High voltage design and manufacturing comes with an additional set of disciplines that must be applied in order to produce products with excellent long term reliability, often in harsh environments. As high voltage power supplies become increasingly compact and miniaturized, mastering these disciplines is essential to ensure long term and trouble-free operation in the field. Failure to properly apply these design and manufacturing principles can result in real-world MTBF well below design expectations. In school, we were taught that materials were either conductors or insulators. Air was said to be an insulator. However, lightning proves that air is not always an insulator. Welcome to the world of high voltage!
High voltage is like a caged animal; it never stops trying to escape. Taming
and controlling it is the job of the high voltage engineer and/or physicist. A wide selection of insulating systems and materials are available today, but there are many factors that can cause these systems to break down and fail at voltages far below expectations. Once they break down, the result is nearly always catastrophic.
Early in my career, I started troubleshooting and repairing avionic high voltage CRT power supplies for first generation GPS systems and other demanding applications. In those early years I learned a lot about failure mechanisms that occur long after the typical warranty period had passed. Designing and building a high voltage power supply that can last through the warranty period is one thing, getting them to survive for years and years of continuous operation is another, especially when in harsh environments.
This article will highlight some of the more important considerations relating to designing a high voltage power supply that is reliable over the long term.
One of the first aspects of achieving reliability is taking account of the thermal cycling that can occur in a power supply. In addition to the effects of temperature changes, thought needs to be given to mismatched or incompatible materials with different thermal expansion coefficients and the mechanical stresses that can lead to insulation cracking over time. Other factors such as poor adhesion, age induced brittleness due to loss of plasticizers, excessive temperature swings, exposure to UV radiation, corona, ozone, mineral oil and harsh PWB cleaners/solvents are other factors that can lead to pre-mature failure.
The combination of material properties, environmental factors and product design can create unplanned side effects. For example, leakage currents can increase over time with the potential to eventually result in a hard-arc and catastrophic failure. Excessive leakage currents may create errors in high impedance feedback circuits resulting in voltage drift and stability issues over time and with changes in temperature. FR4 PWB substrates can be particularly vulnerable to contamination and absorbed moisture. Absorbed moisture lowers the glass-transition temperature (Tg) of FR4, making the assembly susceptible to field failures in applications with dynamic thermal conditions. Impurities, incorrect fillers or incomplete cure in encapsulation systems can cause excessively high leakage currents that are non-linear and erratic over time and temperature, potentially destabilizing the high voltage system. Another example is that high voltage circuits are particularly vulnerable to electrochemical migration. Moisture can facilitate ionic corrosion forming conductive filaments. Dendrite growth may occur from the redistributed metal ions. High voltage stresses accelerate these electro-chemical processes (though tin whiskers can form without the presence of an electromagnetic field). Crystalline microstructures created by ionic migration create very high voltage gradients and electric field intensities, which may lead to premature breakdown between voltage nodes. Proper design and manufacturing controls are critical and typically need to exceed documented industry standards.
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