The Right Approach to Power-Supply Selection and Design

There are three main options, though more complex systems could rely on a combination of all three. The first option that will be most familiar to experienced board-level engineers is the custom power circuit, though this will often involve a combination of off-the-shelf power controllers together with discrete power transistors and passives for filtering and conditioning. The second is a module that bundles the combination of components found in a custom design into a single package, with external pins for the ground, input and output voltage-rail connections. Such modules are often fully encapsulated for environmental protection and easy soldering. Finally, there is the off-the-shelf power supply unit (PSU), which may come in the form of an external front-end unit. Alternatively, if the PSU needs to be integrated into the system enclosure, it can be purchased as an open-frame design or a fully packaged version. The fully enclosed version will often be chosen in systems where user access is required as it provides the greatest level of safety.

The decision of whether to use a custom design, configured modules or entirely off-the-shelf power supplies can often come down to a matter of tradition. ‘It’s the way we’ve always done it’ is not necessarily the wrong answer though, it may be a conclusion won through experience and should not be overturned lightly, but it is one that is worth reconsidering for a new project as circumstances and technologies may have developed in favour of a different approach.

Legislation, for example, is a moving target around the world and legislators have gradually tightened up laws that cover power-supply safety and energy consumption. Traditional rule-based mechanisms for obtaining safety certification for medical power supplies, for example, have shifted towards risk-based assessments that need to be backed up by evidence. Similarly, circuit designs that would be suitable for consumer power supplies five years ago, now draw too much energy in standby mode to be acceptable. Legislation now demands more sophisticated control and techniques in the front-end rectifier section with which in-house designers may be unfamiliar, even if they have years of experience in board-level power architectures.

If a new product needs substantial changes in the power supply, it can make sense for a team that is used to custom designs to switch to an implementation based on configurable modules or a readymade PSU. Manufacturers of external or fully packaged PSUs will typically provide certification to the relevant standards. Modules will generally be designed to comply with standards but the final systems into which they go will need certification.

The issue of time-to-market may lead to a similar conclusion. An off-the-shelf PSU will generally lead to the fastest time to market if one can be supplied that fits within the system’s enclosure. A customised PSU from a third-party supplier will still take time to design, tune and manufacture, which will increase the time-to-market. Where customisation is important, modules provide greater flexibility but will lead to a longer design and integration cycle, particularly where certifications are required. For most design teams, both options will be faster than a fully customised circuit.

Where size is concerned however, a fully custom circuit design can easily offer the best option. A designer familiar with the system’s requirements can build the PSU around the other circuit boards. This may not be the smallest possible design in terms of watts per cubic centimetre, but it can conform to the rest of the system in a manner that minimises volume without compromising airflow for cooling. Modules are often designed for small size and low cost, frequently taking advantage of novel packaging techniques to minimise volume. This can make them excellent choices when the requirements of the target system match the characteristics of the available modules.

There may be situations where modules themselves are individually highly compact but the combination needed for the target application leads to unused rails and capacity; this will increase the solution’s overall size and cost. The situation with modules is constantly changing however, as there are many module options now readily available through distributors such as Farnell, making it easier than ever to pick combinations of modules that fit a target application neatly. Despite the growing number of module options, if a target design has special requirements, the team may have to compromise on size or mix modules with custom design for the specialty rails.

As with module manufacturers, the providers of off-the-shelf PSUs can take advantage of economies of scale and optimised production technologies to build highly compact products. As a result, they often offer designs that provide high power densities when expressed in terms of watts per cubic centimetre. However, the off-the-shelf design will frequently need to be somewhat over-specified to fit a catalogue product to the target application’s peak power usage. The result may be a larger than desired form factor.

Flexibility of design can be an important criterion for teams working on families of products. This is a situation where module-based approaches can often make the most sense as it is relatively easy to swap modules with minimal redesign around the surrounding circuitry. With off-the-shelf PSUs, although larger and smaller variants will often be available, they may not fit the original form factor. When custom circuitry is employed, the team will have to find time to design for each and every variant, which may slow down the project timeline.

In terms of cost, there is often a clear tradeoff between development and unit costs. The unit cost of an off-the-shelf PSU will generally be higher than with module-based or custom designs. But the readymade unit has the advantage of little to no development cost, which can be very important for low-volume projects. Where unit prices are likely to dominate, the custom-design option is likely to offer the greatest advantage though it incurs the largest development costs.

Although custom circuitry involves the longest development time, manufacturers have developed a range of assistance tools that greatly simplify the selection and PCB design of suitable power supplies. An example is the EE-Sim DC-DC Design Tool developed by Maxim Integrated. Through intuitive forms, the engineer can input their application-specific requirements and obtain a circuit schematic, simulation results and a bill of materials to support implementation.

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Figure 2: Maxim Integrated Himalaya uSLIC Power Modules

It is also worth noting there is increasingly a blurring of the lines between different types of power-supply solutions. Suppliers such as TDK-Lambda use modular design techniques to offer the convenience of off-the-shelf PSUs with the flexibility and performance of highly tuned modules. At the other end of the spectrum, component suppliers are taking advantage of package-level integration to offer highly complex power architectures previously only found in fully packaged DC/DC converters.

The result is a wide range of options design teams can consider when choosing the ideal power-supply strategy for their project. Each project needs to be judged on its own merits, using the criteria described above. Close analysis of the requirements will reveal which point along the curve from fully off-the-shelf to fully custom best meets the needs of the application.

Farnell

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