The influence of electronic ballast on the performance of fluorescent lamps

1. Starting method of electronic ballast and its influence on lamp switch life

To reduce the size of the device, the electronic ballast can be made into a high-frequency inverter, with a switching frequency of 30-50kHz. The energy storage of the ballast inductor is much smaller than that of the inductive ballast, and the energy released by the inductor cannot break through the lamp tube. Therefore, the currently popular LC resonance method is used to generate a sufficiently high resonant voltage to break through the lamp tube, so that the lamp tube is started and then ignited by the inductor.

The LC resonant ignition method can force the lamp to break through and ignite when the filament is not heated, that is, "light up immediately after being turned on". This "light up immediately after being turned on" function seems to be very convenient to use, but because the filament is not heated and cannot emit electrons to neutralize the mercury ions, the mercury ions under the action of the high-voltage electric field bombard the filament, causing the substances on the surface of the filament that are conducive to emitting electrons to be bombarded and splashed, which violates the basic principle of adding high-voltage ignition after the filament is heated. During the ignition period before the lamp tube is ignited, due to the high Q value of the LC resonant circuit (10-30). The cold resistance of the filament is low (less than 10Ω in total), so the resonant current will reach about 10 times the normal working current. The switch tube and the resonant capacitor will bear huge currents. Current impact is one of the main reasons for the high failure rate and short life of electronic ballasts. Therefore, electronic ballasts must have the function of ignition after the filament is preheated. The basic requirement for preheating the filament of a fluorescent lamp is: preheat the filament to 600℃~800℃ (in dim light, the two ends of the lamp tube will start to glow red) within the first 2 seconds or so after the electronic ballast is powered on. Then, apply LC resonant high voltage at both ends of the lamp tube to break down the lamp tube and ignite it. Since the filament has the ability to emit electrons after preheating, the mercury ions can be neutralized when the filament is heated to stop accelerating, thereby minimizing the bombardment of the filament and extending the life of the filament. There should also be no glow discharge during the filament preheating process. After doing this, the switching life of the lamp tube can generally exceed 200,000 times, and at least more than 100,000 times. This eliminates the traditional concept that each ignition reduces the service life by half an hour to one hour, so that the switching of fluorescent lamps basically no longer affects their service life. To achieve this effect, the preheating method must be optimized.

The ignition method uses a PTC element connected in parallel at both ends of the resonant capacitor. After the electronic ballast is powered on, the low resistance value of the cold PTC element is used to reduce the Q value of the inductor capacitor resonant circuit, thereby reducing the voltage at both ends of the capacitor, so that the lamp tube is not broken down and ignited, but is in a preheating state. When the PTC element is heated to the transition temperature by the current, it changes from a low resistance state to a high resistance state, which increases the Q value of the resonant circuit, and the voltage at both ends of the capacitor (i.e., both ends of the lamp tube) increases, causing the lamp tube to be broken down and ignited. Theoretically, this preheating ignition method is feasible, but in actual applications, since the PTC element is in a high temperature state, its reliability cannot be guaranteed for a long time, and when the ambient temperature varies greatly, the PTC element will not play a role in preheating and igniting the lamp tube in a high temperature environment. At the same time, it is also easy to cause the PTC to break down due to the overvoltage (several times higher than the normal ignition) during the cold ignition of the lamp tube. In low temperature environment, due to insufficient heating power, the temperature of PTC components cannot reach the turning value, which often makes the lamp unable to be ignited or return to the ignition state after ignition, and cannot maintain normal lighting. Due to the above reasons, PTC components are often damaged in actual applications, and even damaged before the lamp, and the overall effect is not very obvious in improving the switching life of the lamp. We believe that the low-stress preheating starting method is the best, that is, during the preheating process, the stress of the electronic ballast at various locations is no higher than that in the normal starting state, the input power increases slowly as the temperature of the filament rises, and when the filament is heated to have the ability to emit electrons, the LC resonant high voltage is added to both ends of the lamp tube to cause the lamp tube to be broken down and started. As the filament is preheated, the starting voltage of the fluorescent lamp tube is significantly reduced (150V~290V), so that not only can the electrons emitted by the filament neutralize the mercury ions accelerated by the electric field, but also because the breakdown field strength of the lamp tube is much smaller than that of the cold filament lamp tube, the speed of mercury ions is small, further reducing the bombardment of the mercury ions (atoms) on the filament, so that the extension of the lamp life is guaranteed.

2. The impact of lamp failure on electronic ballast

When the inductive ballast is matched with the fluorescent lamp, as the ignition time increases and the number of switching times increases, the electron-emitting material on the surface of the filament in the lamp tube is splashed due to the bombardment of mercury ions and reduced, and the filament's ability to emit electrons is weakened, causing the lamp tube voltage to increase and the current to decrease. Ultimately, the lamp tube voltage cannot be maintained higher than the glow discharge voltage of the starter, causing the starter to repeatedly operate and the lamp tube to fail to ignite normally, that is, the lamp tube has reached the end of its life. At this time, the lamp tube can only be replaced with a new one to resume operation.

When the electronic ballast is used with a fluorescent lamp, since the electronic ballast adopts a resonant ignition method, after the lamp is ignited, the conducting lamp is connected in parallel at both ends of the resonant capacitor, causing the Q value of the resonant circuit to drop sharply. At this time, the terminal voltage of the capacitor is the working voltage of the lamp. When the lamp reaches the end of its life, due to the above reasons, the lamp voltage increases. In this state, the terminal voltage and current of the resonant capacitor increase accordingly, and the filament current increases, thereby alleviating the phenomenon of insufficient electron emission of the filament, but the lamp power increases.

When the lamp tube reaches the end of its life or due to other reasons, the lamp tube may not be activated and ignited. The equivalent resonant inductor and resonant capacitor will flow a current 10 times or even greater than the normal operating current. If the filament cannot burn out within a very short time, the above-mentioned components, especially the switching tube, will be thermally broken down, thereby damaging the electronic ballast.

If the electron emission of the filaments at both ends of the fluorescent lamp is unbalanced, there will be a "rectification" phenomenon of varying degrees. When this "rectification" phenomenon is serious, the potential of the voltage-dividing output point of the two series-connected input filter electrolytic capacitors will shift, which may cause the voltage on one of the capacitors to exceed its rated voltage and break down, thus damaging the electronic ballast.

3 Electronic ballasts should get out of the misunderstanding

At present, many electronic ballast manufacturers have produced electronic ballasts without filament preheating function in order to reduce production costs, or even if there is a preheating function, it is mostly useless. This greatly affects the service life of the lamp tube and the service life of the electronic ballast. Not only does it cause a huge waste of funds and resources, but the discarded old lamp tubes will cause mercury and fluorescent powder to pollute the environment. Since most electronic ballasts do not have their own protection function when the lamp tube cannot be activated, the electronic ballast will inevitably burn out at the end of the lamp life, resulting in a phenomenon that a lamp tube is damaged and an electronic ballast is also damaged. In order to improve the power factor of the electronic ballast, most high-power-factor electronic ballasts use a rectifier circuit as shown in Figure 1, and the power factor can reach 0.96, but the rectifier output voltage ripple is 50%, as shown in Figure 2.

Under normal constant voltage rectifier output voltage, when the electronic ballast is used with a fluorescent lamp, the current waveform factor of the lamp tube is about 1.4 to 1.5. The experimental result after using the rectifier circuit in Figure 1 is that the current waveform factor of the lamp tube increases by 30%. To ensure that the current waveform factor of the lamp tube is less than the requirement of 1.7 specified in the national standard GB10682-89, the waveform factor of the lamp tube must be less than 1.4 when the electronic ballast under constant voltage rectifier output power supply is used with a fluorescent lamp. In fact, the current waveform factor of a properly designed electronic ballast is generally 1.35.

In order to reduce the damage rate of electronic ballasts, some electronic ballast manufacturers often make the lamp input power less than the regulations in GB15143-94 and GB15144-94. Some electronic ballasts even have an input power of only 80% of the rated power of the lamp. This derating method of lamp use not only reduces the luminous efficiency of the lamp by more than one level, but also reduces the service life of the lamp.

Figure 1 Circuit of high power factor electronic ballast

(a) AC voltage and current waveforms

(b) DC voltage ripple Figure 2 Voltage and current waveforms of electronic ballast

Figure 2 Voltage and current waveforms of electronic ballast

To sum up, the misunderstandings about electronic ballasts result in the service life of lamps being significantly shorter than when using inductive ballasts. If a lamp fails, a ballast will burn out, thus losing the advantage of the long life of fluorescent lamps. In addition, since many electronic ballasts are not equipped with fuses, there is a risk of electrical accidents and electrical fires.

An electronic ballast with good performance should have the following features: filament preheating function; lamp failure protection function and anti-"rectification" effect function; and the harmonic components of the electronic ballast input current should meet the IEC555-2 standard. In this way, not only the reliability of the electronic ballast is guaranteed, but also the service life of the fluorescent lamp is significantly longer than that of the fluorescent lamp equipped with the inductive ballast, and also significantly longer than the service life specified by the manufacturer, due to the use of the preheating ignition method and the lamp current waveform factor of about 1.4.