Interleaved converters

In the case of some types of polyesters, the dried treated sheet is very tacky, and must be interleaved with a plastic release film to prevent blocking in the roU prior to use. Certain polyesters and acryhcs can be converted to a gelled state by exposure to actinic radiation. 

Figure 3-27 The effects of interleaving on the electrical waveforms of an off-line flyback converter. (Note spike amplitude and overall degree of "ringing.")...

The first thing we need to do to improve the situation is have opposing flux lines cancel each other. In a forward converter, that is in fact something that tends to happen automatically, because the secondary windings pass current at the same time as the primary, and in the opposite direction. However, even that can prove totally inadequate, especially at the higher power levels that a forward converter is more commonly associated with. So a further reduction in these proximity losses is achieved by interleaving as shown in Figure 3-9. 

Note In the flyback, we had ignored the proximity effect for the sake of simplicity. But in any case, since the primary and secondary windings do not conduct at the same time, interleaving won t help. But interleaving is still carried out in the flyback, in a manner similar to the forward converter. However, the purpose then is to increase coupling between primary and secondary, and thereby reduce the leakage... 

As with any polynomial, these can be factorised to find the roots, but when we do so in this case, we find that because the system is lossless then all the roots lie on Ihe unit circle. If we solve for P(z) and Q z) we find that the roots are interleaved on Ihe unit circle. In essence, the LSF representation converts the linear prediction parameters fi om a set of P/2 pairs of complex conjugate poles which contain frequency and bandwidth information, into a set ofP/2 pairs of frequencies, one of which represents the closed termination and the other the open termination. Note that while the LSF tube is lossless, the loss information in the original LP model is still present. In the process of generating A z) back from P(z) and Q z), the extra 1 reflection coefficient is removed and hence the loss and the bandwidths can be fully recovered. 

Surface insulation tests utilize two interleaved Cu combs with an imposed dc bias across the combs. These combs may be designed into existing boards or a coupon such as the IPC-B-25 test board shown in Fig. 57.24 may be used.The measured resistance (ohms) from the comb pattern can be converted to surface resistivity (ohms per square) by multiplying the measured... 

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