Patent Application
20070090769
The present exemplary embodiment relates to High Intensity Discharge (HID) lamp lighting systems. It finds particular application in conjunction with metal halide lamp dimming systems and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications, including mercury lamps and high pressure sodium (HPS) lamps.
In general, HID lamps suffer from a degradation in light output over time. This degradation in light is commonly referred to as the Lamp Lumen Depreciation (LLD) of a lamp. The LLD of a lamp is defined as the light output vs. time. When MH lamps are operated in the fully dimmed mode, the metal halide vapor pressures drop by a very large amount and the lamp reverts to a mercury discharge. A mercury discharge under these conditions will have a very poor Color Rendition Index (CRI), a low efficiency and poor LLD characteristics.
There is widespread evidence that the LLD during full power operation can be dramatically improved through the use of frequencies higher than 60 Hz. The two frequency domains that are commonly used for this are approximately 100 Hz square waves and higher frequencies of 100 to 200 KHz for 400 W lamps.
In accordance with one aspect of the present exemplary embodiment, a ballast lamp circuit is provided. The ballast lamp circuit comprising a HID lamp full power mode, the ballast lamp circuit configured to generate an ac waveform during the HID lamp full power mode; and a HID lamp reduced power dimming mode, the ballast lamp circuit configured to generate a dc waveform during the HID lamp reduced power dimming mode. The HID lamp reduced power dimming mode providing less power than the ac waveform during the said HID lamp full power mode and the HID reduced power dimming mode configured to provide power to a HID lamp after an initial warm up period wherein the ballast lamp circuit is configured to provide full power to the HID lamp during the HID full power mode.
In accordance with another aspect of the present exemplary embodiment, a method of operating a HID lamp is provided. The method comprising generating a dc waveform during a reduced power dimming mode, the power of the dc waveform being less than the full power rating of a lamp to be driven, and driving the lamp with the dc waveform to generate a dimmed lamp output, wherein the lamp lumen depreciation of the lamp over the life of the lamp is less with the use of the dc waveform compared to the use of an ac waveform of equal power driving the lamp to generate a dimmed lamp output.
In accordance with another aspect of the present exemplary embodiment, a ballast lamp circuit is provided. The ballast lamp circuit comprising a means for generating a dc waveform during a lamp reduced power dimming mode, the power of the dc waveform being less than the full power rating of a metal halide lamp; and a means for driving the lamp with the dc waveform to generate a dimmed lamp output.
As briefly discussed in the background section above, HID lamps and MH lamps in particular suffer from a reduced LLD when operated at a reduced power level or what this disclosure refers to as an ac dimmed mode. The LLD of MH lamps in the dimmed mode is especially inferior when the lamp is operated at 50% or less power levels.
A primary reason for the inferior LLD of a MH lamp operated in ac dimmed mode is the lamp current is significantly less in the dimmed mode which reduces the operating temperature of the electrodes as compared to the temperature of the electrodes while operating in the full power mode. Less lamp current during the dimmed mode leads to difficulty maintaining the electrodes at a high enough temperature for good thermionic emission while the electrodes operate as cathodes. This, in turn, results in a higher rate of tungsten evaporation from the electrodes which causes blackening of the arc tube and decreases the LLD characteristics of the MH lamp.
As stated above, the problem discovered in the ac dimmed mode is the electrodes are too cool to properly support thermionic emission from the cathode electrode during the cathode cycle. This is caused by the reduced lamp current associated with lower power operation and the electrode alternating between being an anode and cathode during ac operation. In addition, the lower electrode of a vertically operated MH lamp tends to operate cooler than the upper electrode due to the fact that hot gases generated by the discharges rise. This rise of hot gases tends to heat the upper electrode while the lower electrode is actually cooled by the cooler gases that come down the arc tube inner surface and then impinges upon the lower electrode, thereby contributing to a poorer thermionic emission and LLD properties of the MH lamp.
DC operation MH lamp allows one electrode to better maintain a hot spot since the electrode will be in a continuous cathode mode. In this mode, most of the energy is dissipated in a very small area, or hot spot of the cathode surface. The electrode operating in a continuous anode mode provides a more uniform dissipation of energy over the entire anode electrode.
By operating the MH lamp using dc in the dimmed mode, there is approximately twice the energy available to maintain a cathode hot spot as compared with using 100% ac in the dimmed mode. In addition, by utilizing the upper electrode as the cathode, a relatively hotter cathode electrode is achieved which enables a stable hot spot to be maintained with relatively less power.
Alternative methods of maintaining a stable hot spot include a multiple cathode and/or multiple anode approach. During the MH dimmed mode of operation, a relatively smaller cathode is utilized, thereby improving the stability of the cathode hot spot as compared to a relatively larger cathode electrode utilized during full power operation of the MH lamp. A multiple electrode configuration also provides an improvement of LLD for ac dimming, whereby a relatively smaller pair of electrodes are utilized during the dimmed ac mode, as compared to a relatively larger pair of electrodes being utilized during the full power mode.
With reference to
The ballast 10 is configured to generate an ac waveform during the MH lamp full power mode and a reduced power dc waveform during the MH dimming mode. The reduced power dc waveform drives the MH lamp 12 during the dimming mode while providing an improved LLD as compared to a reduced power ac waveform driving the MH lamp.
The embodiment of the ballast circuit and MH lamp 12 configuration according to
For reduced power or dimming mode operation of the MH lamp, a dc current component can be added to the lamp 12 by changing the duty cycle of the voltage reversing action of the ballast 10 from the standard 50% positive 50% negative to an asymmetric duty cycle. The dc current component can range from 0% to 100% of the lamp current by controlling the duty cycle of the voltage reversing section.
As illustrated in
Other variations of the ballast configuration include generating an ac component and a dc component during the metal halide lamp reduced power dimming mode. This can be accomplished using various pulse width modulation techniques. In addition, a wide range of frequencies can be used to generate the full power ac waveform. This range includes 50 Hz to 400 Hz; however, other frequencies outside this range are within the scope of this disclosure.
With reference to
With reference to
With regard to the dc dimmed mode of operation, a full wave rectifier bridge was connected to the output of the electronic ballast while operating in the dimmed power mode. To provide 125 W to the MH lamps, two MH lamps were connected in series and connected to the bridge output.
With regard to the ac dimmed mode of operation, the ac output of the electronic ballast, while operating at dimmed power, was connected to two MH lamps in series, thereby providing 125 W at each MH lamp.
In summary,
As
The discussion heretofore has been limited to an exemplary embodiment including a MH lamp dimming apparatus and method of operation. However, other HID lamps such as mercury lamps and High Pressure Sodium lamps are within the scope of this disclosure. Particularly, mercury lamps which can benefit from an improved LLD while being operated in a dimmed mode as described heretofore with reference to MH lamps. In addition, particular reference has been made to CMH lamps, however, other variations of MH lamps, including quartz MH lamps, are within the scope of the disclosure.
Furthermore, the discussion heretofore has been limited to an exemplary embodiment including an electronic ballast configuration. However, magnetic and hybrid electronic/magnetic ballasts can be utilized to provide the necessary ac waveforms and dc waveforms to drive a MH lamp according to the exemplary embodiments described.
The exemplary embodiment has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the exemplary embodiment be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.
