Closed loop feedback control circuits for gas discharge lamps

Abstract

A circuit for controlling low wattage gas discharge lamps comprised of a inductive coupling device, e.g., a transformer, a switching transistor, a current sensor, a comparator receiving a signal from the current sensor and driving the switching transistor, and a protective clamp circuit. The primary winding of the inductive coupling device receives a DC voltage and is also connected to the transistor. The transistor allows current to flow through the primary winding causing energy to be stored therein until the current sensor signals the comparator that a first threshold has been reached, the comparator then turns the switching transistor off and the inductive coupling device then enter a fly-back mode where the stored energy is applied to the lamp for a set period of time. The transistor then turns back on and this process is repeated so that the lamp receives a high frequency alternating voltage. The clamp circuit senses the voltage produced when the inductive coupling device enters the fly-back mode and, when this voltage reaches unsafe levels, the clamp circuit limits the energy stored in the inductive coupling device and thereby limits the voltage to a safe level. In one configuration, a boosting circuit is used to increase the first threshold to permit more energy to be stored in the primary winding and thereby increase the voltage applied to start the lamp.