Confronting the power supply make-or-buy decision

Figure 1. Two views of the UltiMod Series modular power supply.

With so many power supply ICs and reference designs available today, it can be tempting for engineers building all types of systems – from industrial, lighting and medical, to communications to military – to consider designing their power supply in-house rather than purchasing a supply from a commercial power supply vendor. For example, it might appear to be a cost-saving alternative to have an IC manufacturer design something for you since many of them will practically design the circuit just to earn the business.

However, engineers need to consider if designing a power supply in-house really makes sense. This article argues that, when it comes to larger off-line supplies in the range of 250W or above, companies are better off leaving the task to a proven power supply manufacturer who has the engineering knowledge and resources to design the supply to meet specific needs. It explains how buying a well-designed power supply translates into lower unit cost, higher reliability and shorter time to market.

Considerations for designing an in-house power supply

From my personal experience, I have seen companies who have designed their own in-house supply, only to find that they run into costly and time-consuming challenges after heading down that path. I can’t tell you how many times someone has chosen the wrong part, topology or circuit only to have 3000 units in stock with failures mounting. Another problem is that they company fails to conduct full reliability and compliance testing before committing to volume production. For example, I have seen companies attempt to design their own power converters only to have reliability problems in production or in the field after copying a reference design assuming it was production ready.

And while it is not impossible, especially with today’s design tools and software, to create a reliable power supply, the odds often do not favor of the casual power electronics designer. Sometimes companies will go out for quotes on a power supply and then assume internal resources will be are “free” when in reality external solutions can fit the bill at less cost and effort. The engineering time can then best be spent on the companies core product line vs trying to be a power supply company in addition to making their end product as well.

The following are questions that an applications engineer should ask before attempting to design a power supply in house. If an engineer answers “no” to even one of these questions, it is likely that building an in-house power supply will prove to be a time consuming, costly and frustrating endeavor. Even assuming an in-house design can meet all the technical requirements similar to a commercially available supply, it will difficult, or impossible, to make it pay off in the long run. In contrast, power supply manufacturers can offer the products, test equipment, and experienced staff that allows them to answer “yes” to all the questions on the checklist.

Do I have the proper test equipment?

Frequency response analyzers, transient generators, AC sources, IR cameras, EMI-RFI measurement equipment, hi-pot testers, power analyzers, electronic loads, electrical safety testers – these and many other types of specialized equipment are needed when designing a power supply. Many companies simply do not have these items on hand and buying or renting them is inefficient and can be costly. Furthermore, capital approval can take months or even years if it can even happen at all at certain companies.

Do my people have the necessary skill-sets?

To design power supplies, it is necessary to have a workforce with experience in magnetics design, power system PCB layout, as well as high-frequency power and grounding, and shielding techniques. Also, when the auto-router is turned off, manual layout skills will be needed. Additionally, during layout, the proper safety spacing techniques need to be considered in order to meet creepage guidelines.

Do I have thermal simulation software?

FlowTherm and magnetics simulation software such as Magsoft or other specialized CAD tools are necessary when designing a power supply.

Do I have the right expertise and equipment to meet safety standards?

This is a must when performing electrical safety testing on the power supply designs to ensure that they comply with worldwide safety specifications and regulations. It is unlikely that a company will know how to design magnetics for clearances required by safety agencies and approvals. The number of global approvals keeps increasing and changing – do you have the staff to keep up with compliance to them all?

Without the necessary expertise and equipment, it is time-consuming and expensive to try to obtain safety agency approvals. Moreover, products that ship internationally will require large input ranges of voltages and operating frequencies (85-265, 47- 440 Hz), and will require a long list of agency approvals, validations, and certifications for world-wide acceptance. Companies who take on these tasks in-house must be prepared for expense and delays, assuming they have the safety experts on staff who understand the regulatory requirements in the first place.

 Am I prepared to do EMI-RFI testing?

Switching supplies often generate a lot of noise, and they are even nosier if not properly designed. Shipping globally requires meeting electromagnetic interference (EMI) and radio frequency interference (RFI) approvals. This testing can be costly – especially if you don’t pass the first time – and requires highly specialized test equipment as well as the people who know how to do the testing in an anechoic chamber or a test range. The FCC even dictates domestic EMI-RFI compliance – and they have no patience for a poor switching power supply design. Plus, if when shipping globally, the individual standards for each country need to be understood and complied with. Trips to the EMI lab are very expensive.

Can I design in PFC?

In many countries, power factor correction is required for equipment over certain power levels. In-house power supply designs will need to have PFC installed to meet the necessary requirements.

Can I conduct environmental tests?

Do you have temperature test equipment to evaluate designs over minimum/maximum temperature humidity extremes, high/low input voltages and min/ max output loads? What about shock and vibration? Even during transport and shipping this can be severe.

Will my design have inrush current and transient protection, Hi-pot certification etc?

Most commercial power supplies have protection against potentially harmful events. But for an in-house design, the mean time between failures (MTBF) will need to be known. Many manufactured supplies have predicted reliability as well as established reliability as they have been shipping off the shelf models to other customers in high volumes for extended periods of time and have a great deal of demonstrated MTBF data.

Do I understand the transient recovery time and stability over time and temperature?

The temperature coefficient for both current limit and voltage stability for a power supply design must be well understood. Engineers will need to know how to design and optimize control loops in high current power supplies and systems. Additionally, an in-house design must be well behaved under fault conditions, including short circuits, brownouts and over voltages, sags, surges, dips, transients, overvoltage, over current, over temperature.

Modular power supplies

Modular power supplies can provide something between a full custom system and building a power supply from the ground up if volumes do not mandate a full custom power supply.

Conclusion

For many applications, especially off line and isolated telecom, industrial, s
ome military and most medical – industrial or other sophisticated electronics, the majority of power supply designs are best left to established, global manufacturers who focus on power supplies as their sole product. Modular power supplies offer companies an efficient, reliable and cost-effective alternative to trying to cut costs by designing supplies in-house and serve a very significant need vs commissioning a full custom or compromising on gluing several fixed output supplies together.