Foldback

There are two versions of overeurrent proteetion on voltage-mode eontrol ICs. The older version is average current foldback. Here, the output eurrent is... 

Figure 3-47 The high-voltage linear regulator bootstrap start-up circuit (used only at startup and foldback periods).

Fig. 4.3.6 Velocity maps and profiles at differ- mark the NMR foldbacks from the stationary ent heights of the Fano column. The dark ring fluid at the inner surface of the fluid reservoir, surrounding the pipe at z= 1.5 mm (larger In the velocity profiles, the solid curves are the white arrow) is due to a layer of stationary fluid calculated Poiseuille profiles in tube flow, adhering to the pipe exterior following the Velocity images are reprinted from Ref. [20], dipping of the pipe into the reservoir at the with permission from Elsevier, start of the experiment. The small white arrows...

Note that this is an entirely different problem from the I) > 50% foldback issue discussed elsewhere in this book, because that is really related to a rather weirdly designed first level current limit of this family of ICs. 

A. But we truly have serious foldback issues if, for example, we need to reduce the load to much less than the rated maximum. We will likely see startup issues on this converter. 

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. 

March 1, 2006 The condition described is the result of what amounts to a foldback current limit design that s intended to prevent damage to either the regulator or the load under unusual fault conditions. Anyone familiar with foldback current limit will realize that there are always conditions that can be realized that force the foldback to get stuck in a stable, low output voltage operating mode. The solution in general is to reduce the load until the output is allowed to recover. The datasheet clearly advises the user what to look for and how to deal with potential problems that may arise from this. Any implication that the information is deliberately obscured is clearly misleading. 

In the normal mode of the SdFFF operation, a balance is reached between the external centrifugal field, driving the particles toward the accumulation wall, and the molecular diffusion in the opposite direction. In that case, the retention volume increases with particle diameter until steric effects dominate, at which transition point there is a foldback in elution order. 

In general, we have to be very careful in implementing any type of foldback scheme. Foldback is a two-edged sword. It might help control responses to abnormal conditions, but may also end up encroaching on normal responses. 

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