Looking for a power supply that can convert power from either AC or DC to one or more DC levels? Then you need a switch mode power supply. Here’s how they work.
Commonly referred to as “switchers,” these devices switch their power sources on and off many times per second. This switching creates an effective input frequency often reaching into the megahertz range.
What does "switching mode" mean?
The “switch” in a switching power supply is actually a semiconductor – a MOSFET that is either off or on – driven into its saturation range to transfer power across nearly zero resistance. It does this many thousands of times per second, creating the high-frequency AC intermediary. Because the semiconductor has been driven into saturation, it exhibits virtually no resistance, which makes for high efficiency and very little heat generation. A linear power supply, on the other hand, is so named because its rectifiers operate in the linear range. This means that when they conduct power, they do so over resistance, with all of the wasted power and heat generation that is intrinsic in the process.
Whatever the frequency at which the semiconductor is switching on and off, the time between the beginnings of each of the “on” pulses is defined as the switching period. The on time, as a fraction of a switching period, is called the duty cycle. By varying the duty cycle, the output voltage is controlled. Changing the duty cycle on “the fly” keeps the voltage on target.
What are the advantages of SMPS?
High Efficiency – They generate far less heat. Lower-powered units will often not require a heat shield which means that they can be mounted directly onto PCBs.
Compact Form Factor – Because switchers operate at a higher frequency, the value, and thereby the size of their associated filtering capacitors and inductors will be smaller and the overall unit will take up less space.
Versatile Design – Switchers can be designed to step up voltage (Boost) or step-down down voltage (Buck) as the application requires.
How to design for switch mode power supplies
Terminology
Switchers are often described in terms of bricks, half-bricks, and quarter bricks. A full brick has dimensions in inches of 4.6 x 2.4 x 0.5, a half-brick at 2.3 x 2.4 x 0.35, and a quarter-brick measures 2.3 x 1.45 x 0.35. These definitions are widely accepted, but not uniformly adhered to. The specification also includes the pin-out, so when using switchers that adhere to the standard, it is very easy to substitute a product when the designer needs or chooses to. Newer-model quarter-brick power supplies can supply 250 watts and more of electrical power. This is a great improvement over the amount of space previously required for power, freeing up space for more features to be included in the product being designed.
Construction
The equations that describe the physics of switchers are deceptively simple. The practical realities of dealing with amperes, as opposed to the microamps that most electrical engineers typically deal with, can lead to delays, added expense, and even outright product failures. Switcher design is always best left to the power specialist, and this is one scenario in which it is almost always better to buy than to build.
Engineers have two main options for designing with switching power supplies. The first is a single power supply that generates all the voltages needed for the system that’s being powered. The second involves one unit interfacing with the external AC, but with only one DC output, often either 12, 24, or 48 volts. In this case, the voltage produced by the main power supply is the highest that will be needed. If a lower voltage is needed anywhere in the system, buck converter can be employed to do the required DC-to-DC downward conversion.
Buck converters are extremely efficient, with losses of only 5 percent, or even less. They are also sometimes described as switching regulators. In a manner similar to all switchers, the heart of the buck converter is a semiconductor switch tuning the source voltage on and off many thousands of times per second or more.
Because of their design advantages, switch mode power supplies have quickly become the standard in all but the most technical and stringent applications. Designs are still being constantly improved in terms of efficiency, smaller size, and smaller weight. Looking for more power supply tips? Go in depth with our breakdown of the top types of switching DC-to-DC converters. Also, find out when to use an uninterruptible power supply and how to pick the right AC-to-DC solution.