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

    Altium Designer
    |  February 16, 2018

    Man with lightning coming out of his hands

    We often have our old-timer pioneers to thank for laying down the groundwork for our continued success and future growth. These old-school designers had nothing more than theory and confidence to drive their designs, or more specifically, to drive their failures. But as stated by a multitude of wise individuals, each failure is nothing more than a discovery of how not to do something. However, with the discoveries of failures also come discoveries of success!

    When putting in numerous hours, days, even months into a PCB design, you may have skipped over a crucial piece of the ever-evolving, ever-changing puzzle of the PCB; power. Of the complex puzzle, we’re considering a whole PCB design, power distribution might as well be an entirely unique and equally as complex puzzle on its own.

    DC Power and Switching Power Supplies: What You Need to Know

    Taking a moment, we will be reviewing the topology of the switching power supplies. It should be noted that there are certain cases in which a linear power supply may benefit more than switching, but in this age of technological advancement, a switching mode power supply design has significantly decreased in cost and complexity allowing us mere mortals to incorporate these networks into our designs.

    Switching power supplies are usually made up of a number of stages. These stages include an input stage responsible for filtering and rectifying to a DC input, an inverting stage which takes this now DC input and converts it back into a higher frequency AC input, and an output stage which filters and rectifies the output. A transformer can be placed in between the rectifier and output stage if an isolated design is required.

    Now that we have our surface level definition of a switching mode power supplies, we’ll dive into a variety of topologies that you’ll likely be choosing from in order to power your own PCB!

    Types of Topologies for Switching Mode Power Supplies

    There are several topologies for you to choose from depending on the specific needs of your PCB, which all come with their own costs and benefits. Getting to know them individually and their strengths will equip you to better choose the switching mode power supply topology that works for your design needs.

    Buck: The buck converter is one of the cheapest, most and simplest designs around. Although not suited for isolated power supplies, it is ideal for DC-DC step downs. With its high-efficiency levels, it’s well suited for high power applications and only requires the use of a single inductor (for single phase applications), although special inductors can be integrated into the design for multi-phase applications.

    The downside of this topology is its discontinuity in input current which can create higher than desired EMI emissions. This, however, can be mitigated with proper filtering components like mode and filter chokes.

    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.

    SMPS frame and buttons associated

    Your power supply can be as diverse as the needs of your specific designs.

    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 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 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 current applications are required, the non-pulsating outputs are much better suited for currents surpassing 15A.

    Push-Pull: Utilizing two primary windings which creates a dual-driving circuit, the push-pull 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, very large currents 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.

    Whether it’s high-voltage or low, high-power or low, or high-current or low, choosing the right topology for your design can be a matter of knowing all the demands of your design on top of the demands from your production and manufacturing goals.

    Circuit routing design

    Make sure your higher power designs have the right power supply to keep them running.

    PCB Design and Power Supply Factors

    Whatever topology you choose to run your PCB design, obvious factors come into play within the topology itself. General board considerations such as space, cost, complexity to manufacture and procure, testing, chassis requirements etc. should all be included aside from the “simple” act of power requirements.

    If you are feeling particularly adventurous with your design and see yourself as a modern day design pioneer, then I highly encourage you to set out and find your own successes (in addition to failures). The best part is, with modern PCB design software, you’ll have a strong, supportive system to backup your design risks. With smart design rule checking built-in to your layout, and a power distribution network analysis, Altium Designer® is a great choice for your design needs.

    If you are looking to etch your success further into the future and would like to discuss your options with a specialist, talk to an Altium expert today.

    About Author

    About Author

    PCB Design Tools for Electronics Design and DFM. Information for EDA Leaders.

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