The Nine Most Useful Power Topologies
For most power supply applications, a handful of topologies continue to be used after more than 30 years. Simplicity and ruggedness keep these circuits relevant today.
At the beginning of modern power supply design, about thirty years ago, there were a handful of topologies that served the industry well. In the 1980s, an explosion of research into new and advanced power conversion techniques created hundreds of new topologies that could be used. Today, mainstream industry has reverted back to the early topologies. The same handful of circuits provides the best solutions for most applications.
The Nine Most Useful Power Topologies
In the beginning of power supply design, there were three fundamental converters: the buck, boost, and buck-boost. Early analysis papers cover just these topologies. There were also converters that behaved exactly the same as these fundamental topologies. They were considered to be in the buck, boost, and buck-boost families, with isolation being included in the circuit. Included in the buck converter family are the forward, two-switch forward, half-bridge, full-bridge and push-pull. The boost has an isolated version that can be built with a bridge or push-pull circuit. The isolated buck-boost circuit is the well-known flyback converter.
It is an intriguing research exercise to invent new power topologies and study their operation. This formed a large part of research in the past, especially during the 1980s. Some fascinating circuits were invented that stretch the mind to fully understand their operation. A paper from Caltech came up with over 300 new topologies which used an additional switch and diode. For a while it seemed as if the old standby topologies were in jeopardy of being replaced.
It was a very confusing time for many designers who needed to produce products. After reading conference papers, engineers were tempted to try exotic new topologies that promised superior performance but which proved to be very difficult to implement in production.
As a result, the industry came full circle. Now it relies on the original basic topologies for nearly all designs. Exceptions are made for some very high density applications, or for unusual voltage and power ranges, but the working engineer can almost always get the job done with the basic set of circuits.
That is not to say that industry has not made progress. It has come a long way—just not through the usage of fundamentally different circuit topologies. The main advances have been in judicious use of the correct circuit for the right application, partitioning of power into smaller pieces (such as the motherboard VRMs and point of load converters), advanced packaging, new silicon devices, and careful application of low-loss switching for some of the topologies.
1. Buck Converter
The buck converter is the most fundamental of all power supplies. It supplies a lower voltage output than the input, and is used at all power levels where isolation is not required.
As shown in Figure 1(b), the diode of the buck converter can be replaced with an active switch when the output voltage is low. This provides efficiency advantages. Caution is required when using discrete parts since avoiding overlap of the two switches is essential.
Probably more buck converters are built than any other converter. They form the basis for microprocessor power supplies, providing over 100 A to the load. Modern design techniques break the total load into smaller pieces, using several parallel buck converters. This results in a final power system which can switch faster. It is also smaller and more efficient.
When working with these converters, don’t forget that the input is noisy. Good power designers will have input filters on the front end of their buck converters, preventing the large switching currents from creating noise issues elsewhere on the board.
The buck converter becomes less optimal as the input-to-output voltage ratio increases. A 10:1 step down is reasonable as an extreme. Beyond this, the silicon is stressed hard and a transformer topology may be more appropriate.
Figure 1: Buck Converter
Applications: Step down without isolation for power levels from less than 1 W to over 1 MW.
Strengths: Low noise output.
Cautions: Noisy input requires filtering. Don’t try to step down too much.
Advanced application: Synchronous rectifiers, multiphase for current sharing.
2. Forward Converter
If your system requires isolation or a large step down ratio, it can be provided by the forward converter. This inserts a transformer in the circuit and allows appropriate scaling of the input voltage. The transformer also inserts complications – the voltage stress on the switch is increased, and the core must be properly reset when the converter is turned off. In general, forward converters only operate to a 50% duty cycle. The rest of the period is reserved for transformer reset.
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