Switch Mode Power Supply Topologies Compared

The ubiquitous linear power supply of old has been on a steady decline for years, ever since the advent of Switch Mode Power Supplies (SMPS). The clear advantage to SMPS over linear supplies has been size and efficiency, and as the world-wide energy crises looms on the international, commercial and political scene, we have seen the trend to SMPS only accelerate.

As a new technology, the high component count of SMPS made the technology more expensive than linear. But with the birth of the electronic age, component costs have dropped so low that the high raw material content of copper and iron in the linear transformer has made the SMPS technology more cost effective.

Even with the disadvantages of being more complex and requiring more care to control EMI, the advantages of switch mode power supplies far outweigh linear power supplies in all but a few niche applications.

Switching power supplies are made up of a number of different stages. If the input is an AC input, then the input stage needs to include both the input filter and a rectifier to convert to a DC input. DC to DC converters do not need the rectifier. The inverter stage turns around and immediately converts the now DC input back into an AC input by switching the DC input voltage on and off at a much higher frequency than the original AC input. The frequency of operation is often chosen to be in the 20kHz to 150kHz range, which is high enough to be outside the audible range and low enough to keep it outside of the FCC requirements for conducted EMI.

After the inverter stage, the output stage rectifies and filters the output. If an isolated design is required, a transformer is placed between the rectifier and output stage. This transformer can be much smaller, lighter and cheaper than the linear power supply transformer, due to the higher switching frequency.

After the inverter stage, the output stage rectifies and filters the output. If an isolated design is required, a transformer is placed between the rectifier and output stage. This transformer can be much smaller, lighter and cheaper than the linear power supply transformer, due to the higher switching frequency.

When designing power supply, typically the design criteria favors a switch mode power supply over a linear power supply. When it comes to deciding which SMPS topology to use, the decision can often be more difficult.

Selecting the wrong topology can result in a design project that does not meet your cost targets, efficiency goals or a host of other requirements that you might have. Below we discuss some of the more common topologies and their features.

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