Design Article

Pump and dump -- delivering more power than you thought possible!

John Betten, Application Engineer, Texas Instruments

10/15/2008 11:18 AM EDT

Designers are often challenged with creating highly efficient power converters. The reason may be to reduce heating within a confined area, letting other subsystems have more available input power, or just to save power in the quest to be "green." So what is a designer to do when asked to provide 50 or a 100 percent more output power beyond what is available from the input? This seemingly impossible task can be accomplished " but not without practical limits. Some loads require power beyond the input limits, but only for "relatively" short bursts of milliseconds, seconds or even minutes. This article discusses how this can be accomplished by charging a capacitor bank to store energy until needed (Pump), and discharge it into the load (Dump) in a controlled manner.

All input power sources have defined limits, whether it's current, voltage or power. Batteries droop in voltage when loaded heavily, indirectly setting the maximum output current to maintain voltage regulation at the load. Nearly all power adapters or "black boxes" are designed to a maximum output power level, thereby setting the maximum input power. Beyond this level and the power adapter may go into over-current protection mode, or even blow a fuse to protect the input source. The versatile universal serial bus (USB) is a 5V source with an output current of only 0.1A, but when requested can supply a maximum current of 0.5A. That current limits this extremely popular power source to just 2.5W. Additional output power can come only from drawing upon a source of stored energy, such as a capacitor or a battery.

Providing current to the load from a charged capacitor is a matter of providing the correct amount of charge over a defined period of time. Viewed in terms of delivered power, this is defined by Equation 1 where Vi is the capacitor's initial voltage, and Vf is the final, discharged voltage.


Equation 1


Next: Know Your Load


bcarso

10/16/2008 1:14 PM EDT

Check out US 6178514 which uses this technique for sipping power from a bus. The application initially was powered speakers with power from USB, but it's applicable to a far wider set of devices and bus categories.

Brad Wood

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