AC/DC power supply performance and international efficiency standards

July 21, 2015 //By Paul Buckley
AC/DC power supply performance and international efficiency standards
This article discusses  the implications of  new efficiency standards on the power supply controller, how they affect the output voltage integrity and the latest design techniques used to improve performance without adding unnecessary cost or complexity.

New design techniques With the increasing restrictiveness of international power supply efficiency standards, power supply controllers are being pushed to their cost-performance limits.  Meeting these new standards while keeping performance high and cost low, has forced the market to move to disruptive new technologies.  now enable AC/DC converters to achieve the stringent DC efficiency requirements without sacrificing AC performance, in particular, load transient response.

International Power Supply Efficiency Standards

The US Department of Energy’s AC/DC external power supply efficiency standards enacted in 2007 established a strict combination of no-load power consumption and average efficiency at loads from 25% to 100% of rated load current.  The European Union also has enacted similar standards, as have other countries throughout the world, although the DoE’s standards are the strictest mandatory standards.  The DoE released an updated external power supply standard in February 2014, further restricting efficiency and no-load power consumption in off-line power supplies.

By limiting the maximum power consumption of a power supply when operating at no-load, the standard forces power supply manufacturers to reduce the amount of current drawn from the mains input during no-load operation.  Starving a control circuit of current during standby saves energy, but it also hinders the power supply’s ability to transition quickly from no load to full load, a feature that we remarkably take for granted in our always-on, consumer electronics driven world.

Load Transient Response - Large Signal Response and Operating Current

Load transient response time directly impacts the quality of the output voltage; faster response time helps reduce the deviation from the regulated output voltage without having to use excessive amounts of output capacitance, and slower response time does the opposite.  With low power consumption controllers, the response time tends to be slower, forcing the power supply to depend upon the external components to take care of the output current changes.  Load transient response is, in reality, a measure of the large signal response

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