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Power tip: Boost cell phone charger design

Posted: 22 Oct 2012     Print Version  Bookmark and Share

Keywords:low-power chargers  FET  valley switching 

There is an additional challenge when improving the voltage regulation of these types of power supplies. The device being charged is at the end of a cable, which can have significant voltage drop at full load. In this particular implementation, the controller estimates the output power from the power switch peak current, allowing the output voltage to be adjusted to compensate for the voltage drop across the cable.

Figure 4 shows the physical embodiment of the power supply. High-frequency switching and advanced control methods provide significant improvements over previous wall adapter designs. Input voltage capability is increased from a single voltage to universal input. The no-load dissipation is reduced from the 1 watt range to less than 30 mW. Full-load efficiency is improved from the 50 per cent range to about 80 per cent with diode rectification, and 85+ per cent with synchronous rectification. Finally, size and aesthetics have been significantly improved.

Figure 3: Advanced control ICs eliminate the optocoupler with primary voltage sensing.

Figure 4: High-frequency switching and advanced control techniques minimise size and loss in this offline cell phone charger.

Advanced circuit techniques have remade the lowly cell phone charger from a clunky wall wart that consumes significant amounts of power to an innocuous device that is not much larger than a wall plug. The power savings from this improvement is significant. With two billion new cell phones being put into service yearly, the savings is measured in tens of power plants that will not be needed worldwide.

About the author
Robert Kollman is a Senior Applications Manager and Distinguished Member of Technical Staff at Texas Instruments. He has more than 30 years of experience in the power electronics business and has designed magnetics for power electronics ranging from sub-watt to sub-megawatt with operating frequencies into the megahertz range. Robert earned a BSEE from Texas A&M University, and a MSEE from Southern Methodist University.

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