The essence of self-excitation circuit is actually a single-tube self-excited oscillation circuit, adding pulse width control circuit and voltage regulator became the basic sampling circuit medical power supply, of course, but also add the necessary practical circuit over-current protection.

Self-oscillating circuit in addition to the adjustment means to adjust the pulse width, there is a way - modulation, frequency modulation circuit
More complex, have become obsolete, the basic need. I've seen the model seems to have: Telefunken ***, are some European brands.

Self-oscillating circuit has two basic components: a resistor (about 20-47 ohms) and a capacitor in series,
They will switch the transformer is a winding of the pulse signal fed to the transistor base, in order to maintain a stable oscillation state transistors