"I. abnormal protection of electronic ballast
As a recognized green lighting product, electronic ballast fluorescent lamp has many obvious advantages over ordinary inductive ballast fluorescent lamp, such as high luminous efficiency, no stroboscopic, remarkable energy-saving effect and so on; However, some electronic ballasts also have the disadvantage of high failure rate. For end customers, electronic ballasts have become a high-cost (relative to inductive ballasts) disposable product.
Through our research, it is found that one of the main reasons for the above problems is that some electronic ballast manufacturers do not take reliable protection measures for the abnormal state of electronic ballast due to various reasons, so that the electronic ballast will be scrapped at the end of the lamp life.
We know that the basic working principle of electronic ballast is shown in the figure below:
Fig. 1 basic working principle of electronic ballast
Under normal circumstances, after the electronic ballast is powered on, the inverter, together with inductance L, filament 1, capacitor and filament 2, forms a series resonant circuit. Within a certain period of time, high voltage is generated at both ends of the capacitor. This high voltage causes the arc discharge of the fluorescent lamp to start the fluorescent lamp, and then the resonant circuit is detuned, and the fluorescent lamp enters a stable ignition state.
In case of abnormal conditions such as lamp aging or lamp leakage, the fluorescent lamp cannot be started normally, the above circuit is always in resonant state (unless the filament is burned out or the electronic ballast is damaged), and the output current of the inverter increases continuously. Usually, this current will rise to 3-5 times of the normal current. If effective protective measures are not taken at this time, great harm will be caused. Firstly, excessive current will cause the triode or FET and other peripheral components in the inverter to burn due to overload, and even cause accidents such as smoke and burst. At the same time, the lamp pin will form a very high voltage to the ground or neutral line for a long time. For 20W, 36W, 40W and most other electronic ballasts of national standard / non-standard lamps, this voltage will often reach 1000V or higher, which is not only strictly prohibited by the national standard gb15143, but also endanger personal and property safety.
The abnormal state test of electronic rectifier in gb15143-94 "11, 14" and gb15144-94 "5.13" includes: Lamp open circuit, cathode damage, deactivation, rectification effect, etc. at the same time, it is stipulated that the electronic ballast shall not have safety fault and can work normally after the above test.
2、 Abnormal state protection scheme of electronic ballast
At present, electronic ballasts adopt many protection measures, including the following:
1. Connect glass tube fuse in series in AC input circuit. Connecting a fuse in series at this position will make some people mistakenly believe that it will play the role of overcurrent or overload protection; In fact, such protection mode generally does not provide protection under overload conditions such as filament deactivation. It often fuses after the breakdown of switching devices, and can not play a real role in abnormal state protection.
2. The protection circuit with thyristor, bipolar transistor or FET as the core is adopted on the rectifier output circuit. The greatest advantage of this electronic circuit protection mode is that the protection time is short, but at the same time, it has the following disadvantages:
1) Misprotection is easy to occur: if, for some reason, even a very short sharp pulse is formed at the trigger end of the thyristor, the inverter will stop working and the lamp will go out.
2) The design and debugging work is cumbersome: generally, this protection circuit will have at least 6 electronic components including resistance, capacitance and secondary coil of pulse transformer. The use of so many components at the same time, coupled with the discreteness and temperature drift of active devices such as thyristor, will increase the difficulty of debugging and affect the production efficiency.
3) This protection method also has the disadvantages of high cost and large occupation of PCB space, which is also a headache for many electronic ballast manufacturers.
3. A self restoring polymer PTC thermistor is connected in series next to the resonant circuit, i.e. the resonant capacitor. Figure 2 is the circuit schematic diagram of abnormal protection of electronic ballast by using polymer PTC thermistor. When the lamp tube is normal, after the electronic ballast is powered on, the resonant circuit composed of inductance, capacitance and PTC thermistor makes the fluorescent lamp start and work normally. If the lamp is deactivated due to filament aging or air leakage, the PTC thermistor will act within a few seconds to force the LC series resonant circuit to stop vibration, so as to cut off the high voltage and protect the switching devices in the inverter.
Fig. 2 circuit schematic diagram of abnormal protection of electronic ballast by polymer PTC thermistor
The advantages of this protection mode have been recognized by many electronic ballast manufacturers, but so far it has not been applied on a large scale. The main reason is that the PTC components provided in the market can not meet the particularity of the use of electronic ballasts. At present, the main problems are: 1) the action time is too long under the condition of high temperature misoperation or deactivation; 2) When PTC thermistor is in the protection state for a long time (e.g. 24 hours), it is easy to increase the resistance irreversibly and seriously degrade the performance. This is the main reason why most polymer PTC thermistors fail to be successfully applied in electronic ballasts.
In view of the above problems, Shanghai Kete polymer material Co., Ltd. has developed kt120-h series special PTC thermistor for electronic ballast, which overcomes the above defects and can well solve the problem of abnormal state protection of ballast., Technology Zone
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