Small power transformers play a vital role in several products and in the toolbox of electrical engineers. They show up wherever there is a need to reduce line voltage to a lower level such as in electronic devices, industrial equipment, and household appliances. While the basics are well established, many refinements have been made to make small transformers more reliable, safe, and costs-effective. One of the most significant innovations has been the introduction of epoxy encapsulation.

Originally introduced in Europe, where they command 80 percent of the market for transformers under 10 VA, epoxy encapsulated transformers are becoming increasingly popular in North America and the rest of the world. They offer a number of advantages over traditional open-core designs. The most obvious benefits arise from the physical protection that the encapsulant and outer casing provide to the windings, core, and lead connections. With these fragile components sealed inside a tough, waterproof casing, encapsulated transformers are a clear choice whenever moisture, dust, dirt, or other environmental threats are an issue.

Since the casing and encapsulant provide physical support for the internal components, these transformers are highly resistant to mechanical damage, shock, and vibration. In addition, encapsulated transformers are suitable for automated assembly processes because their simple, regular shapes make them easy to grip and manipulate. This allows PC board installation using fast pick-and-place robots.

However, physical toughness and environmental ruggedness are not the only advantages. Keith Robertshaw, head engineer at TransERA Electronics of Waterloo, Ontario, a subsidiary of the German-based ERA Group and a member of the Technitrol family of Philadelphia, PA, explains that encapsulation also has a strong impact on thermal and electrical performance. “The epoxy compound used for potting has a high coefficient-of-thermal conductivity and is very effective at dissipating heat away from the windings and core.” This thermal management reduces winding temperature differentials and allows the transformers to be used in higher ambient temperature environments without exceeding allowable temperatures for the insulation class.

The epoxy encapsulant also increases electrical insulation reliability when compared to tape or paper insulation. All production is subjected to a 4, 000V hi-pot test between primary and secondary windings. With no air around the windings, there is reduced potential for corona under high voltage surge conditions.

Because the encapsulation process eliminates tape or paper wrapping, with its potential to damage very fine windings or connections, it is feasible to build transformers with significantly finer wire than used for open-core transformers. This results in very compact designs: For example, Mini Line transformers are less than an inch square and wide and under a half-inch deep. The use of fine gauge magnet wire also means that these small transformers have high electrical resistance windings. Because of this, they are inherently energy limiting and meet UL/CSA Class 2 requirements for overload current and temperature without fuses or other current limiting devices. Moreover, they will recover from extended overload conditions without damage to the transformer.

The manufacturing process for encapsulated power transformers is highly automated and closely controlled for quality and consistency. Potting is done under vacuum. All products are thoroughly tested at each stage and are designed to meet a wide range of US and international standards. The transformers, which range from 0.08 to 50 VA power ratings, are built in a variety of configurations and shapes to meet different installation requirements.

Given their mechanical ruggedness and electrical and thermal characteristics, encapsulated transformers can be used for a wide range of challenging applications, including industrial equipment or appliances designed for harsh operating conditions, out-door metering equipment, and line-monitoring electronics.

Stefan Aichele is vice president of TransERA Electronics, 7 Colby Ct., Waterloo, Ontario N2V 1Y9, Canada. The company can be reached at www.trans-era.com or by phone at 519-885-5040.