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 Input Filter Placement
One of the most difficult, and least understood aspects of converter design is the requirement that "The output impedance of the input filter must be less than the input impedance of the output filter!" In the Cascade and Cuk Converters, this rule is complicated by the fact that, unlike the case of the Buck, there is a dip, or glitch, in the control to output voltage transfer function. This, it was taught, meant either a bilevel low frequency power loop gain, or otherwise a section of four pole negative slope. As neither of these is desireable, I came up with the idea of incorporating the dip into the two pole rolloff of the control function, resulting in a simple double pole in the vhat/dhat transfer function. These poles are slightly lower in frequency than those of the output filter, but that is of little consequence. Given this constraint, the in/out-put filter placement problem becomes far simpler, and thus easier to solve*. A discussion of the solution is to be found in succeeding pages; but the result is four reactance values, 2 L's & 2 C's, that require only damping to be well behaved. *This is sometimes referred to as the "Engineer's Dilemma," that a more difficult poblem is easier to solve!   Shunt Damping
A very erudite paper exists which demonstrates how best to shunt damp an LC filter. Unfortunately, it is quite involved mathematically for the working engineer. A simplified approach exists if one just "bridges technologies" so that the damping capacitor is an order of magnitude or more larger than the working cap. Using an electrolytic to damp a film cap is an example, and tantalums can be used either damped by a 'lytic, or as damping for a monolithic Redcap. With this simplification, one just chooses a damping resistor r so that Q = r/SQRT(L/C) is about 1 or 1/2. The exact value of the damping cap is irrelevant, just so Cd>>C. Using a damping cap of the same kind as the working cap is NOT recommended, since it is hard to achieve Cd>>C. Note that this rules out electrolytic caps as the working caps in a switcher. That is, the C's of the equivalent input and output filters. This is not really a limitation, since current practice pushes fs up well beyond the region where such large values are appropriate.  Transient Current in the Isolation Transformer
Perhaps the most difficult problem encountered by those designing Cuk Converters in the early days was transient current in the isolation transformer. This would have caused mysterious switch failures due to saturation, in spite of proper operation steady-state. Suppose that a line transient occurs, shifting the input voltage, Vg, from its min to its max. The input-side energy transfer cap stands off Vg in the steady state. When this voltage shifts from one value to a higher one, charge moves off of the cap (V increases, since charge is negative, thanks to Ben Franklin!) which moving charge constitutes a current in the primary winding of the isolation transformer. Meanwhile, on the output side, regulation maintains the output voltage, V, constant. Thus the voltage on the output side half of the energy transfer cap remains constant too. Thus current flows into the dot on the primary side, but none exits the secondary side. What happens? It is simple: the current flows through the magnetizing inductance of the core. [Drawing out the equivalent circuit is helpful, here.] The problem is, how much is the peak current, and does it saturate the core? Obviously, as this current rises, then falls, the area underneath is the change in charge on the ET cap corresponding to the input voltage change, via Q=CV. But, as a little thought will show, a larger Lm will slow down the rise and fall, reducing the peak. But this transformer will saturate more easily, since the core has little gap. Conversely, with a large gap, Lm is small, and the peak occurs quickly, and is high, which may saturate even a gapped core. The question is, how much overhead is needed to accomodate a worst-case line voltage, or load current, transient? This problem is taken up in succeeding pages! | Design Hangups | More Hangups | | The Four Topologies | Other Topologies | Philosophy of Design | Do the Math | | Return Home | Old Home | |
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