Problems with lamp current control using a PWM signal

In electronic ballasts for fluorescent lamps, it is common to use a half-bridge inverter. If the half-bridge switches are not gated with a 1:1 ratio, a PWM signal is generated at the output of the half-bridge to drive the lamp. To filter out the DC component of the lamp drive, a coupling capacitor is used. Despite of having no DC component in the lamp current, the darkening of one end of the lamp can be observed. This is caused by a discovered nonlinear electrophoresis effect of the mercury in the lamp. To avoid this effect, a new driving scheme for the half-bridge switches is presented. It can be described as toggled duty cycle (TDC). Some properties of the TDC are discussed.