Failure mechanism analysis of the power tube of th

Failure mechanism analysis of energy-saving lamp power tube

as an environmental friendly power supply, energy-saving lamp has been widely used all over the world, and the production of domestic energy-saving lamp is even more outstanding. As an important component of energy-saving lamp (including electronic ballast), the quality of high-power switching triode plays a key role in the quality and life of energy-saving lamp. At present, the quality of domestic energy-saving lamp power tubes is not stable except for several imported brands such as Xiantong and St. This paper analyzes the failure mechanism of high-power switching triode in the application of energy-saving lamps, and discusses the factors affecting the failure

2 failure mode

the main reason for the damage and short service life of energy-saving lamps is the failure of high-power switching triodes. Through the anatomical analysis of the failed power tubes, the vast majority of the failed tubes belong to the burning and short circuit of the transmitting junction. When observing the dissected failed tubes with a microscope, it can be seen that there are obvious burned and blackened spots near the welding position in the emission area (see Figure 1 for the extensive business activities in this area long before). This is a typical burning phenomenon

when the triode works, due to the thermal effect of current, it will consume a certain amount of power, which is dissipative power. Dissipative power is mainly composed of collector dissipative power:

Pt vceic, namely Pt PCM

we know that the working current of triode is greatly affected by temperature. The relationship between the forward current of PN junction and temperature is:

I E - (eg qv)/kt

when the triode works, the dissipated power is converted into heat, which increases the collector junction temperature and the collector junction current, which will cause a vicious cycle and burn the tube. This condition is called thermal breakdown. The maximum working temperature to prevent thermal breakdown of the tube is defined as the maximum junction temperature. The maximum junction temperature of silicon PN junction is:

tjm=6400/(10.45 + LN)

in another case, when the tube does not reach the maximum junction temperature, or does not exceed the maximum dissipation power, due to material defects and process heterogeneity, as well as the current tightening effect in the emission region caused by structural reasons, the working current distribution of the triode is uneven. When the current distribution is concentrated at a certain point, the power consumption at that point increases, causing the local temperature to increase, which in turn makes the current at that point further increase, thus forming an over hot spot. If its temperature exceeds the co melting point of metal electrode and semiconductor, it will cause the triode to burn. On the other hand, local temperature rise and high current density will cause local Avalanche (breakdown). At this time, local high current can make the tube burn through, sharply reduce the breakdown voltage, increase the current, and finally cause the tube to burn out. This situation is called secondary breakdown. The characteristic curve of triode secondary breakdown is shown in Figure 2 [1]

secondary breakdown is an important cause of power tube failure. In order to ensure the normal operation of the pipe, the concept of safe working area SOA is proposed. The schematic diagram of SOA is shown in Figure 3. It is composed of collector maximum current ICM line, breakdown voltage BVCEO line, collector maximum dissipation power PCM line and secondary breakdown power PSB line. Since the design of working current and maximum voltage during use will not exceed the rated value of the tube, under normal circumstances, the collector dissipation power and secondary breakdown characteristics are the main factors causing the failure and burning of the tube

3 factors affecting failure

from the above failure mechanism analysis, it is important to reduce the power of the tube during operation and improve the secondary breakdown characteristics in order to reduce failure, which are actually related. It can be seen from the occurrence mechanism of secondary breakdown that the rise of temperature leads to the increase of HFE of the tube, the deterioration of switching performance and the deterioration of secondary breakdown characteristics (more prone to secondary breakdown); The increase of temperature also makes the actual dissipated power parameters of the pipe worse, and the safe working area of the pipe smaller. On the contrary, because the dissipative power of the tube is mainly related to the thermal resistance of the tube, the small dissipative power, in fact, the current and voltage it can withstand is low, and the heat dissipation performance is poor, which also affects the secondary breakdown characteristics. Therefore, the most effective way to improve the quality of the pipe is to prevent the pipe temperature from rising too high and improve the dissipative power of the pipe. 1) When the PN junction temperature exceeds the allowable maximum 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 certain, improving the heat dissipation performance of the tube is to improve the dissipative power of the tube. At the same time, with good heat dissipation performance, the temperature rise of the tube is low, which also reduces the possibility of secondary breakdown, which is an important factor to improve the secondary breakdown characteristics

thermal resistance, as an important parameter of high-power tube, represents the heat dissipation capacity of the tube. 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 ambient temperature are constant, the dissipated power depends on the thermal resistance

in energy-saving lamp products, pipes with the lowest thermal resistance should be selected. In addition to the chip itself, the material, process and quality of post process assembly have a great impact on the thermal resistance. Testing and screening the thermal resistance of tubes is the basis for ensuring the quality of energy-saving lamp power tubes. These building blocks must be combined with billions of Lego building blocks currently in use worldwide

2) switching parameters

when a typical energy-saving lamp line works, the two tubes work in saturation and cut-off states in turn, so the switching parameters of the tubes have a significant impact on their working conditions. 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 switching waveform diagram of the three-stage transistor in Figure 4, the transition time of the tube from cut-off to saturation is affected by the delay time and rise time, and from saturation to cut-off, the transition time is affected by the storage time and fall time. The power consumed by the tube in different working states is:

cut-off: p=vce ICEX

saturation: p=vces IC

because the reverse leakage current ICEX and saturation voltage drop Vces of the three-stage tube are very low, the power consumed by the tube is not large at saturation and cut-off, but in the conversion process of the two states, the tube works in the amplification area for a part of the time. At this time, the current and voltage are larger, and the longer it is in the amplification area, Thus, it is easy to cause cracking and falling off of the facade by consuming work, and the higher the temperature is

it can be seen from the waveform diagram that the switching parameters that affect the tube in the amplification area are mainly the rise time and fall time. Therefore, pipes with the shortest rise time and fall time should be selected

on the other hand, because the two tubes of the energy-saving lamp work in saturation and cut-off states in turn, the relationship between switching parameters is also very important. In addition to the delay time, if the sum of storage time and falling time is too large than the rising time, there is a great chance that the two tubes will be in the conduction state at the same time, which will also lead to adverse results. Similarly, the consistency of the switching parameters of the two tubes is also very important. Because of the switching time of triode, the storage time Ts is the longest, so its influence is also the largest. Pipes with short storage time shall be selected as far as possible, and the consistency of storage time shall be as good as possible

3) high temperature leakage current

in the above description, we know that the power consumption of the pipe when working in the cut-off state is mainly determined by the reverse leakage current ICEX, because most of the spring yellow is determined by the key components on mechanical equipment and cars. Under normal temperature, ICEX is generally small, so the cut-off power of the pipe is not large, but when the temperature rises after operation, ICEX becomes larger, and its power consumption also becomes larger, until normal operation is affected. On the other hand, the increase of reverse leakage current softens the breakdown characteristics of PN junction and makes the tube easy to burn. Therefore, high temperature leakage current is also an important parameter affecting the quality of tubes

iceo= (1 ＋) Icbo (1 ＋) AE Ni xmg/2

its change with temperature is mainly related to material and process. In the test of pipes, the change of leakage current △ ICEO at different temperatures (high temperature and normal temperature) is often used as an index to select, and △ ICEO is required to be as small as possible

4) other parameters of other

power switch triodes are also related to their use. HFE is also one of the factors often considered. Its influence on tube quality is also reflected in the influence on switching time, which is relatively less important than the influence of switching parameters. In addition, ICM and BVCEO are also factors often considered

the failure mechanism analysis of the high-power switch tube of the energy-saving lamp and the parameter selection requirements obtained from it have been proved to be consistent with the actual situation through a large number of grouping tests. Huashan Electronic Devices Co., Ltd. also took corresponding measures in the production of related products, and achieved good results in practical application, meeting the requirements of domestic customers and quickly accepted by the market