By using a gated-oscillator architecture instead of a clocked-PWM one, gated-oscillator-type switching regulators permit high efficiency over extended ranges of output current. This architecture eliminates the housekeeping currents associated with the continuous operation of fixed-frequency designs. Gated-oscillator regulators simply self-clock at whatever frequency is necessary to maintain the output voltage. Typically, loop-oscillation frequency ranges from a few hertz to the kilohertz region, depending on the load.source

A programmable reference diode, such as the Motorola TL431A, can supply constant-current bias for a silicon pressure-sensor bridge. This circuit is simpler than using an op amp and separate reference diode or than using a current diode, which requires temperature compensation.source

This power supply does not really excel in power efficiency, but it is the cheapest and most compact solution for a small power supply.Note that the low voltage output is directly connected to the mains; it can not be used when the low voltage part can be touched (e.g. chargers).With the component values of this circuit, the circuit can supply 12V /15mA max.source

This circuit based on a chip (LTC4412). It has been designed to be used in conjunction with an external P-channel power FET, to form an ideal diode function with a very low 0.05v voltage drop. The chip monitors the voltage on either side of the FET.source