I used to think that high voltage applications were only used in power engineering. Since I had no desire to work at a power plant or transformer station, I was safe from having to learn high voltage PCB design. Alas, my interest in space applications proved me wrong and forced me to confront my laziness. It turns out high voltage applications show up in almost every industry, from manufacturing and power plants to medical and aerospace.
Designing a PCB for high voltage applications requires consideration of a host of details at every level of your design and fabrication. The board will be subject to harsh operating conditions and will be much more sensitive to the lifetime of the parts and materials. If you’re up for the challenge, then there are a few design considerations before you start your board layout.
Consider your operating frequency
The operating frequency of your product affects high voltage design as much as ESD and noise management affect the board. This is because high frequencies will arc at a lower voltage, requiring more stringent spacing around those signal lines.
At the other end of the frequency spectrum, low voltage DC also needs special consideration. In certain environmental conditions, a DC differential can cause etching and electrochemical migration. While neither is desirable, electrochemical migration is the greater risk to performance and lifetime in high voltage designs. This is because conductor pads or traces will “grow” delicate, conductive filaments, sometimes called whiskers, that can eventually create a short between potentials. At a minimum, they will create points that are more likely to arc, and decrease the effective creepage and clearance distances on your board.
Electrochemical migration is most common with tin and silver, but even copper will occasionally produce those little filaments of destruction. To minimize your risk, don’t use pure tin or silver as a finish on your PCBs. If you’re using tin, even a low lead content is recommended, as it will dramatically increase the difficulty of “growing” the conductive filaments.
Many metals can grow whiskers. Tin whiskers tend to have a more 'fractalline' look.
Derate your components
When you are designing for high-stress environments, component tolerances will get “derated.” That means that you reduce the functional maximum value of the current, voltage, or temperature for a component or material to improve its lifetime in your product. Often, it’s calculated simply by taking a percentage of the manufacturer rating. The percentage is usually specified by a MIL-STD, or other specification determined by your customers and the operating environment for your product.
In many cases, materials are derated to the average parameter value they are expected to experience, which makes for less stringent requirements and cheaper production. However, the risk in high voltage designs comes from over-voltage events that cause arcing or corona on the board. You should derate to the maximum voltage, rather than the average, to improve the survivability of your product during an over-voltage event.
Select your components
After derating, you may find some of the initial components you’ve selected aren’t viable for your product’s operating environment. Even if they all pass, you should review each part again. The high voltage environment causes high variability in the electric field across your PCB, but it also causes field stresses within individual components. To ensure the reliability of each component, Sierra Proto Express recommends that you have a minimum margin of 1.5:1, and possibly as high as 2:1.
In some cases, the variability in voltage will not only damage the components but also cause them to become arc points for a discharge across the board. Component packaging should be considered carefully, since the cases and edges may have a sharp enough angle to concentrate the electric field. The same is true of any fasteners, clips, and connectors. Everything should have the maximum possible radius to distribute the electric field and avoid creating sites for over arc on your board.
Casing and solder joints can concentrate the electric field on your PCB and increase the risk of over arc.
Designing your PCB for high voltage applications is a particularly challenging task. However, the applications are so prevalent that it’s hard to avoid ever working on a high voltage design. Fortunately, there is great PCB design software, like Altium Designer, that can help you manage design rules and get your board just right.
Have a question about high voltage designs? Contact an expert at Altium.
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