1 Introduction
As an environmentally friendly power source, energy-saving lamps have been widely used in the world, and the production of domestic energy-saving lamps is even more unique. As an important component of energy-saving lamps (including electronic ballasts), the quality of high-power switching transistors plays a key role in the quality and longevity of energy-saving lamps. At present, in addition to several imported brands such as Fairchild and ST, the quality of domestic energy-saving lamps power tubes is not stable enough. In this paper, the failure mechanism of high-power switch triode in energy-saving lamp application is analyzed, and the factors affecting failure are discussed.
2 failure mode
The main reason for the damage of energy-saving lamps and short life is the failure of high-power switch transistors. Through the anatomical analysis of the failed power tube, most of the failed tubes are shorted by the emitter junction burnout. When observing the dissected failed tube with a microscope, it can be seen that there is a noticeable burnt black spot near the weld zone of the emitter (see Figure 1). This is a typical burning phenomenon.
When the triode is working, due to the current thermal effect, it will consume a certain amount of power, which is the power dissipated. The dissipated power is mainly composed of the collector dissipated power:
PT≈VceIc is PT≈PCM
We know that the operating current of a triode is greatly affected by temperature. The relationship between the forward current of the PN junction and temperature is:
Iâˆe-(Eg-qV)/kT
When the triode is working, the dissipated power is converted into heat, the collector junction temperature is increased, and the collector junction current is further increased, which may cause a vicious cycle to burn the tube. This situation is called thermal breakdown. The maximum operating temperature at which the tube does not thermally break is defined as the highest junction temperature. The maximum junction temperature of the silicon material PN junction is:
Tjm=6400/(10.45+lnÏ)
In another case, when the tube does not reach the maximum junction temperature, or does not exceed the maximum dissipated power, the operating current of the triode is caused by the defect of the material and the non-uniformity of the process, and the effect of the current in the emitter region caused by the structural cause. The distribution is uneven. When the current distribution is concentrated at a certain point, the power consumption of the point increases, causing the local temperature to increase, and the increase of the temperature in turn causes the current at the place to further increase, thereby forming a "hot spot" whose temperature exceeds the metal electrode and The eutectic point of the semiconductor causes the triode to burn out. On the other hand, local temperature rise and large current density cause local avalanche (breakdown), where a large local current can cause the tube to burn, causing the breakdown voltage to drop sharply, the current to rise, and finally the tube to burn. This situation is called secondary breakdown. The characteristic curve of the secondary breakdown of the triode is shown in Fig. 2.
Secondary breakdown is an important cause of power tube failure. In order to ensure the normal operation of the pipe, the concept of a safe working area SOA is proposed. The SOA schematic is shown in Figure 3. It consists of the collector maximum current Icm line, the breakdown voltage BVceo line, the collector maximum dissipation power Pcm line, and the second breakdown power consumption Psb line. Since the operating current and maximum voltage are not designed to exceed the rating of the tube during use, the collector dissipated power and secondary breakdown characteristics are the main factors causing the tube to fail.
3 factors affecting failure
From the above failure mechanism analysis, in order to reduce the failure, it is important to minimize the power of the pipe during operation and improve the secondary breakdown characteristics, which are actually related. It is known from the mechanism of secondary breakdown that the temperature rises, the tube HFE increases, the switching performance deteriorates, the secondary breakdown characteristics deteriorate (more secondary breakdown is more likely); the temperature rise also makes the tube The actual dissipated power parameters deteriorate and the safe working area of ​​the pipe becomes smaller. Conversely, since the dissipated power of the tube is mainly related to the thermal resistance of the tube, the power dissipation is small, in fact, the current and voltage that it can withstand is low, and the heat dissipation performance is poor, which also affects the secondary breakdown characteristics. Therefore, it is the most effective way to improve the quality of the pipe by preventing the temperature rise of the pipe during operation and increasing the power dissipation of the pipe. 1) Thermal resistance
When the tube is working, when the PN junction temperature exceeds the maximum allowable junction temperature, the power consumed by the tube is the maximum dissipated power of the collector of the tube. Since the maximum junction temperature of a certain material is constant, improving the heat dissipation performance of the tube is to increase the dissipated power of the tube. At the same time, the heat dissipation performance is good, the temperature rise of the tube is low, and the possibility of secondary breakdown is also reduced. This is an important factor in improving the secondary breakdown characteristics.
Thermal resistance is an important parameter of high-power tubes and represents the heat dissipation capability of the tubes. The relationship between thermal resistance and dissipated power is:
Pcm=(Tjm-Ta)/RT
Where Tjm is the highest junction temperature, Ta is the ambient temperature, and RT is the thermal resistance. It can be seen that when the maximum junction temperature and the ambient temperature are constant, the amount of power dissipated depends on the magnitude of the thermal resistance.
In energy-saving lamps, tubes with the lowest possible thermal resistance should be used. In addition to the chip itself, the materials, processes, and qualities of the post-assembly assembly have a significant impact on the thermal resistance. The test and screening of the thermal resistance of the tube is the basic requirement for ensuring the quality of the energy-saving lamp power tube.
2) Switching parameters
When the typical energy-saving lamp line works, the two pipes work in a saturated and cut-off state in turn, so the switching parameters of the pipe have a significant impact on its operation. There are four switching parameters of the tube: delay time td, rise time tr, storage time ts and fall time tf. As shown in the waveform diagram of the three-stage tube switch shown in Figure 4, the transition time is affected by the delay time and the rise time when the tube is cut off to saturation. From saturation to cut-off, the transition time is affected by the storage time and the fall time. The power consumed by the pipe during different operating conditions is:
Deadline: P=Vce·Icex
When saturated: P=Vces·Ic
Since the reverse leakage current Icex and the saturation pressure drop Vces of the tertiary tube are both low, the power consumption of the tube is not large at the time of saturation and cutoff, but in the conversion process of the two states, the tube has a part of time to work. In the amplification area, the current and voltage are large at this time, and the longer the time is in the amplification area, the greater the power consumption, and the more the temperature rises.
It can be seen from the waveform diagram that the switching parameters affecting the tube in the amplification region are mainly rise time and fall time. Therefore, the tube with the shortest rise and fall times should be used as short as possible.
On the other hand, since the two tubes of the energy saving lamp operate in a saturated and cut-off state in turn, the relationship between the switching parameters is also important. Except for the delay time, if the sum of the storage time and the fall time is much larger than the rise time, the chances that the two tubes are simultaneously turned on are large, which may also result in undesirable results. Again, the consistency of the switching parameters of the two tubes is also very important. Because the switching time of the triode is the longest, the storage time ts is the longest. Pipes with short storage times should be used as much as possible, and storage time consistency should be as good as possible.
3) High temperature leakage current
In the above description, we know that the power consumption of the tube when it is in the off state is mainly determined by the reverse leakage current Icex. At normal temperature, Icex is generally small, so the cut-off power of the tube is not large, but when the temperature rises after the work, the Icex becomes larger, and its power consumption also becomes larger until it affects normal work. On the other hand, an increase in the reverse leakage current makes the breakdown characteristic of the PN junction soft, which also makes the tube easy to burn. Therefore, high temperature leakage current is also an important parameter affecting the quality of the pipe.
The ce reverse leakage of the silicon triode is:
Iceo=(1+)Icbo≈(1+)Ae×Ni×XMG/2τ
Its temperature change is mainly related to materials and processes. In the test of the pipe, the leakage current change ΔIceo at different temperatures (high temperature and normal temperature) is often selected as an index, and the ΔIceo is required to be as small as possible.
4) Other
Other parameters of the power switch transistor are also related to its use. hFE is also one of the factors that are often considered. Its influence on the quality of the pipe is also reflected in the influence on the switching time, and its importance is relatively large compared to the influence of the switching parameters. In addition, Icm and BVceo are also frequently considered factors.
The above analysis of the failure mechanism of the energy-saving lamp high-power switch tube, and the resulting parameter selection requirements, after a large number of group tests, proved to be consistent with the actual situation. Huasheng Electronic Devices Co., Ltd. also took corresponding measures in the production of related products, and achieved good results in practical applications, meeting the requirements of domestic customers, and quickly accepted by the market.
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