The AC-DC Power Supply: Make It Or Buy It?

Designers have wrestled with the question of whether to design and make their own ac-dc power supply or buy one from an outside vendor for years. Although project criteria, available tools and components, and demands on the supply may have changed, the importance of the decision has not.

A typical ac-dc unit operates from ac-line mains (nominal 120/240 V, 50/60 Hz) and delivers one or several dc rails, usually from a few volts to around 48 V, at less than 1000 W. For example, N2Power offers ac-dc supplies that provide 375 W of output between 12 and 56 V, based on the model (Fig. 1).


1. In the “make versus buy” decision, a commercially available supply such as this 375-W unit from N2Power may provide a performance, packaging, and price combination that is very difficult to achieve and verify.

But today’s supplies must do more than deliver power. They have to meet increasingly stringent safety regulations, electromagnetic/radio-frequency interference (EMI/RFI) standards, efficiency mandates, and power factor correction (PFC) objectives. In some specialty applications, such as medical instruments, they also have to keep leakage below a certain threshold and ensure that component failures won’t cause life-threatening conditions.

Making Design Seem Easy

Today’s ICs make it much easier to design your own power supply. Many ICs embed control and algorithms for PFC, enhancing efficiency, transient response, and load/line performance, while minimizing EMI. Their advanced topologies and operational modes would be difficult to design yourself.

Some ICs support digitally controlled supplies, where your system can monitor many of the supply’s internal parameters and adjust them dynamically for optimal operation depending on internal factors such as loading as well as external considerations like ambient temperature and ac power costs.

Further, vendors offer reference designs and development tools that can, at first glance, make the supply design almost trivial. These materials fall into two categories. In the first, you get a detailed reference design for a specific supply such as 375 W, 48 V dc that includes a schematic, printed-circuit board (PCB) layout, and bill of materials (BOM). In the second, you use the vendor tools to define your specifications. The tools then return with the appropriate IC or ICs, passive components, schematic, layout, and performance curves.

Most often, designers craft their own power supply because the product’s form factor is unusual or unique. Apple’s notebook power supply is a good example (Fig. 2). Sometimes, standard supplies can’t meet the packaging constraints.


2. Sometimes in-house design is preferred due to the unique nature of the product, as in Apple’s MagSafe ac-dc power supply.

In addition, the higher volumes of these consumer products may be a strong justification for custom design. If you’re looking at around 1000 units per month or more, you’ll be amortizing the design and qualification process, and careful BOM analysis may show you can achieve higher profit margins.

Also, your requirements may fall outside of what’s available, or few vendors may meet enough of your requirements, prompting you to design your own supply. Some supplies might require a high dc voltage, like greater than 1000 V, though some suppliers may come close enough to what you need or can modify what they offer.

Power supply design is a balance among tradeoffs and constraints of nominal performance, efficiency, thermal issues, maximum/minimum parameter performance, cost, complexity, reliability, technical risk, and BOM supplier uncertainty. Still, there are some applications where one parameter is so overwhelmingly critical that only a custom, do-it-yourself supply will fit, since no commercially available unit is prioritized for that parameter.

You also may design your own supply if your requirements are looser than commonly available supplies, and you can get away with less. A supply for basic indicator lights may have loose nominal specs for output accuracy, say ±5%, and few or no transient load issues, so a low-cost design may be all that’s needed. At the other end, you may need specs that are far better than what’s available, which is sometimes the case for science applications.

A final reason to design your own is in-house expertise. If you have been designing supplies for years and are familiar with balancing, meeting, and testing to technical and regulatory requirements, then you’re ahead of many design OEMs whose expertise is in digital design.

So Why Not Design Your Own?

Basic supply design may be straightforward, but a fully qualified design that meets all of the performance and regulatory specifications is not. And that’s not even accounting for the cost and sourcing of the many components in the complete design.

Start with the design itself. ICs can implement a complex topology, but every ac-dc supply needs many non-IC components. Defining and sourcing them can be a headache, especially when their secondary characteristics play a role. For example, capacitance and working voltage define a capacitor, but its equivalent series resistance (ESR) affects its operation, especially at higher frequencies.

Even with the right part, you face supplier issues. Your purchasing group may substitute a nominally identical part for an inductor, for instance, to save cost. As a result, field problems may appear months later.

You also need to decide the minimum and maximum operating ranges for your design: will it be for a restricted-range line voltage, such as nominal 120 V ac ±10%, or full range (120/240 V ac)? The former design is somewhat easier and less costly, but it also means you need a second design for the other ac mains if you’re planning to serve worldwide markets.

You need a test plan for the design. How will you ensure that it works in corner cases, such as high/low line plus maximum ambient temperature, plus line/load transients, all happening at once?

Then there's cooling. Are you planning to use convection cooling? Do you have the tools to model your supply and its operating environment to be certain that the available airflow will be sufficient? Which way is the supply mounted in use, anyway? It makes a big difference in the cooling situation. If you find out that you need a fan, how will you size it?

If you’re designing your own supply, you’re most likely using an IC or chipset from a vendor that also provides a reference design. Has the reference design been built, or is it just a schematic supported by simulation? You’ll likely find that actual performance is not where it was supposed to be, as physical layout, routing and size of ground plane, power and control traces, and connectors will make even the best simulation only a rough approximation of what the actual circuit will do.

Even if the reference design includes a printed-circuit board (PCB) layout, you have to be very careful if you make any change to the layout or BOM. An apparently trivial change can adversely affect performance. A power supply is a closed-loop amplifier that can oscillate, have transient-response issues, and both source and be sensitive to EMI/RFI.

The Regulatory And Standards Jungle

Even a well-designed and tested supply faces regulations and standards. These mandates are getting even more challenging as tighter standards are being phased in each year. They cover:

  • Basic safety, which affects insulation, isolation technique, layout spacing, and design topology
  • EMI emissions, which are determined by the supply’s operating frequency, internal waveforms, switching characteristics, and layout
  • Efficiency, which is assessed by the relationship between ac line power used and dc output power
  • Power factor correction (PFC), which defines how resistive the supply “looks” to the ac mains as a load; if the supply is not resistive-looking (they usually are not), the design must employ techniques to yield a power factor close to unity (IEC61000-3-2)

Adding to the challenge is the worldwide nature of regulations, which means you’ll be dealing with many regulatory authorities and their unique ways of testing and doing business.

Conclusion

Today’s ICs, reference designs, and tools make it easier than ever to design your own supply. Yet the combination of specifications in the N2Power unit in Figure 1 would be very hard for a non-expert to achieve, especially when all regulatory and manufacturing issues are added in.

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