Energy-saving lamp power tube failure mechanism analysis - Power Circuit - Circuit Diagram
Introduction:
As an eco-friendly power source, energy-saving lamps have gained widespread adoption worldwide, with China's production standing out uniquely. As a critical component of energy-saving lamps (including electronic ballasts), the quality of high-power switching transistors plays a pivotal role in the lifespan and performance of these lamps. While renowned international brands like Fairchild and ST continue to dominate the market, the domestic production of energy-saving lamp power transistors remains inconsistent in terms of quality. This paper analyzes the failure mechanisms of high-power switching transistors in energy-saving lamps and discusses the factors influencing their reliability.
Failure Mode:
The primary cause of energy-saving lamp failure and shortened lifespan is the malfunction of high-power switching transistors. Upon examining failed power transistors under a microscope, it becomes evident that most failures result from the short-circuiting of the emitter junction due to burnout. A distinct blackened spot near the emitter weld zone is visible under microscopic observation, indicative of a typical burning phenomenon.
When a transistor operates, it dissipates a certain amount of power due to the current thermal effect. This dissipated power primarily comprises the collector dissipated power:
PT ≈ Vce × Ic ≈ PCM
We know that the operating current of a transistor is heavily influenced by temperature. The relationship between the forward current of the PN junction and temperature is:
I ∠e^(-(Eg-qV)/kT)
During operation, the dissipated power converts into heat, raising the collector junction temperature, which in turn increases the collector junction current. This vicious cycle could lead to thermal breakdown. The highest junction temperature at which the transistor does not thermally break down is termed the maximum junction temperature. For silicon material PN junctions, the maximum junction temperature is:
Tjm = 6400 / (10.45 + lnÏ)
In another scenario, when the transistor does not reach the maximum junction temperature or exceed the maximum dissipated power, the operating current is affected by material defects and process non-uniformities. Structural issues can lead to uneven current distribution in the emitter region. If the current concentrates at a specific point, the power consumption at that point increases, causing localized temperature spikes. These temperature increases further amplify the current at those locations, creating a “hot spot†where the temperature surpasses the metal electrode and semiconductor eutectic point, ultimately leading to transistor burnout. Additionally, localized temperature increases and high current densities can trigger localized avalanches (breakdowns), where large currents can burn out the transistor, drastically reducing the breakdown voltage and causing current surges, eventually leading to transistor burnout. This situation is referred to as secondary breakdown. The characteristic curve of secondary breakdown in transistors is illustrated in Figure 2.
Secondary breakdown is a critical cause of power transistor failure. To ensure the proper operation of the transistor, the concept of a Safe Operating Area (SOA) is introduced. The SOA schematic is depicted in Figure 3, consisting of the collector maximum current (Icm) line, the breakdown voltage (BVceo) line, the collector maximum dissipation power (Pcm) line, and the secondary breakdown power consumption (Psb) line. Given that the operating current and maximum voltage are not intended to exceed the transistor’s ratings during usage, the collector dissipated power and secondary breakdown characteristics are the primary factors contributing to transistor failure.
Factors Influencing Failure:
From the aforementioned failure mechanism analysis, minimizing the power dissipation during transistor operation and improving secondary breakdown characteristics are crucial. These are, in fact, interrelated. It is understood from the secondary breakdown mechanism that rising temperatures increase the transistor's HFE, degrade switching performance, and worsen secondary breakdown characteristics (making secondary breakdown more likely). Temperature increases also deteriorate the transistor's actual dissipated power parameters, reducing its safe operating area. Conversely, since the transistor's dissipated power is mainly related to its thermal resistance, lower thermal resistance implies lower current and voltage tolerance, as well as poorer heat dissipation performance, which also impacts secondary breakdown characteristics. Thus, the most effective way to enhance transistor quality is by preventing temperature rise during operation and increasing the transistor's power dissipation capabilities.
1) Thermal Resistance:
When a transistor operates, if the PN junction temperature exceeds the maximum allowable junction temperature, the power consumed by the transistor equals its maximum collector dissipated power. Since the maximum junction temperature of a given material is fixed, improving the heat dissipation performance of the transistor effectively increases its dissipated power. Moreover, better heat dissipation reduces the temperature rise, thereby decreasing the likelihood of secondary breakdown. This is a vital factor in enhancing secondary breakdown characteristics.
Thermal resistance is a crucial parameter for high-power transistors, representing their heat dissipation capability. 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 is clear that when the maximum junction temperature and ambient temperature are constant, the amount of dissipated power depends on the thermal resistance's magnitude.
In energy-saving lamps, transistors with the lowest possible thermal resistance should be used. Besides the chip itself, the materials, processes, and post-assembly quality significantly affect thermal resistance. Testing and screening the thermal resistance of transistors is the fundamental requirement for ensuring the quality of energy-saving lamp power transistors.
2) Switching Parameters:
During the typical operation of an energy-saving lamp circuit, the two transistors alternately work in saturated and cut-off states. Hence, the switching parameters of the transistor have a significant impact on its performance. A transistor has four switching parameters: delay time (td), rise time (tr), storage time (ts), and fall time (tf). As shown in the three-stage transistor switching waveform diagram in Figure 4, the transition time when cutting off to saturation is affected by the delay time and rise time. From saturation to cut-off, the transition time is influenced by the storage time and fall time. The power consumed by the transistor during different operational conditions is:
Cutoff: P = Vce · Icex
Saturation: P = Vces · Ic
Since the reverse leakage current (Icex) and saturation voltage drop (Vces) of the transistor are both low, the power consumption during saturation and cutoff is not significant. However, during the transition between the two states, the transistor spends some time in the amplification region, where both current and voltage are high. The longer the time spent in the amplification region, the greater the power consumption and the higher the temperature rises.
From the waveform diagram, it is evident that the switching parameters affecting the transistor in the amplification region are mainly the rise time and fall time. Therefore, transistors with the shortest possible rise and fall times should be prioritized.
On the other hand, since the two transistors in the energy-saving lamp alternately operate in saturated and cut-off states, the relationship between their switching parameters is also crucial. Apart from the delay time, if the sum of the storage time and fall time is much larger than the rise time, there is a higher chance that the two transistors might be simultaneously turned on, potentially leading to undesirable consequences. Additionally, the consistency of the switching parameters of the two transistors is very important. Since the switching time of the transistor is the longest, the storage time (ts) is the longest. Transistors with shorter storage times should be used whenever possible, and storage time consistency should be as good as possible.
3) High-Temperature Leakage Current:
As described earlier, the power consumption of the transistor in the off state is primarily determined by the reverse leakage current (Icex). At normal temperatures, Icex is generally small, so the cutoff power of the transistor is not significant. However, when the temperature rises during operation, Icex becomes larger, and its power consumption also increases until it affects normal operation. Additionally, an increase in reverse leakage current softens the breakdown characteristic of the PN junction, making the transistor prone to burnout. Therefore, high-temperature leakage current is also a crucial parameter affecting transistor quality.
The collector-emitter reverse leakage current of a silicon transistor is:
Iceo = (1 + β) Icbo ≈ (1 + β) Ae × Ni × XMG / 2τ
Its temperature changes are primarily related to materials and processes. In transistor testing, the change in leakage current (ΔIceo) at different temperatures (high and normal) is often selected as an indicator, and ΔIceo should be minimized.
4) Other Factors:
Other parameters of the power switch transistor are also relevant to its use. HFE is another factor often considered. Its impact on transistor quality is also reflected in its influence on switching time, and its importance is relatively large compared to the impact of switching parameters. Additionally, Icm and BVceo are frequently considered factors.
The above analysis of the failure mechanisms of high-power switching transistors in energy-saving lamps, along with the resulting parameter selection requirements, was proven consistent with actual conditions through numerous group tests. Huasheng Electronic Devices Co., Ltd. also implemented corresponding measures in the production of related products, achieving excellent results in practical applications, meeting the needs of domestic customers, and quickly gaining market acceptance.
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