脉宽调制电压对环氧树脂电树引发及生长特性影响研究

Impact of Pulse Width Modulation Voltage on Electrical Tree Characteristics in Epoxy Resin

工作在脉宽调制电压下的电力电子器件承受高频脉冲电应力,其绝缘封装可靠性已成为制约电力电子器件向高频、高功率密度发展的重要因素。作为绝缘封装的重要材料,环氧树脂在脉宽调制电压下的绝缘性能值得关注。对此,在峰峰值12 k V的脉宽调制电压下,试验研究了脉冲上升时间、载波频率和热效应对环氧树脂电树引发和生长特性的影响规律。在温度20℃、上升时间70~150ns、载波频率0.5~2 k Hz条件下的大量电树引发和生长特性表明:一定电压幅值下环氧树脂电树引发概率受电压上升时间影响显著,上升时间越短,电树引发概率越高;上升时间减小可能引起电极注入载流子能量增加,从而提高了电树引发概率。电树引发后生长特性主要受载波频率影响,载波频率增大,单位时间内载流子复合次数可能增加,从而加速了电树生长。研究同时发现:上升时间对电树形态影响显著,较小上升时间下电树呈丛林状,随着上升时间增大逐渐演化为树枝状。在上升时间110 ns、载波频率1 k Hz、温度20~50℃的结果表明:温度升高既会增加单位时间内载流子入陷或复合释放的能量;也会降低聚合物局部链段松弛性,致使材料破坏应力阈值下降,从而影响电树引发、生长和形态。与相同频率和峰峰值的正弦电压相比,脉宽调制电压下电树引发概率和生长速度明显增大。表明相较于传统正弦电压,高频脉宽调制电压更易引发、促进绝缘电树老化,必须根据脉宽调制频率、上升时间和热效应加强绝缘或优化绝缘设计,以提升电力电子系统绝缘可靠性。

The electronic devices for power supply working in pulse width modulation(PWM) voltage bear high frequency pulse electric stress, so the reliability of their insulation package has become the major factor restricting the development to high frequency and power density. As an important material of insulation package, the insulation performance of epoxy resin under PWM voltage is of great attention. Consequently, the influences of pulse rise time, carrier frequency, and thermal effect on the initiation and growth characteristics of epoxy resin tree are studied under the pulse width modulation voltage of 12 k V peak-to-peak value. A large number of electrical tree initiation and growth characteristics at a temperature of 20 ℃, a rise time of 70~150 ns, and a carrier frequency of 0.5~2 k Hz show that the initiation probability of epoxy resin electrical tree under a certain voltage amplitude is significantly affected by the voltage rise time.The shorter the rise time is, the higher the probability of initiation will be. The decrease of rise time may increase the carrier energy which is injected into the electrode and thus may increase the initiation probability of electrical tree. The growth characteristics after electrical tree initiation are mainly affected by the carrier frequency, the higher frequency may lead to more recombination times per unit time, accelerating the engendering of electric trees. The study also reveals that the rise time plays a significant role in the shape of the tree. When the rise time is small, the tree is like a jungle, and gradually evolves into a tree branch with the increase of the rise time. The results obtained at PWM voltages with 110 ns rise time, 1 k Hz carrier frequency and 20~50 ℃ temperature show that the increase of temperature will increase the energy released by carrier sink or recombination in unit time, and reduce the relaxation of polymer local chain segment,resulting in the decrease of material failure stress threshold and considerable influences on the initiation, growth and morphology of electrical tree. Compared with the sinusoidal voltage with the same frequency and peak value, the initiation probability and growth speed of the tree will increase obviously under the PWM voltage, which indicates that high frequency PWM voltage is more likely to occur and promote the tree aging in the insulation. Therefore, it is necessary to strengthen the insulation and optimize the design according to the PWM frequency, rise time and thermal effect so as to enhance the insulation reliability of the power electronic system.