Overview of mercury migration mechanisms in cold cathode fluorescent lamps caused by asymmetrical driver waveforms

Certain CCFL driver topologies produce an asymmetrical output voltage which causes migration of the mercury to one side of the CCFL tube. It is known that a DC voltage component across the CCFL leads to this effect. However, even when no DC component is present across the lamp, migration takes place if the voltage is asymmetrical. Eventually, in the absence of a series ballast capacitor, a DC current starts to flow through the CCFL, due to the interaction between the non-linear lamp characteristic and the voltage asymmetry. The direction of this external DC current is opposite to the direction of internal ion displacement (migration direction). This paper summarizes the mechanisms causing these effects. Different circuit configurations are investigated experimentally. A DC servo circuit is introduced, regulating the residual DC voltage across the lamp to zero. It is shown that a series ballast capacitor effectively eliminates the DC lamp current, but, under certain conditions, allows for a relatively high DC voltage to build up across the lamp terminals again leading to migration.