Patent Application
20100102757
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. JP2008-271378, filed Oct. 21, 2008.
Not Applicable
Not Applicable
The present invention relates generally to an electronic ballast for powering a high-intensity gas discharge lamp such as a high-pressure mercury lamp or a metal halide lamp. More particularly, the present invention relates to circuitry within an electronic ballast for powering a lamp that prevents operation below a resonance frequency.
Electronic ballasts for powering high-intensity discharge lamps such as high-pressure mercury halide lamps or metal halide lamps are conventionally known in the art. Referring to
The voltage step-down circuit 1 of the present example further includes an inductor L, a capacitor C1, a diode D1 and a switching element Q1 such as a synchronous rectifier Q1. The voltage step-down circuit 1 steps down the DC voltage from the DC power supply by switching on and off the switching element Q1 and provides the stepped-down DC voltage across either end of capacitor C1. A current sensor such as a resistor Ra is connected to one end of the DC power supply for detecting a load current flowing through the discharge lamp La. The voltage step-down control circuit 4, which may be a microprocessor, applies a driving signal to the switching element Q1 for controlling the switching rate of the switching element Q1. The voltage step-down control circuit 4 changes a frequency of the driving signal according to the load current detected across the resistor Ra, thereby controlling the voltage output from the voltage step-down circuit 1 to a predetermined voltage level.
The inverter circuit 2 is configured such that a series connected circuit including the switching elements Q2, Q3 is coupled in parallel to a series connected circuit including the switching elements Q4, Q5. The resonant circuit 20 and the discharge lamp La are coupled to a node between the switching elements Q2, Q3 and to another node between the switching elements Q4, Q5. The resonant circuit 20 in this example is an LC circuit including an inductor L2 and a capacitor C2.
A voltage detection circuit 5 that detects the high frequency voltage supplied to the discharge lamp La is coupled to a node between the inductor L2 and the capacitor C2. The voltage detection circuit 5 is configured to include capacitors C3, C4 and diodes D2, D3 that rectify the high frequency voltage, resistors R1-R4 that divide the rectified voltage, and a capacitor C5 that smoothes the rectified voltage. Further, the voltage detection circuit 5 is configured to apply a voltage across either end of the capacitor C5 and to the driving circuit 3.
The driving circuit 3, which may include a microprocessor, supplies a driving signal to each of the switching elements Q2-Q5 so as to alternately switch on and off a first pair of switching elements Q2, Q3 and a second pair of switching elements Q4, Q5, thereby switching the switching elements Q2-Q5 at a high frequency. The driving circuit 3 utilizes frequency sweep control to modulate the driving signal frequency within a predetermined range and to secure proper starting operation even where the inductance or capacitance of components L2, C2 of the resonant circuit 20 vary, or even if the discharge lamp La is at the end of its life and the voltage necessary to start the lamp increases.
Operation of the conventional discharge lamp ballast will be described with reference to
During this predetermined period of time, a high frequency voltage of several tens of kilohertz (kHz) to several hundreds of kHz is supplied to the discharge lamp La, and the discharge lamp La is ignited and lit. If the discharge lamp La is not ignited during this predetermined period of time, the driving circuit 3 changes the driving frequency to an initial frequency from the time of activating the ballast and sweep controls the driving frequency again. After the discharge lamp La is ignited, the driving circuit 3 controls the driving frequency to supply a low frequency voltage of several tens of hertz (Hz) to several hundreds of Hz to the discharge lamp La, and maintains steady-state operation of the lamp.
In accordance with this example, if the driving frequency of the driving circuit 3 is close to the maximum frequency in the predetermined range, the high frequency voltage supplied to the discharge lamp La decreases to be almost identical to a voltage detected immediately after lighting of the discharge lamp La. While it is difficult to promptly determine whether or not the discharge lamp is lit, it will nevertheless be determined within a finite period of time whether the discharge lamp is lit or not. Therefore, the discharge lamp La may be lit in a state in which the high-frequency voltage is low, and if so the driving circuit 3 repeatedly sweep controls the driving frequency from a maximum frequency to a minimum frequency.
In the above-stated case, the high frequency voltage changes according to a resonance characteristic, not at a point where the lamp is unlit, but at a point where the lamp is lit. Due to this, even if the driving circuit 3 sweeps the driving frequency to make the frequency lower, the high frequency voltage is of a reduced magnitude and therefore does not reach the predetermined voltage. As a result, conditions for stopping the driving circuit 3 from sweeping are not met and the driving circuit 3 continues sweeping the driving frequency to below the resonant frequency. No problems occur if the discharge lamp La remains lit. However, when the discharge lamp La is turned off, the resonant circuit 20 operates at a frequency lower than the resonant frequency according to the resonance characteristic present at the time the discharge lamp La was extinguished. The below resonance operation may place excessive and potentially destructive stress on the various switching elements of the ballast circuitry.
The present invention has been conceived in view of the prior art as stated above. It is an object of the present invention to provide an electronic ballast capable of preventing below-resonance switching operation.
An embodiment of a lamp ballast of the present invention includes a voltage step-down circuit reducing a DC voltage from a DC power supply, and outputting the reduced DC voltage. An inverter circuit includes at least one high frequency switching element to periodically invert a polarity of the DC voltage from the voltage step-down circuit and output a high frequency voltage. A resonant circuit ignites a discharge lamp by a resonance effect, with the high frequency voltage from the inverter circuit supplied to the resonant circuit.
A voltage step-down control circuit controls the DC voltage output from the voltage step-down circuit. A driving circuit supplies a driving signal for switching the switching elements of the inverter circuit. The driving circuit also changes a driving frequency of the driving signal and thereby controls the high frequency voltage. The driving circuit further includes a frequency setting mode for setting the driving frequency to a frequency near to and above a resonance frequency, and a starting mode for changing the driving frequency to the frequency set in the frequency setting mode and applying the high frequency voltage to the discharge lamp at a level sufficient for ignition of the lamp. The voltage step-down control circuit controls the voltage step-down circuit to reduce the DC voltage output and prevent ignition of the discharge lamp in the frequency setting mode.
The lamp ballast includes a voltage detection circuit detecting the high-frequency voltage. The driving circuit adjusts the driving frequency until the high-frequency voltage detected by the voltage detection circuit in the frequency setting mode reaches a predetermined voltage. The driving circuit then controls the high-frequency voltage so as to fix the driving frequency upon reaching the predetermined voltage.
In the lamp ballast of the present invention, the driving circuit may further control the driving frequency from a preset initial frequency to the frequency set in the frequency setting mode, fix the frequency for a predetermined period of time in the starting mode, and then repeat the control at least once.
The driving circuit may further control the driving frequency from a preset initial frequency to the frequency set in the frequency setting mode, again adjust the driving frequency to the preset initial frequency in the starting mode, and repeat the control at least once.
A lighting fixture further includes the lamp ballast according to various aspects of the present invention herein described, and a fixture main body accommodates therein the discharge lamp lighting device.
A projector further includes the lamp ballast according to various aspects of the present invention herein described.
a is a circuit diagram showing a lamp ballast according to an embodiment of the present invention.
b is a graphical view showing waveforms of a high-frequency voltage, a driving frequency, and a voltage output from a voltage step-down circuit, respectively.
a-3c show other operation examples in a starting mode according to the embodiment of
a is a circuit diagram showing a prior art lamp ballast.
b is a graphical view showing waveforms of a high-frequency voltage, a driving frequency, and a voltage output from a voltage step-down circuit at starting time, respectively.
c is a graphical view of waveforms of a high-frequency voltage, a driving frequency, and a voltage output from the voltage step-down circuit if the discharge lamp is started at low voltage, respectively.
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.
Referring generally to
A ballast according to an embodiment of the present invention will now be described with reference to the drawings. It is to be noted, however, that a basic structural topology of the lamp ballast according to this embodiment is similar to that of the conventional lamp ballast as shown in
The driving circuit 3 of this embodiment includes a first mode of operation herein referred to as a frequency setting mode 24 for setting a driving frequency close to and above a resonance frequency F_res for a resonance circuit 20 while simultaneously preventing the lamp from starting. The driving circuit 3 further includes a second mode of operation, herein referred to as a starting mode 22 for adjusting the driving frequency Fdrv to the frequency set in the frequency setting mode 24, and for applying to a discharge lamp La a high frequency voltage Vhf high enough to start the lamp.
The voltage step-down control circuit 4 controls a voltage step-down circuit 1 to reduce a voltage output Vout_sd from the voltage step-down circuit 1 so as to inhibit starting of the discharge lamp La. In this embodiment, a voltage division circuit including a series connected resistive network R6, R7 is provided at the output of the voltage step-down circuit 1. By supplying a reference voltage from the voltage step-down circuit 1 output to the voltage step-down control circuit 4 from a node between the resistors R6, R7, the voltage output Vout_sd from the voltage step-down circuit 1 may be controlled to be equal to a predetermined voltage.
As shown in
However, as stated in the conventional example, if the discharge lamp La is lit in a state in which the high-frequency voltage is low, the high-frequency voltage cannot reach the predetermined voltage. As a result, the driving circuit 3 cannot stop the sweep-control and possibly cause the resonant circuit 20 to operate at a frequency 28 below the resonant frequency. Therefore, this embodiment solves the problems by setting the frequency setting mode 24 prior to the starting mode 22.
Operation of a ballast according to this embodiment may now be described with reference to the drawings. As shown in
Once the driving circuit 3 sets the driving frequency Fdrv in the frequency setting mode 24, the driving circuit 3 may then move to the starting mode 22. At the same time, the voltage step-down control circuit 4 controls the voltage output Vout_sd from the voltage step-down circuit 1 to return to the output voltage Vout_sd for normal operation.
In various embodiments, the difference between the high-frequency voltage in the frequency setting mode 24 and that in the starting mode 22 may be relatively large. In light of this, in the voltage detection circuit 5 a series-connected circuit that includes a resistor R5 and an npn transistor Tr may be coupled in parallel to a resistor R4. In this configuration, the transistor Tr may be turned on to increase a voltage division ratio in the frequency setting mode 24, and the transistor Tr may be turned off to reduce the voltage division ratio in the starting mode 22, thereby improving detection accuracy of the voltage detection circuit 5.
As stated above, after setting the driving frequency Fdrv to the frequency F_fixed near to and above the resonance frequency for the resonant circuit 20 in the frequency setting mode 24, the driving circuit 3 may then move to the starting mode 22. It is thereby possible to stop sweep-controlling the driving frequency Fdrv at the preset driving frequency F_fixed whether the discharge lamp La is lit or not. It is, therefore, possible to prevent the driving frequency from sweeping to a frequency 28 lower than the resonant frequency F_res of the resonance circuit 20, and accordingly further possible to prevent the resonant circuit 20 from operating at a frequency 28 lower than its resonant frequency F_res.
In the embodiment as shown in
Alternatively, as shown in for example the embodiment of
In yet another alternative embodiment as shown in
In yet another alternative embodiment as shown in
A ballast according to another embodiment of the present invention will now be described referring to
The filter circuit 6, which eliminates noise in an AC voltage from the AC power supply AC, may be configured to include capacitors C6 and C7 and a line filter LF. The rectifying circuit DB, which may include a diode bridge, rectifies the AC voltage from the AC power source AC and outputs a pulsating voltage. The step-up circuit 7 may be configured to include capacitors C8 and C9, an inductor L3, a diode D4, and a switching element Q6. The step-up circuit 7 is capable of switching the switching element Q6 on or off to thereby raise the pulsing voltage from the rectifying circuit DB, cause the capacitor C9 to smooth the raised pulsing voltage, and output a desired DC voltage across both ends of the capacitor C9.
A detection resistor Rb for detecting a current flowing through the switching element Q6 may be connected in series to the switching element Q6. The step-up control circuit 8 may be a microprocessor and supplies a driving signal for controlling the switching of the switching element Q6. The step-up control circuit 8 changes a frequency of the driving signal according to the current detected by the detection resistor Rb, thereby controlling the voltage output from step-up circuit 7 to be equal to a predetermined voltage. Since the respective circuits are well known, they are not described herein in detail.
Each of the embodiments described herein may be applied to an illumination fixture, including a fixture main body accommodating therein the lamp ballast or a projector including the lamp ballast, and can exhibit similar effects to those stated above.
The previous detailed description has been provided for the purposes of illustration and description. Thus, although there have been described particular embodiments of the present invention of a new and useful “HID Lamp Ballast with Controlled DC Step-Down Circuit,” 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 |
|---|---|---|---|
| JP2008-271378 | Oct 2008 | JP | national |
