Maybe this question is perhaps too broad for the site: however, after looking on Google, searching the literature and asked to a friend who's a specialist in the design of SMPS without getting any useful answer, I decided ask it here, apologizing in advance if it is "Off-topic".
Question: do there exists switching power supply topologies which do not rely on the standard PWM paradigm but work, for example, by producing a variable, voltage controlled, frequency sine oscillation? And in case the answer is yes, what are their most used structures?
A little bit of background
- I got interested in the topic after seeing the timeline of the switching power conversion as given in the wikipedia: it spans at least 50 years, so it seems plausible that a great deal of different ideas have been developed. So why focusing on designing fixed frequency PWM modulators/demodulators?
- In general manuals and application notes and about switching power supplies, it is occasionally said that "the frequency is kept fixed in order to improve the EMC compliance". However, it seems more that this is a design trend and not a design requirement imposed by deep physical limitation, as it is seen for example in , so why always choose the PWM way of design?
 Mikko Kuisma, "Variable Frequency Switching in Power Supply EMI-Control: An Overview" January 2004IEEE Aerospace and Electronic Systems Magazine 18(12):18 - 22, DOI: 10.1109/MAES.2003.1259021.