HID electric light source overview
The HID lighting system consists of a HID electric light source, a driving power source (ballast) and a luminaire. The core is the light source. The design direction of ballasts and luminaires is always extended around how to achieve the best lighting effect. At present, according to the difference of luminescent substances, HID electric light sources are divided into three categories, namely high pressure mercury lamps (HPL), high pressure sodium lamps (HPS) and metal halide lamps (MH), among which metal halide lamps are divided into quartz metal halide lamps (QMH). And ceramic metal halide lamps (CMH or CDM).
Previously HPL lamps were mainly used for outdoor lighting, but due to its low light efficiency, HPL is now being replaced by HPS lamps. Although the LED streetlight concept was initially put into practice, the most cost-effective outdoor lighting is still the traditional inductive ballast-driven HPS lamp. The HPS's initial light efficiency can be above 100lm / W, and can achieve 20% dimming, which is very suitable for lighting energy saving, which is the main advantage of HPS. However, the color rendering index (CRI) of HPS is small, only about 20, and the unsightly yellow light is the main defect of HPS lamps. The white sodium lamp (SDW), which belongs to the sodium lamp series, can greatly increase the CRI to more than 80 by sacrificing the light effect (near 50 lm /W), which is very useful in the meat window of the supermarket.
The two indicators of light efficiency and color rendering index are not easy to achieve in the sodium lamp series electric light source, but they are well balanced in the metal halide lamp. For example: the initial luminous efficacy of the metal halide lamp is 80lm /W. Above, the color rendering index is above 70. Compared with QMH, the initial color temperature consistency of the mass-produced CDM is good, and the color temperature stability after long-term work is good. The CDM discharge tube wall material adopts PCA, which has strong corrosive endurance to the halogen salt, thus avoiding the QMH discharge tube. The danger of explosion under certain conditions. The same power CDM compared with QMH (such as 70W), the color rendering index and light efficiency have increased by more than 10 points, the life expectancy is about 1.5 times QMH. Also, CDM lamps that can be dimmed to 50% are also available. In the near future, CDM lamps that are comparable in efficacy to HPS lamps, have good color rendering, and are dimmable will become a new force in outdoor lighting.
The initial light effect of the HID lamp is the light effect measured after the lamp has been in operation for 100 hours. Its height is determined by the characteristics of the lamp itself, and has nothing to do with the performance of the ballast. However, the long-term workability of HID lampsâ€”lumen (light efficiency) retention and CRI stabilityâ€”is greatly affected by the performance of the drive ballast. Since the core of the lighting system is an electric light source, it is the basic criterion for optimal design of the ballast under the premise of ensuring safety and meeting the requirements of cost control and maximizing the lighting effect of the lamp. Regardless of the market factor of the ballast, if only from the performance point of the lighting system, the most important indicator for evaluating the quality of the electronic ballast is the lamp driver interaction or interface-LDI. Good or bad. LDI is a comprehensive indicator. Firstly, according to the working characteristics of the lamp, the ballast is required to have suitable parameter matching to provide the best driving effect for the lamp. Secondly, it requires the town to work in different working stages and abnormal working conditions of the lamp. The flow device has good reliability. The matching of HID ballasts and lamps and the reliability of the ballast itself are at the heart of this article.
2 Interface between HID lamp and electronic ballast
The operation of the energy indicator (LDI) HID lamp is a complex process of gas discharge and atomic illumination. The HID lamp should be reliably lit and stable and efficient for a long time.
The work must standardize the electrical parameters such as the size, shape, frequency and harmonic content of the lamp voltage, lamp current and lamp power, and the HID electronic ballast circuit topology is continuously developed and the control method is continuously improved. Better meet these normative requirements. The various stages of the HID lamp are directly related to the design of the HID ballast, which are:
(1) The relationship between the current breakdown of the discharge tube and the ignition spike voltage of the ballast: When the ignition spike voltage generated by the ballast is sufficiently large (> 3kV), the neutral atom in the HID discharge tube (such as mercury atom) Starting to be ionized, positive ions bombard the electrode and generate a large amount of electrons, resulting in electron avalanche and self-sustaining discharge. This is the current breakdown phase of the discharge tube. The discharge at this stage is glow discharge, requiring the ballast to be given between the discharge tube electrodes. Provides voltage spikes of sufficient height and width. If the peak value of the spike voltage is not large enough, or the cumulative effective pulse width in the spike voltage is not large enough, the discharge tube is not easy to self-sustain discharge; if the peak value of the spike voltage is too large (such as > 5kV), then the HID bulb is thermally ignited at the HID bulb. Arcing and sparking may occur between the two electrode leads. In addition, based on safety considerations, the ignition process of the ballast should be controllable. When the ignition time exceeds 30 minutes, the ignition circuit must be stopped.
(2) The relationship between the voltage breakdown of the discharge tube and the open circuit voltage (OCV) of the ballast: After the glow discharge occurs, if a sufficient voltage is maintained between the two electrodes of the discharge tube, the degree of electron avalanche will increase. Finally, the voltage across the discharge tube electrode jumps from open-circuit voltage (OCV) to about 10V. This process is called glow discharge to arc discharge transfer (GTA), which is the voltage strike of the discharge tube. The wear phase, which requires the ballast to maintain a sufficiently large OCV between the two electrodes of the discharge tube. If the OCV is not large enough, the voltage breakdown of the discharge tube is not easy to achieve, the GTA is not smooth, and it is difficult to transition to a stable arc discharge phase, and the glow discharge sustaining time is elongated; if the OCV is too large, the mercury positive ion counter electrode The bombardment is too intense and can cause transient sputtering of the electrode material. The glow discharge is maintained for a long time, and the sputtering of the electrode material is severe, which is the main reason for the shortened life of the HID lamp and the sudden decrease in the lumen maintenance rate. Therefore, the OCV is too small or too large, and is not allowed.
(3) The relationship between the Runup process of the discharge tube and the size of the Runup current of the ballast: The Runup process of the HID lamp is the process of vaporizing the amalgam and halide salt in the discharge tube, and gradually increasing the voltage across the electrode and the lamp power. When the lamp power reaches full power, the Runup phase is completed. During the Runup phase of the CDM lamp, the ballast is preferably capable of outputting a constant-sized Runup current, and the Runup current is set to meet the lamp specifications. If the Runup current is too small, the discharge tube halogen salt cannot be vaporized, and the lamp will be extinguished. If the Runup current is too large, the electrode material evaporation loss is too large, and the discharge tube wall is prematurely blackened, which may cause the lumen maintenance rate to drop sharply. .