A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the reproduction of the patent document or the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.
This application claims benefit of the following patent application(s) which is/are hereby incorporated by reference: Japan Patent Application No. JP2008-277400, filed Oct. 28, 2008.
Not Applicable
Not Applicable
The present invention relates generally to electronic ballasts for powering a discharge lamp. More particularly, the present invention relates to an electronic ballast for suppressing discharge lamp flicker during a transition from lamp startup to steady-state operation.
Conventionally, an electronic ballast for lighting a hot-cathode discharge lamp such as a high pressure discharge lamp (also called HID (high-intensity discharge lamp) is provided with a power converter for converting input DC power to output AC power, a resonant circuit connected between output terminals of the power converter, along with the discharge lamp, and a controller for controlling the power converter.
In an example of such an electronic ballast as known in the art, the controller executes a starting operation for raising an output voltage of the power converter relatively higher to start the discharge lamp, and then begins a steady-state operation in which the power converter is controlled to output to the lamp the AC power for maintaining the lighting of the discharge lamp.
More specifically, the starting operation makes the discharge lamp output a high starting voltage by setting an output frequency of the power converter (hereinafter referred to as an “operating frequency”) to a resonant frequency of the resonant circuit and the discharge lamp (hereinafter referred to as a “load circuit”) with the discharge lamp unlit, or to approximately 1/(n) of the resonant frequency over a predetermined starting time, where “n” is an odd number greater than 3.
Here, the resonant frequency of the load circuit changes in accordance with the beginning of the discharge of the discharge lamp, i.e., the starting thereof. Then, when an operation frequency during the starting operation is far from the resonant frequency of the load circuit after the starting of the discharge lamp, the electric power supplied to the discharge lamp by the end of the starting operation relatively decreases, thereby relatively lowering the temperature of each electrode of the discharge lamp. Therefore, the discharge becomes unstable at the time of beginning the steady-state operation, which may generate lamp flicker and an imperfect lighting.
In view of foregoing, an object of the present invention is to provide an electronic ballast capable of suppressing a flicker and an imperfect lighting at the time of shifting from lamp startup to steady-state operation.
An aspect of the present invention is characterized by including a power converter receiving DC power input thereto and outputting AC power, a resonant circuit coupled to the discharge lamp, the resonant circuit being further connected between output terminals of the power converter, and a controller controlling the power converter. The controller executes a starting operation to make the discharge lamp start discharging by setting an output frequency of the power converter to a predetermined start frequency when starting the discharge lamp, followed by shifting to a steady-state operation by setting the output frequency of the power converter to a predetermined steady-state frequency lower than the start frequency. The steady-state operation makes the discharge lamp output the alternating current power for maintaining lighting of the discharge lamp.
The start frequency is set to a frequency identical or close to 1/(n) of the resonant frequency of the resonant circuit with the discharge lamp unlit, to an extent capable of causing the lamp to light. The start frequency is also identical or close to the resonant frequency of the resonant circuit with the discharge lamp lit, to an extent capable of sufficiently raising the temperature of each electrode of the discharge lamp after the starting of the discharge lamp and by the end of the starting operation.
The temperature of each electrode of the discharge lamp is preserved more effectively by the end of the starting operation, as compared to the case where the start frequency is far from the resonant frequency of the resonant circuit and the discharge lamp is lit, so that it is possible to suppress lamp flicker at the time of shifting to the steady-state operation.
Another aspect of the present invention is characterized wherein the controller executes a starting operation to by periodically changing an output frequency of the power converter within a predetermined start frequency range when starting the discharge lamp, followed by shifting to a steady-state operation by setting the output frequency of the power converter as a predetermined steady-state frequency lower than a lower limit of the start frequency range, the steady-state operation making the discharge lamp output the alternating current power for maintaining lighting of the discharge lamp. The start frequency range includes 1/(n) of the resonant frequency of the resonant circuit with the discharge lamp unlit, the “n” being any odd number, and the start frequency range further includes the resonant frequency of the resonant circuit with the discharge lamp lit.
The temperature of each electrode of the discharge lamp is thereby preserved more effectively by the end of the starting operation, as compared to the case where the start frequency range is far from the resonant frequency of the resonant circuit with the discharge lamp lit, so that it is possible to suppress flicker at the time of shifting to the steady-state operation.
Another aspect of the present invention is characterized wherein the start frequency range includes 1/(an odd number) of the resonant frequency of the resonant circuit with the discharge lamp unlit, does not include the resonant frequency of the resonant circuit with the discharge lamp lit, and is also set to a frequency close to the resonant frequency of the resonant circuit with the discharge lamp lit, to an extent capable of sufficiently raising temperature of each electrode of the discharge lamp after the starting of the discharge lamp by end of the starting operation.
Accordingly, the temperature of each electrode of the discharge lamp is preserved more effectively by the end of the starting operation as compared to the case where the start frequency range is far from the resonant frequency of the resonant circuit and the discharge lamp lit, so that it is possible to suppress flicker at the time of shifting to the steady-state operation.
Another aspect of the present invention is characterized in that the start frequency range is phase-shifted in relation to the resonant frequency of the resonant circuit with the discharge lamp lit.
Another aspect of the present invention is characterized in that the resonant circuit includes an inductor connected in series to the discharge lamp.
Another aspect of the present invention is characterized in that the resonant frequency of the resonant circuit with the discharge lamp unlit is greater than or equal to five times the resonant frequency of the resonant circuit with the discharge lamp lit.
Another aspect of the present invention is characterized in that the duration of the starting time is greater than or equal to a sum of a minimum time required for making the discharge lamp start discharging and a minimum time required for heating each electrode after the discharge lamp starts discharging.
Another aspect of the present invention is characterized in that the controller detects the starting of the lamp during the starting operation, and the operation shifts to the steady-state operation after a lapse of a certain period of electrode heating time subsequent to the detection of the starting of the lamp. The duration of the starting operation is thereby reduced to relieve the electrical stress applied on the discharge lamp, so that the life of the discharge lamp can be extended compared to the invention according to the previous aspects of the present invention.
Another aspect of the present invention is characterized in that the controller determines whether a half-wave discharge (rectification) is generated at the discharge lamp during the starting operation, and the operation shifts to the steady-state operation when it is determined that the half-wave discharge is not generated at the discharge lamp.
Throughout the specification and claims, the following terms take at least the meanings explicitly associated herein, unless the context dictates otherwise. The meanings identified below do not necessarily limit the terms, but merely provide illustrative examples for the terms. The meaning of “a,” “an,” and “the” may include plural references, and the meaning of “in” may include “in” and “on.” The phrase “in one embodiment,” as used herein does not necessarily refer to the same embodiment, although it may. The term “coupled” means at least either a direct electrical connection between the connected items or an indirect connection through one or more passive or active intermediary devices. The term “circuit” means at least either a single component or a multiplicity of components, either active and/or passive, that are coupled together to provide a desired function. The term “signal” means at least one current, voltage, charge, temperature, data or other signal. Where either a field effect transistor (FET) or a bipolar junction transistor (BJT) may be employed as an embodiment of a transistor, the scope of the terms “gate,” “drain,” and “source” includes “base,” “collector,” and “emitter,” respectively, and vice-versa.
Various embodiments for carrying out the present invention will be described below with reference to the drawings.
Referring now to
One of the output terminals of the full bridge circuit, i.e., the connection point of the switching elements Q2 and Q3 forming one of two series circuits which include two of the switching elements Q2-Q5 and are connected between the output terminals of the DC power source 2 in parallel with each other, is connected to one of the terminals of the discharge lamp DL (i.e., one of the electrodes) via a first inductor PT. The other output terminal of the full bridge circuit, i.e., the connection point of the switching elements Q4 and Q5 constituting the other series circuit, is connected to the other terminal of the discharge lamp DL (i.e., the other electrode) via a second inductor L2. The first inductor PT is an autotransformer having a tap that is connected to the ground via a series circuit of the first capacitor C4 and a resistance R1. Moreover, a second capacitor C3 is connected in parallel with the series circuit of the first inductor PT and the discharge lamp DL. The inductors PT and L2 and the capacitors C3 and C4 constitute a resonant circuit (hereinafter referred to as a “load circuit”) together with the discharge lamp DL.
The DC power source 2, in which a well-known step-up chopper circuit (a boost converter) is connected to an output terminal of a diode bridge DB for full-wave rectifying of the AC power input from the AC power source AC, is provided with a series circuit of an inductor L1 connected between the output terminals of the diode bridge DB, a diode D1, and a capacitor C1, a switching element Q1 connected in parallel with the series circuit of the diode D1 and the capacitor C1, and a driving circuit 21 for controlling the switching element Q1 to turn on and off, which uses both ends of the capacitor C1 as the output terminal thereof.
The driving circuit 21 controls a duty ratio for turning on/off the switching element Q1 so that the output voltage, i.e., the voltage between both the ends of the capacitor C1, is set to be constant. The driving circuit 21 having features as described above can be realized by various well-known techniques, and any detailed illustrations and descriptions will be therefore omitted.
The ballast 1 of the present embodiment is provided with a controller 3 which drives the switching elements Q2 to Q5 respectively forming the full bridge circuit to turn on/off. The controller 3 drives the switching elements Q2 to Q5 to turn on/off so that the switching elements out of Q2 to Q5 located diagonally to each other are simultaneously turned on, and the switching elements out of Q2 to Q5 connected in series with each other are alternately turned on/off. This converts the DC power provided from the DC power source 2 into AC power, and the frequency of this AC power is the polarity inversion frequency due to the on/off driving (hereinafter referred to as an “operating frequency”). In the controller 3 as described above, a microprocessor such as for example ST72215 available from ST can be used, but the controller 3 is of course not limited to this specific example.
When power is turned on and the electronic ballast 1 begins starting, the controller 3 executes the starting operation for a certain period of starting time to set the operating frequency to a predetermined start frequency for starting the discharge lamp DL. In the present embodiment, the start frequency is set to the frequency nearly 1/11 of the resonant frequency of the load circuit with the discharge lamp DL unlit (hereinafter referred to as a “resonant frequency in the extinguished condition”), and to a frequency slightly higher than the resonant frequency in the lighting condition.
In the present embodiment, the resonant frequency in the extinguished condition is the resonant frequency of an LCR series resonant circuit of a primary winding portion of the first inductor PT as the autotransformer (i.e., the portion between the connection point of the switching elements Q2 and Q3 and the tap), the first capacitor C4, and the resistance R1, which is 440 kHz in the present embodiment. Therefore, the resonance voltage generated at the primary winding portion of the first inductor PT is increased by the first inductor PT to be applied to the discharge lamp DL. This voltage makes the discharge lamp DL start discharging at a starting time point t1 shown in
After completing the starting operation at an operation switching time point t2 shown in
The relationship between the amplitude of the lamp current Ila and the operating frequency f in the present embodiment is shown in
According to the structure described above, lamp flicker and an imperfect lighting at the time of shifting to the steady-state operation are suppressed compared to the case where the start frequency is far from the resonant frequency under the lighting condition (for example, the case where the start frequency is set to 100 kHz).
Instead of setting the operating frequency f during the starting operation to be constant as described above, the operating frequency f may be changed periodically within a certain start frequency range during the starting operation, as shown in
Further, instead of setting the duration of the starting operation to be the constant starting time as described above, a process may be implemented in which the controller 3 always or regularly determines whether the discharge lamp DL is started during the starting operation, and the operation shifts from the starting operation to the steady-state operation after a certain period of electrode heating time (for example, 500 ms) subsequent to the determination (detection) of the starting operation of the discharge lamp DL.
For example, a method for determining the starting of the discharge lamp DL includes detecting the amplitude of the voltage (refer to
As shown in
In addition, as depicted in
More specifically, a method for determining whether the half-wave discharge is generated, for example, detects peak values (absolute values) of both positive and negative polarities of the lamp current Ila, compares the difference between the detected peak values for each polarity (hereinafter referred to as an “asymmetric current value”) to a predetermined symmetric threshold, and determines that the lamp current Ila is symmetric with respect to positive and negative polarities thereof. Thus half-wave discharge is not generated if the asymmetric current value is lower than the symmetric threshold, while determining that the lamp current Ila is asymmetric with respect to positive and negative polarities thereof and thus the half-wave discharge is generated if the asymmetric current value is higher than or equal to the symmetric threshold. Employing this structure can reduce the duration of the starting operation to relieve the electrical stress applied on the discharge lamp DL, so that the life of the discharge lamp DL can be extended compared to the case where the duration of the starting operation is set to be constant, or the case where the operation shifts to the steady-state operation after the elapse of a certain period of time subsequent to the detection of the starting of the discharge lamp DL.
Further, as a method for determining the time to terminate the starting operation to shift to the steady-state operation, the starting time, the detection of lighting of the lamp, and the detection of the half-wave discharge may be used in combination. For example, the operation will shift to the steady-state operation at the latest among the elapsing of a predetermined starting time, the elapsing of a predetermined electrode heating time after detection of discharge lamp DL startup, and a time in which it is determined that the lamp current Ila is symmetric with respect to positive and negative polarities thereof and the half-wave discharge is not generated. The structure controller 3 which performs each operation as described above may be realized by various techniques as well known to one of skill in the art, and detailed illustrations and descriptions will be omitted as unnecessary.
In addition, any other well-known DC power source, such as a battery, may be used as the DC power source 2.
The various embodiments of electronic ballasts as described above can be used in, for example, lighting fixtures 5 shown in
Thus, although there have been described particular embodiments of the present invention of a new and useful Electronic Ballast with Lamp Flicker Suppression During Start-to-Steady State Transition it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.
Number | Date | Country | Kind |
---|---|---|---|
JP2008-277400 | Oct 2008 | JP | national |