This is a step up converter. It functions by driving the capacitor with the transformer and its load as a resonance circuit. If the load is changed it affects the backwards inductivity and thus changes the frequency. This circuit is designed to always be resonant. In application this circuit can be controlled to move away from resonance, decreasing the transmitted power. In this case it wouldn't have a direct feedback loop but a sensing circuit connected to the secondary side that would control the driver frequency.
