Worst-case Input Voltage

Therefore, for a buck, it is always preferable to start the inductor design at Vinmax (i.e. at iin). 

Therefore, for a buck-boost, we should always start the inductor design at Vinmin (he- (d Duxx)- 

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Worst-case Input Voltage for Switching Losses... 

The worst case operating condition is at the high input voltage. 

From Figure 2-4, we see that Iac increases at high input voltages for both the buck and the buck-boost. For a buck, the general inductor design calculation above was carried out at Vinmax and that just happens to be the point at which the core loss is a maximum too. Therefore, calculating the core loss at Vinmax as we did in the previous example does coincidentally also give us the worst-case core loss. 

Note If for the boost, the input range of the given application does not include the D = 0.5 point, we need to identify which voltage end of the range provides a duty cycle closest to D = 0.5. And we need to then do the worst-case core loss calculation at that end (if so desired). 

For the buck, as the input voltage is raised, the duty cycle falls, and because the average inductor current II remains fixed at lo, the average diode current increases. That means we get the worst-case diode current (and dissipation) at Vinmax for a buck. So we can just use the numbers we already have derived from carrying out the general inductor design procedure (at Vinmax)-... 

The rule-of-thumb is to pick an output capacitor with a ripple current rating equal to or greater than the worst-case RMS capacitor current calculated above. Its voltage rating is usually picked to be at least 20 to 50% higher than what it will see in the application (i.e. Vin.max for all topologies). The input voltage ripple of the converter is also usually a concern because a small part of it does get transmitted to the output. There can also be EMI considerations involved. In addition, every control IC has a certain (usually unspecified) amount of input noise and ripple rejection, and it may misbehave if the ripple is too much. Typically, the input ripple needs to be kept down to less than 5% to 10% of the input... 

Having verified the selection of Vz and Vor at highest input, now we need to get back to the lowest input voltage, because we know from the previous discussions about the buck-boost (see the general inductor design procedure in the previous chapter) that Vinmin is the worst-case point we need to consider for a buck-boost inductor/transformer design. 

The most basic question in design invariably is — what input voltage represents the worst-case point at which we need to start the design of the magnetics (from the viewpoint of core saturation) For the forward converter choke, this should be obvious — as for any buck converter, we need to set its current ripple ratio at around 0.4 at Vinmax- But coming to the transformer, we need some analysis before we can make a proper conclusion. 

Though we can pick any specific input voltage point for assuring ourselves that the core does not saturate anywhere within its input range, since the copper loss is at its worst at Vinmin, we conclude that the worst-case for a forward converter transformer is at Vinmin For the choke, it is still Vinmax-... 

We must return now to the all-important question — when we have a wide-input voltage range, what specific input voltage point represents the worst case for calculating switching losses ... 

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