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EDN's Best of Power...Feature Articles
These articles are in-depth looks at a targeted area of power-system design from both EDN technical editors and contributing experts in the field.
By Staff -- EDN, 5/15/2008
These articles are in-depth looks at a targeted area of power-system design from both EDN technical editors and contributing experts in the field.
Wireless power transmission: no strings attachedReceiving electrical power without the use of wires has long been an ideal for electronic devices. How feasible is it, and what are some of the other options?
Portable power: New lithium-ion-battery chemistries allow designers to trade off energy capacity and power
As recently as two years ago, lithium-ion cells might not have met your system's power requirements. But take another look: With new batteries featuring iron-phosphate cathodes, you might be pleasantly surprised by what's available now.
Permanent-magnet motors boost efficiency and power density
Sensorless versions of these highly efficient motors reduce cost and parts count, but the motors still require complex control algorithms. Match the right motor type and controller to your application for the best performance and cost.
Stealing USB-port power
With a design that conserves current drain, you can power your peripheral device directly from the host computer's USB port.
Reducing ground bounce in dc/dc-converter applications
Electrical ground, which looks simple on a schematic, can become complex depending on how you lay out the PCB (printed-circuit board). Unfortunately, ground-node analysis is difficult. However, understanding the physics of ground noise helps to reduce the problem.
The box below originally appeared in "Reducing ground bounce in dc/dc-converter applications," and provides an intuitive understanding of how current moves within a PCB (printed-circuit-board) as well as the effect the PCB layout has on the current.
| PCB-layout configurations affect ground bounce |
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Conductors that cross at right angles do not interact magnetically: The magnetic field from the vertical trace induces positive and negative voltages that cancel in the horizontal trace (Figure A). Magnetic-field lines around parallel wires with equal currents cancel everywhere between the wires, so the total stored energy is less than what you would find for the individual wires. Wide PCB (printed-circuit-board) traces have less inductance than narrow traces (Figure B). Magnetic-field lines around parallel conductors with opposite current flow cancel everywhere outside the conductors and add everywhere inside. If you make the inside loop area small, then the total magnetic flux and, therefore, the inductance will also be small (Figure C). This behavior is the reason that the ac ground-plane return current always flows under the top-trace conductor. Corners have more inductance because both the vertical and the horizontal traces see a magnetic field from themselves as well as from the perpendicular trace (Figure D). A current flows into a top trace, down a via, into a ground plane, and back up a via to the bottom of the source (Figure E). The return current flows, with dc current taking the path of least resistance and ac current taking the path of least impedance. Because top-trace corners and ground-plane cuts increase impedance, you can expect ground bounce. The change in magnetic flux at those points induces the bounce. The upper trace in Figure F shows good layout practice; the capacitor is in line with the current flow, creating a minimal loop size. The bottom trace, with the capacitor at right angles to the current flow, creates an unnecessarily large loop, resulting in ground bounce. |
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