The circuit described here is called a boost converter. It can take a low DC source voltage and step it up to a much higher DC voltage, a process also called DC to DC conversion. The source voltage can be anything from batteries to solar panels. Since the total power output (P = V I) must be conserved, the output voltage will be higher and the current will be lower than the source current.
The boosted output can be used to power xenon flash lamps, solid state laser diodes, Geiger counters, high voltage capacitors and other circuits that require a high voltage to operate.
The basic operating principle of a boost converter is the ability of an inductor to resist a change in current flow by creating and collapsing a magnetic field. When current flows through an inductor, the inductor stores some of the energy by generating a magnetic field.
If the current flow is suddenly interrupted, the magnetic field will collapse and in the process, creates a current flow towards the load. This results in an output voltage that when added to the original source voltage, will be higher than the source voltage alone.
To take advantage of this property of an inductor, the inductor must be cycled at a high enough rate to prevent it from fully discharging. The inductor output is then routed through a diode and then to a capacitor that is connected in parallel to the load. The capacitor will then charge to a voltage that is higher than the source voltage. The diode is necessary to prevent the capacitor from discharging backward through the inductor and source. The load can then utilize the current and increased voltage stored in the capacitor.
In the example below, a simple transistor oscillator (Q1, Q2) is used to drive a buffer transistor (Q3). The output of the buffer transistor is used to drive the gate of a MOSFET (Q4) that switches the 1mH choke (L1) on and off at a rapid rate. The boosted output from the choke is routed through a steering diode (D1) and then stored in a 0.1uF capacitor (C3) that supplies power to the load. The two one megohm resistors (R7, R9) act as a minimal output load and as a bleeder resistor (R7) that will safely discharge the storage capacitor when the circuit is not in operation.