Energy-saving lamp power tube failure mechanism analysis - Power Circuit - Circuit Diagram

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1 Introduction

As an environmentally friendly power source, energy-saving lamps have gained widespread popularity globally, with domestic production being particularly notable. As a critical component of energy-saving lamps (including electronic ballasts), the quality of high-power switching transistors plays a pivotal role in determining the overall quality and lifespan of these lamps. While global brands like Fairchild and ST offer reliable options, the quality of domestically produced energy-saving lamp power transistors remains inconsistent. In this article, we analyze the failure mechanisms of high-power switching transistors in energy-saving lamps and discuss the factors influencing their failures.

2 Failure Modes

The primary cause of energy-saving lamp damage and shortened lifespan is the failure of high-power switching transistors. Upon analyzing failed power transistors, most show signs of emitter junction burnout, leading to short circuits. Under microscopic observation, a noticeable burnt black spot can be seen near the weld zone of the emitter (refer to Figure 1). This is a classic example of junction burnout.

When a transistor is operational, it consumes a certain amount of power due to the current thermal effect, referred to as the dissipated power. The dissipated power primarily consists of the collector dissipated power:

PT ≈ Vce * Ic ≈ PCM

We understand that the operating current of a transistor is significantly influenced by temperature. The relationship between the forward current of the PN junction and temperature is given by:

I ∝ e^(-(Eg - qV)/kT)

During operation, the dissipated power is converted into heat, raising the collector junction temperature. This increase in temperature further elevates the collector junction current, potentially creating a vicious cycle that burns out the transistor. This scenario is termed thermal breakdown. The highest junction temperature at which the transistor does not thermally break down is defined as the maximum junction temperature. For silicon material PN junctions, the maximum junction temperature is:

Tjm = 6400 / (10.45 + lnρ)

In another instance, even before reaching the maximum junction temperature or exceeding the maximum dissipated power, defects in the material or non-uniform processes can lead to uneven current distribution in the emitter region due to structural reasons. When the current becomes concentrated at specific points, the localized power consumption increases, causing the temperature to rise. This rise in temperature further amplifies the current at those points, creating a "hot spot" where the temperature exceeds the melting point of the metal electrode and semiconductor, ultimately causing the transistor to burn out. Additionally, the localized temperature rise and high current density can trigger localized avalanche (breakdown), where a large current can burn out the device, causing a sharp drop in breakdown voltage, an increase in current, and eventually the destruction of the transistor. This situation is known as secondary breakdown. The characteristic curve of the secondary breakdown of a transistor is illustrated in Figure 2.

Secondary breakdown is a major contributor to power transistor failure. To ensure proper operation of the device, the concept of a Safe Operating Area (SOA) has been introduced. The SOA schematic is depicted in Figure 3. It comprises 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. Since the operating current and maximum voltage are not intended to exceed the rated values during use, the collector dissipated power and secondary breakdown characteristics are the primary factors contributing to transistor failure.

3 Factors Affecting Failure

From the above failure mechanism analysis, minimizing the power consumption of the transistor during operation and enhancing its secondary breakdown characteristics are crucial. These aspects are interrelated. Based on the mechanism of secondary breakdown, temperature rise leads to an increase in transistor HFE, deteriorating switching performance and secondary breakdown characteristics (making secondary breakdown more likely). The temperature rise also degrades the actual dissipated power parameters of the transistor, shrinking its safe operating area. Conversely, since the dissipated power of the transistor is mainly related to its thermal resistance, lower thermal resistance implies lower current and voltage endurance, as well as poorer heat dissipation performance, which also impacts secondary breakdown characteristics. Thus, the most effective method to improve transistor quality is to prevent temperature rise during operation and enhance the power dissipation capacity of the transistor.

1) Thermal Resistance

When a transistor is operational, if the PN junction temperature exceeds the maximum allowable junction temperature, the power consumed by the transistor equals its maximum collector dissipated power. Given that the maximum junction temperature of a particular material is constant, improving the heat dissipation performance of the transistor effectively increases its dissipated power. Furthermore, better heat dissipation reduces the temperature rise of the transistor, thereby decreasing the likelihood of secondary breakdown. This is a vital factor in enhancing secondary breakdown characteristics.

Thermal resistance is a key parameter for high-power transistors and represents 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 can be observed that when the maximum junction temperature and ambient temperature are constant, the dissipated power depends on the magnitude of the thermal resistance.

In energy-saving lamps, transistors with the lowest possible thermal resistance should be utilized. Beyond the chip itself, the materials, processes, and post-assembly assembly quality significantly impact thermal resistance. Testing and screening the thermal resistance of transistors is a fundamental requirement for ensuring the quality of energy-saving lamp power transistors.

2) Switching Parameters

When a typical energy-saving lamp circuit operates, the two transistors alternately work in saturated and cutoff states. Therefore, the switching parameters of the transistor have a substantial impact on its operation. There are four switching parameters for a transistor: delay time (td), rise time (tr), storage time (ts), and fall time (tf). As shown in the waveform diagram of the three-stage transistor switch in Figure 4, the transition time is influenced by the delay time and rise time when transitioning from cutoff to saturation. From saturation to cutoff, the transition time is affected by the storage time and fall time. The power consumed by the transistor during different operating conditions is:

At the cutoff phase: P = Vce * Icex

At 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 minimal. However, during the transition between the two states, the transistor spends some time in the amplification region, where the current and voltage are high. The longer the time spent in the amplification region, the higher the power consumption and temperature rise.

From the waveform diagram, it is evident that the switching parameters impacting the transistor in the amplification region are primarily the rise time and fall time. Hence, transistors with the shortest possible rise and fall times should be selected.

On the other hand, since the two transistors of the energy-saving lamp alternately operate in saturated and cutoff states, the relationship between the switching parameters is also significant. Besides the delay time, if the sum of the storage time and fall time is much greater than the rise time, the likelihood of both transistors being simultaneously on increases, potentially leading to undesired outcomes. Additionally, the consistency of the switching parameters of the two transistors is crucial. Given that the switching time of the transistor is the longest, the storage time (ts) is the longest. Transistors with shorter storage times should be prioritized, and storage time consistency should be as good as possible.

3) High-Temperature Leakage Current

As mentioned 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 usually small, so the cutoff power of the transistor is not significant. However, when the temperature rises after 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 an important parameter affecting transistor quality.

The collector-emitter reverse leakage current of a silicon transistor is:

Iceo = (1 + κ) * Icbo ≈ (1 + κ) * Ae * Ni * XMG / 2τ

Its temperature change is mainly related to materials and processes. In transistor testing, the change in leakage current (ΔIceo) at different temperatures (high and normal) is often chosen as an indicator, with ΔIceo required to be as small as possible.

4) Other

Other parameters of the power switching transistor are also relevant to its use. hFE is also one of the factors that are frequently considered. Its impact on the quality of the transistor is also reflected in its influence on the switching time, and its importance is relatively significant compared to the influence of switching parameters. Additionally, Icm and BVceo are also frequently considered factors.

The above analysis of the failure mechanism of energy-saving lamp high-power switching transistors, along with the resulting parameter selection requirements, was proven to align 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.