Current limit second level

In a particular case I handled, a major Japanese customer was managing to blow up his simple Buck switcher under short-circuits on the output. I knew that these parts had not only the usual cycle-by-cycle current limiting (their first line of defense), but in fact, a hidden second level of current limit protection, which if ever encountered, caused protective... 

My suggestion is to open the pdf datasheet of any prospective switcher IC and carry out a text search (Ctrl + F) for the world foldback. If you find it, question the vendor about its full impact before you select the part for your application. Foldback is, in general, a good idea in terms of protecting the converter under abnormal conditions, but it should be used very judiciously so as not to impact normal behavior. For example, the Simple Switcher family has a hidden second-level current limit protection at which frequency foldback (or skipped pulses) occurs. But that trip level can only be encountered under very severe conditions—namely, a sudden overload with a completely incorrectly sized inductor that hard-saturates in the process. At other times it is not encountered and doesn t therefore interfere. It is considered transparent to all but the most novice engineers. And that is what I consider the right type of foldback. 

The second level (1 mA) of 50/60 Hz is due to the direct electric excitation of nerves. The level increases as frequency is increased above 1 kHz (Figure 10.18). For example, 10 mA at 100 kHz is without perception, but temperature rise caused by the current may then be a limiting factor. Interpersonal variations... 

Recently, Veith et al. (1985) developed a second toxicity QSAR from data on 29 industrial esters. This equation, log LC50 = -0.535 (log Kq ) -2.75 r2 = 0.828, or narcosis II model, predicted the activity of polar-narcotic chemicals. Interestingly, the slopes of our equation 3 and Veith s polar-narcotic equation are not different in absolute value, although they are different in sign. This difference is due to the reciprocal transformation of the Tetrahymena toxicity data. Further comparison of these two equations reveals strikingly similar coefficients of determination. Veith and co-workers (1985) note that the polar narcotics QSAR may be improved with the addition of an electronic descriptor. These studies with para-substituted phenols show the field or polar electronic parameter, F, to be the best currently available second descriptor. While being a substituent constant may limit its universality, its addition to the current data set, as seen with equation 4, markedly improves the coefficient of determination, and F becomes significant at the P = 0.05 level. 

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