Inductor design

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)-... 

As mentioned, all the steps involved in the general inductor design procedure below are being carried out at a certain Vin — which is the maximum input voltage for a buck, and minimum input voltage for a boost or a buck-boost. 

This completes the general inductor design procedure. 

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. 

For a buck The general inductor design procedure was carried out at Vinmax, that is, Dmin. So for example, the r we have set to 0.3-0.4 (and possibly re-calculated with the selected inductor) is actually Tdmin to be precise. Similarly, the voltseconds, Et, we have calculated so far is actually EtoMiN- ... 

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)-... 

For the boost and buck-boost, the general inductor design procedure is at Dmax in any case. So we can directly use the numbers derived from that, to find the switch RMS current using the equation below ... 

Coincidentally, the worst-case output capacitor RMS current for all three topologies occurs at the same point at which the general inductor design procedure for each of them is carried out. In other words, this point is Vinmax for the buck, and Vinmin for the boost and buck-boost. So we should have no trouble, directly using the numbers derived from the general inductor design procedure, to find the worst-case RMS current of the output capacitor, using the equations below. 

For a buck — peak to peak capacitor current is Io x tdmin- This is the same point at which the general inductor design procedure would have been carried out, and so rDMiN is already known. 

For the buck-boost, things are much simpler, since the worst-case input capacitor RMS current occurs at Dmax, which is also the point at which we carry out the general inductor design procedure. So all the numbers available from that procedure can be used directly in the equation below... 

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. 

Here we should actually check the IC datasheet to see the MAX value of the current limit range. For example, for a 5 A switcher the MIN value may be say 6 A (set high enough simply to guarantee full 5 A load capability with the usual 20% inductor design criterion mentioned above), but the MAX value of the current limit may be say 7 A over temperature and process variations, and depending upon how it may have been trimmed in production. 

McLyman, W. T. Transformer and Inductor Design Handbook , 2nd Edition, 1988, ISBN 0824778286 Marcel Dekker, Inc. 

Transformer and Inductor Design Handbook Second Edition, Revised and Expanded, Colonel Wm. T. McLyman... 

Miniature power inductors designed for SMT assembly consist of a coil wound on a ferrite bobbin core that sits on a base injection moulded from Zenite LCP. 

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