Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Impedance or current regulator in the supply circuit
Reexamination Certificate
1999-02-12
2001-01-09
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Impedance or current regulator in the supply circuit
C315S307000, C315SDIG004
Reexamination Certificate
active
06172466
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an electronic ballast for a fluorescent lamp that provides dimming control of the lamp.
BACKGROUND OF THE INVENTION
Fluorescent lamps are a very popular form of illumination, especially in offices and workplaces, but also increasingly in domestic applications. However, one drawback with fluorescent lamps in comparison with incandescent lamps, is that it is much harder to provide a dimmable lamp. The main reason for this is that a certain minimum voltage must be applied to the lamp in order for the discharge that is the source of illumination to occur. If the applied voltage goes below this minimum, the discharge may not strike and the lamp will simply go out. Thus controlling the lamp's power output by simply varying the applied voltage (as is done for incandescent lamps) is generally unsuccessful.
Conventionally fluorescent lamps are driven by ballast circuits. Originally such ballast circuits were electromagnetic, but more recently electronic solid-state ballast circuits have become preferred. Such electronic ballasts are effectively switched mode power electronic circuits and have a number of advantages including improved efficiency of the overall system, higher lumen output per watt and longer lifetime of the lamps.
PRIOR ART
A number of approaches have been taken to providing dimming control for a fluorescent lamp, but to date none have been commercially successful as can be seen from the fact that dimmable fluorescent lamps are surprisingly rare given their desirability in terms of energy saving, cost saving and in the greater versatility that such a lamp would possess.
For example, U.S. Pat. No. 4,370,600, U.S. Pat. No. 4,392,087 and U.S. Pat. No. 4,441,054 all disclose fluorescent lamps in which the dimming control is accomplished either by pulse width modulation of the inverter drive, or by changing the supply AC voltage to the rectifying circuit that in turn supplies the DC voltage to the inverter. In U.S. Pat. No. 4,523,131 the dimming is achieved by changing the primary inductance of a transformer system characterized by a variable inductance primary winding. In U.S. Pat. No. 5,315,214 dimming is controlled through sensing the lamp current with a reference signal to control either pulse width or frequency of the inverter drive.
Known methods of providing dimming control of a fluorescent lamp include varying the duty cycle or the switching frequency of the two switches that generally comprise the inverter drive of a conventional electronic ballast. Higher switching frequency leads to lower light power output. Conventionally, however, to achieve this control it is necessary to provide a separate controller with a pair of control wires leading to the ballast separate from the input power supply wires. This means that for a fluorescent lamp provided with such a dimming control four wires are required—two to carry input power, and two to control dimming—and this is clearly undesirable for a number of reasons including cost and the general inconvenience of requiring separate switches to control the power on/off and the dimming of the lamp.
U.S. Pat. No. 4,492,897 shows a method for providing dimming control in which only a pair of wires are required. In this arrangement a dimmer control is provided in the power supply line before the electronic ballast. This dimmer clips off part of the trailing or leading edge of the power supply curve so as to reduce the average power supply to the lamp. In this way the dimming control operates in a way common to the dimming control provided to incandescent lamps. However, since this means that the average voltage applied also falls, this patent does not overcome the problem of the average voltage falling so low that the lamp fails to discharge.
Some of the problems that prior art proposals have faced include factors such as: (1) stable control of the light may be difficult to obtain, eg changing the supply voltage for control makes the lamp susceptible to power line fluctuations, (2) the proposed method may not be practicable due to cost constraints, this is true for example of variable inductance winding proposals, and (3) the control scheme may require a separate pair of control wires to be connected from the dimming unit to the ballast.
There remains a need for a simple and reliable means for providing dimming control for a fluorescent lamp and which overcomes or at least mitigates many of the aforesaid problems.
SUMMARY OF THE INVENTION
According to the present invention there is provided dimming control means for a fluorescent lamp comprising, means for removing a part of the phase of an applied input voltage, an electronic ballast means for driving the fluorescent lamp, and means for varying the switching frequency of said electronic ballast means in response to the phase removed.
By means of this arrangement while part of the phase is removed from the input supply voltage to control the dimming, this control is not performed directly from the reduced supply voltage, but rather information of the amount of phase removed is used to control the switching frequency of the ballast which in turn controls the light power output. This means that only a very small part of the phase of the input supply need be removed (say up to 15%) and the supply voltage does not fall below that required for the lamp to illuminate. Indeed the dimming control means may be provided with means for limiting the amount of phase that can be removed to ensure that this does not happen.
Preferably the dimming control means comprises (a) means for generating pulses of a width dependent on the amount of phase removed from the input supply voltage, (b) means for integrating the pulses to generate a voltage dependent on the pulse width, and (c) means for generating a switching signal for driving the ballast means, the switching signal having a switching frequency dependent on the voltage.
The integrating means may generate a peak voltage which is preferably then filtered by a lowpass filter to produce a filtered voltage.
The switching signal generating means may comprise means for converting the filtered voltage to a current varying at a frequency dependent on the voltage.
The pulse generating means may comprise a comparator for comparing the part phase removed applied input voltage to a reference voltage.
Preferably the dimming control may be either an open loop control or a closed loop control, for example in the latter case the lamp current may be sensed and provided as a feedback to the switching signal generating means.
Viewed from a further aspect the invention provides dimming control means for a fluorescent lamp including an electronic ballast, comprising: (a) means for removing part of the phase of an applied input supply voltage, (b) comparator means for generating pulses of a width dependent on the amount of phase removed from the input supply voltage, (c) means for integrating the pulses to generate a peak voltage dependent on the pulse width, (d) lowpass filter means for filtering said peak voltage, (e) means for converting the filtered voltage to a current varying at a frequency dependent on the voltage, and (f) means for generating a switching signal from the varying current for driving the ballast.
Viewed from a still further broad aspect the present invention provides a fluorescent lamp comprising, a gas discharge tube, an electronic ballast for driving the discharge tube, and means for providing dimming control of the lamp, wherein the dimming control means comprises means for removing part of the phase of an applied input supply voltage, and means for varying a switching frequency of the electronic ballast in response to the phase removed.
Viewed from yet another broad aspect the present invention provides a method for providing dimming control of a fluorescent lamp, comprising removing part of the phase of an applied input supply voltage, and varying a switching frequency of an electronic ballast in response to the amount of phase removed.
REFERENCES:
patent: 4350933 (1982-09-01), Agarwal
Ki Wing Hung
Mok Kwok Tai Philip
Sin Kin On Johnny
Burns Doane Swecker & Mathis L.L.P.
The Hong Kong University of Science and Technology
Vo Tuyet T.
Wong Don
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