 Design Considerations |
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Design Considerations |
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  If the source resistance R1 is too large relative to R2, our simple resistive divider power supply will not work so well! As current is drawn by Io, Vo dips rapidly due to the IR drop across R1. Meanwhile, R2 is (relatively) low, and current leaks away due to V/R losses through R2. The result is as shown in the graph. Regulation is not maintained very long as Io>0, and we soon drop below Vreg! Our design is now good only for very small values of output current, and the transition to current limit is too slow. It is for this reason that a power supply must work from a low impedance voltage source, while its series resistances must be kept small! Likewise, currents shunted to ground must be minimized, as must housekeeping and drive consumption. As a footnote, you will find it disquieting to calculate the power lost in a Shottkey diode due to leakage in the OFF state--particularly when it is running hot! Such dissipation can rival forward losses if one is not careful....   A similar, but somewhat more complex analysis of the typical switching converter yields essentially the same results as did our DC analogy! This work was done by Caltech's Dr. R. D. Middlebrook, and involved the construction of his "Canonical Model." From it, one can derive his impedance conditions for stability and high performance. These say that the output impedance of the converter's input filter must be much less than the input impedance of its output filter! This condition is really just the AC equivalent of the result we found for the resistive divider above! There, we found that R1 << R2 gave us a good design. Here, we must design our converter so that Zi << Zo. If we do not, the converter will 'choke' under load transients. That is, a sudden demand for output current will not be met, since increased current through the input filter causes an excessive voltage drop, and the output is left starved for power! | In the Frequency Domain | Background | Tips | Engineering vs. Science | | Isolation of the Cuk Converter | Historical Perspective | | Return Home | Safety First | Lesson in Ethics | The Consequences of Bad Design | Real World Applications | Deals | e-mail me! | |
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