For Your DC Power Application, Which Switching Power Supply Topology is Right?

Switching power supplies are the mainstay of power for digital electronics, offering a passive way to control output voltage and current over wide input voltage ranges. With so many options available for switching power supplies, how does one pick the best option for their system? There are a few considerations and ways to approach this problem which will be outlined in this article.

Some of the key questions to consider for topology selection include:

• How much power is needed?
• Is galvanic isolation required?
• Does the design need to step up or step down the input voltage?
• How much circuit design effort is needed?
• How many outputs are needed?

We won't be going over every possible topology in this article, but many common switching power supply topology options and design resources will be presented.

List of Common Switching Power Supply Topologies

The table below outlines many of the common switching power supply topologies. These options can be used in a broad range of designs requiring different voltages and power levels.

Boost: Similar to the buck topology, boost circuits are not suited for isolated power supplies when their main function is the stepping up of DC-DC power rather than down. However, unlike buck topologies, boosts have continuous input power making them more ideal for power factor correction circuits. Again, the use of special inductors can be implemented in order to cater to multi-phase projects.

Buck-Boost: As the name may imply, the buck/boost is a mix of the two aforementioned topologies allowing for either a step up or down of DC power. This is ideal for battery powered applications which require variable voltage input networks. The downside to this topology is the fact that the output voltage is inverted, but with a little magic, accommodations can be made to the design. Additionally, complications in the drive circuit incur with a lack of ground in the switch in which greater care should obviously be taken.

SEPIC and Cuk: Again, ideal for battery powered applications, this network can step up or down DC power, but unlike the Buck-Boost topology, the SEPIC and Cuk topologies do not invert the output stage. Capacitors, as well as two inductors, are used for energy storage. These inductors can either be two separate components or a single coupled inductor. Additionally, the capacitors can act as a limited isolated design offering a bit of protection.

Flyback: Essentially acting as an isolated version of a buck-boost design, the flyback topology uses a transformer as the storage inductor. Integrating a transformer into the design can also adjust the output voltage by “simply” adjusting the turn ratio of the secondary winding(s). Multiple outputs are then possible given enough room on the transformer.

This simple and isolated DC power supply is ideal for low power applications. Since the transformer here acts as the storage inductor, there is no need for additional inductors which makes this a very popular and cost-effective design.

Forward: The forward DC power supply design is simply a buck design with the use of an isolated transformer. But again, this design is better suited for lower power applications. Utilizing a separate inductor on the output stage, the design is not well suited for higher voltage outputs. Although not suited for high voltages, when high direct current applications are required, the non-pulsating outputs are much better suited for a direct current surpassing 15A.

Push-Pull: Utilizing two primary windings which creates a dual-driving circuit, the push-pull DC power supply design offers greater efficiency than the flyback or forward design. This topology can be scaled up to higher power applications but greater care must be taken with the switching control. If both switches are on simultaneously, a very large direct current can shoot through the design which may otherwise damage or destroy (never words you want to hear in PCB design). If implemented correctly, however, stresses of switching are still very high which makes the design undesirable for high voltage and power factor correction circuits.

Half-Bridge: Similar to push-pull designs, half-bridge topologies can be scaled up for higher power applications as well (and are based on forward topologies), similar switching issues can occur. However, and advantageously, switching stressors are equal to the input voltage making it much better suited for the higher voltage applications. On the flip side, the output currents are much higher than the push-pull topology making it less suited for high current applications.

Resonant LLC: Using resonant techniques to reduce switching losses, the resonant LLC topology scales up well with higher power levels. Although not suited for stand-by power mode applications due to the resonant tank needing to be continuously energized, the advantage comes with the range of input voltages. The disadvantage of this design though come with the increase in complexity as well as the cost associated.

There are some variants on these topologies which extend the power output, efficiency, and number of outputs to higher levels. Typically, these topologies are the starting point for selecting a switching power supply design for a PCB. The simplest approach is to evaluate a typical buck, boost, or buck-boost topology, and then determine whether a more advanced or isolated topology is needed.

Isolated DC/DC Converters

In systems requiring galvanic isolation, or when there is an element of user safety at play, an isolated switching power supply is generally used. These converters provide isolation up to high voltage values in accordance with many industry standards. A glance at the above table shows several options for isolated switching power supplies.

Switching power supplies which are isolated require a transformer (or equivalently coupled inductors), which allows some control over the number of outputs and the step-up/step-down factors involved.

Extending to Multiple Outputs

Multiple outputs in a switching power supply can be achieved by using additional or auxiliary windings in the transformer. For example, a basic multi-rail output transformer with a single-line input is shown below.

Here, the center tap acts as the GND reference connection and is bridged back to the primary side ground (PGND) using a Y-type safety capacitor.

Some topologies readily extend to multiple outputs by adding more rails a transformer, or by using coupled inductors instead of a single inductor. For example, we have some of the common topologies which can be extended to multiple rails in this way:

• Fly-buck (or fly-boost)
• Multi-output flyback
• Isolated multi-output push-pull
• Isolated multi-output full-bridge

Each of these rails is galvanically isolated from the input (assuming they are not sharing a center tap for a ground reference), so each isolated rail needs its own ground net. This can be challenging to control high-frequency currents (and thus EMI) between each of the grounds with safety capacitors; only some of the rails may actually require the safety capacitor connections shown above. This requires testing to determine which rails (if any) require the safety capacitor connections.

Finally, the tertiary rail (Out1 in the above image) allows for feedback to the switching/gate drive controller or generation of another supply output. Another use case is generation of a logic level output which could be used for powering a specific sub-section of the design.

PCB Design Tips For Isolated DC/DC Converters

It’s difficult to give general points that apply to every topology, but there are a few points that matter in multiple topology options:

• In designs that use transformer inductance, make sure you know the actual inductance of your transformer, which can be measured with an LCR meter
• When multiple ground polygons are used, lay them out laterally instead of overlapping them, otherwise capacitive coupling between overlapping planes creates unpredictable return paths
• For rectifying diodes/FETs, the most important component specification is almost always the reverse recovery time as this will determine how much power is lost during switching/rectification
• In bridge/push-pull topologies, do not run the controller near 50% duty cycle as shoot-through can occur; either use a controller with configurable dead time or redesign to provide duty cycle margin
• Higher current handling can always be achieved with multiple FETs in parallel, but more copper may be needed; consider prioritizing more plane/rail layers instead of heavy copper on fewer layers
• Switch node ringing in isolated designs is most easily controlled with RCD snubber/clamp circuits; use actual measurements of the ringing waveform to design these circuits, don’t try to estimate from simulation

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