Buck-boost transformer and PWM topology help OEMs operate equipment in countries with mains fluctuating outside design limits

Magnetics Selection A major advantage of VRp topology is the fact that buck-boost transformer is not larger than a 2kVA isolation transformer for a system with output of 10kVA and with regulation of +-20%. Conversely a similar transformer with a winding ratio of 10:1 would be able to handle 20kVA. The result is an economical approach as magnetic components are kept to a minimum.

Conventional transformers with CGO cores can be used at the lower power levels where losses due to harmonics from the inverter section are not high enough to impact efficiency. Of course, core materials such as thin-gauge, Hi-B materials that tolerate higher frequencies will improve efficiency by reducing AC losses due to eddy current and hysterisis of the material. Losses due to high-frequency skin-effect can be controlled by appropriate selection of copper conductor thickness; such as copper foil and litz wire.

Systems Efficiency The typical VRp has an efficiency of 96%, but this can be improved by the selection of better magnetics and slower response time.

Conclusion

The VRp offers a reasonable improvement on the slow, electro-mechanical AVR, the electronic tap-switching AVR, and the dual-conversion AVR, as it offers many of the attractive features of these technologies at a reasonable price without trading off performance parameters. VRp does not cause di/dt problems, as it doesn’t switch the power path, and it is highly efficient and attractively priced.

End footnotes

TSI Power Corporation, based in Wisconsin, USA, develops and manufactures electronic AVRs, DC-AC power inverters, and automatic transfer switches as well as customized equipment to suit specific needs. Customers include OEMs, telecoms, and governments.

Romarsh Ltd. is a major UK based manufacturer of transformers and inductors. It counts among its customers major blue-chip OEMs throughout the world. Romarsh manufactures TSi AVRs for sale in EMEA.

TSi/Romarsh have co-developed a range of AVRs suitable for countries with very poor mains stability.

About the authors Peter Nystrom is the president of TSI Power Corporation and the developer of two generations of AVR products. Nystrom is the founder of three companies and has served as a consultant to a major telecommunications infrastructure manufacturer. He is a director of two UK companies and one US company. Peter can be contacted by email: peter.tsipower@gmail.com (TSi website: www.tsipower.com) Paul Ling is the Technical Sales Director of Romarsh Ltd and is a committee member of the UK. Magnetics Society. Paul can be contacted by email: paul@romarsh.co.uk (Romarsh website: www.romarsh.co.uk)