Increasing Active-Mode Efficiency And Reducing Standby Power for Energy Star

by Majid Dadafshar, Fairchild Semiconductor, Carlsbad, Calif., How2Power Today, Jun 29 2010

Focus:
This article discusses requirements for achieving Energy Star certification in set-top box (STB) applications, and how power supply performance can be optimized to meet the Energy Star requirements. Power supply requirements for STBs and the applicable Energy Star requirements are described. Sources of loss in ac-dc power supplies are identified and techniques for reducing these losses during active and standby modes are explained. The loss-reducing techniques include architectural-level improvements as well as component-level optimization. The power supply in this application typically includes an ac-dc adapter based on a flyback converter, plus a dc-dc converter stage. In discussing the ac-dc supply, the article looks at losses associated with the transformer, MOSFET, output diode, and start up resistor. Use of PWM, PFM, and burst modes to reduce power consumption is discussed. Loss issues relating specifically to the design of a synchronous buck converter for the dc-dc converter stage are discussed. These issues include MOSFET selection, shoot-through losses, deadtime losses, and parasitic losses. Other power-saving techniques include the use of load-switches to reduce standby power and dynamic voltage scaling of the CPU supply voltage.

What you’ll learn:

• How to meet Energy Star requirements for set-top box power supplies
• How to improve active-mode efficiency and reduce standby-mode power consumption in set-top box power supplies

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