subject: Sequential feedback control system for an electronic ballast [print this page] Sequential feedback control system for an electronic ballast
Feedback control systems are used to control electronic ballast because they provide effective control despite irregular characteristics of lamp loads, and because they are effective for reducing energy consumption and extending lamp life.
A sequential feedback control system that can implement soft start and dimming operations provides further benefits in terms of energy efficiency and extended lamp life. During a soft start operation, a lamp is driven at a reduced power level during a preheat cycle before the entering a discharge mode. This reduces the stress on the filament and extends the lifespan of the lamp. The power to the lamp is then increased until it enters a discharge mode. After a sufficient period during which the discharge is maintained, the power can be reduced during a dimming operation to adjust the brightness of the lamp commensurate with the ambient lighting conditions, thereby conserving power. A sequential control system employing feedback can adapt the power flow to the changing load conditions presented by a lamp as it is operated in the various modes.
A prior art feedback control system for an electronic ballast is shown in FIG. 1. An electronic ballast 12 drives a lamp 11 and generates a current consumption signal (ifb) which is indicative of the current consumed by the ballast. The current consumption signal (ifb) is multiplied with a direct link voltage signal (E) by multiplier 13 which generates a control current signal (imo) representative of the power consumption of the ballast 12. The direct link voltage signal (E) is converted into a direct link current signal (Ie) by a resistor (1/RL).
A resistance block (Rmo) converts the current signal into a voltage signal (Vmo) which is subtracted from a reference voltage signal (Vref) from a reference voltage generator 14 by an adder 15. The adder 15 generates an error signal (Verr) which is converted to an amplified current signal (Iin) by an error amplifier 16 having a transconductance (Gm). The amplified current signal (Iin) charges the capacitor (C) to generate an integrated voltage signal (Vin) which is changed into a integrated current signal (i1) by a voltage controlled current source (VCCS).
A second adder 18 subtracts the integrated current signal (i1) from the sum of the direct link current signal (Ie) and a standard current reference signal (Iref), thereby generating a composite current signal (it). The composite current signal (it) is used by an oscillator output driver 19 to charge a capacitor (Ct) and generate a frequency signal (f1) which controls the power consumption of the electronic ballast (12).
One aspect of the present invention is a sequential control system for an electronic ballast comprising: a feedback control portion that generates a feedback signal responsive to the power consumption of the ballast And receives a control signal for controlling the power consumption of the ballast; a time controller that generates a time signal; a dimming controller coupled to the time controller, the dimming controller generating a dimming signal responsive to the time signal; and an adder coupled to the feedback control portion and the dimming controller, the adder generating the control signal responsive to the feedback signal and the dimming signal.
The system further includes: a soft start controller coupled to the time controller and the adder, the soft start controller generating a soft start signal responsive to a second time signal; and an adder that generates the control signal responsive to the feedback signal, the dimming signal and the soft start signal.
The dimming controller includes a first resistor that determines the level of the dimming signal during a steady portion of a dimming cycle and a second resistor that determines the rate at which the dimming signal changes during a sloping portion of the dimming cycle.
Another aspect of the present invention is a method for sequentially controlling an electronic ballast comprising: generating a feedback signal responsive to the power consumed by the ballast; generating a time signal; generating a soft start signal responsive to the time signal; generating a dimming signal responsive to the time signal; generating a control signal responsive to the feedback signal, the soft start signal, and the dimming signal; and controlling the power consumed by the ballast responsive to the control signal.
Generating the soft start signal includes: maintaining the value of the soft start signal at a level sufficient preheat a load connected to the ballast during a preheat cycle; and decreasing the value of the soft start signal to a level sufficient to cause the load to discharge during a discharge cycle.
