The smallest/cheapest DC-DC converters are just multivibrators with a transformer hanging off it. No protection, no control, only raw AC. They tend not to go into this aspect in much detail, in the datasheets, but it is apparent from the characteristics they must provide (e.g., output regulation is sloppy and PSRR is nil).
You'll find circuits can in fact be simpler, but even more compromises must be made.
![](https://www.seventransistorlabs.com/tmoranwms/Circuits_2010/Blocking_Oscillator.png)
This is the basic plan of the blocking oscillator. The transistor turns on hard, due to positive feedback, then turns off due to rising inductor current and/or falling base current (due to the relatively small value of Cbb, which discharges through Rb); in either case, hFE rising until it comes out of saturation. When it switches off, the voltage flies back, where you employ a clamp diode on the collector (for a boost or SEPIC converter) or secondary (for an isolated flyback supply).
Like the Royer, a feedback winding is needed, which itself may be more expensive (i.e., custom magnetics) than throwing a few more transistors at it (especially transistors with nearly zero incremental cost, i.e., on an IC).
Discrete circuits are entertaining, but do not fool yourself into thinking they are in any way size- or cost-effective except in very rare cases today.
![Smiley :)](https://www.eevblog.com/forum/Smileys/default/smiley.gif)
(I'd say, the case of making a million qty run is rare enough to count.)
Tim