Dim the lights. Problems with lamp current control using a PWM signal

It is the state of the art to drive a fluorescent lamp with a ballast that mainly consists of a high-frequency generator. High frequency, in this context, means a frequency that is substantially higher than the mains frequency. Most ballasts on the market are working at an operating frequency of about 50 kHz. It is common to realize the high-frequency generator with a half-bridge inverter. This inverter has an output waveform with a rectangular shape, and the operating frequency f_o is equal to the frequency of the oscillation of the half-bridge. The fluorescent lamp is connected to the output of the inverter via a matching network. The matching network has two main tasks. The first task is to transform the output impedance of the inverter to an impedance the lamp needs for stable operation. The second task is to cut off the DC component of the output signal that is delivered by the inverter. This task can easily be accomplished by a capacitor that is connected in series to the lamp. Of course, a lamp needs a certain value for the lamp current. One technique to control the lamp current is variation of the pulsewidth of the inverter output signal. The main purpose of this article is to discuss the problems of the pulsewidth modulation (PWM) control of the lamp current and to propose a new driving scheme for a PWM-controlled half-bridge inverter.