Here is a app note from Microchip describing how to implement a high-voltage boost DC-DC converter intended for Nixie tube designs. The basic principle of Nixie tubes is that they require more than 170V to function properly.
This design uses the internal PWM generator of a PIC16 microcontroller to drive the DC-DC converter. A comparator controls the PWM pules to the converter. One side of the comparator is connected to a internal DAC, and the feedback output voltage from the power supply is connected to the other. Slow output voltage increase is achieved by varying the DAC output.