires the selection of the right power semiconductors and magnetics that can operate efficiently at the necessary frequencies.

Another important thing to learn is the output-to-input transfer function of a converter. The DC gain is calculated based on the fact that in steady state, the net volt-seconds across any inductor and net amp-seconds through any capacitor over one switching cycle must be zero. The diagram to the left illustrates this concept for a buck converter.

Below you will find theory, SMPS schematics and design guides, power electronics tutorials and other useful online resources for the engineers and hobbyists. If you don’t like to randomly surf the web, this list will save you hours of research.

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