SWITCHING MODE POWER SUPPLY (SMPS) TOPOLOGIES

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Let's first clarify what is a power supply topology. Switch mode power supply (SMPS) circuits contain networks of energy storage inductors and capacitors as well as power handling transistors and rectifiers. Their particular configuration is referred to as a topology. Here I will help you select the right one for your application. Although there is a wide variety of converter topologies, only about a dozen basic ones are used in practical power design. They can be classified into two major families: buck and flyback depending on the method of operation. In the buck family energy is transferred from the input to the load during the conduction cycle of a switching transistor. In the flyback family the energy is accumulated during "on-state" of a switch and delivered to the output during the "off-state". Any practical circuit belongs to one of these two configurations or their combination. Examples of the buck type are forward and bridge converters. Examples of the flyback type (besides transformer-isolated flyback) are buck-boost and boost regulators.

A complete off-line PSU normally contains several conversion stages. Usually it is comprised of a rectifier section followed by a PFC boost and one or more downstream DC-DC converters. Each of these stages is usually controlled by a separate controller IC, although there are also PFC-PWM combo ICs. The table below summarizes and compares electrical features and characteristics of the main single-stage switching regulator circuits. This chart is followed by the DC to DC regulator selection guide. For complete information about the main topologies as well as all essential SMPS design information get my handbook.

TOPOLOGY SELECTION CONSIDERATIONS

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There is no single topology, which is best for all applications. The right switching power supply topology for a given application should be selected based on specific requirements for the power supply design including cost, size, time factors, and expected production volume. For example, for low-volume designs, the engineering expenses may be more important than BOM cost. In this case, you may want to choose a straightforward "textbook-based" approach in which you are most experienced. Well executed trivial design is better than poorly executed "fancy" one. Of course, while a basic design may be straightforward, meeting all of the safety and EMC regulatory specifications may still be a very time consuming task. For a high-volume production, you'll want to put extra engineering efforts in developing new solutions, minimizing component cost and assembly labor. When the functional requirements are pretty much conventional, the power level is usually the main factor that determines the topology selection. As an illustration, the table below shows the configurations I would normally prefer for a downstream DC-DC converter in an off-line switching power circuit depending on its output power level. This selector guide is given for the power sources with output voltages below 60V running off 120 to 400V DC-link (which are typical levels for rectified AC input line voltage or the output of a PFC boost). At power levels above 3000W I would consider using multiple interleaved converters to spread heat dissipation. Note, all the information here of course, is just a general guidance based on the author's personal view.

	0-100 W, Iout<10 A	0-100 W, Iout>10 A	100-400 W	400-1200 W	1200-3000 W
Single-switch flyback		-	-	-	-
2-switch flyback		-	-	-	-
Active clamp flyback		-	-	-	-
Single-switch forward		-	-	-	-
2-switch forward				-	-
Active clamp forward				-	-
Half bridge	-				-
LLC Half Bridge	-				-
Full bridge	-	-	-		-
Phase shifted ZVT full bridge	-	-	-

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