Electronic – Explaining high voltage inverter principle (KIKUSUI COS5020 oscilloscope)

The question appears to be "How do DC Capacitors get UL recognized?"

Or more generally "How does a product get UL Recognized or Listed?"

If the product fall under existing standards (UL and non UL standards) then the manufacturer applies to UL.

If the product does not fit the above standards or a change to the standard is required then the manufacture contacts the relevant standards committee representative (this could be UL ). For example ISO (http://www.iso.ch) or ANSI (http://www.ansi.org). (The committees are composed of “interested parties” like UL, consumer groups, manufacturer’s, insurance representatives etc. )

Another route is to participate as a member of the UL Standards Technical Panel, I believe it takes substantial time and effort. But if you want to participate try here ... http://www.ul.com/global/eng/pages/solutions/standards/developstandards/participation/callformembers/index.jsp

The OP wanted specifically the history of how how the standards were developed for the capacitors. The work of the committees / panels is recorded in minutes / bulitins however I don't know how to get hold of these.

In answer to your second question (which is the easiest), your 5% scheme produces a system which is almost guaranteed to display a certain degree of instability. Let's say that (just as an example) a duty cycle of 50% gives a slightly low output, and 52.5% (1.05 times 50) gives a slightly high output. Then the system will "hunt", shifting back and forth between the two duty cycles. This is a classic problem with feedback systems with quantized outputs. The solution is also classic - establish a "dead zone" in which small errors around the desired output are ignored. Generally the dead zone must be equal or greater than twice the output quantization; in your example, +/- 5%.

The first question is a little harder, since it depends on how much error you can tolerate. A proportional controller provides an error proportional to the inverse of the feedback gain - the higher the gain, the less the error. In principle this error can be made arbitrarily small by increasing the gain. However, since there is always delay around such a loop (in this case the obvious one being the rectification and filtering of the output of the feedback transformer, but the filtering required to remove harmonics from the bridge output also counts), at some point increasing gain will cause the loop to become grossly unstable. If this limit is reached while the error is too large, more sophisticated measures are called for. This is also a classic problem, and you are aware of the classic solution - a PID controller. As to whether or not you need this solution - well, that's up to you. There's no way anybody can tell without a detailed system analysis.