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For almost all electronic products and systems, high reliability should be expected. Still, in some markets, reliability cannot be compromised or a trade-off for other performance factors. In particular, mission-critical industrial and medical applications, which require almost constant use, are expected to provide reliable operation over many years. So, a fundamental design requirement is choosing underlying subsystems and modules that can deliver on this expectation.

In terms of overall reliability, power supplies are especially important. A failure of these critical assemblies means a complete system failure and downtime that, at best, can impact cost and, at worst, could literally be the difference between life and death. 

A wide range of high-voltage DC-DC power supplies are available including products that are optimized for industrial and medical applications.

In this article, we will look at some key factors to help ensure optimized reliability when choosing and designing-in precision DC-DC high-voltage power supplies.

High-voltage power solutions
In today’s competitive markets, reliability is vital. However, the cost of unexpected failure can be exceptionally high in specific mission-critical applications. Many applications that require high-voltage power fall into the “mission-critical” category. In the world of semiconductor manufacturers, for example, failure of a high-voltage power system used for plasma generation would be expensive in terms of system downtime and the need to scrap work in progress. The same is true in other industrial applications ranging from factory automation to glass manufacture. In the medical field, where high-voltage supplies have become a fundamental element of equipment used for monitoring, diagnosis, and treatment, the consequences of failure can range from misdiagnosis to serious patient harm or even death.

With the wide range of potential applications for high-voltage power supplies, it is no surprise that many types are available. These include everything from large multi-kilowatt rack-mount designs to lower power solutions with output voltages in the tens of kilovolts. In addition, there are smaller, modular supplies as well as PCB-mount DC-DC converters for delivering high voltage to the point of use.

Selecting high-voltage power supplies
While specifications are essential in selecting an appropriate solution for a given application, it is only part of the story. Equally important when considering reliability is an understanding of the design process behind the power supply and the margins employed during that process. Six of the most important areas to consider are:

Ideal operating range
Ensuring a power supply is used within its ideal operating range is important to maintain long life and reliability. Some suppliers will place restrictive caveats on these operational windows, while others design their solutions to operate indefinitely at their maximum voltage and current ratings. Achieving this requires careful planning of design margins. 

For example, critical components in the high-voltage section (such as capacitors, diodes, and magnetics) must be selected with ample voltage margin. Also, the geometry of traces on circuit boards must be able to carry more than the required current levels while keeping metal content to a minimum. Components with elevated levels of thermal dissipation need to be kept far away from temperature-sensitive components to reduce internal heating effects, which can affect temperature coefficient, stability, and overall product reliability.

Encapsulation & thermal considerations
Modern, compact, high-voltage DC-DC power supplies use some form of encapsulation to increase power supply reliability. Simply relying upon the physical spacing of high-voltage components in an open-air design drives up the size of the product to unmanageable scale. Over time, creepage paths will become compromised due to the inevitable ingress of dust, contaminants, and moisture. 

Therefore, selecting the right encapsulant is critical — and challenging. Suitable materials exhibit extremely high dielectric strength, but physical properties are also essential. Designers must choose materials with the highest combination of thermal conductivity and dielectric strength for their products. This ensures that these products will last through extended voltage breakdown resistance and long-term resistance to thermal degradation.

Power dissipation
The power dissipation of a typical high-voltage DC-DC supply is closely related to its power density. A supply designed to make it as compact as possible, using the best creepage path and encapsulation design principles, will have a small volume, which can be desirable in space-constrained applications. However, this can complicate thermal dissipation because there is less available surface area from which to extract heat. 

Additionally, care must be taken to ensure that components that dissipate relatively elevated heat levels (such as MOSFETs and driver amps) are physically located as close as possible to thermally conductive planes to allow heat to escape. This must be considered during the initial design phase of all products, including volume, footprint, and location of thermally active components. Achieving the ideal balance of size and thermal characteristics gives the user the smallest volume possible while ensuring long-term reliability.

Electrical connections
Using connectors and conductors that are properly sized for the applied voltage and the current level they must carry will also increase product lifespan and reliability. For low-voltage conductors, a comfortable margin is applied during the selection process to keep contact resistance to a minimum. This translates into less heat dissipation, which delivers higher reliability.

Conversely, if superior design principles are not applied, high-voltage output cables and connectors will be subjected to punishing attacks from ionization. The effects of ionization can become a serious reliability factor within the power supply. The most common contributors are sharp points and edges of trimmed component leads and poorly designed printed circuit board (PCB) circuit traces, which create high-voltage gradients. Over time, these gradients will break down the dielectric properties of even the best encapsulation compounds, cause “tracking” (PCB surface arcing), and lead to irreversible failure of the power supply. Techniques such as “ball soldering” can be applied to prevent these potential failures during the construction of the high-voltage sections. Unique to the high-voltage power supply industry, ball soldering increases reliability by eliminating sharp points and narrow edges on the PCB.

Robust product development based on time-proven construction techniques and the careful selection of high-voltage cables and interconnections will ensure long-term reliability by eliminating ionization-induced degradation.

Arc & short circuit protection
It is important to consider that arcing may occur outside the power supply – an unpleasant facet in nearly all high-voltage applications. With that in mind, designing a power supply to withstand arc occurrences is integral to the design cycle. This often means strategically placing arc-clamping components at all key connections, including but not limited to the DC input, I/O, and high-voltage return paths. After installation, these safeguards must be exhaustively evaluated by simulating arcing conditions at all expected intensity and frequency levels. This makes for a robust, reliable package that can be deployed in even the most demanding applications.

Design verification
After the initial design phases, products need thorough design verification testing. This step verifies that all the above design considerations will contribute to optimized reliability in the field. Once the power supply has passed this development phase, it is considered ready for manufacture.

Enhanced system reliability through power supply monitoring
Choosing a well-designed and optimized power supply helps mitigate potential reliability issues in the field. In addition, choosing the right power supplies can contribute to overall system reliability, as power supplies can help monitor overall system health and predict and prevent failures.

Abrupt failures rarely happen without any degradation or warning, provided you know where to look or listen. For example, a worn bearing in a fan or other mechanism will produce louder audible noise and may have more resistance to turning, which will increase motor current. A failing capacitor may lead to more ripple and noise on the supply line. 

Monitoring the power supply can detect these types of fault trends before failure or out-of-specification operation happens. One of the most significant advances in power supply intelligence is the Power Management Bus — or PMBus. 

This standard provides a means of controlling and monitoring a power supply through digital communications. Electrically compatible with an Inter-Integrated Circuit, or I2Cv(while offering more features and overcoming noise-related and other issues sometimes associated with the latter), PMBus, in conjunction with suitable software, allows for monitoring of the telltale signs of change that often occur before a failure. As a result, preventative maintenance can be implemented to minimize or eliminate potential system downtime.

Summary
High-voltage power is essential to many mission-critical systems where the consequences of failure are high. Selecting a suitable unit with sufficient margin for the application is a good starting point to reduce the risk of failure. Furthermore, understanding the design process in critical areas can also ensure longevity. In addition, intelligent power supplies incorporating technologies such as PMBus can support monitoring that provides insight into key aspects of the wider system, providing advance warning of degradation / impending failure and allowing for low-cost preventative maintenance.

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