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How to select the proper power supply topology?
Most switching power supplies convert a DC voltage into another DC voltage. I say ‘most’ because there is one special type of switching converter that operates directly from AC, the Power Factor Corrector. Therefore, all switching power supplies that operate from AC have an input stage that converts AC into DC before the DC/DC conversion stage. This AC/DC conversion stage could be a simple rectifier and filter (the most common) or a Power Factor Controller (PFC).
So, when comparing switching power supply topologies, one must be aware that there will always be an AC/DC rectifier stage (simple rectifier or PFC) in front of the switching converter.
With regard to switching power supplies, terminology is not used consistently, so one has to be careful about using a particular term as a means of conveying specific information. However, for the purpose of this paper, here is the terminology I will use:
Other than being isolated or non-isolated, switching converters can be:
Another important consideration for switchmode converters is the power level, as some topologies are more suitable to certain power levels. A rough break-down of power levels can be as follows, keeping in mind that there can be significant overlap, depending on the application and the topology:
Finally, the output voltage (and to a lesser extend the input voltage) can have also a significant impact on topology selection. The typical ranges below also can have significant overlap.
Topologies are presented roughly in order of increasing complexity. For instance, the one transistor Flyback and the one transistor Forward converters are similar, but the Forward topology requires an additional power inductor compared to the Flyback. In some cases, a Forward converter can be used in resonant mode in such a way that a separate inductor may not be required (leakage inductance in the transformer is used).
| Topology | Isolation | Conversion | Power | Output Voltage |
|---|---|---|---|---|
| Buck | no | Step down | Low-Medium | Low |
| Boost | no | Step up | Low-medium | Low or Medium |
| Buck-Boost | no | Step up or down | Low | Low or medium |
| SEPIC | no | Step up or down | Low | Low or Medium |
| Flyback (1 transistor) |
yes | Step up or down | Low | Any |
| Flyback (2 transistors) |
yes | Step up or down | Medium | Any |
| Forward (1 transistor) |
yes | VStep up or down | Low-Medium | Any |
| Forward (2 transistors) |
yes | Step up or down | Medium | Any |
| Half-Bridge | yes | Step up or down | Medium | Any |
| Full Bridge | yes | Step up or down | High | Any |
Note that there are other more subtle variations within each topology. For instance, most topologies can be used in a resonant fashion, which is normally done to minimize switching losses. Resonant topologies can use a variable switching frequency or a fixed switching frequency.
Many circuit variations have been designed to improve certain characteristics of the switching converters, particularly to reduce the size and/or to improve efficiency and also to reduce Electromagnetic Interference. Unlike what the marketing literature would make you believe, smaller size converters are not naturally more efficient than larger ones, as a smaller size usually requires a higher switching frequency which increases switching losses. Also, a smaller size reduces the size of the conductors and increases conduction losses.
Historically, all switching converters were designed with analog technologies: a Pulse Width Modulator composed of a clock, a sawtooth generator and a comparator generates a variable duty cycle pulse train that is used to control the switching elements.
More recently, digital control has started to replace the analog control circuit, but this has not fundamentally changed the design of the power stages. While digital control gives the designer more flexibility to adjust the optimum pulse width, switching frequency and precise timing of the various switches depending on operating conditions, it does not fundamentally change the design or operation of the switching stages.