Method to control striations in a lamp powered by an electronic ballast

摘要

A high efficiency electronic ballast or driver circuit provides striation control at reduced power levels. The driver circuit includes a controller operating a half-bridge inverter to drive a resonant tank circuit. The controller provides asymmetric on-times to upper and lower switches of the half-bridge inverter when operating at low duty cycles (e.g., duty cycles of 50% or less). The resulting asymmetric output current eliminates striation in lamps driven by the ballast. In order to maintain light output (i.e., output current), the controller reduces the operating frequency of the half-bridge inverter to increase the gain and impedance of the resonant tank circuit.

主权项

1. A driver circuit configured to provide power to a load from a power source, the driver circuit comprising:
a controller configured to provide an upper switch drive signal and a lower switch drive signal, wherein an on-time of the upper switch drive signal is less than an on-time of the lower switch drive signal; a half-bridge inverter coupled to the controller and configured to provide an output signal as a function of the upper drive switch signal and the lower drive switch signal; and a resonant tank circuit coupled to the half-bridge inverter and configured to receive the output signal from the half-bridge inverter and provide power to the load as a function of the received output signal; and wherein the controller comprises
a signal generator configured to provide a frequency modulated signal having a duty cycle of 50% or less, wherein the frequency is modulated as a function of an output current or output voltage provided to the load,a delay circuit coupled to the signal generator and configured to receive the frequency modulated signal, and delay a turn on-time of the frequency modulated signal to generate a delayed frequency modulated signal, anda half-bridge driver coupled to the delay circuit and configured to receive the delayed frequency modulated signal and provide the upper switch drive signal and the lower switch drive signal as a function of the delayed frequency modulated signal,the half-bridge driver further comprising:
a first dead time delay configured to delay a turn on-time of the upper switch drive signal from a turn on-time of the delayed frequency modulated signal,a NOT gate configured to receive the delayed frequency modulated signal and invert the delayed frequency modulated signal to generate an inverted delayed frequency modulated signal, anda second dead time delay configured to delay a turn on-time of the lower switch drive signal from a turn on-time of the inverted delayed frequency modulated signal.